UNITED STATES DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

Geology and Hydrocarbon Resources of 

Onshore Basins in Eastern China

by

Gregory Ulmishek* 

c U.S. Geological Survey

Open-File Report 93-4

This report is preliminary and has not been reviewed for conformity with U.S. 

Geological Survey editorial standards (or with the North American Stratigraphic 

Code). Any use of trade, product or firm names is for descriptive purposes only and 

does not imply endorsement by the U.S. Government.

1 Denver, Colorado

1992
CONTENTS

Page 

Purpose and Scope of Study..................................................................................... 1

Ordos Basin

Introduction................................................................................................................. 3

Stratigraphy.................................................................................................................. 3

Tectonics....................................................................................................................... 16

Petroleum Geology and Potential Exploration Plays.......................................... 17

Sichuan Basin

Introduction................................................................................................................. 29

Stratigraphy.................................................................................................................. 29

Tectonics....................................................................................................................... 40

Petroleum Geology and Potential Exploration Plays.......................................... 44

Songliao Basin....................................................................................................................... 47

Introduction................................................................................................................. 47

Stratigraphy.................................................................................................................. 47

Tectonics....................................................................................................................... 51

Petroleum Geology and Potential Exploration Plays.......................................... 57

North China Basin

Introduction................................................................................................................. 65

Stratigraphy.................................................................................................................. 65

Tectonics....................................................................................................................... 72

Petroleum Geology and Potential Exploration Plays.......................................... 73

Nanxiang Basin

Introduction................................................................................................................. 99

Stratigraphy.................................................................................................................. 99

Tectonics....................................................................................................................... 103

Petroleum Geology and Potential Exploration Plays.......................................... 103

Jianghan and Dongting Basins

Introduction................................................................................................................. 110

Stratigraphy.................................................................................................................. 110

Tectonics....................................................................................................................... 115

Petroleum Geology and Potential Exploration Plays.......................................... 118

Erlian (China) and Southeast Gobi (Mongolia) Basins

Introduction................................................................................................................. 122

Stratigraphy.................................................................................................................. 122

Tectonics....................................................................................................................... 129

Petroleum Geology and Potential Exploration Plays.......................................... 139

References............................................................................................................................... 142
ILLUSTRATIONS

Page 

Figure 1. Major petroleum basins of China............................................................... 2

2-19. Ordos basin

2. Location map of the Ordos basin................................................................. 4

3. Fades of the Cambrian-Ordovician section of the Ordos basin............ 5

4. Isopach map of Cambrian-Ordovician rocks ........................................... 6

5. Fades of Middle Ordovician rocks of the Ordos basin........................... 7

6-8. Isopach maps of the Ordos basin:

6. Carboniferous-Permian rocks...................................................................... 9

7. Lower Triassic rocks....................................................................................... 10

8. Middle-Upper Triassic rocks........................................................................ 11

9. Jurassic paleogeography of the Ordos basin.............................................. 13

10-11. Isopach maps of the Ordos basin:

10. Jurassic rocks.................................................................................................... 14

11. Cretaceous rocks.............................................................................................. 15

12. Paleostructural cross sections showing formation of the

foredeep in the western Ordos basin in Cretaceous time.................. 18

13. Structure of the Ordos basin on base of the Yanan-6 member

of the Jurassic Yanan Formation............................................................ 19

14. Cross section through the western thrust belt of the Ordos basin....... 20

15. Isopach map of members 9 and 10 of the Jurassic Yanan Formation.. 21 

16-18. Cross-sections through the Ordos basin:

16. Zhiluo oil-gas field......................................................................................... 22

17. Maling oil field................................................................................................ 24

18. Tiachi gas field................................................................................................. 25

19. Thermal maturity of upper Paleozoic rocks of the northern

Ordos basin.................................................................................................. 26

20-30. Sichuan basin

20. Main structural units of the Sichuan basin.............................................. 31

21. Isopach map of lower Paleozoic rocks of the Sichuan basin................. 32

22. Subcrop geologic map of the pre-Premian unconformity..................... 35

23-27. Isopach maps (in meters) of the Sichuan basin:

23. Upper Triassic rocks....................................................................................... 36

24. Lower Jurassic rocks ...................................................................................... 37

25. Middle Jurassic rocks..................................................................................... 38

26. Upper Jurassic rocks....................................................................................... 39

27. Cretaceous rocks.............................................................................................. 41

28. East-west cross section through the Sichuan basin................................. 42

29. Cross section through the thrust zone of the Longmenshan,

Sichuan basin.............................................................................................. 43

30. Oil and gas fields of the Sichuan basin...................................................... 45

11
31-44. Songliao basin

31. Columnar section of the Songliao basin showing main producing

reservoirs..................................................................................................... 48

32. Isopach map of the Denglouku Formation.............................................. 49

33. Thickness (in kilometers) of the middle Cretaceous section of

the Songliao basin...................................................................................... 50

34. Depositional conditions of the lower Qingshankou Formation......... 52

35. East-west cross section through the Songliao basin................................ 53

36. Thickness (in meters) of the Upper Cretaceous Sifangtai and

Mingshui Formations............................................................................... 54

37. Main structural units of the Songliao basin............................................. 55

38. Location of the Daqing oil field within the Central depression

of the Songliao basin................................................................................. 56

39. Prinicpal oil fields of the Songliao basin................................................... 58

40. Oil pools in different types of traps in the Putaohua reservoir............ 59

41. Fuyu-Xinli oil field......................................................................................... 60

42-44 Cross sections:

42. The Fuyu field................................................................................................. 61

43. Daqing oil field................................................................................................ 64

44. Songfangtun oil field..................................................................................... 64

45-65. North China basin

45. Main structural units of the North China basin..................................... 66

46. Restored thickness of rocks of the North China basin........................... 67

47a.Thickness of Upper Jurassic-Lower Cretaceous rocks............................. 70

47b.Thickness of Neogene rocks......................................................................... 71

48. Structural units and oil and gas fields of the Dongpu-Kaifeng

depression.................................................................................................... 72

49-51. Cross sections, North China basin:

49. Dongpu-Kaifeng depression........................................................................ 76

50. Wenliu oil-gas field....................................................................................... 77

51. Weichen oil-gas field..................................................................................... 78

52. Structural units and oil fields of the Jizhong depression...................... 80

53. Distribution of Lower Tertiary source rocks and related oil

fields in the Jizhong depression............................................................. 81

54. Types of oil and gas pools in the Langfang-Guan sag of the

Jizhong depression.................................................................................... 82

55. Cross section through the Renqiu oil field............................................... 83

56-57. Structural units and oil and gas fields:

56. Huanghua depression................................................................................... 85

57. Jiyang depression............................................................................................ 86

58. Facies of the Shahejie Formation............................................................... 88

59. Oil field of the Shengli producing complex.............................................. 89

60. Cross section through the Gudao oil field in the Jiyang depression... 90

61. Oil field in onlapping strata of the Shahejie Formation....................... 91

62. Structural units of the Bohai Gulf and drilled wells.............................. 93

in
	Page

63. Contour map and oil fields of the onshore part of the Liaohe

	depression.................................................................................................... 94

64. Cross section through the Liaohe depression.......................................... 95

65. Fades of the third member of the Shahejie Formation......................... 97

66-73. Nanxiang basin

66. Main structural units and oil fields of the Nanxiang basin.................. 100

67. Tertiary stratigraphy and facies of the Biyang depression..................... 101

68. Sedimentary facies of the third member of the Hetaoyuan

	Formation in the Biyang depression..................................................... 102

69. North-south cross section through the Biyang depression................... 104

70. Contour map on top of the third member of the Hetaoyuan

	Formation.................................................................................................... 105

71-72. Cross sections of the Biyang depression:

71. Shuanghe oil field.......................................................................................... 106

72. Xiaermen oil field........................................................................................... 106

73. Thickness of source rocks and reservoir rocks in the third

	member of the Hetaoyuan Formation.................................................. 108

74-80. Jianghan and Dongting basins:

74. Main structural units of the Jianghan and Dongting basins................. 112

75. Depositional facies of the Qianjiang Formation...................................... 113

76. Extension of evaporites of the Qianjiang Formation............................. 114

77. Cross section through the Qianjiang depression..................................... 116

78. Salt domes in the Qianjiang depression.................................................... 117

79. Oil fields of the Qianjiang depression........................................................ 119

80. Contour map and cross section of the Zhongshi field........................... 120

81-91. Erlian (China) and Southeast Gobi (Mongolia) basins

81. Late Mesozoic structure of the Erlian basin.............................................. 124

82. Late Mesozoic structure of southeastern Mongolia................................ 125

83-86. Paleogeographic maps of eastern Mongolia

83. Neocomian (Tsagantsab time)....................................................................! 128

84. Early Cretaceous, beginning of Aptian-Albian time

	(Khukhtyk time)........................................................................................ 130

85. Early Cretaceous, end of Aptian-Albian time (Barunbayan time)....... 131

86. Late Cretaceous time...................................................................................... 132

87. Cross sections through the Saihantala graben......................................... 134

88-91. Lithology and thickness of formations in the Dzunbain field area:

88. Tsagantsab Formation................................................................................... 135

89. Bituminous subformation of the Shinhuduk Formation.................... 136

90. Shale-sandstone subformation of the Shinhuduk Formation............ 137

91. Khukhtyk Formation.................................................................................... 138

IV
	TABLES

Table 1. Paleozoic-Triassic stratigraphy of the Sichuan basin.................................. 30

2. Stratigraphy of Mesozoic-Tertiary rocks of the Sichuan basin.................. 34

3. Stratigraphy of the North China basin........................................................... 69

4. Stratigraphy of the Jianghan basin.................................................................. Ill

5. Stratigraphy of the Erlian basin....................................................................... 123

6. Upper Mesozoic stratigraphy of eastern Mongolia...................................... 127
GEOLOGY AND HYDROCARBON RESOURCES OF ONSHORE BASINS IN

EASTERN CHINA

By Gregory F. Ulmishek

PURPOSE AND SCOPE OF STUDY

The report presented here covers the geology and petroleum resources of 

eastern China (fig. 1). Only onshore basins are considered except for parts of the 

Bohai Gulf that are described in the chapter on the North China basin. The 

petroleum resources of western China were analyzed for the U.S. Geological Survey 

World Energy Resource Program by Ulmishek (1984). A report on offshore basins of 

China, which also covers basins of southeast Asia, is now in preparation by K. 

Robinson.

The purpose of this study is to provide the reader with a review of the 

petroleum geology which underpins the assessment of undiscovered recoverable oil 

and gas resources made by participants of the World Energy Resource Program, as 

reported by Masters and others (1990). The report considers all significant basins of 

eastern China (Lee and Masters, 1988) except for the unexplored Hailar basin (fig. 1) 

for which no sufficient data have been found in published literature. Additionally, 

the report contains a description of petroleum basins in the Gobi region of 

Mongolia. Data on a few very small basins in the southeastern part of China are not 

included because their negligible resources cannot affect the resource assessment. 

Metric units, except for quantities of oil (barrels) and gas (cubic feet) are used 

throughout the report.
Figure 1.-Major petroleum basins of China. Dotted are basins analyzed in the previous study

(Ulmishek, 1984). Basins described in this study are hatched. Petroleum basins: 1, Tarim; 2

Zhungaer; 3, Chaidamu; 4, Tulufan; 5 Jiuquan; 6, Zhaoshui; 7, Lunpola; 8, Ordos; 9, Sichuan; 

Nanxiang; 11, Jianghan; 12, North China; 13, Songliao; 14, Erlian; 15, Southeast Gobi; 16, 

Tamtsag; 17, Hailar; 18, North Yellow Sea; 19, Subei - South Yellow Sea; 20, East China Se

21, Pearl River Mouth; 22, Beibu Gulf; 23, South China Sea.
ORDOS BASIN

INTRODUCTION

The Ordos (Eerduos, Shan-Gan-Ning) basin is located in the western part of 

the Sino-Korean (North China) craton. The basin is bordered by the Helanshan and 

Liupanshan on the west, by the Qinling Mountains on the south, by the Yinshan 

and Langshan on the north, and by the Shanxi plateau on the east (fig 2). Area of

the basin is approximately 250,000 km2- If the peripheral Tertiary graben system is 

included (see fig. 13), the area increases to 300,000-320,000 km2- The surface of the 

basin is a loess plateau with an altitude ranging from 800 to 1200 m.

The first cable-tool hole in China was drilled to a depth of 76 m in the 

Yanchang area of the Ordos basin in 1907. The well produced 60 barrels of oil per 

day (Meyerhoff, 1982). The peak of production before World War IE was reached in 

1916 when 2700 barrels of oil were produced. At low level, exploration resumed in 

1948 with the technical assistance of Soviet geologists. The exploration was 

intensified in the 1970's and a number of oil fields were discovered in Upper 

Triassic and Jurassic rocks. The Ordos basin is also noticeable for its rich coal 

reserves mainly concentrated in the Upper Triassic-Lower Jurassic section (Lee, 

1986).

STRATIGRAPHY

The oldest sedimentary rocks that overlie the Archean-Lower Proterozoic 

basement of the Ordos basin are of Middle-Late Proterozoic age. These rocks are 

poorly known; supposedly they occur in an aulacogen (Jingshan aulacogen) in the 

eastern part of the basin and are slightly metamorphosed (Sun Zhaocai and others, 

1989). Another aulacogen was located in the Helanshan area. Outside the 

aulacogens, Proterozoic sedimentary rocks are thin or absent.

Unconformably overlying Proterozoic rocks or crystalline basement is the 

Cambrian-Middle Ordovician (and possibly partly Upper Ordovician Sun Zhaocai 

and others, 1989) section (fig. 3). In the interior of the basin, the section is 200-600 m 

thick and is dominantly composed of shallow-water carbonate rocks (mainly 

dolomites). On the Dongsheng uplift, the section may be completely absent. The 

thickness increases abruptly on the western and southern margins of the basin 

toward the Qilian and Qinling geosynclines and the Helanshan aulacogen (fig.4) 

The increase of thickness is accompanied by the appearance of marine elastics in the 

section including turbidites and slump facies. This transition into more deep-water 

facies may also be seen in figure 5, which illustrates the facies distribution of the 

Middle Ordovician rocks. Dark graptolitic shales in the upper part of the Middle 

Ordovician (Pingliang Formation) impinge upon the platform slope and may reach 

400 m in thickness (Guan and others, 1981). Carbonate reefs may be expected in the 

transition zone from shallow to more deep-water facies.
105 

I

110

35

PLATEAU

 40

Bo i n Boundar y

'35'

Figure 2. Location map of the Ordos basin
Plot form focies 

Ptotform morgin

Tranftitioruil facit* 

Rocks absent

Yonan Yon^hong \ -, * >Eugeosynclinal focies 

\\\\Miog*osyndinal

Basin Boundary

Figure 3.-Facies of the Cambrian-Ordovician section of the Ordos basin. (After Sun and others, 1983.)
IO no

40*-

35

OON6SHEN6

S Oongsheng

ORDOS PLATFORM

 Yulin

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75 150 £4O KM

80 sin Boundary

'35'

Figure 4.-Isopach map of Cambrian-Ordovician rocks of the Ordos basin. (After Zhang, 1982.)
IO5

40*-

35

 4O'

Rock* obsent 

~ Morine elastics 

l_j_I Open-shelf fimestone

X/Logoonal dolomites 

limestone s

'35'

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IXion'

is- I

10

75 150 £40 KM

Bos i n Boundary

Figure 5. Fades of Middle Ordovician rocks of the Ordos basin. (After Zhang and others, 1982.)
The Silurian-Devonian section is absent from the Ordos basin as well as from 

the entire Sino-Korean craton, except possibly for a narrow westernmost zone where 

Silurian marine elastics with graptolites and Devonian red beds were deposited in 

connection with the Caledonian tectonics in the Qilianshan (Sun and others, 1989). 

Sedimentation in the basin resumed in Carboniferous time when the sea 

transgressed the western and eastern basin margins. Marine clastic and carbonate 

rocks of the Yanghugou and Benxi Formations are hundreds of meters thick in the 

Helanshan trough and thin to a few tens of meters on the eastern margin. The 

central part of the basin and the Dongsheng (Ulanger) uplift on the north (fig. 6) 

remained exposed and are devoid of sediments of this age. The paralic coal-bearing 

Taiyuan (Upper Carboniferous) and Shanxi (Lower Permian) Formations occur 

above Lower-Middle Carboniferous rocks on the western and eastern basin margins 

and overlap lower Paleozoic and basement rocks on the central basin uplift and on 

the Dongsheng uplift correspondingly. Marine limestones are present among coalbearing elastics in the Taiyuan Formation and become increasingly uncommon in 

the Shanxi Formation. The overlying Upper Permian Shihezi and Shiqianfeng 

Formations do not contain significant coal beds. Most of the rocks are of continental 

origin; their dominant red color indicates arid climatic conditions. Marine layers 

are present only in the lower portion of the section and largely in the southern part 

of the basin. Thickness of the Carboniferous-Permian section of the Ordos basin 

ranges from less than 200 m to about 600 m (fig. 6). The section is much thicker in 

the Helanshan trough west of the basin margin.

The Lower Triassic Liujiagou and Heshanggou Formations conformably 

overlie Permian rocks. Both formations are mainly composed of red elastics. The 

Liujiagou Formation chiefly consists of sandstones with conglomerates whereas 

shales dominate in the Heshanggou Formation. Thickness of the Lower Triassic 

varies from 400-500 m on the west and south of the basin to 700-800 m on the east 

(fig. 7). A detailed lithologic description of the Lower Triassic and younger sections 

can be found in Lee (1986).

Major subsidence of the Ordos basin occurred during Middle and Late Triassic 

time. The total thickness of the Middle Triassic Ermaying and Tongchuan 

Formations (this section is alternatively identified as the Zhifang Formation in Sun 

Zhaocai and others, 1989) and the Upper Triassic Yanchang Formation exceeds 2000 

m in the depocenter in the southeastern part of the basin (fig. 8). Red, purple, and 

greenish colors dominate in elastics of the Middle Triassic section, especially in its 

lower part, indicating continuation of the arid climate. In contrast, the Upper 

Triassic Yanchang Formation consists of mainly gray elastics and include coal beds. 

This transition to humid climatic conditions at the end of the Middle Triassic is 

characteristic also of all basins in western China where rocks of this age are present 

(Ulmishek, 1984).

In the northern half of the basin, the sedimentation during Late Triassic time 

occurred on a swampy alluvial plain. Coarse-grained piedmont deposits were laid 

down along the western basin margin reflecting uplift in the Helanshan and
105 

I

1 1C

35'

DONG3HEN6 UPLI

Dengkou

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i

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Bati n Bou ndary

Figure 6.-Isopach map of Carboniferous-Permian rocks of the Ordos basin. (After Zhang, 1982.)
40*-

35

is75 

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'35'

Bo « i n Boundary

Figure 7.-Isopach map of Lower Triassic rocks of the Ordos basin. (After Zhang, 1982.)

10
105 

I

40*-

35'

DONGSHENG UPLIFT

 OS. -^

0.4 Dongshcnq

0.6

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'35'

^ B D««p lok«, source rock

factes in Upper 

Tnossic Hujiocun 

Formation 

(lower Yonchong)

80 « i n Boundary

Figure 8.-Isopach map of Middle-Upper Triassic rocks of the Ordos basin. (Compiled from Zhang, 

1982, and Han and Yang, 1980.)

11
Liupanshan. The depocenter in the Tongchuan depression (fig. 8) was occupied by a 

lake. Periodically the lake deepened, anoxic conditions developed over the bottom, 

and bituminous black muds were deposited. Organic-rich black shales appear in the 

Middle Triassic Tongchuan Formation, but they become abundant in the Upper 

Triassic. These black shales are the main source rocks for oil fields of the Ordos 

basin.

At the end of the Triassic, the Ordos basin was uplifted and tilted eastward. 

The western areas were deeply dissected by rivers; depth of the river valleys there 

exceeded 200 m and decreased eastward (fig. 9). Sedimentation resumed in the 

beginning of Jurassic time and the river valleys were gradually filled with deposits 

of the Fuxian and the lower part of the Yanan Formations (Baotashan Sandstone). 

Coarse-grained elastics, including conglomerates, dominate in axial zones of the 

ancient rivers, and mudstones and sandstones form the river banks (Song, 1984).

The overlying Jurassic strata (upper part of the Yanan Formation, Zhiluo and 

Anding Formations) consist of coal-bearing elastics deposited in fluvial conditions. 

A lake with intermittent deposition of anoxic black shales existed in the 

southeastern portion of the basin during Yanan time (fig. 10). The Upper Jurassic is 

regionally absent, except maybe in the Liupanshan (southwest of the basin margin) 

where more than 2000 m of red and violet conglomerates and sandstones overlie a 

relatively thin (150-200 m) coal-bearing section that can be correlated with the Lower 

and Middle Jurassic of the Ordos basin. The maximum thickness of the Jurassic 

exceeds 1000 m in the western part of the basin and decreases eastward, partially 

because of synsedimentary thinning, but primarily due to Cretaceous and later 

erosion. At present, Jurassic strata crop out on the east of the basin and are 

completely eroded in its southeastern part (fig. 10).

