Sylhet-Kopili/Barail-Tipam 

Composite Total Petroleum System, 

Assam Geologic Province, India

By C.J. Wandrey

U.S. Geological Survey Bulletin 2208-D

U.S. Department of the Interior

U.S. Geological Survey

Petroleum Systems and Related Geologic 

Studies in Region 8, South Asia

Edited by Craig J. Wandrey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

This publication is only available online at:

http://pubs.usgs.gov/bul/b2208-d/

Posted online June 2004, version 1.0
Foreword

This report describing the petroleum resources within the Sylhet-Kopili/BarailTipam Total Petroleum System in India was prepared as part of the World Energy 

Assessment Project of the U.S. Geological Survey. For this project, the world was 

divided into 8 regions and 937 geologic provinces, which were then ranked according 

to the discovered oil and gas volumes within each (Klett and others, 1997). Of these, 76 

“priority” provinces (exclusive of the United States and chosen for their high ranking) 

and 26 “boutique” provinces (exclusive of the United States and chosen for their 

anticipated petroleum richness or special regional economic importance) were selected 

for assessment of undiscovered oil and gas resources. The petroleum geology of these 

priority and boutique provinces is described in this series of reports.

The purpose of the World Energy Project is to assess the quantities of oil, gas, and 

natural gas liquids that have the potential to be added to reserves within the next 30 

years. These volumes either reside in undiscovered fields whose sizes exceed the stated 

minimum-field-size cutoff value for the assessment unit (variable, but must be at least 1 

million barrels of oil equivalent) or occur as reserve growth of fields already discovered. 

The total petroleum system constitutes the basic geologic unit of the oil and gas 

assessment. The total petroleum system includes all genetically related petroleum that 

occurs in shows and accumulations (discovered and undiscovered) and that (1) has been 

generated by a pod or by closely related pods of mature source rock, and (2) exists within 

a limited, mappable geologic space, along with the other essential, mappable geologic 

elements (reservoir, seal, and overburden) that control the fundamental processes of 

generation, expulsion, migration, entrapment, and preservation of petroleum. The 

minimum petroleum system is that part of a total petroleum system encompassing 

discovered shows and accumulations along with the geologic space in which the various 

essential elements have been proved by these discoveries.

An assessment unit is a mappable part of a total petroleum system in which 

discovered and undiscovered fields constitute a single, relatively homogeneous population 

such that the chosen methodology of resource assessment based on estimation of the 

number and sizes of undiscovered fields is applicable. A total petroleum system may 

equate to a single assessment unit, or it may be subdivided into two or more assessment 

units if each unit is sufficiently homogeneous in terms of geology, exploration 

considerations, and risk to assess individually.

A graphical depiction of the elements of a total petroleum system is provided in the 

form of an events chart that shows the times of (1) deposition of essential rock units, (2) 

trap formation, (3) generation, migration, and accumulation of hydrocarbons, and (4) 

preservation of hydrocarbons. 

A numeric code identifies each region, province, total petroleum system, and 

assessment unit; these codes are uniform throughout the project and will identify the 

same type of entity in any of the publications. The code is as follows: 

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The codes for the regions and provinces are listed in Klett and others (1997).
Oil and gas reserves quoted in this report are derived from Petroconsultants’ 

Petroleum Exploration and Production database (Petroconsultants, 1996) and other area 

reports from Petroconsultants, Inc., unless otherwise noted.

Figure(s) in this report that show boundaries of the total petroleum system(s), 

assessment units, and pods of active source rocks were compiled using geographic 

information system (GIS) software. Political boundaries and cartographic representations 

were taken, with permission, from Environmental Systems Research Institute’s ArcWorld 

1:3,000,000 digital coverage (1992), have no political significance, and are displayed 

for general reference only. Oil and gas field centerpoints shown in these figures are 

reproduced, with permission, from Petroconsultants (1996). 
Contents

Foreword ....................................................................................................................................................... iii

Abstract.......................................................................................................................................................... 1

Introduction ................................................................................................................................................... 1

Acknowledgments........................................................................................................................................ 1

Regional Geologic History........................................................................................................................... 3

Stratigraphy ................................................................................................................................................... 8

Exploration and Production History......................................................................................................... 10

Composite Petroleum System................................................................................................................... 11

Source Rock ....................................................................................................................................... 11

Maturation .......................................................................................................................................... 14

Generation and Migration ................................................................................................................ 14

Reservoir Rocks ................................................................................................................................. 14

Traps and Seals.................................................................................................................................. 16

Assessment Unit ......................................................................................................................................... 16

Assessment of Undiscovered Oil and Gas ............................................................................................. 17

Summary ...................................................................................................................................................... 17

Selected References.................................................................................................................................. 18

Figures

1. Map showing location of Assam geologic province .............................................................. 2

2. Photograph showing Himalaya mountain range, Brahmaputra River, 

and Assam Shelf........................................................................................................................... 3