A pronounced uplift of the Shanxi block east of the Ordos basin in the 

Cretaceous and intense thrusting along the western basin margin resulted in 

formation of a foredeep and strong tilt of the basin westward. Thickness of 

Cretaceous rocks in the axial zone of the foredeep along the thrust belt exceeds 1400 

m and decreases rapidly eastward (fig. 11). The Lower Cretaceous (Zhidan Group) is 

dominated by fluviodeltaic red elastics in its lower part and by gray elastics in the 

upper part; the latter unit includes thin coals near the top. In the Liupanshan, the 

thickness is more than 2,000 m; the sequence includes a 150 m-thick conglomerate at 

the base and black shale and coal beds in the upper 800 m-thick section. The 

thickness of the Lower Cretaceous in the Helanshan varies widely from 50 to 1,000 

m and more. Basic and intermediate volcanics are present at the top of the 

sequence.

Upper Cretaceous rocks are known only in the northwestern part of the basin 

where they are represented by red and brown mudstones, siltstones, and some 

sandstones. This suite of rocks is known as the Tegaimiao Formation.

12
IO5

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isno

1

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Basin Boundary

'35'

Figure 9.-Jurassic paleogeography of the Ordos basin. (After Sun and others, 1989.)

13
10 5

40'

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 40'

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Iovr«r-middle Yanan Fm.

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Figure 10. Isopach map of Jurassic rocks of the Ordos basin (Compiled from Zhang, 1982, Sun and 

others, 1983, and Han and Yang, 1980.)

14
10 5' 

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35'

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'35'

05*

Xi Q n 

NORTH QINLING FOLD SYSTEM

75 

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Figure 11. Isopach map of Cretaceous rocks of the Ordos basin. (After Zhang, 1982.)

15
Cenozoic rocks of the Ordos basin are confined to grabens surrounding the 

basin on the north, west, and south (Hetao graben system, Yinchuan graben, and 

Weihe graben respectively see figure 13). The grabens are the westernmost part of 

the Shanxi rift system that stretches into eastern China. The rifting originated in 

middle Paleogene time and continued through the Quaternary. At present, the 

grabens control the position of the Yellow River (Huanghe) Valley and its 

tributaries. The lower part of the Cenozoic sedimentary section in the grabens is 

mainly composed of coarse continental clastic rocks; more fine-grained fluvial and 

lacustrine facies dominate in the upper part. Thickness of the section in the 

Yinchuan graben is almost 3 km, in other grabens it is probably less (Wang, 1969). 

An alternate assessment suggests significantly larger thicknesses of Tertiary rocks: 

9,000 m in the Hetao graben and 6,000 m in the other two grabens (Zhang, 1982).

TECTONICS

In the view of Chinese geologists, the Ordos basin (as well as the Sichuan 

basin) occupies the intermediate position between the compression-dominated 

basins of western China and the extensional rifted basins of eastern China (Li, 1983a, 

1990; Chen and Dickinson, 1986). The basin occupies the western part of a crustal 

block that was characterized by a stable tectonic regime during the Paleozoic and 

Mesozoic and was rifted in the Tertiary. The Qilian fold system southwest of the 

basin was formed during Caledonian orogeny at about the end of Silurian time. The 

Inner Mongolian fold system north of the basin and the Qilian fold system south of 

it were formed during late Hercynian-early Kimmerian tectonic events in the course 

of agglomeration of the Asian continent (Watson and other, 1987; Ji and Coney, 

1985). During much of Paleozoic time, the stable block was separated from more 

western structures of the Sino-Korean craton (Alashan, Beishan) by the Helan 

aulacogen (fig. 6) which was structurally inverted and deformed probably during 

Triassic time in response to transpression along the north-south zone of dextral 

shearing (Watson and others, 1987; Sun and Liu, 1983).

The lower Paleozoic section of the basin unconformably overlies the 

Archean-Early Proterozoic crystalline basement rocks and Middle-Late Proterozoic 

quartzites and siliceous limestones occurring in grabens (Sun Zhaocai and others, 

1989). Stable platform conditions characterized most of the basin; but along its 

southern and western margins, turbidites and other sediments characteristic of 

continental slopes on passive tectonic margins were deposited (figs. 3-5). A relative 

uplift occupied central and northern areas of the basin; the thickness increased 

toward the present Shanxi high.

During Silurian and Devonian time, the Ordos basin was uplifted along with 

the entire Sino-Korean craton. Only in a narrow zone of the western basin margin 

were Silurian marine deposits laid down. They were overlain by Devonian 

continental red bed molasse derived from the Caledonian orogen of Qilian.

16
A new stage of basin subsidence began in Middle Carboniferous time. The 

Hercynian structure is characterized by uplifts in the northern and southern parts of 

the basin and by depressions on the western and eastern margins (fig. 6). The latter 

depression clearly extended into the present-day Shanxi high. Beginning in Late 

Permian time, the sea regressed southward and continental sedimentation, 

characteristic of the rest of geologic history of the Ordos basin, was established.

The Early Triassic structure (fig. 7) resembles that of the CarboniferousPermian, but the western basin margin was uplifted in connection with partial 

structural inversion of the Helanshan aulacogen. A significant tectonic 

reorganization occurred during Middle-Late Triassic time with formation of the 

deep Tongchuan depression that embraced a large southeastern portion of the basin 

(fig. 8). The depression continued eastward of the Ordos basin, along the foredeep of 

the Qinling fold system (Sun Zhaocai and others, 1989). At the end of Triassic time, 

the basin, together with the Shanxi platform, were uplifted and tilted 

northeastward. River valleys were deeply cut into Triassic sediments; the depth of 

erosion decreased eastward (fig. 9).

Thrusting along the western basin margin, formation of the western foredeep 

due to thrust loading, tilting of the basin westward, and uplift of the Shanxi block 

began in Late Jurassic time and continued into the Cretaceous (figs. 10-12). The 

eastern margin of the present-day Ordos basin was formed during this time as a 

result of erosional removal of thick older rocks from the Shanxi high. Tertiary 

rifting along the basin margins completed the tectonic development of the Ordos 

basin.

The present-day structure of most of the basin is a simple, almost 

unstructured homocline that dips westward toward the Liupanshan and Helanshan 

(fig. 13). The western basin margin is deformed by a series of imbricate thrust faults 

that have transported rocks eastward over a distance of 20-40 km (fig. 14). The thrust 

faults flatten with depth and are believed to merge in a nearly horizontal 

decollement zone in the Carboniferous coal measure (Guo and Zhang, 1989).

PETROLEUM GEOLOGY AND POTENTIAL EXPLORATION PLAYS

Oil has been produced for centuries from hand-dug wells in the Yanchang 

area (fig. 13). Drilling began in 1907 and the first well produced 60 b/d of oil from a 

depth of 76 m (Meyerhoff, 1982). However, significant exploration efforts did not 

begin until 1951. Since that time, a number of oil and gas fields have been found; 

most discoveries were made during the 1970s-1980s.

Most of the discovered oil fields are found in traps connected with an 

erosional unconformity and an incised paleodrainage system at the boundary 

between the Triassic and the Jurassic (fig. 15). These paleogeomorphic traps are 

variable; oil pools are found at the top of the Triassic in buried hills (fig. 16), in

17
W END OF ZHIDAN-3 AGE

KiZ4 YANWU CHENHAO-E. HUACHI-FUZIEN-GANQUAN

1200-

240X). END OF JURASSIC 

BASE OF CRETACEOUS

600-

1200

END OF YANAN 

BASE OF ZHILUO FORMATION

500- 

1000-

400-

eoa

0

400

END OF YANAN-9 AGE 

BASE OF YAN-8 STRATUM

BASE OF T3Y4

END OF YANCHANG-3 AGE

meters

0, 10 20 /° km.

Figure 12. Paleostructural cross sections showing formation of the foredeep in the western Qrdos basin 

in Cretaceous time. (After Song, 1984.)

18
05 I I 0

35*'

 4O

is-

'35'

Basin boundary

Contour lines, meters

Erosional boundary of 

Jurassic

Faults

Oil and gas fields

Batin Boundary

Figure 13.-Structure of the Ordos basin on base of the Yanan-6 Member of the Jurassic Yanan Formation. 

(After Song, 1984.)

Oil and gas fields: \, Yongping; 2, Yanchang; 3, Yanan; 4, Maling; 5, Qizi; 6, Chenghao; 7, 

Zhiluo; 8, Nanliang; 9, Mafang; 10, Tianchi; 11, Qican

19
w

0

2500-

5000-

7500- 

(m)

K3

o

Figure 14.-Cross section through the western thrust belt of the Ordos basin. (After Tang and others, 1988.) The p

of the cross section is not indicated. T<j - T|2 are stratigraphic units of the Triassic section.
o

0 8 16 24 32 km 

till j

,20' Isopach. m 

OH Field

YANAN

FUXIAN

Figure 15.-Isopach map of members 9 and 10 of the Jurassic Yanan Formation. (After Song, 1984.) These two basal 

members of the Jurassic fill erosional relief, which was developed by incision of a river system at the 

Triassic/Jurassic unconformity. Numerous oil fields are controlled by paleomorphic traps.

21
500n

450

400

Figure 16.-Cross section through the Zhiluo oil-gas field, Ordos basin. (After Song, 1984.) These 

stratigraphic pools are in an erosional hill in Triassic rocks beneath the Triassic/Jurassic 

unconformity.
pinch-outs and drape structures over buried hills in basal Jurassic sandstones (fig. 

17), and in sandstone lenses of various origin (bars, distributary channels, deltaic 

sand lenses). Reportedly, some hydrocarbon pools (mainly gas) have been recently 

found in Permian through Jurassic rocks in anticlinal traps of the western thrust 

belt (i.e. Majiatan field - Lee, 1986) and in upper Paleozoic rocks in stratigraphic traps 

on the north of the basin (Sun Zhaocai and others, 1989). No detailed data on these 

pools are available from the literature, and their precise position cannot be 

indicated.

Two gas pools (Tianchi and Qican, fig. 13) were recently found in Ordovician 

rocks of the basin. The pools are in simple structural traps (fig. 18). Gas was tested 

from the Permian Shihezi Formation and from the Lower Ordovician Majiagou 

Formation on the Tianchi structure at 2608-2618 and 3676-3959 m correspondingly. 

Both formations were tested simultaneously and produced 328,000 m3 /day (12 MM 

cfd). Different isotopic compositions of gas from the two formations is interpreted 

to reflect different sources for gas: Carboniferous-Permian coals for the upper pay 

and lower Paleozoic carbonate rocks for the lower pay (Dai and Xia, 1990). Two more 

gas shows were obtained from wells east of the Qican structure.

Organic-rich shales deposited in lakes under anoxic conditions are potential 

petroleum source rocks of excellent quality. These shales are thickest in the Upper 

Triassic Yanchang Formation. They are also present in the Middle Triassic 

Tongchuan Formation and in the Middle Jurassic Yanan Formation. The 

distribution of these lacustrine deposits is shown in figures 8 and 10. However, 

oil/source rock correlation studies indicate that oil accumulations in both Upper 

Triassic and Jurassic rocks were derived from Triassic source rocks whereas Jurassic 

shales are immature (Huang and others, 1988; Song, 1984; Lee, 1986). Average TOC 

in Triassic black shales is 2.4%, and they contain 1060 ppm of hydrocarbons (Sun 

Zhaocai and others, 1989).

In the Paleozoic section of the basin, potentially rich gas source rocks are 

present in the widespread Carboniferous and Lower Permian coal-bearing 

formations, especially in the Taiyuan Formation. The coal-bearing formations 

cover more than 250,000 km2 of the basin area. Some oil shows are believed to be 

related to these source rocks (Wang and Liu, 1983). Carboniferous source rocks have 

been studied in the northern part of the Ordos basin (Sun and others, 1983; Zhu 

Zongqi and others, 1981). The rocks are in the oil window in a narrow zone along 

the northern and western basin margins; the maturity increases rapidly toward the 

basin center (fig. 19).

Ordovician and possibly older lower Paleozoic carbonate rocks are believed to 

be a source for gas in the Lower Ordovician Majiagou Formation (Dai and Xia, 1990). 

Geochemical data on these rocks are very limited. The average TOC content in 63 

carbonate samples ranging in depth from 1139 to 3959 m from 12 wells in the Ordos 

basin is only 0.21%. However, this average concentration probably does not 

characterize the organic richness of all strata; for example, TOC values as high as

23
Figure 17. Cross section through the Maling oil field, Ordos basin. (After Hu and others, 1984.) Oil 

pools are in basal Jurassic rocks above the Triassic/Jurassic unconformity. The pools are 

controlled by stratigraphic pinch cuts and by compaction structures over the erosional hills. 

The scale is not indicated.

24
wsw ENE

1500-

2000

2500

Tih-H

Figure 18.-Cross section through the Tianchi gas field, Ordos basin. (After Dai and Xia, 1990.) A gas 

pool in Permian rocks is not shown. C2b-Carboniferous Benxi Formation; PIS, Lower Permian

Shanxi Formation; Pj^sh, Lower-Upper Permian Shihezi Formation; P2S, Upper Permian

Shiqianfeng Formation; T, h +1, Lower Triassic Heshanggou and Liujiagou Formations. The 

scale is not indicated.

25
Figure 19. Thermal maturity of upper Paleozoic rocks of the northern Ordos basin. (After Sun and others, 1

1, borehole; 2, coal mine; 3, fault; 4, boundaries of areas of different maturity (I, low; II, intermedia

high); 5, vitrinite reflectance (in numerator) and depth of a sample and stratigraphic unit (in demo

C^t, Upper Carboniferous Taiyuan Formation; PjS, Lower Permian Shanxi Formation; P^sh, Lowe

Permian Shihezi Formation.
0.64% have been recorded. Although oil-prone kerogen type II dominate in the 

rocks, the organic matter is strongly overmature. In a Qican field well, the R0 value 

at 2835 m is 2.31. Possibly, rocks with better source potential can be expected in the 

Middle Ordovician section deposited in the upper part of the continental slope on 

the basin margins and in the Helanshan aulacogen where graptolitic shales are 

present. However, the source rock potential of lower Paleozoic rocks of the basin is 

poorly known and more studies are necessary for a reliable evaluation.

The average thermal gradient in the Ordos basin is 2.75°C/100 m (1.5° F/100 

ft) (Zhai and others, 1988). However, significant removal of strata, especially in the 

eastern areas of the basin, preclude the assessment of thermal maturity based on the 

geothermal gradient. Available assessments suggest that Triassic source rocks are in 

the oil window over most of the basin whereas Jurassic rocks are immature 

everywhere except probably in the western foredeep where Jurassic strata do not 

contain anoxic lacustrine source rocks.

A rapid increase of maturity of the Carboniferous-Permian coal measure from 

the northern margin basin ward is shown in figure 19. These rocks, as well as older 

Paleozoic rocks, are probably overmature throughout most of the basin.

Reservoir properties of Mesozoic continental rocks of the Ordos basin are 

generally poor which is true of most continental reservoir rocks in China. The best 

reservoirs in the Mesozoic section are basal Jurassic sandstones and conglomerates 

deposited in a high-energy environment in rivers deeply incised into underlying 

Triassic rocks. Porosity of these sandstones is rather high and permeability averages 

10-50 md (Sun Zhaocai and others, 1989). Most of the other Mesozoic reservoir 

rocks are of fluvio-lacustrine origin. Their porosity ranges up to 20-25%, but the 

permeability commonly is only a few millidarcies or lower. Similar reservoir 

properties are characteristic of Permian sandstones.

A limited number of measurements indicate poor reservoir properties for 

Ordovician carbonate rocks in discovered fields. Measured porosity varies from 0.5 

to 3.5% and permeability is lower than 1 md (Sun Zhaocai and others, 1989). 

However, significant yields of gas in tested wells probably indicate an important role 

of fracturing.

At present, the Ordos basin is not a rich hydrocarbon province. The single 

productive oil play is in paleomorphic traps of the basal Jurassic river system. The 

oil reserves are not disclosed; however, they certainly do not exceed a few hundred 

million barrels and probably are significantly less. Future potential of this play 

seems to be limited. Although many more oil pools can certainly be found, 

especially on the west side of the river system, they will be dominantly of small to 

very small size. On the east side of the system, the incision of rivers was much less 

and did not cut into the Triassic, therefore, the Triassic black shales remained to be 

sealed from Jurassic reservoir rocks. Mild tectonics prevented fracturing of the seal 

and vertical migration. The Cretaceous-Tertiary uplift ceased oil generation in the

27
Triassic source rocks, and the generative potential of the Jurassic black shales has not 

been realized.

The western thrust belt is considered a highly potential play by Chinese 

geologists (Tang and others, 1988; Guo and Zhang, 1989). However, the reasons for 

the high expectations for this play are not obvious. Triassic and Jurassic black 

lacustrine shales do not extend into the thrust belt except for a small area in its 

central part (fig. 8). Alluvial facies of these strata can be expected to contain mainly 

terrestrial organic matter with gas-prone type HI kerogen. Gas source rocks are also 

characteristic of the Paleozoic section of the belt due to both organic matter type and 

overmaturation. Sealing conditions are an obvious problem in the highly faulted 

thrust sheets (fig. 14). Better sealing can probably be expected in the subthrust 

structures, but the tectonics is complex, the seismic resolution is poor, and the 

exploration is highly risky. In general, this play does not seem to be very promising 

and is clearly gas-prone.

A potentially rich gas play is associated with the Carboniferous-Lower 

Permian coal measure that is present over most of the Ordos basin. The best 

reservoirs are in the Upper Permian Shihezi Formation (Sun Zhaocai and others, 

1989); interlayered and overlying shales provide the seals. Structural traps (drapes 

over basement highs) similar to that in the Qican field (Dai and Xia, 1990) are most 

promising, but their abundance remains to be demonstrated. Pinch-out zones of 

Middle Carboniferous marine elastics against the Dongsheng uplift and the 

paleohigh in the central part of the basin (fig. 6) may contain significant stratigraphic 

traps. This play has a greater potential in the north part of the basin where source 

rocks are at lower stages of thermal maturity.

Gas discoveries in Ordovician carbonates highlight the lower Paleozoic 

prospects of the basin. The problem with the play is the uncertainty in quality and 

distribution of source rocks. The interpretation that assigns organically lean 

Ordovician carbonates as the source rock for gas discoveries in this section (Dai and 

Xia, 1990) is suspect. The gas could have been sourced from upper Paleozoic coalbearing rocks and migrated across the pre-Carboniferous unconformity into 

underlying carbonates. More data are needed to understand the importance of this 

unconformity in prospecting for gas in the basin. Structural drapes over basement 

highs, analogous to those in discovered fields, possess the highest potential.

The southern margin of the basin has a good potential for hydrocarbons in 

the Ordovician section, especially in the Middle Ordovician reef play. The 

paleogeographic situation (fig. 5) indicates the presence of basinal facies (Pingliang 

Formation) in the Qinling fold belt and on the craton edge, a narrow zone of openshelf limestones, and lagoonal facies farther basinward. This situation suggests a 

high probability for the presence of reefs on the basin margin. Source rocks for this 

play may be present in lagoonal and basinal facies. The latter are now strongly 

deformed in the Qinling geosyncline, but they are also present in an unmetamorphosed zone along the southern and southwestern slopes of the craton (fig. 3).

28
SICHUAN BASIN

INTRODUCTION

The Sichuan (Red) basin is located in the western part of the Yangtze craton 

in southern China (fig.l). The basin has a rhombic shape which was acquired after a 

series of compressional deformations that took place during Mesozoic and Tertiary 

time. Along all its perimeter, the basin is surrounded by mountain chains (fig. 20). 

The basin area is 230,000 km2 (88,000 mi2).

Wells with bamboo pipes were drilled in the basin for salt brine production 

thousands of years ago. Later, some wells were drilled for gas which was used for 

heating the brine and precipitating salt (Meyerhoff, 1982). One well reached the 

depth of more than 1000 m as early as in the early 19th century (Wang and others, 

1989). Modern drilling began in 1937 on the Weiyuan structure, and during 1944- 

1954, a number of other anticlines were involved in exploration with the technical 

assistance of Soviet geologists. At present, the basin is the major producer of gas in 

China, but the discovered amount of oil is small.

STRATIGRAPHY

The basement of the Sichuan basin in its central, northeast-trending zone 

consists of an Early Proterozoic metamorphic terrane. On the northwest and 

southeast, the upper Sinian through Tertiary sedimentary cover is underlain by 

diagenetically strongly altered lower Sinian elastics which may be considered an 

economic basement (Yin, 1985).

The upper Sinian (latest Proterozoic) section covers the basement rocks. Basal 

elastics, apparently of glacial origin, are present mainly on the east-southeast side of 

the basin. Overlying rocks are marine shales, chert, and algal (often vuggy) 

dolomite (Denying Formation). The dolomite is productive in several gas fields. 

The maximum thickness of the formation exceeds 1,000 m. Climatic conditions 

during deposition of the shales and carbonates were arid as indicated by inclusions 

of salt and gypsum in contemporaneous rocks of neighboring regions (Liu and 

others, 1973).

The lower Paleozoic overlies Sinian rocks with significant unconformity 

(table 1). The Cambrian, Ordovician, and Silurian are composed mainly of 

carbonate rocks with some shales, such as a graptolitic shale at the base of the 

Silurian (Johnson and others, 1985). Thickness of this section in the basin varies 

from 500 to 2,500 m. A flysch trough filled with as much as 10,000 m of lower 

Paleozoic rocks was located in the Longmenshan. Another trough with a 5-km 

thick lower Paleozoic section is found southeast of the basin (fig. 21).

Devonian and Carboniferous rocks are absent from most of the Sichuan 

basin. Only on the east side of the basin, thin (50-200 m) Upper Carboniferous sandy 

and vuggy dolomites and limestones are present and contain several gas fields

29
Table l.-Palcozoic-Triassic stratigraphy of the Sichuan basin. (After U.S. Energy Information Administration, 1987.) Wade-Giles 

SYSTEM SERIES GROUP/FORMATION LTTHOLOGIC CHARACTERISTICS

TRIASSIC

PERMIAN

UPPER

MIDDLE

LOWER

UPPER 

LOWER

o

UPPER

CARBONIFEROUS MIDDLE 

LOWER 

UPPER

Leikooupo 

(Trenchiangshan)

Chialingchiang

(=Tzialintszian

=Patung)

Fenghuangshan

(Yehlang

Feihsienkuan =

Tayeh)

Changhsing (=

Yuehping)

Loping

(=Lungtangtsu)

Maokou (=Yangsin)

Chihsia

Tungkunkchi

Maping

Weining

Tsungchangkou

Tangwangchai

Light-grey limestone and dolomite in the middle and 

bottom units of grey-green clays 

Light-grey limestone and dolomite with clay, 

anhydrite and rock salt units

Violet and black shale and marl with limestone 

parting

Grey siliceous limestone

Black carbonaceous shale with sandstone and 

limestone basalt in the Omi Mountains 

Grey limestone with shale at the bottom

Light-grey limestone with black carbonaceous shale

and sandstone at the bottom

Limestone

Limestone and calcareous marl

Thick beds of limestone banded with flint

DEVONIAN

SILURIAN

ORDOVICIAN

CAMBRIAN

MIDDLE

LOWER

UPPER

MIDDLE

LOWER

UPPER

MIDDLE

LOWER

MIDDLE

LOWER

Kuanwushan

Yangmapa

Pingipu

Shamaoshan

Hsiavhopa

Lungmachi

Wufeng

Aichia/Pagoda

Aichia/Yangtzupei

S anhui ch ang /Upper

Sanhuichang/Middle

Sanhuichang/Lower

(Chiulaotung Shale)

Thin beds of limestone banded with reef limestone

Interbedded unpure limestone and hard sandstone

Quartz sandstone

Shale

Limestone

Limestone and unpure limestone

Limestone and dolomitic limestone

Limestone containing crystalline pyrite

Shale

THICKNESS 

(in meters)

40-100 

500-650

300-500 

(Average 500)

80-150

60-110 

50-80

400 

250-300

6-15 

20-40

130 

90-100

600

500

320

200-1900

500-600

150-200

100-150

10

50

200

100

200

100

300

O

O

PRECAMBRIAN BASEMENT
I05e

Tong jiong \ Depression

105

Figure 20.-Main structural units of Sichuan basin. (After Wang and others, 1983.)

31
105

IO5

Figure 21. Isopach map (in meters) of lower Paleozoic rocks of the Sichuan basin. (After Wang and 

others, 1983.)

32
(Chen, 1982). These carbonate rocks occur between two unconformities that 

enhanced their reservoir properties. They were deposited in a long and narrow bay 

that penetrated into the basin from the epicontinental sea of southern and eastern 

regions of South China (Li and Li, 1988).

In pre-Permian time, the Sichuan basin area was strongly uplifted and deeply 

eroded. Permian rocks unconformably overlie older rocks ranging in age from the 

Sinian to the Carboniferous (fig. 22). Carbonate rocks dominate in the Lower 

Permian section. Algal reef facies are widespread, and they are mainly concentrated 

around the ancient uplifts. Black and gray clayey limestones of the Yangxin 

Formation were probably deposited in more deep-water environments. These 

limestones are believed to be the major source rock for gas in the basin (Huang, 

1984). Upper Permian rocks were deposited in paralic conditions, and the section is 

composed of carbonate and clastic beds with layers of coal. On the west and east, the 

section includes widespread basalt flows at the base (Emei Basalt).

Unlike in the North China domain, marine sedimentation in the Sichuan 

basin continued into Early and Middle Triassic time. Carbonate sediments were 

deposited in the eastern part of the basin during the Early Triassic. Westward, 

carbonate rocks pass into variegated shales and sandstones of the Feixianguan 

Formation (Sun Shu and others, 1989). The Middle Triassic is mainly composed of 

carbonate rocks on the periphery of the basin. Shallow-water algal banks and oolitic 

shoals are common. Dolomite, shale, gypsum, and salt dominate the Middle 

Triassic section in the central areas of the basin.

A marked change of depositional environments began in Late Triassic time. 

Marine sedimentation continued until Norian time in the northwestern part of the 

basin. Pre-Norian Upper Triassic rocks, that consist of reefal and oolitic carbonates 

and very thick deltaic and lacustrine elastics containing coals, unconformably onlap 

onto the uplifted southeastern basin area. At the end of the Norian, the 

Longmenshan was folded and uplifted and began to pour abundant coarse clastic 

material into the forming foredeep. In the foredeep, thickness of the Upper Triassic 

exceeds 3 km, and the section rapidly thins southeastward (fig. 23). Formation of the 

Longmenshan belt cut the connection of the Sichuan basin with the Tibetan sea and 

brought an end to marine sedimentation. This event started the formation of the 

Sichuan basin in its present-day boundaries.

The Early Jurassic subsidence was not as intense as that of Late Triassic time. 

Maximum thicknesses of sediments are found in the central area of the basin (fig. 

24) where lacustrine facies, including dark shales, fresh-water limestones, and 

sandstones, dominate in the section. Fluvial, commonly red elastics compose the 

section on the basin margins (table 2). Shallow-lake, mainly clastic deposits 

dominate in the Middle Jurassic in the northeastern half of the basin which was 

characterized by intensive subsidence (thickness to 3 km) (fig. 25). Westward, the 

thickness decreases and alluvial facies are most common. In the Late Jurassic, the 

depocenter again shifted to the northwestern part of the basin (fig. 26) evidently in

33
Table 2.-Stratigraphy of Mesozoic-Tertiary rocks of the Sichuan basin. (After Yin, 1985.)

SYSTEM FORMATION 

Quaternary Pleistocene

Paleogene Eocene

Upper

Cretaceous

Lower

Upper

Middle

Jurassic

Lower

MEMBER THICKNESS (m)

Triassic Upper

Middle

Lushan 

Mingshan

Guankou

Jiaguan

Gudian

Qiqusi

Bailong 

Cangxi

Penglaizhen

Suining 

Upper Shaxi

Lower 

Shaximiao 

Qianfoya

Daanzhai

Ziliujing Maanshan 

(Baitianba) Dongyuemiao 

Zhonzhuchong

Upper

Xujiahe Middle

Lower

Xiaotangzi

Maantang

>650 

500-1,200

400-1,100

150-800

>40-150

200-500

100-400 

200-500

700-1,500

300-800 

350-2,900

100-700 

100-350

80-120

50-70 

30-60 

100-350

100-500

0-1,360

200-650

0-900

0-450

Leikoupo

MATNLrTHOLOGY 

Yaan Gravel Bed 

Dayi Conglomerates

Red fluvio-lacustrine mudstone and siltstone X 

Red fluvio-lacustrine sandstone, siltstone and mud

marlite and coarsening westward.

Lacustrine mudstone, siltstone, marlite, gypsum, g

area, and red fluvio-lacustrine moderate-fme-grainte

Fluvial sand-conglomerate rocks

Interbeds of red siltstone and mudstone, intercalate

conglomerates.

Greyish green and purplish grey moderate-grained 

mudstone, fining upward and coarsening downward

Cyclothems consisting of sandstone and red mudst

Missive sandstone intercalated with red mudstones

Interbeds of red fluvio-lacustrine sandstones and m

fluvial sand-conglomerate beds in front of Longm

Formation).

Red mudstone intercalated with siltstone and fine s

Red flood mudstone intercalated with fluvial sands

Interbeds of red-green fluvio-lacustrine sandstone a

turning into dark mudstone and shale.

Red fluvial sand-and-mud-stones in the south, and 

shale intercalated with siltstone and fine sandstone 

In the south, red and greyish green mudstone inter

middle, dark mst and sh intercalated with shelly Is

In the south, red ms intercalated with ss; in the mi

Dark grey mist and sh intercalated with shelly Is. 

In the south and middle, red ms intercalated with q

northwards.

In the lower part, massive sand-conglomerate rock

into ss and mst with coal stringers.

Cyclothems of dark ms and sh, and quartzose ss in

stringers.

Feidsphthic quartzose ss and polymictic quartzose 

cong.; in the middle part, intercalated with argillac

Predominantly marine clastic rocks, intercalated w

and coal stringers.

In the upper part, dark grey sh intercalated with si

lower part, oolitic Is and bioclastic Is. 

Limestone and dolomite.
105' iio105

||| Devonian

^-v. Simon

 " ""_ Ordovicion

m Combrion

 _ Faults

'" &>££" Carboniferous

</>. Intermediates oc«dic inlrustves 

f j Silurian

^ Basin boundary 

^^: Pr«-Sinion

Figure 22.-Subcrop geologic map of the pre-Premian unconformity. (After Wang and others, 1983.)

35
105' 110'

30° 304

60 120km

105' 110°

Figure 23.-Isopach map (in meters) of Upper Triassic rocks of the Sichuan basin. (After Yin, 1985.)

36
30

105* T~ no <

0 60 120 km

30°

HO'

 Limit of predominantly lacustrine facies 

Figure 24. Isopach map (in meters) of Lower Jurassic rocks of the Sichuan basin. (After Yin, 1985.)
30' 

O 60 l2Okm

105

........ Western limit of predominantly lacustrine facies

Figure 25.-Isopach map (in meters) of Middle Jurassic rocks of the Sichuan basin. (After Yin, 1985.)

38
105' 110*

30'

120km

105 I 10'

Figure 26.-Isopach map (in meters) of Upper Jurassic rocks of the Sichuan basin. (After Yin, 1985.)

39
connection with continuing thrusting in the Longmenshan. The rocks are mainly 

red fluvio-lacustrine sandstones, shales, and conglomerates.

Fluvial and, to a lesser extent, lacustrine clastic rocks compose the Cretaceous 

section (fig. 27). In the Early Cretaceous, deposition occurred mainly in the northern 

and western areas of the basin. Uplift of these areas in pre-Late Cretaceous time 

shifted the depocenter to the south. Paleocene-Eocene rocks are thick and are 

composed of similar lithologies. Maximum thicknesses of Paleocene-Eocene rocks 

occur in approximately the same areas as during Late Cretaceous time. At the end of 

the Eocene, the basin was uplifted and sedimentation resumed only in the 

Pleistocene with deposition of coarse clastic material.

TECTONICS

The Sichuan basin did not exist until Late Triassic time. Deposition of the 

upper Sinian-lower Paleozoic and Permian-Middle Triassic sequences occurred on 

the Yangtze craton near its western passive margin. Tectonic movements had a 

platform character with block-type basement-involved uplifts and depressions. 

Even the regional uplift at the end of Silurian time (probably related to compression 

and folding in the Cathasian belt) resulted in rather mild deformation with 

formation of a large gentle uplift in the western part of the basin (fig. 22). Massive 

eruptions of middle Permian Emei plateau basalts indicate regional tension and 

associated fracturing of the basement.

The first compressive deformation occurred in middle Late Triassic time in 

connection with folding and thrusting in the Longmenshan. This Indosinian 

deformation marks the collision of South China with the Songpan-Ganzi terrane on 

the northwest (Ji and Coney, 1985). A foredeep filled with thick Upper Triassic 

sediments was formed in response to loading by thrust sheets. The second episode 

of thrusting and formation of a foredeep probably occurred in the Dabashan in 

Middle Jurassic time (Bally and others, 1986). In pre-Late Cretaceous time, folding 

and regional uplift took place in the eastern Sichuan basin. Himalayan movements 

in post-Eocene time resulted in three episodes of folding in western Sichuan and 

thrusting and folding in eastern Sichuan (Wang and others, 1989). In the latter 

region, Tertiary (late Neocathasian) folding with strikes at 20° NE overprinted the 

earlier and gentler Late Jurassic-Cretaceous folds which strike 60° NE (Zheng, 1980).

The present-day structure of the Sichuan basin is characterized by a generally 

uplifted central zone (Central and Weiyuan-Longnusi uplifts) and by a series of 

depressions along the basin margins adjoining the mountainous fold belts (fig. 20). 

The basement occurs at depths of 4-5 km on the uplifts and as deep as 7-10 km in the 

depressions. The northwestern and southeastern margins are most strongly 

deformed. The main mode of deformation is folding1 and thrusting with 

decollements at the basement level on the northwest and at the Silurian level on 

the southeast (figs. 28 and 29). A secondary decollement surface is probable in 

Triassic evaporites.

40
105 110

30*

0 60 120km

Limit of Cretaceous rocks

Figure 27.-Isopach map (in meters) of Cretaceous rocks of the Sichuan basin. (After Yin, 1985.)

41
NW

OROGENIC FOREDEEP 

BELT THRUSTBELT

CENTRAL STABLE AREA 

1 v'-i

BASEMENT SINIAN-OROOVICIAN SILURIAN DEVONIAN 

CARBONIF

UPPER TRIASSIC JURASSIC CRETACEOUS METAMOR

Figure 28. East-west cross section through the Sichuan basin. (After Wang and others, 1989.) The precise 

the cross section is not indicated.
NW

2000-

4000-

6000H

8000-

Figure 29.~Cross section through the thrust zone of the Longmenshan, Sichuan basin. (After Liu, 1986.) The p

position of the cross section is not indicated. Subscripts with letter T indicate various units of Triassic 
PETROLEUM GEOLOGY AND POTENTIAL EXPLORATION PLAYS

The Sichuan basin is the largest gas producer in China. More than 60 gas 

fields have been found, most of them of small to medium size. The larger fields are 

shown in figure 30. The largest gas field is the Weiyuan field with reserves of about 

1.3 TCP. Major gas production is from the Permian-Middle Triassic section; 

however, gas pools are present in several fields in the Sinian-lower Paleozoic 

carbonates. The Weiyuan field contains the largest pool in this lower carbonate 

section (Tang and Zhan, 1988). At least five gas fields were discovered in recent 

years in Carboniferous leached dolomites in the eastern Sichuan basin. The precise 

positions of these fields are not available.

Most of the oil fields in the basin are on the central uplifts. The largest of 

them (on Petroconsultants data) is the Lungnussu field (fig. 30) with initial 

recoverable reserves of 500-600 million barrels. The oil of the field is in Jurassic 

rocks at depths of 300 to 1,500 m. The oil is light, with a high paraffin content. 

Initial reserves of other fields range from 25 to 200 million barrels. It seems probable 

that these numbers are too high and that the reserves are overevaluated by 

Petroconsultants. The main oil production is from Upper Jurassic continental 

sandstones and from Middle Jurassic fresh-water limestones. Triassic rocks are also 

productive in at least two fields. Most of the oil fields were discovered in the 1950's 

and are now significantly depleted.

Little data on source-rock characteristics of the Sichuan basin are available 

from publications. Seemingly, the best source rock is in the Lower Permian Yangxin 

Formation. The formation is about 100 m thick and is composed of dark, clayey and 

siliceous limestones which were probably deposited in deeper parts of the carbonate 

shelf (Li and Li, 1988). The average value of TOC in these limestones is 0.97% and 

the average amount of chloroform-extracted bitumen is 0.07%. The organic matter 

is overmature throughout the basin area (Huang, 1980). Gas in the Carboniferous 

through Triassic section is thought to be sourced from this Lower Permian 

limestone (Huang, 1984). However, other authors believe that Upper Permian and 

younger coals also generated gas for the reservoir rocks (Xu and others, 1984).

Older Paleozoic rocks seem to be devoid of the source potential, except 

perhaps, for a thin graptolitic shale at the base of the Silurian (no geochemical data 

on this shale are available). A source rock for gas accumulations in the Sinian and 

lower Paleozoic carbonates is probably a stromatolitic dolomite of the Sinian 

Denying Formation (Tang and Zhan, 1988). The present TOC content in the 

dolomite is only 0.12%, but it is believed to be significantly reduced because of strong 

overmaturation (RQ > 3.0).

The only oil source rocks which are not overmature are lacustrine black 

shales, mainly concentrated in the Lower-Middle Jurassic section (figs. 24 and 25). 

Probably, these source rocks are responsible for oil pools in Mesozoic continental 

elastics and carbonates.

44
o

V

Basin Boundary 

Outlines of major 

Oil fields 

Gas field

units

Figure 30.--Oil and gas fields of the Sichuan basin. List of hydrocarbon fields of the Sichuan basin

(Wage-Giles transliteration)

1. Yinshan 7. Chiliuchin

2. Nanchung 8. Huanchiachan

3. Lungnussu 9. Shengtengshan

4. Penglaichen 10. Tengchingkuan

5. Hochuan 11. Huangkuanshah

6. Loutuhsi 12. Shihyoukou

13. ?

14. Yenkaohsi

15. Nahsi

16. Chanyuanpa

17. Kaomutin

18. Khouba

19. Khaytanpu

20. Louchuanching

21. Hoerhkan

22. Wutungchiao

23. Shuichiatsao

24. Weiyuan

45
The best reservoir rocks that contain most of oil reserves are basal Upper 

Jurassic sandstones characterized by high porosity (to 25%) and permeability. The 

porosity of the other oil reservoir, fractured fresh-water carbonates of Middle 

Jurassic age, is much lower (10-12%). Marine carbonates of the Sinian through 

Middle Triassic section contain many types of sufficiently good reservoir facies 

including fractured dolomites, reefs, detrital and oolitic limestones and dolomitic 

limestones. Distribution of gas in this section is controlled by the availability of 

seals rather than by the quality of reservoirs. Production in the lowest stratigraphic 

level (Sinian-lower Paleozoic) is controlled by Cambrian and basal Silurian shale 

seals (Tang and Zhan, 1988). Gas pools in the Carboniferous and Permian are sealed 

by the Upper Permian paralic section. Finally, gas accumulations in the LowerMiddle Triassic are capped by evaporites at the top of this section.

The Sichuan basin is strongly structured. Structural traps control all known 

hydrocarbon fields. No data on the presence of stratigraphic traps is available from 

the literature.

The basin is probably rather maturely explored. Most traps are young and 

easy to map. The majority of them have probably been tested during the decades of 

exploration history. Chances for very significant new discoveries (at least in 

Permian and younger rocks) in young (Himalayan) traps are slim. Sinian and lower 

Paleozoic carbonates are probably less explored due to great depths. The potential for 

new discoveries in these rocks, especially on paleouplifts of older (Caledonian and 

Hercynian) origin is higher.

The overthrust play in the Longmenshan is assessed highly by Chinese 

geologists. However, as it is true for any overthrust play, the integrity of seals and, 

thus, the trapping conditions may present a problem. Of certain interest may be 

traps hidden beneath decollements along Middle Triassic evaporites, especially in 

the eastern part of the basin. Stratigraphic traps in the basin are possible at various 

levels, especially under and above the main unconformities. However, in a 

strongly structured basin, such as the Sichuan basin, significant reserves in 

stratigraphic traps can hardly be expected.

46
SONGLIAO BASIN

INTRODUCTION

The Songliao basin (fig. 1) is located in the northeastern part of China just 

north of the Sino-Korean craton; the basin overlies the Paleozoic Tianshan-Xingan 

fold system and has an area of about 260,000 km2 (101,000 mi2). Along its periphery, 

the basin is surrounded by Hercynian ranges that consist of folded and 

metamorphosed Paleozoic rocks.

For over 20 years the Songliao basin has been the main oil producer in China. 

Deep drilling began in 1957, and in 1959, the super giant Daqing oil field was 

discovered. At present, the basin produces about a million barrels of oil per day 

from eighteen fields, but the Daqing field remains the major producer and contains 

the bulk of discovered reserves.

STRATIGRAPHY

The Hercynian basement of the basin is cut by a system of Jurassic rifts. The 

rifts are filled with volcanic rocks interlayered with coal-bearing continental elastics 

which are believed to be mainly or entirely of Late Jurassic age (Zhou and others, 

1985).

Jurassic rocks in the rifts and basement rocks on some adjoining horsts are 

unconformably overlain by the Lower Cretaceous (Hauterivian-Barremian?) 