3. Generalized geologic map of Assam geologic province ....................................................... 4

4. Map showing Sylhet-Kopili/Barail-Tipam Composite 

Total Petroleum System (803401) ............................................................................................... 5

5–10. Paleogeographic maps from perspective of study region, showing:

5. Middle Jurassic (approximately 166 Ma) ...................................................................... 6

6. Early Cretaceous (approximately 130 Ma)..................................................................... 6

7. Late Cretaceous (approximately 94 Ma)........................................................................ 6

8. Latest Cretaceous (approximately 69 Ma)..................................................................... 6

9. Middle Eocene (approximately 50 Ma) .......................................................................... 7

10. Late Oligocene (approximately 27 Ma) .......................................................................... 7

11. Generalized cross section showing development of Assam Shelf...................................... 8

12. Generalized stratigraphic diagram of Assam Shelf................................................................ 9

13. Sketch map showing oil and gas fields and identified prospects in 

Assam geologic province.......................................................................................................... 10
14–17. Graphs showing:

14. Cumulative number of new-field wildcat wells versus well completion year....... 11

15. Grown field sizes grouped in thirds by age of discovery and ranked by size........ 12

16. Cumulative grown oil volume versus cumulative new-field wildcat 

wells and cumulative grown oil volume versus field discovery year in 

Assam geologic province............................................................................................... 12

17. Reservoir depth versus oil field discovery year ......................................................... 13

18. Map showing Kopili Formation source rock richness.......................................................... 13

19. Gas chromatograms and a composition plot for two Eocene oil crude types 

(heavy and normal) collected at three fields ......................................................................... 14

20. Diagram showing temperature, maturity, and subsidence history for the 

Geleki field well Geleki-174 ....................................................................................................... 15

21. Pie diagram showing temporal and lithologic distribution of producing 

reservoirs by youngest age and predominant lithology....................................................... 15

22. Pie diagram showing distribution of trap types..................................................................... 16

23. Events chart summarizing stratigraphy, source rocks, reservoirs, seals, 

traps, and petroleum information for the Sylhet-Kopili/Barail-Tipam 

Composite Total Petroleum System......................................................................................... 16

Table

1. USGS estimates of undiscovered oil and gas resources, Assam, province 8034; 

assessment results summary—allocated resources .......................................................... 17
Abstract

The Sylhet-Kopili/Barail-Tipam Composite Total Petroleum system (TPS) (803401) is located in the Assam Province 

in northeasternmost India and includes the Assam Shelf south 

of the Brahmaputra River. The area is primarily a southeastdipping shelf overthrust by the Naga Hills on the southeast 

and the Himalaya Mountain range to the north. The rocks that 

compose this TPS are those of the Sylhet-Kopili/Barail-Tipam 

composite petroleum system. These rocks are those of the 

Eocene-Oligocene Jaintia Group Sylhet and Kopili Formations, the Oligocene Barail Group, and the Oligocene-Miocene 

Surma and Tipam Groups. These groups include platform 

carbonates, shallow marine shales and sandstones, and the 

sandstones, siltstones, shales, and coals of deltaic and lagoonal 

facies. 

Source rocks include the Sylhet and Kopili Formation 

shales, Barail Group coals and shales, and in the south the 

Surma Group shales. Total organic carbon content is generally 

low, averaging from 0.5 to 1.8 percent; it is as high as 9 percent in the Barail Coal Shales. 

Maturities are generally low, from Ro (vitrinite reflectance) 0.45 to 0.7 percent where sampled. Maturity increases 

to the southeast near the Naga thrust fault and can be expected 

to be higher in the subthrust. Generation began in early Pliocene. Migration is primarily updip to the northwest (< 5 to 15 

kilometers) along the northeast-trending slope of the Assam 

Shelf, and vertical migration occurs through reactivated basement-rooted faults associated with the plate collisions.

Reservoir rocks are carbonates of the Sylhet Formation, 

interbedded sandstones of the Kopili Formation and sandstones of the Barail, Surma, and Tipam Groups. Permeability 

ranges from less than 8 mD (millidarcies) to as high as 800 

mD in the Tipam Group. Porosity ranges from less than 7 percent to 30 percent.

Traps are primarily anticlines and faulted anticlines with 

a few subtle stratigraphic traps. There is also a likelihood of 

anticlinal traps in the subthrust. Seals include interbedded Oligocene and Miocene shales and clays, and the thick clays of 

the Pliocene Gurjan Group. 

Introduction

Among the 76 priority provinces identified by the U.S. 