Denglouku Formation (fig. 31). The formation was deposited mainly in grabens in 

the central area of the basin, but its upper portion overlaps some low-standing 

horsts between the grabens (fig. 32). The formation consists of gray and variegated 

elastics ranging from conglomerates at the bottom to shales and sandstones higher 

in the section.

The sag stage of basin development began with deposition of dominantly red 

clastic rocks of the Aptian-Albian Quantou Formation (fig. 31). Maximum 

thicknesses of the formation reaching 1600 m are still confined to the underlying rift 

structures, but its upper members overlap the highest basement uplifts.

The overlying Upper Cretaceous (Cenomanian-Coniacian?) Qingshankou, 

Yaojia, and Nenjiang Formations constitute the upper portion of the sag sequence. 

During deposition of these formations, the sedimentation area constantly increased 

with time and reached its maximum of about 200,000 km2 (fig. 33). The formations 

compose the main oil-producing section of the basin. The section consists of gray 

and green clastic rocks in the Qingshankou and Nenjiang Formations and mainly 

red and brown elastics in the Yaojia Formation. The deposition occurred in a lake 

that varied from anoxic deep-water (first member of the Qingshankou Formation 

and first and second members of the Nenjiang Formation) to shallow and oxic.

47
System 

Series

Slooe

Quaternary

h/eo^e

Pa/eofene

3 

Q

U 

<C

O

LU

a 

a.

U/

o

Mopsion

Companion

Coniacian

Turonion

Cenomonian

Alb 

Apt

Barr 

Haut

ion 

ion

emian 

erivwr

i

Fornn o i i o n

Taikang

Daon

Yion

Mmgshui

Sifangtoi

Nenj iang

Yaojio

Qingshankou

Quantou

Denglouku

T

Member

2

1

5

A

3 

2

1

2+3

1

2 +3

1

A

3

2

1

A 

3

2

1

SI rali. - 

graphic

colurnn

0 x^^^^~>-

\^jT^i,

~ ^, &,

 O 0

-o-s^SxJLxX

-^-V -^-i-.-~.

v ; / ._; ^=

-^ :' - '-~r.

=zd^z^i:

 

^r_ 

^r^>3^^s~^r^

 ^=.~-^-^=^-

 ," 

_

_T-^_

-3-

_ _

===

L'_! L

 -.' i c

=r^r±_^

tf'jjf

 * ' 

I- _* _

 

_. ^_: .___

 

7~_ __ 

* . * *

. _ _._

 - 

. . . - .

 

-=* ' -L-

 - . -., ,

 ' 

~= = :^ 

 ....

 _

 - ....

 

 - 

- 

.- .« nun

&?c

- o . o

J«^ 

 

 

Thickness 

(m)

O - 143

O - 165

O - 123

O -222

0 -333

0 -243

O -413

0 -355

155-334

47-131 

50-252

27 -222

17 -14O

0-78

263 -503

36-131

65-126

451 -672

212-417

356-651

134-212

250-621

309-700

119-220

1OOO

Reservoir

Heidimiao

Shaertu

Oil &.

90S

i 

.

o

Putaohua T *

Gaotaizi

Fuyu

YangDacheng

Nongan

ri

o 

o i

Q

Pi

r

COMMERCIAL OIL COMMERCIAL GAS

OIL SHOW GAS SHOW

Figure 31. Columnar section of the Songliao basin showing main producing reservoirs. (After Ma and 

others, 1989).

48
/2T

(27*

Figure 32. Isopach map (in meters) of the Denglouku Formation, Songliao basin. (After Zhou and 

others, 1985).

1, limits of the Denglouku Formation; 2, basin boundary

49
Figure 33. Thickness (in kilometers) of the middle Cretaceous section (Quantou through Nejiang 

Formations) of the Songliao basin. (After Zhou and others, 1985).

50
Shales dominate in the central area of the basin whereas deltaic sandstones occur 

primarily on the northern, southern, and western margins (fig. 34). Various kinds 

of sedimentary environments, including braided river, deltaic, littoral, and other 

environments, controlled the deposition (Tian and others, 1983; Xu and Wang, 

1983).

The rest of the Upper Cretaceous section consists of the red-colored Sifangtai 

and mainly gray-colored Mingshui Formations (fig. 31). The sedimentation was 

restricted to the western part of the basin where the thickness exceeds 1000 m. Thin 

Tertiary rocks complete the sedimentary sequence.

TECTONICS

The Late Jurassic rifting in the Songliao basin is believed to be connected with 

a mantle plume that caused uplift and erosion during the Triassic-Middle Jurassic 

and subsequent fracturing of the crust. The rift system consists of three grabens and 

separating horsts trending north-northeast. An unconformity at the base of 

Cretaceous rocks that overlie folded Jurassic strata (Ma and others, 1989; Zhou and 

others, 1985) probably indicate a compressional episode at the end of the Jurassic.

The crustal extension resumed in the Early Cretaceous during deposition of 

the Denglouku Formation. The Early Cretaceous rifting followed the northeastern 

Jurassic trend; however, the Cretaceous structures did not coincide in detail with 

those of the Late Jurassic (fig. 35). In particular, Jurassic rifts developed over a much 

larger area than the Cretaceous rifts. The Denglouku Formation has a transitional 

character; its lower, conglomeratic part is dearly restricted to grabens whereas the 

upper part overlaps horsts suggesting the beginning of the sag development stage.

The principal sag stage, connected with crustal cooling, includes the time of 

deposition of the Quantou through Nenjiang Formations. The thickness of this 

section exceeds 3 km (fig. 33). The tectonic setting of the overlying Upper Cretaceous 

Sifangtai and Mingshui Formations is not clear. On the one hand, the thickness of 

this section is significant (more than 1000 m) despite the smaller depositional area 

(fig. 36); on the other hand, Chinese geologists indicate that deformation began 

during this time which suggests the transition to compressional regime (Ma and 

others, 1989). However, judging from the increase of closure and area of the Daqing 

central basin high (Yang, 1985), it seems that the main compression and 

deformation occurred during the Paleogene.

The present-day basin structure of middle Cretaceous and younger strata is 

rather simple and consists of a central depression surrounded by uplifted zones 

along the basin margins (fig. 37). The strata are gently folded forming a number of 

faulted synclinal and anticlinal structures. The largest of them is the Daqing arch in 

the northern part of the central depression (fig. 38). The trend of the majority of the 

structures is from south-southwest to north-northeast which is generally parallel to 

the long axis of the basin. However, the east-west trending faults are also present,

51
Figure 34. Dep>ositional conditions of the lower Qingshankou Formation which includes one of two 

main source rocks in the Songliao basin. (After Zhou and others, 1985.)

1, limit of the rocks; 2, basin boundary

52
o - 

2. -

Figure 35. East-west cross section through the Songliao basin. (After Ma and others, 1989). Numbers 

on the cross section denote stratigraphic units.

1, Quaternary; 2, Mingshui and Sifangtai Formations; 3, Nerjiang Formtion; 4, Yaojia 

Formation; 5, Qingshankou Formation, 6, Members 3 and 4 of the Quantou Formation; 7, Members 

1 and 2 of the Quantou Formation; 8, Members 3 and 4 of the Denglouku Formation; 9, Members 1 

and 2 of the Denglouku Formation; 10, Jurassic.

53
Figure 36. Thickness (in meters) of the Upper Cretaceous Sifangtai and Mingshui Formations. (After 

Zhou and others, 1985.)

1, limit of the rocks.

54
Qi'qi'.kar$ / ,'^

Figure 37.-Main structural units of the Songliao basin. (After Ma and others, 1989.)

55
IZ4

46-

45'

46

Figure 38. Location of the Daqing oil field within the Central depression of the Songliao basin. (After 

Yang, 1985.)

1 7 oil-productive area; 2, gas productive area; 37 local depressions; 4, structural terraces. 

Numbers on the map denoting separate pools of the Daqing field:

1, Lamadian pool; 2, Saertu pool; 3, Xingshugang pool; 4, Taipingtun pool, 5, Gaotaizi pool; 6, 

Putaohua pool; 7, Aobaota pool.

56
especially on the south. The main stage of formation of the structures was probably 

pre-Tertiary time. Tertiary rocks unconformably overlie the Upper Cretaceous. 

Judging from published cross sections (e.g., fig. 35), very few or none of the faults 

penetrate above the top of the Upper Cretaceous.

PETROLEUM GEOLOGY AND POTENTIAL EXPLORATION PLAYS

In 1985, 18 oil fields were in production in the Songliao basin. The main 

fields are shown in figure 39. The most important is the supergiant Daqing oil field 

that contains the majority of the basin's oil reserves. The oil accumulation of the 

field is in a structural trap on a large (2,800 km2) arch located in the northern part of 

the central basin depression. The arch comprises seven local anticlinal structures 

separated by structural saddles. All of the anticlines contain oil pools; in the 

northern part of the field, the pools are connected with one another through the 

structural saddles (fig. 38). The pays are in deltaic sandstones of the Qingshankou, 

Yaojia, and Nanjiang Formations (fig. 31). Most of the oil reserves are in the 

northern part of the field where the sandstones are thick. Some of the oil pools 

there have gas caps, but the gas reserves are insignificant (Yang, 1983). The thickness 

of reservoir rocks gradually decreases and the reservoir properties deteriorate 

southward where separate sandstones are only 1-3 m thick (Ma and others, 1989). 

Significant variations in permeability of sandstone reservoir rocks result in poor 

sweep efficiency of waterflooding which creates production problems (Xu and 

Wang, 1983). The producing sandstones are sandwiched between two main source 

rocks of the basin which are the first member of the Qingshankou Formation and 

the first member of the Nenjiang Formation (fig. 31). However, the source rocks are 

immature to marginally mature on the crest of the arch and thus, lateral migration 

from adjacent depressions needs to be involved to explain the formation of the 

giant field. The upper of the two source rocks is also a regional seal. Oil in the field 

is of medium gravity with high paraffin (23-26%), low resin (6-14%), and low 

sulphur contents (Yang, 1985).

The Songfangtun and Gaoxi fields located just south of the Daqing field 

(fig. 39) contain oil pools in the same deltaic complex. Production is from the 

Putaohua reservoir (figure 31), but the pools are controlled by pinch-outs of 

sandstones rather than by structures (fig. 40). The reservoir thickness is 50-60 m and 

the net pay is 5-10 m.

The Fuyu-Xinli oil field is possibly the second largest field in the basin (fig. 

41). Oil pools of the field are in the top portion of the Quantou Formation just 

below source rocks of the first member of the Qingshankou Formation. The oil is 

trapped at updip pinch out of sandstones on a faulted structural nose (fig. 42). Oil 

pools in at least some adjacent fault blocks do not have a hydrodynamic connection.

57
Figure 39.-Principal oil fields of the Songliao basin. (After Xu and Wang, 1983.)

58
Figure 40.-Oil pools in different types of traps in the Putaohua reservoir of the north-central Somdiao 

basin. (After Ma, 1985.)

1, structural contours; mb.sJ.; 2, structural pools; 3, combination pools; 4, stratigraphic pools.

59
Figure 41. Fuyu-Xinli oil field, Songliao basin. (After Ma and others, 1988.) 

I, productive area; 2, structural contours; m b.s.l.; 3, fault.

60
',00 -

Figure 42. Cross section through the Fuyu field, Songliao basin. (After Ma and others, 1988.)

61
Source rocks of the basin are deep-lake deposits. They were extensively 

studied by Chinese geochemists as a prime example of source rocks of continental 

origin. Two main source rock intervals are in the lower Qingshankou Formation 

(member 1) and the lower Nanjiang Formation (member 1); they were deposited 

when the ancient lake reached its maximum depths and extents. Black anoxic 

shales are also present at some other stratigraphic levels, but they are only locally 

developed and are of secondary importance. The TOC content of the black shales 

commonly exceeds 2%. The kerogen is of mixed type, however, type I, rich in lipids, 

greatly predominates (Yang and others, 1985).

The present geothermal gradient in major oil-producing areas of the basin is 

high; it averages 4.2°C/100 m and decreases to 3.5°C/100 m toward the basin 

margins. The ancient geothermal gradient was even higher and reached 5.6°C/100 

m based on vitrinite reflectance data (Ma and others, 1989). The high geothermal 

gradient resulted in a shallow occurrence of the oil window; the top of the oil 

window is found at a depth of 1100-1300 m and its bottom (top of the gas window) is 

at 2700-2900 m. The peak oil generation occurs at 1700-1900 m. Oil generation in the 

deepest depressions could have begun as early as time of deposition of the Nenjiang 

Formation (Yang and others, 1985), but the main oil generation probably took place 

during the latest Cretaceous. Some rocks of this age were probably removed by preTertiary erosion.

Besides the middle Cretaceous sources, presently unstudied source rocks may 

be present in the Jurassic section that fills the rifts. Dark shales are present among 

Jurassic coal-bearing rocks and are believed to have an oil and gas-source potential 

(Ma and others, 1988,1989). The potential of these rocks as a source for oil is 

doubtful because of their coaly character and over-maturation throughout most of 

the basin. However, these rocks could have generated gas, and gas shows were 

recorded in some wells (fig. 31).

Reservoir properties of rocks in the main producing intervals of the middle 

Cretaceous vary significantly depending on facies characteristics of sandstones. The 

best reservoirs are associated with the main bodies of the progradational deltas 

deposited closer to the basin margins away from the area of source rock deposition. 

However, these sandstones are largely devoid of oil, probably due to the lack of seals 

and to flushing. Most oil production is from the transitional zone between the 

main part of a delta and the deep-lake source facies. Sandstones in this zone were 

deposited in distributary mouth bars, in channels, and as sand sheets in the delta 

front (Xu and Wang, 1983). Reservoir sandstones in this zone are interlayered by 

lacustrine shales that acted as seals and occur in contact or in close proximity to 

source rocks. Reservoirs are chiefly composed of feldspathic and graywacke 

sandstones. The loss of porosity with depth is significant and below 2-2.5 km, the 

reservoir properties are poor (Du and others, 1984). Farther into the paleo-lake, 

sandstone bodies are thin and erratically distributed; they are absent in central areas 

of the paleo-lake. Typical reservoir porosities in the oil fields are about 20% and 

permeabilities are a few tens to a few hundreds of millidarcies (Ma and others, 1988).

62
Oil pools in the main middle Cretaceous reservoirs are sealed by interlayered 

shales. However, the main regional seal is a shale in the second member of the 

Nenjiang Formation. This lacustrine shale, from 50 to 250 m thick, covers a variety 

of underlying fades over most of the basin area. The lacustrine shale that composes 

the first member of the Qingshankou Formation, one of the two major source rocks 

in the basin, also acts as a regional seal for the underlying Fuyu and Yangdachengzi 

reservoirs (fig. 31).

Structural traps control the majority of oil reserves of the basin, but many 

reservoirs are actually combination traps because extensions of oil pools in these 

reservoirs are controlled by lithologic changes. Fault planes also seal many pools as 

can be seen in the Fuyu (fig. 40) and Daqing (fig. 43) fields. Purely stratigraphic traps 

are also known, as, for example, in the Songfangtun field (figs. 39 and 44) in the 

Sanzhao depression east of the Daqing field (fig. 38).

The Songliao basin is a mature exploration area although data on the actual 

amount of exploration are not available. Many thousands of wells have been 

drilled and, although most are development wells, the number of wildcats is 

probably large. All significant structural traps in the middle Cretaceous section 

probably have been drilled and the largest fields have been found. However, the 

complex fades pattern of the delta systems presents numerous opportunities for 

exploration for stratigraphic traps outside of the structural uplifts. Most fields in 

these traps will be small, although their number may be significant. The total 

amount of reserves in these fields probably will not exceed several percent of those 

already discovered in the basin.

Published data suggest that there has been very little exploration for deeper 

Cretaceous reservoirs beneath the main producing intervals of the Qingshankou 

through Nenjiang Formations. Oil fields are known in the upper part of the 

Quantou Formation, but the oil migrated downward from the lower Qingshankou 

source rocks (Ma and others, 1988). Productivity of older stratigraphic units depends 

on the presence of deeper source rocks. It seems unlikely that significant oil-source 

rocks are present in the Lower Cretaceous section. Coal-bearing dastics in the Upper 

Jurassic rifts are probably adequate source rocks for gas, as confirmed by observed gas 

shows in wells. Probably, very little exploration has been done in these rifts, which 

occur at depths of 4-5 km and more. Poor reservoirs and especially the scarcity of 

seals are limiting factors that will affect the productivity of this section.

63
Figure 43.-Cross section through the Daqing oil field. (After Ma and others, 1989.)

1750 -

1800 -

1850 -

Figure 44. Cross section through the Songfangtun oil field. (After Ma and others, 1988.) 

1, oil pay; 2, oil saturated low permeable sandstone; 3, tight sandstone.

64
NORTH CHINA BASIN

INTRODUCTION

The North China (Bohai Gulf, Bohaiwan) basin (fig. 45) is in the eastern part 

of the Sino-Korean craton. The basin area is 196,000 km2 (76,500 mi2). On the north, 

west, and south, the basin is bounded by the Yanshan foldbelt and the Taihang and 

Taikang uplifts respectively. The Luxi uplift flanks the southeast side of the basin. 

On the east, the basin boundary is a system of faults, including the large Tanlu 

strike-slip fault. The basin extends partly offshore, occupying shallow waters of the 

Bohai Gulf.

For many years, the North China basin has been a major oil producing basin 

in China, second only to the Songliao Basin. Oil was first discovered in the early 

1960's in the Shengli productive area of the Jiyang depression. At present, this area 

is the second (after the Daqing field) largest producer in China; in 1989, it produced 

at a rate of 664,000 b/d. This constitutes about two-thirds of production from the 

North China basin.

STRATIGRAPHY

The Archean-Early Proterozoic basement is overlain by a poorly known 

Middle-Late Proterozoic sequence. Sinian carbonate rocks, consisting dominantly of 

stromatolitic dolomite, occur at the top of this sequence. Several hundred meters of 

these dolomites have been penetrated by wells drilled in depressions of the North 

China Basin.

The Paleozoic section of the basin is similar to the Paleozoic section of the 

western areas of the Sino-Korean craton. Lower Cambrian siliciclastic rocks 

unconformably overlie the Proterozoic and grade upward into Middle-Upper 

Cambrian shallow-water, partly oolitic carbonates. The Lower-Middle Ordovician is 

composed of limestones and dolomites containing gypsum beds in the upper part of 

the section. Sedimentation during early Paleozoic time occurred in platform 

conditions; thickness of these rocks reaches 1.5 km in the south part of the basin and 

decreases to a few hundred meters northward (figs. 46, A and B). Sinian and lower 

Paleozoic rocks, especially carbonates, constitute important reservoirs in the "buried 

hill" structures.

Uplift at end of Middle Ordovician time exposed the North China basin as 

well as the entire Sino-Korean craton to subaerial erosion. Sedimentation did not 

resume until Middle Carboniferous time. The Carboniferous-Permian section 

consists of a paralic coal measure that grades upward into red beds with some 

evaporates. Thickness of the upper Paleozoic section is commonly several hundred 

meters (figs. 46, C and D), but in places, it may reach as much as 1,500 m (Hu and 

others, 1989) mainly due to thickening of the red beds.

65
* , x / it -wt v l /

s t__ /T

LQda

Teikang 

upland

EXPLANATION

Uplifted area

Basin boundary

Boundary of structural unit Dashed 

where approximately located, dotted 

where Inferred

0 100 200 300 KILOMETERS 

1_______I_______I______|

Figure 45.-Main structural units of the North China basin. (After Lee, 1989.)

66
Figure 46.-Restored thickness of Cambrian (A), Ordovician (B), Carboniferous (c), and Permian 

Shihezi Formation (D) rocks of the North China basin. (After Li, 1983b.)'

67
The thick (to 1,000 m) Triassic section is known only on the southwest of the 

basin where it is composed of red elastics. Only a few wells penetrated thin Triassic 

rocks north of this area. Commonly, the Triassic is absent due to either 

nondeposition or later erosion (Zhang, 1985).

The Jurassic-Cretaceous stratigraphy is also poorly known because a very 

limited number of wells penetrated significant (several hundred meters) thickness 

of these rocks (Zhang, 1985). According to available data, the Lower-Middle Jurassic 

in different areas of the basin is formed by either coal-bearing, commonly tuffaceous, 

clastic rocks or thick volcanics (Hu and others, 1989). Structures in Upper JurassicCretaceous rocks are different from those in the Lower-Middle Jurassic. The rocks 

are gray and dark-red elastics and abundant volcanics. The thickness of Upper 

Jurassic-Lower Cretaceous rocks varies greatly in different fault blocks and may be as 

large as 4 km (fig. 47a).

Regional uplift and erosion during the Paleocene preceded the Tertiary stage 

of rifting that began in Eocene time. A description of the Tertiary sedimentary 

Figure 47a succession is shown in table 3. The facies and thicknesses of stratigraphic 

subdivisions of the Eocene-Oligocene section are controlled by synsedimentary rift 

development. The entire sequence consists of continental rocks. Coarse-grained 

facies are mainly confined to flanks of the grabens where clastic fans and deltas were 

formed. Basinward, these facies grade into fine-grained rocks deposited in lakes and 

on alluvial plains. Deep lacustrine facies intermittently appear among lacustrine 

rocks; most of them are concentrated in the Shahejie Formation, especially in its 

third member. The deep lacustrine rocks are characteristically enriched by organic 

matter and are the main source for hydrocarbon accumulations. The preservation 

of organic matter was primarily controlled by water stratification in saline and 

brackish lakes. Interbeds of gypsum and salt are abundant in the Eocene-Oligocene 

section, and in places, they form diapiric structures (Wang and others, 1985). 

Detailed models of facies distribution in rift basins of eastern China are described by 

Chen and others (1984).

Intensive rifting during Eocene and Oligocene time was accompanied by 

significant volcanic activity. Tuffaceous material is widespread in sedimentary 

rocks of this age and thick basalt flows are common in the lower part of the 

Kongdian Formation. Basalts are locally present also in younger units of the 

Eocene-Oligocene section, especially in the northeastern part of the basin.

The Neogene Guantao and Minghuashen Formations unconformably overlie 

various older rocks both in rifts and on intervening horsts and form a sag over the 

Eocene-Oligocene graben system. The thickness of the formations is rather even 

(fig. 47b) and ranges from 1 to 2 km over most of the basin area and to 3 km and 

more in the Bozhong depression. Coarse clastic rocks dominate in the lower part of 

the Neogene sequence and grade upward into alluvial shales and sandstones. 

Quaternary sands and gravel, 100-300 m thick, form a blanket of sediment on the 

surface.

68
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120- 

300

800- 

1.200

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1.000

100- 

1.200

ISO600

400- 

1.000

200- 

500

300- 

1.200

60- 

1.900

L t 6Ao-f?«* v

YcUov/ clay and pale fjcy, pcyuh ycUo^ wnd and ^f»vcl bcdi

e-rownith yellow clay rock interbcddcd with clutonc and sandy conglomerate

Upper part: reddish brown cby rock inter bedded with light brown and greyish 

white sandstone and sandy conglomerate, 

lower pan: greyish green, greyish white massive undstonc ind sandy conglomerate

Jlark brown, brownish red and greyish green mudstonc interbcddcd with pale brown 

and greyish green sandstone.

&ey mudslonc intercalated with yellowish brown oil ihxk. greyish white dolomite 

and biolithitc.

Upper pan: greyish white gypsum and rock alt intercalated with grey mudslonc. 

and a few dolomite. 

middle and lower part: inter beds of brownish red mudslorve and siltstonc. 

intercalated with brownish grey sandstone and sandy conglomerate.

.Jfcaik grey mudsXonc intercalated with yellowish brown oil ihaJc, grey marl and 

greyish white dolomite

Upper part: greyish white gypsum and rock salt (not extensively distributed) 

intercalated with dark grey mudstonc. dolomite and yellowish brown oil shale, 

middle and lower pan: inteibcds of dark grey mudstonc and sandstone.

Upper and lower pan: interbeds of red. brownish rod mudstone. argillaceous 

si It stone and sandstone, 

middle part: greyish black mudstonc intercalated with greyish white sUtttonc 

and fine sandstone

Table 3.-Stratigraphy of the North China \*sta(2Ji**9t

69
Figure 47a.-Thickness of Upper Jurassic-Lower Cretaceous rocks of the North China basin. (After Hu 

and others, 1989.)

70
Figure 47b.~Thickness (in kilometers) of Neogene rocks of the North China basin. (After Hu and 

others, 1989).)

71
TECTONICS

During the Paleozoic, the tectonic development of the area now occupied by 

the North China basin was very similar to that of the rest of the Sino-Korean craton. 

Gentle platform-type depressions and uplifts controlled the deposition. Long 

periods of non-deposition and subaerial exposure from Late Ordovician through 

Early Carboniferous time did not result in a significant angular unconformity and in 

change of the generally east-west structural trend.

The Early Kimmerian (Indosinian) deformation at the end of the Middle 

Triassic resulted in folding, local thrusting, and granite intrusions. The general 

structural grain of this phase of deformation had a northeast trend. The tectonic 

character of the following Late Triassic and Early-Middle Jurassic deposition remains 

unclear. Li (1983b) and Zhang and others (1983) suggest that rifting began during this 

time. However, a model of clastic and volcanic molasse deposition in postdeformational subsiding blocks and intermontane basins (Hu and others, 1989) 

seems more probable.

The tensional regime and widespread rifting began in Late Jurassic time and 

continued through the Early Cretaceous. The strike of the rift system was 

northeastern to north-northeastern, similar to the younger Tertiary rifts. The rifting 

event was not confined to the North China basin, but embraced large areas of 

Mongolia and southern Siberia (Zhang and Zhang, 1988). Volcanic material played 

an important role in sedimentary filling of the rifts.

The Late Jurassic-Early Cretaceous rifting apparently was not followed by a 

significant sagging stage. Possibly, the sagging could have begun in the Late 

Cretaceous. A few tens of meters of Upper Cretaceous clastic rocks unconformably 

overlie Lower Cretaceous elastics and volcanics. A deformation event in the Late 

Cretaceous (?)-Paleocene resulted in compression, regional uplift, and folding of 

Mesozoic rocks. Sinistral transpression and wrenching are believed to have resulted 

in echelon folding (Liu, 1986).

The Eocene-Oligocene rifting stage inherited the general northeastern 

structural grain of the Mesozoic rifts. However, particular Tertiary and Mesozoic 

grabens do not necessarily coincide. The rifting is believed to have been caused by a 

mantle plume, which seems to be confirmed by a high heat flow and thinned 

continental crust beneath the rift system (Lee, 1989). The Eocene-Oligocene rifting 

stage was followed by sag subsidence that began in Miocene time and evidently 

continues at present.

The present-day basin structure consists of more than 60 separate grabens. 

Most of them are half grabens (wedge-shaped, or dustpan-shaped depressions of 

Chinese geologists), but some are more or less symmetrical grabens with faults on 

both sides. Normal faults that control half grabens are synsedimentary and listric in 

shape. Facies characteristics of synrift sedimentary rocks may change from graben to

72
graben; actually, each graben represents a separate sedimentary and petroleum 

system. Seven depressions (subbasins), each consisting of a number of grabens, are 

recognized (fig. 45). Six of these depressions are productive; oil and gas have not 

been found only in the Linqing depression. The depressions are separated by uplifts 

on top of which the Paleogene section is absent. Smaller horsts with thin or no 

Paleogene rocks separate individual grabens.

PETROLEUM GEOLOGY AND POTENTIAL EXPLORATION PLAYS

The petroleum geology of the North China basin is complex because the basin 

consists of a large number of laterally disconnected petroleum systems. Despite 

many similarities in the structural development of the different grabens, local 

characteristics of sedimentary environments resulted in different facies and 

dissimilar distribution of both source rocks and reservoir rocks.

The main source rocks in the basin are deep-lacustrine shales deposited under 

anoxic or suboxic conditions on the lake bottom under a stratified water column. 

Both variations in salinity and temperature could have resulted in the stratification. 

The TOC content in the shales is not very high and averages about 2%; however, the 

kerogen in the organic matter is rich in hydrogen (dominantly of type I). High heat 

flow and geothermal gradient (average 3.4°/100m) enhanced the early maturation of 

organic matter at rather shallow (about 2,700 m) depth. Most source rocks are 

concentrated in the Oligocene Shahejie Formation and are mature. Geochemical 

data suggest that some gas could also have been generated from coaly source rocks of 

Carboniferous-Permian age (Zhou and others, 1988; Zhu and Xu, 1988).

Reservoir rocks in the basin are clastic rocks of various origin (fluvial, deltaic, 

lakeshore, clastic fans, turbidites) in the Eocene through Miocene section. Also 

productive in this section are fresh-water carbonates. Important production in the 

basin also comes from the weathered top portion of various pre-Tertiary rocks, from 

the Proterozoic to the Mesozoic, in the "buried hill" structures. Factors controlling 

reservoir properties of rocks in these structures are reviewed in Zhai and Zha (1982). 

Reservoirs of the basin are sealed by shales and evaporates in the Paleogene synrift 

sequence; however, the main regional seal that covers the entire basin area is the 

upper Miocene-Pliocene Minghuashen Formation composed of shales with some 

layers of coarser clastic rocks.

A variety of traps structural, stratigraphic, and a combination of these are 

productive in the basin. A review and classification of traps are provided by Li 

(1983b) who recognizes four main types: structural, combination, stratigraphic, and 

buried-hill traps. All trap types include a number of subtypes, each of which is 

controlled by a major structural and/or stratigraphic feature (e.g., roll-over anticline, 

updip pinch out, fault-sealed block, et cetera).

73
The following is a concise review of the six productive depressions (subbasins 

of the North China basin. The seventh, Linqing, depression does not contain oil 

and gas fields. The reasons for the lack of production in this depression are not clear 

beacase almost no data are present in the literature. Relatively thin and partially 

eroded Cenozoic rocks in this depression are underlain by thick Mesozoic sequences 

(Zhang, 1985).

The Dongpu-Kaifeng depression is in the southwest part of the basin (fig. 45). 

The depression is a horst-and-graben system about 7,000 km2 in area (fig. 48). In a 

cross section through the Wenliu field, the structure is basically formed by two 

grabens separated by a horst (fig. 49). Thickness of Tertiary rocks exceeds 5.5 km in 

the deepest grabens. The Tertiary is underlain by Mesozoic red beds about 1 km 

thick, by a Carboniferous-Permian coal measure, and by Cambrian-Ordovician 

carbonates (Zhu and Xu, 1988). The petroleum geology presented here is mainly 

after Lee (1989).

The main source rocks in the depression are shales and interlayered 

evaporites (salt and anhydrite) of the lower Shahejie Formation (Zhu and Xu, 1988). 

The thickness of the oil-generative part of the formation may reach 1,000 m. The oil 

window is located at a depth between 2,400 and 4,100 m. Based on isotopic data and 

thermal modeling, some dry gas in subsalt reservoirs in the Wenliu and Weicheng 

fields (figs. 50 and 51) is believed to have been generated by the CarbonifeousPermian coal measure (Zhu and Xu, 1988; Zhou and others, 1988).

Reservoir rocks are sandstones in the synrift sequence, mainly in the two 

lower members of the Shahejie Formation. Oil and gas pools are sealed by 

evaporites. Most pools are controlled by faulted structural traps, but some of them 

seem to be located on monoclines and sealed by faults (Lee, 1989).

Gas dominates in hydrocarbon reserves of the Dongpu-Kaifeng depression. 

This may be due either to the additional charge of gas from Paleozoic coals or 

overmaturation of Tertiary source rocks in the deepest grabens where the base of the 

Shahejie Formation occurs at 5-6 km. Probably, the large amount of gas also is due 

to the presence of evaporite seals that prevented escape of gas through faults and 

fractures.

The largest fields are the Pucheng and Wenliu fields, both controlled by the 

central horst in the north-central part of the depression (figs. 48 to 50). Production 

from the Pucheng field was slightly less than 14 million barrels in 1983 (Lee, 1989). 

No data on the amount of exploration in the depression are available. It seems 

likely that all significant structural traps associated with the central horst have been 

tested. Probably, much less exploration was targeted at stratigraphic traps, especially 

at pinch outs against the outer faults of the depression. The Baimiao gas field (fig. 

48) is in a trap of this type. The southern part of the depression is reportedly less 

explored; gas is expected to dominate in undiscovered resources (Lee, 1989).

74
Linqing 

depression

Faulted boundary of primary sag

Secondary sag

Oilfield

Gas field

Oil and gas field

®Qingfeng

PuchenQ oil field 

0 Puyang 

Huzhuangj

10 20 X <0 KILOMETERS

Taikang upland

Rgure 48. Structural units and oil and gas fields of the Dongpu-Kaifeng depression, North China 

basin. (After Lee, 1989.)

75
w 

c

METERS Wenliu oil and gas field

Ooo

Figure 49.~Cross section through the Dongpu-Kaifeng depression, North China basin. (After Lee, 1989.)

Location of the cross section is shown in fig. 48. Pes 1-4 stays for units 1 to 4 of the Paleogene Shahej

Formation. Fed is the Dongying Formation.
1000-

2000-

3000-

4000-

5000-

(M)

Figure 50.-Cross section through the Wenliu oil-gas field, North China basin. (After Zhu and Xu, 

1988.)

Letter S with numbers indicate units and subunits of the Shahejie Formation. Ed is the 

Dongying Formation. 1, carbonate rocks; 2, evaporites; 3, coal-bearing rocks; 4, oil; 5, gas.

77
Figure 51.-Cross section through the Weichen oil-gas field, North China basin. 

(After Zhu and Xu, 1988.) For explanation see figure 50.

78
The Tizhong depression is in the northwestern part of the basin, just 

southeast of the Taihang uplift (Fig. 45). The area of the depression is 25,000 km2. 

The depression is an important oil producer; it contains the giant Renqiu oil field, 

the second largest field in the North China basin.

The depression consists of two chains of half grabens separated by the central 

uplifted zone (Fig. 52). Twelve individual grabens may be distinguished. All 

production is found in the southeastern zone of the grabens. The grabens northwest 

of the central uplift are nonproductive, supposedly because of the absence of 

adequate source rocks (Fig. 53).

Source rocks are deep-lacustrine shales with the TOC content varing from O.4 

to a little more than 1 % and the hydrocarbon content of 250 to 700 ppm (Wu and 

Liang, 1988). Large thicknesses of source rocks reaching many hundreds of meters 

and a high expulsion efficiency owing to the close juxtaposition of source and 

reservoir rocks evidently make up for the relatively low richness of the source 

rocks. Most source rocks are concentrated in the third member of the Shahejie 

Formation, but they are present also in other parts of the section. Lower Cretaceous 

lacustrine shales (TOC 0.8-1.1%) and Carboniferous-Permian coal-bearing rocks are 

present in the pre-Tertiary sequence (Lee, 1989). Gas derived from the 

Carboniferous-Permian source rocks have been reported in some buried-hill pools, 

mainly in Ordovician carbonates (Dai and Xia, 1990).

Reservoir rocks in the depression are Tertiary elastics of various origin 

(deltaic fans, lakeshore beaches and bars, channel and turbidite sandstones). They 

are sealed by either shales or evaporites and are productive from a large variety of 

structural, fault-sealed, and stratigraphic traps (Wu and Liang, 1988; Li Disheng and 

others, 1988, Zha, 1984) (Fig. 54). Most petroleum reserves in the depression are in 

pre-Tertiary reservoirs in the buried-hill type structures. The best reservoir rocks 

are Proterozoic and lower Paleozoic carbonates (Zhai and Zha, 1982) with reservoir 

properties strongly enhanced by leaching, massive karstification, and fracturing (Fei 

and Wang, 1984). The two-billion-barrel Renqiu oil field produces from strongly 

fractured upper Proterozoic carbonates surrounded and overlain by source rocks of 

the Tertiary Shahejie Formation (fig. 55). Other lithologies at the eroded top of preTertiary rocks have much poorer reservoir properties.

The Jizhong depression is maturely explored. At least 35 fields produce oil. 

The largest production is from the Renqiu field which produced more than 75 

million barrels in 1985 (U.S. Energy Information Administration, 1987). A number 

of other buried-hill fields have been discovered. Although data on the amount of 

drilling are not available, it seems likely that the buried-hill play, as well as other 

structural plays in this depression, have been thoroughly explored.

79
Rgure 52,-Stmctural units and oil fields of the Jizhong depression, North China basin. 

(After Wu and Liang, 1988.)

1, limit of Lower Tertiary rocks; 2, oil field in Tertiary rocks; 3, oil field in 

pre-Tertiary rocks of buried hill; 4, fault.

80
Figure 53.-Distribution of Lower Tertiary source rocks and related oil fields in the Jizhong depression, 

North China basin. (After Wu and Liang, 1988.)

1, oil field; 2, outline of mature source rocks; 3, isolines of quantity of generated oil in 10,000 tons 

per km2; 4, limit of source rocks; 5, fault.

81
v v - BasaK

Figure 54.-Types of oil and gas pools in the Langfang-Guan sag of the Jizhong 

depression, North China basin. (After Li and others, 1988.)

82
Figure 55. Cross section through the Renqiu oil field, North China basin- (After Yan and Zhai, 1981.) 

Ek,Kongadian Formation; Es, Shahejie Formation; Ed, Dongying Formation; N, Neogene.

83
The Huanghua depression is in the central part of the North China basin, 

between the Jizhong depression to the northwest and the Jiyang depression to the 

southeast (fig. 45). The Huanghua depression consists of eight half-grabens. The 

northeastern area of the depression lies offshore in the Bohai Gulf (Fig. 56). 

Thickness of Tertiary rocks in the deepest grabens of the depression may exceed 8 

km of which Paleogene rocks constitute about 5.5 km.

Principal source rocks in the depression are deep-water saline lake beds in the 

three upper members of the Shahejie Formation and near the base of the overlying 

Dongying Formation. The TOC conent varies from 0.5 to 2.8% and averages 1.26%. 

The maximum thickness of source rocks (to 1.5 km) is found in the Qikou and 

Bangiao grabens (sags in the terminology of Chinese geologists). The intervening 

Dagang uplift controls the principal oil fields in the depression known as the 

Dagong field complex. The complex consists of the Bangiao, Tangjiahe, Gangdong, 

and Gangxi fields shown in Fig. 56 (Lee, 1989). The combined oil productions from 

these fields exceeded 23 million barrels in 1985 (U.S. Energy Information 

Administration, 1987).

Tertiary silicilastic reservoirs contain the main oil reserves in the depression. 

Productive clastic rocks in the synrift Paleogene sequence are of various origin: 

clastic fans, turbidites, channel and beach sandstones. They are described in detail by 

Zhao and others (1988). They produce hydrocarbons from fault-block traps, rollover 

anticlines, erosional and synsedimentary pinch outs, and faulted anticlines with salt 

and/or mud-flow cores (Hu and Qiao, 1983). Important reservoirs are also present 

in post-rift Miocene rocks, especially in basal sandstones of the Guantao Formation, 

in drape structures over horsts. Smaller production is from lower Paleozoic 

carbonates in the buried-hill structures and from Paleogene bioclastic limestones.

The productive central depression area is densely explored. The absence of 

fields in the relatively shallow southwest part of the depression is probably due to 

the absence or poor development of source rocks. Some exploration offshore has 

been conducted by the Japan China Oil Development Corporation (JCODC), and by 

1985, four wells had been drilled on the southern flank of the Qikou graben 

(Matsuzawa, 1988). One of these wells tested 2000 bid of oil from the Shahejie 

Formation at 2168-2184 m, but probably, the field has not been developed. The other 

three wells were dry.

The Tiyang depression is the principal oil producer in the North China basin. 

The depression is located southeast of the Huanghua subbasin, north of the Luxi 

uplift (fig. 45). The depression area is 17,000 km2 onshore and about half as much 

offshore. A system of central uplifts separates the deepest part of the depression, the 

Dongying sag (5,700 km2), on the southeast from the Zhanhua and Chezhen sags on 

the north and the Huimin sag on the west (Fig. 57). The Dongying sag is probaly the 

best studied structure in the North China basin. The sag is a half graben tilted 

northward toward the central uplifts. Thickness of the Tertiary sequence exceeds 7 

km.

84
Van Shan (old bell

Secondary sag

Oilfield

Depression f^uU boundary

o

Linqing 

depression

Figure 56. Structural units and oil and gas fields of the Huanghua depression, North China 

basin. (After Lee, 1989.)

85
00

Uplift

Buried-hill oil field 

Oil and gas field 

Fault 

Pingfangwang 

oil and gas field

Boxing oil 

and gas field

Figure 57.-Structural units and oil and gas fields of the Jiyang depression, North China basin. (After Lee, 1989.)
The principal source rocks are located in the third and first members of the 

Shahejie Formation. The average geothermal gradient is 36°C/km and the oil 

window is at 2,200 to 3,000 m. Main production in the basin is from Tertiary elastics 

of both synrift (Paleogene) and postrift (Neogene) sections. Also productive are 

Tertiary carbonate bank limestones and Ordovician carbonates in the buried-hill 

structures. The distribution of source and reservoir rock facies in the Dongying sag 

is shown in fig 58. These facies are typical (with certain variations) of all Paleogene 

grabens of the basin.

The well known "Shengli field", the second largest oil producer in China 

(after the Daqing field), is in the Jiyang depression. Actually, the Shengli is not a 

field, but is a large producing complex which includes 59 oil and gas fields (Scott, 

1990). Forty eight of these fields are on production; in 1989, they produced 664,000 

b/d. Most fields are in the central and northern Dongying sag, in the Zhanhua sag, 

and on the flanks of adjacent uplifts (figs. 57 and 59). The largest fields of the 

complex are the Shengtuo (3.7 billion barrels), Gudao (2.5 billion barrels), and 

Gudong (1.6 billion barrels) fields. Probably, the numbers represent in-place 

resources. A major offshore discovery was reportedly made in 1988.

A large variety of trap types, both structural and stratigraphic, control the oil 

fields. Structural fields dominate; they contain about 77.5% of oil reserves of the 

Jiyang depression (Shaui and others, 1988). The structural traps are primarily tilted 

fault blocks and overlying drape anticlines (fig. 60). Anticlinal structures with a 

diapiric cores (shale or salt) are also important (Wang and others, 1985). Among 

stratigraphic pools, those that are found immediately below and above 

unconformites are the largest; these pools account for more than three-quarters of 

the reserves in stratigraphic traps. An example of a pool in strata overlapping the 

unconformity is shown in fig 61. Pools in other types of stratigraphic traps (seven of 

them are described in Shaui and others, 1988), including buried-hill pools, are far 

less important in the Jiyang depression.

The onshore portion of the Jiyang subbasin is probably very maturely 

explored. More than 2,000 wells are drilled annually, about 250 of them are 

exploratory holes (Scott, 1990). A large variety of stratigraphic traps, many of which 

are subtle (Ma and others, 1982), suggest that discoveries will continue for a long 

time. However, only a relatively small fraction of already discovered oil can 

probably be added to the reserve base. No data on the amount of offshore 

exploration are available. The offshore area of the subbasin is probably explored 

much less, and significant discoveries can be expected, especially in the offshore 

continuation of the Zhanhua sag in which the presence of rich source rocks has 

been proved.

The Bozhong depression is almost completely offshore in the Bohai Gulf. It 

is northeast of the Jiyang depression and is bounded on the southeast by the Tanlu 

fault system (fig. 45). The area of the depression is approximately 16,000 km2 .

87
oo

00

8

Figure 58.-Facies of the Shahejie Formation in the Dongying sag of the Jiyang depression, North

China basin. (After Hu and others, 1989.) Sj to S4 denote members of the Shahejie Formatio

1, gypsum; 2, micritic limestone; 3, detrital limestone; 4r deep lake; 5, moderately deep lake 

prodelta; 6, shallow lake; 7, alluvial cone; 8, lake-floor fan; 9, delta; 10, direction of 

transportation of clastic material.
DAWANGZHUANG 7H,, AfjfiXi

CHENGDONG 80/7a/GUff

WUHAOZHUANG 

CHANGDI 

GUNAN 

GUDONG 

HONGLIU

k 

XINTAN

Laizhou Bay

DONGXIN 

,NGLI 

_ WANGJIAGANG

QIA02HUANG 

CHUNHUA 

BOXING

LINPAN

WEIBEI

o Yucheng

o Jinan

Figure 59.-CH1 field* of the Shengli producing complex. (After Scott, 1990.)

89
NW 

A

S

o

METERS 

0

1000

2000

3000

4000

5000

Gpdao oil field

N

2200-*- 

Minimum 

depth for 

maturation

Figure 60.~Cross section through the Gudao oil field in the Jiyang depression, Nort

China basin. (After Lee, 1989.) PeSj to PeS4 denotes first to fourth members 

of the Shahejie Formation. Lt>ca.it<>» o[-tu. cross .sccf»o^ U
A

I00(?

A'

Figure 61 .-Oil field in onlapping strata of the Shahejie Formation, S-G-S area of 

the Jiyang depression, North China basin. (After Shaui and others, 1988.)

91
Little information has been published on the geology of the Bozhong 

depression. By 1980, Chinese Bohai Oil Corporation had drilled about 40 wells in 

the Bohai Gulf. Since 1980, the offshore exploration was conducted by a joint 

venture, the Japan-China Oil Development Corporation (JCODC). By July of 1984, 

the JCODC had drilled 22 wells in the area, most of them in the Bozhong subbasin. 

The drilling results are described by Matsuzawa (1988). The main structural units 

offshore and locations of drilled wells offshore in the Bohai Gulf are shown in Fig. 

62. The wells marked CFD in the northwestern corner of the map are in the 

Huanghua depression. The Chengbei field was discovered by Chinese in 1972. The 

field produces from a 25-m-thick sandstone in the Dongying Formation draping 

over a Mesozoic buried hill. It is not dear whether this field is located in the 

Bozhong or Huanghua depression.

The Paleogene Bozhong depression underlies the deepest part of the 

superjacent Miocene-Pliocene sag. Thickness of the sag sequence exceeds 3 km in 

the central trough (Bozhong depression of fig. 62). Therefore, the drilling was 

concentrated on the Bonan uplift where thickness of the Miocene-Pliocene section is 

approximately 2.5 km, and was targeted at buried hill structures. Among 12 drilled 

wildcats, oil was tested in 7, and 6 out of 10 appraisal wells yielded oil. All tests but 

one produced oil from Tertiary sandstones in drape anticlines over basement highs. 

Only the BZ-28-1 well tested oil from Cambrian-Ordovician carbonates of a buried 

hill. Yields of oil varied from 1,800 to 12,000 b/d.

Source rocks are located in the first and third members of the Shahejie 

Formation and also at the bottom of the overlying Dongying Formation. The oil 

window is in the depth interval of 2,000 to 4,700 m. Judging from high oil yields in 

the tests, the quality of the reservoir rocks is rather good.

The undiscovered petroleum potential of the Bozhong depression is probably 

high, but large drilling depths over much of the area will hamper exploration. A 

significant amount of gas may be expected in deeply buried traps because much of 

the source rocks should be overmature in the central areas of the depression.

The Liaohe depression, located northeast of the Bozhong depression, occupies 

the northeastern part of the North China basin along the lower reach of the Liao 

River and the adjacent Bohai Gulf (Liaodong Bay) (fig. 45). The area of the onshore

part of the Liaohe depression is 12,400 km2 . An area almost twice as large lies 

offshore. No information is available on this offshore portion of the depression, 

supposedly because of the lack of exploration.

A northeast trending central uplift marks the axis of the long (200 km) and 

narrow (65 km) onshore part of the depression (figs. 63 and 64). Archean granite 

underlies Tertiary rocks on the uplift. The Eastern and Western sags flank the 

uplift. The small (800 km2) Damintun sag is in the far northeastern corner of the 

depression.

92
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CHENOZIKOU -.. ^ .. OA 

"ii*. '' ' 

UPLIFT '-.. «J'o.

8/JZ-l-l/t

BONAN ° /UPLIFT

 -M.

HUANQHEKOU " H 

DEPRESSION

KENDONO UPLIFT

"o* "' 0' 

o° ! /

MIAOXI 

UPLIFT

vN^

Figure 62.~Structural units of the Bohai Gulf and drilled wells. (After Matsuzawa, 1988.)
Damlntun Depression

 Ansha

Liao Dong Ba

Figure 63.--Contour map and oil fields of the onshore part of the Liaohe depression, North China 

(After Zheng, 1988.) Contours are on base of Tertiary rocks.

1, contour line, m; 2, buried hill; 3, oil field in buried hill; 4, major faults; 5, other faults.
Shu 25 Gu 523 Xlng 86 Leng 2

Figure 64.--Cross section through the Liaohe depression, North China basin. (After Zhen

Location of the cross section is shown in figure 63.

My denotes Archean granites; Z, middle and upper Proterozoic rocks; Es and Ed ar

Shahejie and Dongying Formations correspondingly.
Major oil reserves of the depression are in buried-hill structures. Tertiary 

elastics are also productive in stratigraphic traps along the northwestern margin of 

the depression and in drape structures over buried hills, but the reserves are 

significantly smaller. Reservoirs in the buried hills are weathered and fractured 

Archean granites, leached Upper Proterozoic carbonates and quartzites, and JurassicCretaceous volcanics. A detailed description of the fields is available in Zheng 

(1988), Lee (1989), and U.S. Energy Information Administration (1987). Apparently, 

all the discovered fields are in the Western and Damintun sags. The reasons for 

lack of productivity in the Eastern sag and the offshore area are not clear; possibly, 

little exploration has been conducted in those areas. In 1985, a major new discovery 

(730 million barrels) in the Liaohe depression was announced (Oil and Gas Journal, 

1985), but the location was not indicated. Probably, this discovery is not shown in 

figure 63.

Source rocks in the depression are mainly in the Shahejie Formation; as in 

the other depressions, they are deep lacustrine shales with the TOC content varying 

from 1.75 to 2.8%. The shales are present in all three sags onshore (fig. 65), and thus, 

the nonproductivity of the Eastern sag is not related to the absence of source rocks. 

The geothermal gradient in the Liaohe depression is higher than in other

depressions of the North China basin, it varies from 33 to 46.5 °C/km (Lee, 1989). 

Therefore, the oil window is rather shallow; the zone of maximum oil generation 

occurs at 2,000-2,400m.

The Liaohe depression, especially its offshore part, seems to be less explored 

than the other depressions of the North China basin except for the Bozhong 

depression. From available data, the conditions of oil generation and entrapment 

seem to be good. Probably, the depression possesses a significant potential for 

undiscovered petroleum.

Summary

The undiscovered petroleum potential of the North China basin is difficult to 

evaluate based exclusively on published data. The difficulties are related to 

significant variations in stratigraphy (source rocks, reservoirs) and structure from 

one graben to another. Actually each graben (sag) is a separate petroleum system not 

connected with petroleum systems of the other grabens. Thus, stratigraphic, 

structural, and drilling data are needed for each graben to make a detailed resource 

assessment. These data are absent from the available literature.

The above discussion on six subbasins (depressions) of the North China basin 

suggests that four of these subbasins (Dongpu-Kaifeng, Jizhong, Haunghua, and 

Jiyang) are rather maturely explored. Probably, little oil remains to be found in 

large, easily mappable structural traps. Most of the future discoveries may be 

expected in subtle, chiefly stratigraphic, traps. Data on the most thoroughly explored 

Jiyang subbasin indicate that stratigraphic traps contain about one-quarter of its 

petroleum reserves. A part of these reserves in stratigraphic traps remains to be 

discovered in the other three subbasins.

96
Figure 65.--Facies of the third member of the Shahejie Formation in the Liaohe depression, North China basin. (A

Hu, 1985.) The deep lacustrine fades is the main source rock.

1, deep lacustrine facies; 2, shallow lacustrine facies; 3, fluvial plain fac ies; 4, limit of rocks of the third 

member of the Shahejie Formation; 5, basin boundary.
The Bozhong and offshore Liaohe subbasins are probably very lightly 

explored. Significant new discoveries may be expected in both subbasins. No data 

are available on the geology of the Linqing subbasin. Apparently, no exploration is 

being conducted there. The lack of exploration and the absence of discoveries 

probably suggest that there is little or no petroleum potential in the subbasin. 

Original recoverable oil reserves in the North China basin are believed to be 13-14 

billion barrels. Reserves of gas are comparatively small (about 8 TCP). 

Undiscovered hydrocarbon resources will probably amount to about 30-40% of those 

already discovered.

98
NANXIANG BASIN 

INTRODUCTION

The Nanxiang basin is located in the southern Henan Province southwest of 

the North China basin and north of the Jianghan basin (fig. 1). On the northwest, 

northeast, and southeast, the basin is bounded by mountain ranges, the Qinling, 

Funiu, and Tongbai Mountains respectively (fig. 66). On the southwest, the Xinye 

uplift separates the basin from the shallow nonproductive Xiangfan and Zaoyang 

depressions.

The basin consists of two depressions (subbasins): the Biyang (in some 

publications, Miyang) depression on the east and the larger Nanyang depression on 

the west. The depressions are separated by the Tanghe uplift. Published data are 

limited to the Biyang depression which contains the bulk of the basin's oil reserves. 

Therefore, the following description concerns the Biyang depression; the geology 

and petroleum potential of the Nanyang depression are assumed to be similar to 

those of Biyang.

Exploration in the basin began in 1974 and the largest field of the basin, the 

Shuanghe field (fig. 66), was discovered in 1976. By 1979, the oil production in the 

basin reached its maximum of about 40,000 b/d (Hu, 1985)

STRATIGRAPHY

The basin's sedimentary fill is as thick as 8 km in the depocenter near the 

southern boundary fault. The Tertiary sequence is shown in figure 67; presently 

unknown Upper Cretaceous rocks may occur between the Proterozoic basement and 

the Tertiary (Li Chunju and others, 1988).

The Paleocene-Eocene section is 3-4 km thick and consists of fluviolacustrine 

red-colored elastics. Unlike in the Jianghan basin to the south, the Eocene does not 

contain organic-rich deep-lake facies. In the Biyang depression, these facies appear 

in the third member of the Oligocene Hetaoyuan Formation. This member contains 

both the source rocks and the major reservoirs of the basin. Paleogeography and 

depositional environments of the rocks were studied in great detail by Li Chunju 

and others, 1988 and by Zhu, Shuian and others (1981). The association of deep-lake 

black shales in the depression center and sandstone bodies of deltaic, turbiditic, and 

other origin on the margins (fig. 68) resulted in favorable conditions for oil 

entrapment.

Two younger members of the Hetaoyuan Formation and the overlying lower 

part of the upper Oligocene Liaozhuang Formation reflect gradual shallowing, 

shrinking, and final dessication of the lake. The rocks are various elastics of shallow 

lake to alluvial origin. Evaporites (mainly gypsum) in the upper Liaozhuang 

Formation indicate that deposition occurred in a brackish-water to saline lake.

99
Figure 66.-Main structural units and oil fields of the Nanxiang basin. (After U.S. Energy 

Information Administration, 1987.)

100
Brownishyellou;

and

other

colors

<£D I Calcareous sediments 

Gypsum

Gypsum crystals in sediments 

Gypsum vein

Figure 67.-Tertiary stratigraphy and fades of the Biyang depression. (After Li u cW dU&rs,

101
© Tangh*

' ^ r. .7 :.-'',<-'. x ' '

 .,' * v

, . y » . ° Xiaercnen .-.-/-. < .-y

.- /' W

\'/S) Anpeng'/ 

o Shuanghe ^K^rv-:-. ^::; 

; \^^-i»-.-- Pi,,gzhi o-] ^ ^f~° , -

Yangqiao. ° Liyuan

Figure 68. Sedimentary facies of the 3rd member of the Hetaoyuan Formation in the Biyang 

depression. (After Li, Zhu, and Zhu, 1988.)

1, subaqueous fluvial fan; 2, subaqueous gravity-slide sediments, 3, sand sheet; 4, deltaic fan; 5, 

delta; 6, turbidites; 7, mudflow cone; 8, barrier bar.

102
Pre-Neogene uplift and erosion were strongly expressed in the Nanxiang 

basin. In the northern part of the Biyang depression, Neogene sediments 

unconformably overlie various older rocks up to the third member of the 

Hetaoyuan Formation. The Neogene sag sequence is only about 200 m thick and 

consists of loose sand and clay.

TECTONICS

The Nanxiang rift was initiated at the end of Late Cretaceous or in the 

beginning of Tertiary time. The rifting began earlier than in the Tertiary North 

China basin to the north and later than in the Late Cretaceous Jianghan basin to the 

south. Unlike the North China and Jianghan basins in which two consecutive 

stages of rifting occurred, the Nanxiang basin developed in a single stage of rifting. 

Supposedly, the basin was formed on a Proterozoic metamorphic terrane cut by 

Mesozoic granites. These rocks are exposed in the adjacent Tongbai and Funiu 

Mountains and are covered by thin Neogene sediments on the Tanghe uplift.

Structurally, the Biyang depression is a simple half-graben with a great (5-7 

km) downthrow along the southern and eastern boundary faults (fig. 69). From the 

deepest part of the depression near the faults, the basement surface gently rises 

northward and westward. Basically, the same simple regional structure is seen in a 

contour map on the top of the third member of the Hetaoyuan Formation (fig. 70). 

A few structural noses and a single roll-over anticline complicate the monocline.

The southern boundary fault evidently continues into the Nanyang 

depression. Presumably, general structural characteristics of this depression are 

similar to those of the Biyang depression.

PETROLEUM GEOLOGY AND EXPLORATION PLAYS

Four oil fields have been discovered in the Biyang depression. Total reserves 

of the fields are reported to be 104 million tons (760 million barrels) (Ma and others, 

1982). This number probably indicates in-place resources (judging from a 

production rate of about 40,000 b/d). The largest field of the depression is the 

Shuanghe field (fig. 66) that contains 78% of the total reserves. The field is in an 

updip shale-out stratigraphic trap on a structural nose (fig. 71). The productive area 

of the field is 30 km2 . The reservoir rocks are sandstones of a subaqueous alluvial 

fan in the third member of the Hetaoyuan Formation. The average thickness of the 

sandstone beds is 60-70 m. Reservoir properties of the sandstones are probably 

rather good because the initial yields of wells range from a few hundred barrels a day 

to as much as 6,600 b/d.

Oil in the Xiaermen field is in a structural trap (rollover anticline) near the 

southeastern boundary fault (fig. 72). The field area is 4.3 km2. Productive 

sandstones of the third and second members of the Hetaoyuan Formation were 

deposited in a delta. Two other fields of the Biyang depression are small; their

103
Figure 69. North-south cross section through the Biyang depression. (After Li. Chunju 

Location of the cross section is shown in Figure 70.

104
Figure 70.-Contour map on top of the third member of the Hetaoyuan Formation, Biyang depression. 

(After Li Cfocty u~ a*# others ^ (<?&&}

\, oil field; 2, fault; 3, drillhole.

105
H-J 135" 1 ' < MM Big

Figure 71. Cross section through the Shuanghe oil field, Biyang depression, (after L» C^un^u 

^, 1988). Location of the cross section is shown in Figure 73.

Figure 72.-Cross section through the Xiaermen oil field, Biyang depression.(After #q; /WJ.

106
productive areas are 2.5 and 3.0 km2. The Wangji field is not in production. No 

data on fields of the Nanyang depression are available. Probably, all three 

discovered fields (fig. 66) are small.

Oils in the Biyang depression are characterized by an extremely high paraffin 

content (19-42%) and a high pour point (to 43°C). This causes severe testing and 

production problems (Hu, 1985).

Source rocks of the Biyang depression are deep-water lacustrine shales in the 

third member of the Hetaoyuan Formation (fig. 73). Thickness of the shales ranges 

from several hundreds to 1,300 m (Li Chunju and others, 1988). Unlike in the 

Jianghan basin, no signs of high salinity of the lake water are present. The TOC 

content is relatively high; it averages 1.66% as indicated by Li Chunju and others 

(1988) or 1.18% as indicated by Hu (1985). The kerogen is of dominantly sapropelic 

type. The mass-balance calculations (Ma and others, 1982) show that 260 million 

tons of oil were expelled from the source rocks and from these 104 million tons 

were entrapped in the discovered fields. If these calculations are correct, the 

entrapment efficiency is 40% which is probably the highest known efficiency in the 

world. The geothermal gradient is high (41°C/km) and the threshold of oil 

generation is at a depth of 1800 m. Over 80% of the source rocks is mature (Li 

Chunju and others, 1988).

Reservoir rocks are deltaic and subaqueous alluvial fan sandstones. The 

main reservoir rocks are in the third member of the Hetaoyuan Formation, and 

thus, they are contemporaneous to, and interfinger with, source rocks. Such 

relationships resulted in the very high efficiency of entrapment. The sandstones are 

thickest on the depression margins, close to the sources of clastic material. The 

thickness decreases in the central area of the depression, in the deep-lake zone, 

where only fine-grained turbidite sandstones are present (fig. 73).

Due to the small size and simple structure of the Biyang depression, 

structural traps are uncommon. Only one such trap has been found (Xiaermen 

anticline). As a result, the main oil reserves are concentrated in stratigraphic traps.

The Biyang depression, considering its small size, is a very rich petroleum 

system. No data on the amount of drilling are available, but probably the depression 

is maturely explored. The stratigraphy and facies distribution are well studied, and 

the most potential areas for large stratigraphic accumulations have been tested. 

Some new stratigraphic pools can certainly be found, but their sizes will be small.

Almost no data are available for the Nanyang depression. Source rocks are 

obviously present as indicated by the discovered pools. It seems likely that the 

problem of this depression is the lack of reservoir rocks. Productive sandstones of 

the Biyang depression were derived from the metamorphic and granitic terrains of 

the Tonbai and Funiu Mountains. The sources of clastic material for the Nanyang 

depression only could have been the narrow Xinye uplift (assuming that the 

Xiangtan basin trapped clastic material from possible southern sources, figure 66),

107
o

00

Figure 73.~Thickness of source rocks and reservoir rocks in the third member of the Hetaoyuan Formation 

depression. (After Hu, 1985.)

1, oil fields; 2, isopach of sandstones; 3, isopach of dark shales; 4, drillholes; 5, direction of trans

clastic sediments.
and the Qinling Mountains. The latter are formed mainly by folded geosynclinal 

formations, such as volcanics and graywacke, that probably produced poor quality 

reservoir rocks in the Nanyang depression.

109
JIANGHAN AND DONGTING BASINS

INTRODUCTION

The Jianghan basin is located south of the North China basin in the Hubei 

Province (fig. 1). The basin is surrounded by exposures or shallow occurrences of 

pre-Cretaceous rocks (fig. 74). On the north, a shallow graben system connects the 

Jianghan basin with the previously discussed Nanxiang basin. The area of the

Jianghan basin is 28,000 km^. The Dongting basin (fig. 74) is about half as large. It is 

separated from the Jianghan basin by the Huayong uplift on the top of which the 

basement is exposed at the surface. Some authors consider the Dongting basin to be 

a part of the Jianghan basin (e.g., Lu, 1982). However, the stratigraphic successions 

in these two basins are different.

Drilling in the Jianghan basin began in 1959, and the first oil field 

(Wangchang) was discovered in 1961. In the Jianghan basin, all (or almost all)

known oil fields are concentrated in the rather small (2,500 km^) Qianjiang 

depression. The oil production in the basin was about 30,000 b/d in 1985. No 

commericial discoveries have been made in the Dongting basin.

STRATIGRAPHY

The basement of the Jianghan basin consists of Precambrian crystalline rocks 

and of deformed and partly metamorphosed rocks of Sinian through Jurassic age. 

Lower Cretaceous rocks have never been drilled; supposedly, because thermal uplift 

and erosion occurred during this time (Xie and others, 1988).

Upper Cretaceous-Lower Paleogene rocks form the lower rift/sag complex. 

The Upper Cretaceous may be 3-4 km thick and mainly consists of red beds with 

gypsum and salt (table 4). The salt is interlayered with elastics and is no more than 

400 m thick. The evaporite deposition continued into the Paleocene-middle Eocene 

Shashi, Xingouzui, and Jingsha Formations. The Xingouzui Formation (lower 

Eocene) includes organic-rich shales that are believed to be petroleum source rocks 

(Chen and others, 1988).

The second cycle of rifting and evaporite sedimentation began in early upper 

Eocene time (fourth member of the Qianjiang Formation). Thickness of this 

member exceeds 2,200 m and almost one-half of it is composed of salt. However, the 

areal extent of evaporites in this second cycle is much smaller; the evaporites are 

present only in the eastern depressions of the basin and reach their maximum 

thickness in the Qianjiang depression (fig. 74). The evaporites there are interlayered 

with deep-lake organic-rich shales that constitute the main source rocks of the basin 

(fig. 75). Northward and westward, evaporites grade into nearshore lacustrine 

elastics (fig. 76). The salt-bearing section reportedly contains a poor association of 

marine forams (Chen and others, 1989); this indicates that the lake had a connection

110
Sttiti graphic Sequence

System

Quaternary

.Neogenc

Eogenc

Crctaceoos

Scries

Holoccnc 

Pleistocene

Pliocene 

Miocene

Oligoccnc

Eoccnc

Upper

and 

Middle

Lower

Upper

Middle

Lower

Palcocenc

Upper

Lower

Formation

Plain

Guanhuashi

Jtnghezhcn

Qianjiang

Jingsha

Xingouzui

Shashi

Member

1

Upper

Middle

Lower

2

3

4

Upper

Lower

Upper

Lower

Uthologic Character

Gray clay, sill, fine-grained sand, gravel 

beds

Moolod oiudstonc with intercalated sandstone and conglomerate

Dark -grey mudstooe and silutonc with 

intercalated oil shale and glaubcritc-bcaring 

mudstonc

Referred to as "mod-gypsum layer"; consists of gray to dark -gray mudjionc. 

gypcum-fnudslonc. oil shale v/iih intercalatcd salt

Intercalation of gray mudnonc and silutonc

Intercalation of gypsum, salt, sandstone and 

mudstonc. with some oolitic marlite

Consists of 24 rhythmic units, each composed of salt, gypsum -mudstonc. gjaubcritc 

mudstone. oil-bearing mudstonc. marine 

occasionally, silfy fine-grained sandstone 

occurs at the bottoms of the units

Gray to dark-gray mudstonc, sDistonc and 

oolitic marl lie; three rhythmic units and 

two suites of sandstone

Consists of 14 rhythmic units, each composed of dark-gray mudstooe. gypsum - 

mudstonc and salt, with some silry finegrained sandstone

Gray and dark gray mudstone. glaubcritcmudstonc. salt, oil-bearing mudstonc and 

silly fine-grained sandstone

Gray and dark-gray mudstone. gUubcriicmudstonc. salt, oil-bearing mudstonc

Brown-purple-red mudstonc. gypsum - 

bearing mudstone and siltstone

Purple-rod and gray-green mudstonc. mudgypsum, gypsum -bearing siltsionc. oilbearing mudstone

Dark-gray and brown-red mudstonc. gypsum, gypsum-bearing mudstonc. siltstonc

Brown-purplc-rcd mudstonc. mud-gypsum. 

salt, silutonc. gypsum, rod sandy mudstonc 

with intercalated conglomerate

Thickncj! 

(m)

5O-150

300-900

0-1000

12XM50

110-700

150-640

100-700

173-2218

600-1000

600-2000

200-1900

1200-2800

Table 4.-Stratigraphy of the Jianghan basin. (After Xie and others, 1988.)

Ill
Figure 74.~Main structural units of the Jianghan and Dongting basins. (After Lu, 1982.)

1, metamorphic basement rocks; 2, Sinian through Jurassic rocks; 3, Cretaceous-Lower 

Tertiary rocks; 4, Quaternary; 5, boundary of structural units.

112
Figure 75. Depositional fades of the Qianjiang Formation in the Jianghan basin. (After Chen and 

others, 1989.)

1, formation limits; 2, fault; 3, pinch out of sandstones; 4, deltas; 5, bay mudflat facies; 6, 

turbidites; 7, deep saline lake sediments; 8, fan and bar facies; 9, alluvial plain; 10, shallow 

saline lake shale; 11, shallow-lake nearshore shale; 12, banks and bars of shallow saline lake; 

13, shallow-lake, fresh to brackish-water sediments

113
Figure 76.-Extent . of evaporites of the Qianjiang Formation in the Qianjiang depression. (After Xie 

and others, 198*.)

1, pinch out of sandstones; 2, direction of paleocurrent.

114
with the open sea on the east and suggests that influx of marine water was the 

source for the salt. Younger subdivisions of the Qianjiang Formation and the 

Oligocene section are thinner than the fourth member and are mostly formed by 

clastic rocks varying from alluvial plain to lacustrine in origin. Some evaporites are 

present throughout the sequence. This younger part of the upper Eocene-Oligocene 

section apparently contains organic-rich rocks only in its upper part.

Upper Tertiary and Quaternary rocks are thin in the Jianghan basin and form 

a flat-lying blanket unconformably overlying older faulted rocks.

The stratigraphy of the Dongting basin differs from that of the Jianghan basin 

in that only the lower, Upper Cretaceous-middle Eocene, rift/sag cycle is well 

developed. Evaporite rocks are apparently absent or thin. The cycle includes several 

tens to a few hundreds of meters of dark shales in the Paleocene Taoyuan 

Formation. Upper Eocene rocks are thin (a few hundred meters) as compared to the 

Jianghan basin. The Oligocene is absent in the Dongting basin, and the Eocene is 

unconformably overlain by thin Neogene and Quaternary sediments similar to 

those in the Jianghan basin.

TECTONICS

Both the Jianghan and Dongting basins were initiated during Late Cretaceous 

time as a rift system. The rifts formed on the heterogeneous terrane that consisted 

of basement rocks of the Yangtze craton and its Sinian through Jurassic sedimentary 

cover. The sedimentary rocks were significantly deformed at the end of Jurassic 

time. The rifts had a dominant northeastern trend that is similar to the trend of the 

Tertiary rifts of the North China basin. Downfaulting in the basins ceased by Eocene 

time; the lower Eocene Xingouzui Formation (and maybe the middle Eocene) fills a 

sag developed over this early rift system.

The second rifting event began in the middle or late Eocene. The rifting 

occurred only in the Jianghan basin and left intact the more southern area of the 

Dongting basin. The western to northwestern trends of faulting dominated at this 

second rifting stage, although northeastern elements were also present in the 

structural pattern (fig. 74). The rifting evidently continued to the end of the 

Oligocene. Miocene and younger sediments unconformably overlie Paleogene rocks 

in grabens and on horsts and fill a sag developed over the rift system.

The present-day structure of the Jianghan basin is dominated by the western 

to northwestern trend. The general structure of the basin is a half-graben with a 

steep faulted flank on the north and a monocline on the south (fig. 77). The basin is 

separated into several depressions by subordinate uplifts. The Qianjiang depression 

occupies the central position and is the deepest part of the basin. The basement 

occurs as deep as 10 km. Supposedly this Tertiary depression was formed over a Late 

Cretaceous depocenter (Lu, 1982). Salt flowage in the depression complicates its 

structure (fig. 78). The Dongting basin consists of a series of northeast-trending

115
Wangchanj? 

salt dome

d\t\9

c\

£*uAV.

Figure 77.-Cross section through the Qianjiang depression of the Jianghan basin. (After Chen a
Figure 78.~Salt domes in the Qianjiang depression. (After Xie and others, 1988.) 

1, salt domes; 2, syndines; 3, boundary of the depression, 4, fault

117
grabens and horsts that formed during the Late Cretaceous-Paleocene and remained 

basically unchanged during the Eocene-Oligocene tectonic stage.

PETROLEUM GEOLOGY AND EXPLORATION PLAYS

The commercial petroleum productivity of the Jianghan basin is largely, if 

not exclusively, confined to the Qianjiang depression with an area of only 2,500 

km2. Several oil fields have been discovered in the depression and on slopes of the 

adjacent uplifts (fig. 79). The largest oil field is the Wangchang field which is 

controlled by a salt-dome structure. The oil is trapped in turbidite sandstones 

pinching out against the salt dome and in younger sandstones, in the draping 

structure over the salt. Productive stratigraphic traps are either updip pinch outs of 

sandstones (on the southern flank of the depression) or onlap on an unconformity 

surface (on the northern flank) (fig. 80). In addition, there is a number of oil pools 

(probably small) in fractured shale reservoirs (Ma and other, 1982), but detailed data 

are not available.

The oil in all commercial fields in the basin has been generated by upper 

Eocene source rocks, primarily in the fourth member of the Qianjiang Formation. 

The fields are in or just around the area containing these source rocks (fig. 79). The 

rocks were deposited in a deep part of the saline lake. Their thickness exceeds 2 km 

in the depocenter. The kerogen is of mixed sapropelic and humic types as indicated 

by biomarkers and microscopic examination (Xie and others, 1988). The TOC 

content is not particularly high and averages 0.61%; the content of hydrocarbons is 

964 ppm. The geothermal gradient is low compared to other rift basins in eastern 

China because of the presence of thick salt; it varies from 23 to 30° C/km in 

evaporite sections and increases to 30-40° C/km in clastic formations (Xie and 

others, 1988; Jiang and Zhang, 1982). The threshold of oil generation is at a depth of 

2,200 m and the peak of generation occurs at about 3,700 m.

Another source rock section may be present in the lower Eocene Xingouzui 

Formation, although references regarding the quality of these source rocks are 

contradictory (Chen and others, 1988,1989; Yuan and others, 1983) and no hard data 

are available. The possible presence of source rocks in the evaporite-bearing Upper 

Cretaceous section is suggested by Chen and others (1988), but has not been 

demonstrated by drilling. The presence of source rocks in the Dongting basin 

remains to be proven. Dark shales with TOC varying from 0.2 to 1.4% are known in 

the Paleocene Taoyuan Formation (correlative with the Shashi Formation of the 

Jianghan basin), but the hydrocarbon content is rather low (100-380 ppm). Some oil 

shows observed in the Dongting basin may be related to this source rock.

Similarly to other rift basins in China, oil in the Jianghan basin is produced 

from sandstone reservoirs of various origin. Most of the sandstones are 

concentrated in the northern part of the basin close to the sources of clastic material.

118
Figure 79.-CH1 fields of the Qianjiang depression. (After Zhai and others, 1988.) 

1, main oil-generating area; 2, subordinate oil-generation area.

119
'1 2 16 4 17 60

Rgure 80.-Contour map and cross section of the Zhongshi field. (After 

Hu and others, 1988.)

1, pay in the second unit of the Qianjiang Formation; 2, pay 

third unit; 3, pay in the fourth unit.

in the

120
Deltaic, turbidite, alluvial plain, sand bar, and nearshore facies are present (figs. 75 

and 76). Fractured mudstone reservoirs are known (Ma and others, 1982), but no 

further data are available.

The potential for significant new discoveries in the productive upper EoceneOligocene sequence of the Jianghan basin seems to be rather low. Although data on 

the amount of drilling are not available, one can suppose that the richest salt dome 

play in the Qianjiang depression has been thoroughly explored since the discovery 

of the Wangchang field in 1961. These structures have high closures (800 m in the 

Wangchang). They are relatively easy to map and are present in a limited area. 

Stratigraphic traps may be better exploration targets in the depression, but expected 

reserves should be small. The fields are less likely to be discovered on the southern 

basin margin because of the paucity of reservoir rocks.

Significant source rocks are absent from the upper Eocene-Oligocene sequence 

of the Jiangling depression (fig. 74) which is largely composed of alluvial elastics and 

does not contain salt. Thickness of this sequence in the Jiangling depression and 

also in the Xiaoban depression on the east is 1800-2200 m, which is probably 

insufficient for maturation (Xie and others, 1988). Only insignificant shows have 

been observed in exploratory holes in these depressions. The upper EoceneOligocene sequence is thin in the Dongting basin and does not have petroleum 

potential.

The petroleum potential of the Upper Cretaceous-middle Eocene sequence is 

more difficult to access. Probably, the sequence is significantly less explored than the 

younger rocks. The distribution, quality, and even the presence of adequate source 

rocks are uncertain. Source rocks may be associated with salt that is present over 

about one-half of the Jianghan basin. Maximum thickness, of the salt and probably 

the Cretaceous depocenter are situated in the Jiangling depression where salt 

tectonics is known. A few wells reportedly have produced oil flows in tests (Chen 

and others, 1989). Some source rocks are probably present in the lower Eocene.

Data on the reservoir quality of Upper Cretaceous-middle Eocene elastics are 

absent from the literature. Considering the molasse-type character of the red beds, 

rather poor reservoirs may be expected. Structural traps characteristic of rifts are 

prime exploration targets. The hydrocarbon potential of this sequence in the 

Dongting basin is probably lower. Evaporites are absent there and chances for the 

presence of adequate source rocks among the red beds are slim. Some oil shows 

were observed in tests in Paleocene rocks of the basin, but the geochemical 

characteristics of the possible source rocks (dark-gray shales of the Taoyuan 

Formation) seem to be poor.

Gas generation in deformed pre-Upper Cretaceous rocks (e.g., from Permian 

and/or Jurassic coals) and entrapment of gas below the Upper Cretaceous evaporite 

seal is possible, but highly speculative.

121
ERLIAN (CHINA) AND SOUTHEAST GOBI (MONGOLIA) BASINS

INTRODUCTION

The Erlian (Eren, Xiling) basin in northern China (fig. 81) and the Southeast 

Gobi (East Gobi) basin across the border in Mongolia (fig. 82) are two adjacent rift 

systems of Jurassic-Cretaceous age. Both basins developed on the Hercynian foldbelt 

north of the Sino-Korean craton. The area of the Erlian basin is approximately 

220,000 km2, that of the Southeast Gobi basin is about half as much. Like other rift 

basins of China, both basins consist of a large number of individual grabens 

separated by horsts.

Very little data are available on the geology of either basin. Significant 

exploration efforts in the Erlian basin were probably undertaken in recent years, but 

almost nothing has been published. As reported by Oil and Gas Journal (1984), the 

undiscovered petroleum potential of the basin was believed to be rather high (7.3 

billion barrels); since that article, at least two discoveries have been made (H.D. 

Klemme, personal communication).

Exploration for oil in the Southeast Gobi basin began in the 1940's in an area 

of surface seepages. The Dzunbain oil field was soon discovered. A state-owned oil 

company was formed in 1948. However, despite continuous exploration efforts, no 

new commercial discoveries have been made. Probably, very little exploration has 

been done during the last 15-20 years. At present, the Mongolian petroleum 

administration is trying to establish joint exploration enterprises with Western 

companies.

STRATIGRAPHY

A stratigraphic chart for the Erlian basin is shown in table 5. The Mesozoic 

section of the basin unconformably overlies various deformed and metamorphosed 

upper Paleozoic and older rocks of the Hercynian fold belt. The fold belt was formed 

by collision of the northern Mongolian Caledonian block with the Sino-Korean and 

Tarim cratons. No data on pre-Middle Jurassic Mesozoic rocks of the Erlian basin are 

available. In Mongolia, Mesozoic rifting began as early as in Late Triassic time, but rifts 

originating in the Early and Middle Jurassic are also known (Nagibina, 1970). The first 

stage of rifting was completed by the Late Jurassic. Thickness of the Upper TriassicMiddle Jurassic in rifts of eastern Mongolia ranges from 2 to 7 km. Continental 

lacustrine and alluvial, commonly coal-bearing rocks associated with volcanics 

dominate, but marine rocks are known in the extreme northeast part of the rift system. 

Early Mesozoic rifts are discordant to the trend of the late Paleozoic structure. The 

younger, Late Jurassic-Cretaceous, generation of rifts in Mongolia is also discordant 

relative to both late Paleozoic structures and early Mesozoic rifts. Large thickness of 

Middle Jurassic rocks in the Erlian basin, their occurrence outside the younger rifts 

directly on the Paleozoic basement (Fan and others, 1983), and the presence of 

volcanics in the lower part of the section may suggest their rift nature analogous to the 

early Mesozoic rifts of Mongolia; however, this supposition is not certain.

122
Table S.-Mesozoic and Cenojoic Stratigraphy of the Eriian Basin 

(Symbols are related to the local stratigraphic nomenclature)

LITHOLOGY

THICKNfcSS 

(m) DISTRIBUTION

Sand, day, gravel, locally basalt 340

Mudslone, sandstone, conglomerate, Iccaty basalt 461 In depressions and party on uplifts

Mudslone, sandsione, ODngkxnerate 151 On (he west o< fie basin

Mudstone, sittstone, sandstone, conglomerate bads 274

Vihouguoto zone ot depressions

Basalt, andesite. tultaceous agglomerate 165-1215

Sandstone,conglomerate, sdtsione witnmudslone layers 392 675

Mudslone. sittsione, fine sandstone, localy coal beds 6061130

Widespread in the western pan ol 

the YftougUalei and HansharxJake 

toughs and adjacent area

Mudslone. siltstone, fine sandsione. nerbedsol 

conglomerate

32-540

IS Shale, interbeds ol coarse sandsione and conglomerate 465

s Shale, irterbeds ol oi shale, dolomite, sandstone, and 

gypsum 686

Sporadically present in the central 

and western parts ol the basin

Conglomerate with inlerbeds ol sandstone 417

Acidic tuft, tulfaceous agglomerate, zhyolite 187-2608

Basalt, andesite, basaltic tun 382-1129 widely

Acidic iu« with rnyolite and andesite 1470-3430

Sandstone, modstone, carbonaceous shale, coal beds 390

Sandsione and conglomerate 154

Sandsione. sdtsione, mudslone, coal beds 267

Sporadically distributed in the 

uplifted belt and on its 

margins

Conglomerate

Mudslone, shate. sandstone, thin beds ol marl >455

Mudslone, tuflaceous sandsione, lull, basalt 295

Distributed in |he uplilleo Den 

and on its margins

TuH, coarse sandsione, silsione 328

Tutlaceous sandsione, sandsione. conglomerate, acidic 

tuft. Occurs unconlormabty on Permian granites

123

597
MONGOLIA

EXPLANATION

Boundaries of main structural units MAIN STRUCTURES:

Main faults

Grabens I - Northern zone of uplifts

Shows in wells II - Yihuoquolei zone of depressions

Show on surface III - Central zone of uplifts

Drilled wells V - Hunshandake zone of depression

Identified deep-lake facies V- Southern zone of uplifts

Figure 81.-Late Mesozoic structure of the Erlian basin. (After Fan and others, 1983.)

124
Figure 82.--Late Mesozoic structure of southeastern Mongolia (After Khasin and others, 1973

1, late Mesozoic and Cenozoic depressions (1-Tamtsag, 2-East Gobi, 3-Unegetin, 4-Dz

5-Hanbogdin, 6-Holoi, 7-Nemegetin, 8-South Gobi, 9-Dlinoozer); 2, pre-late Mesozoi

partly beneath Upper Cretaceous deposits; 3, depth to the folded basement; 4, areas 

Cenozoic volcanism; 5, faults in upper Mesozoic rocks.

125
A new rifting stage in both the Erlian and Southeast Gobi basins began in the 

Late Jurassic and continued into the Cretaceous. In the Erlian basin, the early, 

volcanic, fill of the rift is very thick; it was formed during early Late Jurassic time 

(table 5). By late Late Jurassic time, the volcanic activity had largely ceased and 

continued subsidence in grabens was compensated by clastic deposition. The synrift 

clastic section of the Erlian basin consists of two formations, the Upper Jurassic 

Badalahu Formation and the Lower Cretaceous Bayanhua Formation. The 

formations are subdivided into members based mainly on the dominant grain size. 

Deep-lake organic-rich shales are abundant in the middle member of the Badalahu 

Formation and are also present in the upper member. They are associated with 

gypsum and some dolomites which suggest arid or semi-arid climate. In contrast, 

coals are widespread in the Bayanhua Formation indicating more humid 

environments.

Upper Cretaceous and younger rocks form the postrift sequence of the Erlian 

basin. They unconformably occur on various older rocks including the basement 

and are relatively thin. The maximum thickness (about 1,000 m) is known on the 

west of the basin. Basalt flows, locally rather thick, are common at the base of the 

section and are genetically connected with pre-Late Cretaceous uplift and erosion.

The Upper Jurassic-Lower Cretaceous synrift sequence of the Southeast Gobi 

basin overlies the Hercynian basement. Only on the northeast of Mongolia, in the 

Tamtsag basin (fig. 82), is this sequence underlain by Lower-Middle Jurassic coalbearing elastics and volcanics of the Khamarkhuburin Formation. The synrift 

sequence of the Southeast Gobi basin begins with the Kimmeridgian-lower 

Valanginian (on other data Kimmeridgian-Tithonian - Shuvalov, 1969) Sharilin 

Formation (table 6). Coarse-grained clastic rocks dominate the composition of this 

formation. Finer-grained rocks including grayish-green lacustrine sandstones, 

effusive rocks, and basalt flows are present in the upper part of the section. 

Thickness of the formation may reach 1,500-2,000 m.

The upper Valanginian (or Tithonian-Valanginian) Tsagantsab Formation 

overlies the Sharilin red beds in grabens or basement rocks on flanks of the grabens 

and on horsts. The formation is to 1,000-1,200 m thick and is composed of volcanic 

material (basalt flows at the bottom and tuffs at the top) with interbeds of sandstones 

and shales. Sedimentation occurred in lakes and on alluvial plains in more humid 

climatic conditions (fig. 83).

Volcanics and elastics of the Tsagantsab Formation are conformably overlain 

by gray clastic rocks with a few volcanics of the Shinhuduk Formation (table 6). 

Decreased supply of clastic material at the beginning of deposition resulted in 

deepening of the lakes and widespread deposition of organic-rich bituminous shales 

(paper shales) in grabens. These paper shales are most probably the main source 

rock of the basin. Their thickness commonly does not exceed 100 m. Upward in the 

section, the paper shales pass into dark gray and gray shales with beds of sandstone

126
Table 6.-Upper Mesozoic stratigraphy of eastern Mongolia.

UPPER 

CRETAC.

00 

D

O

LU

CRETAC

LOWER

oo 

oo

cc ^

D -f 

-5 _J

AGE

APTI ANALBIAN

HAUTERIVIANBARREMIAN

UPPER 

VALANGINIAN

KIMMERIDGIANL. VALANGINIAN

FORM 

EAST GOBI

NEMEGETIN 

BURUNGOIOT 

BAINSHIREIN 

SAINSHANDIN

BARUNBAYAN

KHUKHTYK

SHINHUDUK

TSAGANTSAB

SHARILIN

KHAMARKHUBURIN

A T 1 0 N

NORTHEAST 

MONGOLIA

DZUNBAIN SERIES

CHOIBALSAN

SERIES

LITH

Red c

Gray and greenish sa

Gray shales and sand

shales, marls, few vo

Conglomerates and s

sandstones and shale

volcanic flows and tu

noQ COcuS 6 g r a 1 n 6Q 

sandstone and shale 

Continental gray, co

rocks, coal
Figure 83. Paleogeographic map of eastern Mongolia. Neocomian (Tsagantsab time) (After Martinson, 

1982):

1-3, areas of denudation (1-low mountains, 2-hilly topography, 3-denudational plains); 

4-6, areas of accumulation (4-lakes, 5-alluvial, lacustrine-alluvial, and lacustrine-proluvial 

plains, 6-volcanic relief, mainly basalt plateaus); 7, centers of volcanic eruptions; 8, thickness 

of sediments, m; 9, direction of sediment transportation (a-main, b-local); 10, inferred river 

valleys; 11, possible junction of lakes; 12, inferred channels; 13, fossil fauna; 14, fossil flora; 

15, gypsum; 16, phosphorites; 17, oil.

128
and marl. These non-bituminous rocks constitute the bulk of the Shinhuduk 

Formation whose total thickness reaches 700-1,000 m.

Commonly, the Khukhtyk Formation (table 6) rests unconformably on older 

rocks. The formation consists of gray clastic rocks with rare coal and limestone 

layers. On flanks of the grabens, Khukhtyk lake deposits in places overlap preNeocomian rocks because of significant increases in lake areas (fig. 84). The 

Barunbayan Formation at the top of the Lower Cretaceous is characterized by an 

abrupt decrease in areas of lacustrine deposition and by the appearance of beds of 

red-colored elastics indicating a transition to arid environments of Late Cretaceous 

time (fig. 85).

Upper Cretaceous and Lower Tertiary rocks unconformably overlie elastics of 

the Khukhtyk and Barunbayan Formations in graben-rifts and basement rocks 

outside the grabens. Coarse red and variegated elastics alternate with less abundant 

shales. The Late Cretaceous deposition occurred mainly in numerous shallowwater oxic lakes (fig. 86). Based on dominant grain size and color, the Upper 

Cretaceous section is subdivided into four formations (table 6). Total thickness of 

the section does not exceed 500 m. Lower Tertiary sediments are thin and present 

locally.

A comparison of the stratigraphic successions of the Erlian and Southeast 

Gobi basins suggests certain similarities. In both basins, the rift development 

occurred during Late Jurassic-Early Cretaceous time. Upper Cretaceous and younger 

rocks occur near horizontally above the rifts and on intervening uplifts. The main 

difference is that the initial volcanic stage was expressed much stronger in the Erlian 

basin than in the Southeast Gobi basin and terminated earlier. However, this 

apparent difference may be due to uncertainties in the age determination of 

continental rocks.

TECTONICS

The late Mesozoic rifting stage began in Late Jurassic time in both the Erlian 

and Southeast Gobi basins. The rift system was formed on substrates that ranged 

from Hercynian basement to lower Mesozoic rocks. These latter rocks fill an earlier 

rift system in Mongolia (Khasin and others, 1973) and probably also in the Erlian 

basin of China judging from the abundance of volcanics in the Middle Jurassic 

section (table 5). Probably the Late Jurassic rifts were initiated simultaneously in 

both basins, but uncertainly exists because of difficulties in age determination. The 

initial stage of rifting was pronounced in the Erlian basin where basal volcanics are 

thousands of meters thick. Volcanism was more moderate in the Southeast Gobi 

basin and was preceded by dominantly clastic sedimentation (Sharilin Formation). 

The change from mostly volcanic deposition of the early rifting stage to clastic 

deposition of the late stage apparently occurred earlier (Late Jurassic) in the Erlian 

basin than in the Southeast Gobi basin (Valanginian).

129
0 45 #7 735 750 A*

Figure 84. Paleogeographic map of eastern Mongolia. Early Cretaceous, beginning of Aptian-Albian 

time (Khukhtyk time) (After Martinson, 1982):

1-5, areas of denudation (1-medium high mountains, 2-low mountains, 3-hilly plain, 

4-denudational plains on Upper Jurassic and Neocomian rocks, 5-remnants of dissected basalt 

plateaus); 6-7, areas of accumulation (6-lakes, 7-alluvial, lacustrine-alluvial, and alluvialproluvial plains); 8, gypsum; 9, coal shows; 10, coal fields; 11, centers of volcanic eruptions; 

12, directions of sediment transportation; 13, inferred rivers; 14, thickness of rocks, m; 15, fossil 

fauna; 16, fossil flora.

130
/"~">.

Cholbalsan jr^i:-' ~ :/ ' .".'~yY

, r- -> /. -: - 100. .:. ;'0

0 4-5 Stf 135 180

12

13

Figure 85.-Paleogeographic map of eastern Mongolia. Early Cretaceous, end of Aptian-Albian time 

(Bamnbayan time) (After Martinson, 1982):

1-4, areas of denudation (1-medium high mountains, 2-low mountains, 3-high denudational 

plains, 4-low denudational plains); 5-6, areas of accumulation (5-alluvial-proluvial plains, 6- 

lakes); 7', thickness of rocks, m; 8, volcanoes; 9, main directions of sediment transportation; 

10, local sediment transportation; 11, inferred river valleys; 12, fossil fauna: 13, fossil flora.

131
12

Figure 86. Paleogeographic map of eastern Mongolia. Late Cretaceous time (After Martinson, 1982): 

1-3, areas of denudation (1-low mountains, 2-hilly plains, 3-low denudational plains); 

4-5, areas of accumulation (4-alluvial-proluvial plains, 5-lakes and lacustrine-alluvial 

plains); 6, gypsum; 7, thickness of rocks, m; 8, main directions of sediment transportation; 

9, local sediment transportation; 10, inferred rivers; 11, fossil fauna; 12, fossil flora.

132
The termination of rifting was simultaneous in both basins and was 

obviously associated with a significant compressional event at the Early-Late 

Cretaceous boundary. In the Southeast Gobi basin, the compression brought about 

regional uplift, partial inversion, and moderate folding in the grabens, and deep 

erosion of underlying rocks (Nagibina and others, 1977). In the Erlian basin, the 

uplift resulted in partial erosion of upper members of the Bayanhua Formation 

(fig. 87) and eruption of plateau basalts. Folding and thrusting in grabens of the 

Erlian basin can be seen from the lower cross section in figure 87.

Because of pre-Late Cretaceous compression, a well developed postrift sag was 

never formed in the Southeast Gobi basin. Here, Upper Cretaceous-Lower Tertiary 

rocks are thin and have a platform character. They are not restricted to underlying 

rifts, but cover much wider areas. Beginning in the Oligocene, the basin experienced 

regional uplift and Upper Tertiary rocks are absent. The Late Cretaceous-Tertiary 

geologic development of the Erlian basin after eruption of basalts was similar. Only 

in the west part of the basin, the thickness of overlying elastics reaches several 

hundred meters whereas it is less on the east of the basin (Fan and others, 1983).

The present-day structure of the Erlian basin is shown in figure 81. The basin 

consists of about 40 individual grabens. It is subdivided into two major zones of 

depressions bounded and separated by uplifted zones. The largest and deepest 

grabens are found in the zones of depressions; with a few exceptions, grabens on 

uplifts are shallow and small. One of these exceptions is the Saihantala graben in 

the western part of the southern zone of uplifts (fig. 81). Cross sections of this 

graben in figure 87 indicate a typical half-graben structure with the boundary fault 

on the southeast. As discussed above, the lower cross section clearly indicates a 

compressional event which resulted in thrusting (probably along a pre-existing 

listric fault) and the formation of an anticline.

The Southeast Gobi basin (fig. 82) includes the Dzunbain and East Gobi 

depressions (subbasins), each of which consists of a number of half-grabens. The 

half-grabens are tilted to the south and southeast in the East Gobi depression and to 

the northwest in the Dzunbain depression (Martinson, 1982). The structure of one 

of the grabens in the Dzunbain depression is shown in figures 88-91. The main 

graben fill is the Tsagantsab Formation, the largest thickness of which is along the 

northern boundary fault. The Bain-Mongol-Tsagan-Els horst separates the narrow 

northwestern graben from the rest of the Dzunbain subbasin. The horst is reflected 

in thicknesses of all units of the sedimentary cover. The scale for figures 88-91 is not 

provided; probably the area shown is about 60-80 x 15-20 km and is located in the 

northwestern part of the northeastern Dzunbain subbasin. Unfortunately, cross 

sections of the Southeast Gobi basin are not available; judging from a description by 

Marinov and others (1977), a compressional anticline that controls the Dzunbain 

field is located near the northwestern boundary fault, northwest from the East 

Dzunbain field.

133
Figure 87.-Cross sections through the Saihantala graben. (After Chen, 1985.) Location of the 

shown in figure 81. For stratigraphic indices, see table 5.
Figure 88. Lithology and thickness of the Tsagantsab Formation in the Dzunbain field area. (After 

Kuznetsova, 1966):

1, alternation of shale, sandstone, siltstone, and less commonly, limestone with conglomerate at 

the base; 2, alternation of shale and sandstone; 3, shale to conglomerate in the lower part and 

shale and siltstone with limestone layers in the upper part; 4, shale with sandstone layers; 

5, shale with infrequent beds of siltstone, sandstone, and conglomerate; 6, shale with infrequent 

siltstone layers; 7, isopachs, m; 8, rocks absent; 9, faults.

135
Figure 89.-Lithology and thickness of the bituminous subformation (paper shale unit) of the 

Shinhuduk Formation in the Dzunbain field area. (After Kuznetsova, 1966):

1, alternation of siltstone, sandstone, marl, limestone, shale, and bituminous shale; 2, shale 

(dominant) and sandstone; 3, sandstone (dominant) and shale; 4, shale in the lower part, 

alternation of shale and sandstone in the upper part; 5, alternation of shale, siltstone, 

limestone, and sandstone; 6, isopachs, m; 7, rocks absent; 8, faults.

136
Figure 90. Lithology and thickness of the shale-sandstone subformation (above paper shales) of the 

Shinhuduk Formation in the Dzunbain field area. (After Kuznetsova, 1966):

1, shale with sandstone beds in the middle part of the section; 2, shale with infrequent 

sandstone and siltstone layers in the lower part and rapid alternation of shale, siltstone, and 

sandstone in the upper part; 3, shale and marl in the lower part, sandstone with shale layers in 

the upper part; 4, shale in the lower part, shale and sandstone in the upper part; 5, shale and 

sandstone with limestone and marl beds at the base; 6, shale with infrequent sandstone and 

siltstone beds; 7, shale with a few siltstone layers; 8, isopachs, m; 9, faults; 10, rocks absent.

137
Figure 91. Lithology and thickness of the Khukhtyk Formation in the Dzunbain field area. (After 

Kuznetsova, 1966):

1, alternation of shale, sandstone, and conglomerate; 2, alternation of shale and sandstone with 

conglomerate at the base; 3, shale and sandstone; 4, shale with infrequent sandstone and 

siltstone layers; 5, isopachs, m; 6, rocks absent, 7, faults.

138
The Tamtsag basin on the northeast is a continuation of the Hailar basin in 

northern China. Probably, the general geology of the Tamtsag basin is similar to that 

of the Southeast Gobi basin; however, no data are available in literature.

PETROLEUM GEOLOGY AND EXPLORATION PLAYS

The Dzunbain field, discovered in the 1940's, is the only field that produced 

oil in the Southeast Gobi basin. At its peak in 1955, the field produced about 400,000 

b/year. In 1963, the production decreased to 140,000 b/year and was terminated soon 

afterwards. Data on the geology of the field are provided in Marinov and others 

(1977).

The field is controlled by an anticline about 10 x 4 km in dimensions. The 

steep (15-20°) limbs of the fold suggest a compressional origin for the structure. The 

northwestern flank is faulted down 500-600 m relative to the crest of the fold. 

Several transverse faults with a strike-slip component are present. The fold is 

mapped on Cretaceous horizons; a syncline underlies the fold at the level of the 

lower Tsagantsab Formation which is 750-1,200 m thick.

Production in the field is from sandstones of the Shinhuduk and the upper 

part of the Tsagantsab Formations. The latter contains about 30 pays at depths of 

580-850 m and is the main producer. Each pay is from 1 to 11 m thick. Initial flows 

reached 100-140 b/d but decreased abruptly to a few barrels. The oil is paraffinic, and 

its gravity is 29-29.5° API. Original oil reserves of the field are about 15 million 

barrels, but because of low rates of production the field is apparently uneconomic. 

Porosity of the sandstone reservoirs is 10-24% and the permeability is only 0.2-0.5 

md. Fracturing is significant. The low gas-oil ratio (30-40 ftVb) indicates that the 

field was subjected to degassing.

A few other discoveries in the basin are noncommercial, primarily because of 

very low oil flows. They include the Tsagan-Els and East Dzunbain fields (figs. 88- 

91). Both fields are located on the Bain-Mongol horst and the reservoir properties of 

oil-saturated rocks there are worse than those in the Dzunbain field. Oil saturation 

averages only 5-10 % and nowhere exceeds 25-30%. In addition to noncommercial 

discoveries, several wells on different structures of the basin encountered 

sandstones saturated with heavy oil and bitumen.

Recent oil discoveries in the Erlian basin have not been described in the 

literature. Supposedly, they are significant and may amount to several million 

barrels of oil (H.D. Klemme, personal communication).

Source rocks in both basins are located in the lower part of the clastic section 

that overlies volcanics of the early rifting stage. In the Southeast Gobi basin, the 

source rocks are paper shales at the bottom of the Shinhuduk Formation. The 

source-rock section is 50-100 m thick (fig. 89); paper shales in the section alternate 

with sandstones and siltstones. Geochemical data are not available; however, the

139
lithological character of the shales and a high content of hydrogen in the organic 

matter (8.7%) indicate their excellent quality as a source rock. Evidently, the shales 

were deposited in anoxic deep-lacustrine conditions.

The main source rocks in the Erlian basin are deep-water lacustrine shales in 

the middle member of the Badalahu Formation. The TOC of the shales averages 

2.8%, they contain 0.27% of chloroform extractable bitumen and slightly less than 

1000 ppm of hydrocarbons (Fan and others, 1983). The organic matter is of 

dominantiy sapropelic type. Organic-rich (TOC 1.5%) shales are also present in the 

middle member of the Bayanhua Formation. However, this organic matter is coaly 

in character and should be far less important as a source rock.

No data are available on maturation of source rocks in the Southeast Gobi 

basin. Source rocks seem to be mature at rather shallow depths. Total thickness of 

the overburden calculated from figures 90 and 91 (plus a few hundred meters of 

Upper Cretaceous-Tertiary rocks) shows that lower Shinhuduk paper shales are 

mature at depths of not more than 1,000-1,300 m. A part of the Lower Cretaceous 

section was removed by pre-Late Cretaceous erosion; thus, maximum temperatures 

and maturation were most probably achieved by Late Cretaceous time. Basalt 

eruptions in the Erlian basin and the abundance of volcanic material in rocks of the 

Southeast Gobi basin suggest high heat flow and resulting shallow maturation 

depth. In the Erlian basin, the Badalahu Formation source rocks also achieved 

maturity at depths between 1,000 and 1,500 m (Fan and others, 1983).

Reservoir rocks in both basins are characterized by dirty lithologies due to 

their arkosic composition, poor sorting, and abundance of volcanic material. 

Typically, porosity of sandstones is not very low because of the relatively shallow 

depth of burial and moderate compaction, but permeability is commonly poor.

The main factor which greatly reduces the petroleum potential of the 

Southeast Gobi basin is poor development of the sag sequence over the grabens. 

Upper Cretaceous and Tertiary rocks are thin and sandy and apparently have low 

ability to hydrodynamically seal the highly faulted grabens. Relatively early, preLate Cretaceous, time of maturation and following uplift and erosion resulted in 

partial (Dzunbain and Tsagan-Els fields) or complete destruction of fields. From this 

point of view, exploration in highly uplifted compressional folds and horsts has 

little chance for success. Exploration in the basin was conducted by Soviet geologists 

and/or by Soviet-trained Mongolians and was directed toward targets of this type. 

Probably this was the reason for the lack of success. Stratigraphic updip pinch-out 

traps along boundary faults and other subtle types of traps in deeper parts of grabens 

probably bear significantly more potential. Application of high quality seismics to 

map deep structure of the grabens is critical; however, modern seismic surveys have 

never been conducted in Mongolia. Considering the poor quality of reservoir rocks 

and inadequate sealing of the entire petroleum systems, the petroleum potential of 

the Southeast Gobi basin is probably low to moderate.

140
The stratigraphic succession and structure of the Tamtsag basin on the 

northeast of Mongolia (fig. 82) are quite similar to those of the Southeast Gobi basin. 

Several drill tests (also on highly uplifted structures) did not find significant oil 

shows; only some inclusions of asphalt were recorded in cores (Marinov and others, 

1977). Small oil fields were recently discovered across the border in China, in the 

Hailar basin (H.D. Klemrne, personal communication). The potential of the basin is 

probably similar to that of the Southeast Gobi basin.

Conditions for preservation of oil are probably significantly better in the 

Erlian basin. Although the postrift sag sequence is also poorly developed, thickness 

of the synrift elastics overlying source rocks of the Badalahu Formation is much 

larger (to 2.5 km) than this thickness in the Southeast Gobi basin. Thickness of the 

source-rock section in the Erlian basin is also significantly larger than that in the 

Mongolian basins. Widespread oil shows observed in the early 1980's are described 

by Fan and others (1983). Oil-saturated sandstones at shallow (less than 500 m) 

depths suggest that although sealing is not perfect, pools in deeper horizon have not 

been completely destroyed. No data on recent discoveries are available, but probably 

they are in hundreds of million barrels range (H.D. Klemme, personal 

communication). The available data indicate a significant undiscovered potential 

for the basin. Imperfect sealing suggests a high probability for dominance of oil in 

the resources.

141
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