Geological Survey World Energy Assessment Team (2000) 

was the Assam geologic province. Located in northeastern 

India, Assam geologic province is an onshore province covering approximately 74,000 km2 (figs. 1 and 2). The geologic 

province is bounded on the north and west by the Brahmaputra 

River, and on the south and east by the Indo-Burman Ranges 

and the Central Burma Basin (fig. 3). Major features within the 

Assam geologic province include the Assam Shelf, Brahmaputra River valley, Shillong Plateau, Mirkir Hills, and a foreland 

portion of the Indian Shield. The Assam Shelf consists of a 

portion of the Paleocene to Eocene continental shelf of the 

Indian plate which became emergent and which is being overthrust by the Himalayas on the northwest and by the Burma 

micro-plate on the southeast. 

Structurally, the Assam geologic province consists of two 

primary, subparallel features trending southwest to northeast, 

which developed just prior to, and as a result of plate collision. 

The northernmost structural feature is the Assam Shelf, which 

is presently being subducted beneath both the Eurasian plate 

to the north and the Burma plate to the southeast. The southernmost feature consists of en-echelon folds of the Naga Hills 

overthrust belt and northernmost Indo-Burman Ranges, which 

are continually developing as the shelf is subducted. At present, most hydrocarbon production is northwest of, and parallel 

to, the Naga thrust fault.

Figure 2 shows Brahmaputra River flowing from east 

(right) to west (left) and tributaries flowing from the Himalayan ranges at the top of the view (north) and from the Naga 

Hills in the bottom of the view (south). The producing area 

lies south of the Brahmaputra River. 

Acknowledgments

Insight into the geology and data presented in this report 

were provided by scientists of the Oil and Natural Gas Corporation (ONGC), Dehra Dun, India.

Sylhet-Kopili/Barail-Tipam 

Composite Total Petroleum System, 

Assam Geologic Province, India

By C.J. Wandrey

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Assam

8001 Southeast Afghanistan8002 Himalayan8003 Indian Shield8004 Sri Lanka8005 Indo-Burman8006 Tenasserim-Shan8021 Makran8022 Baluchistan8023 Central Afghanistan8024 Afghan8025 Sulaiman-Kirthar8026 Kohat-Potwar 8028 Himalayan Foreland8030 Chindwara8031 Satpura-Brahmani8032 Damodar8033 Pranhita-Godavari8034 Assam8035 North Burma8042 Indus8043 Bombay8044 Cauvery8045 Krishna-Godavari8046 Mahanadi8047 Ganges-Brahmaputra Delta8048 Irrawaddy8061 Maldives8062 Lakshadweep8063 Konkan

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Figure 1. Location of Assam geologic province shown in greenish-blue (8034); other assessed provinces in region 8 shown in yellow.

2 Petroleum Systems and Related Geologic Studies in Region 8, South Asia 
Regional Geologic History

The Sylhet-Kopili/Barail-Tipam Composite Total Petroleum System (TPS) and Assessment Unit (AU) of the same 

name (fig. 4) acquired their primary structural and stratigraphic features from events associated with plate movements 

that occurred from latest Paleozoic to the present.

From Permian through Middle Jurassic time, the Indian 

plate was located in the southern hemisphere between the African, Antarctic, and Australian plates, and constituted part of 

southern Gondwana (fig. 5). The Lower Permian tillites on the 

northwestern part (Shah, 1977) and other basal Permian glacial 

deposits on the southern Indian plate are indicative of a much 

cooler paleoclimate. The area that is today the Indus Basin and 

Brahmaputra River

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Figure 2. Himalaya mountain range at top of picture (north), Brahmaputra River flowing east to west, and Assam Shelf (NASA, 1998).

Sylhet-Kopili/Barail-Tipam Composite Petroleum System, Assam Geologic Province, India 3
Figure 3. Generalized geologic map of Assam geologic province (Wandrey and Law, 1999).

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4 Petroleum Systems and Related Geologic Studies in Region 8, South Asia 
northern India was a shallow continental shelf on which carbonates, sandstones, and shales were accumulating. These depositional conditions persisted at least intermittently on the western 

and northern parts of the shelf through Late Jurassic. Jurassic 

carbonates have not been reported in the area of the Assam geologic province or in the eastern Gondwana basins of Damodar 

and Krishna Godovari (fig. 1). The Jurassic was generally a 

period of nondeposition (Datta and others, 1983). During the 

Late Jurassic the Indian, Australian, and Antarctic plates began 

to break away from Africa, forming the Somali Rift basin (figs. 

5, 6). Late Jurassic rifting also initiated separation of Australia 

and Antarctica from India (Scotese and others, 1988).

During Early Cretaceous time, the Indian plate drifted 

northward entering warmer latitudes (fig. 6). Along the northeastern portion of the Indian plate, volcanic activity occurred 

as the northeast corner of the Indian plate passed over the 

Kerguelen hot spot (fig. 7); this resulted in the expulsion 

and deposition of the Rajmohal Trap volcanics. Through the 

Late Cretaceous, the Indian plate continued drifting northward toward the Asian plate and the seafloor of the Bengal 

Basin began to form. As the northward movement of the 

Indian plate continued, a transform fault became active along 

the Ninety East Ridge (figs. 7, 8). In the Assam area, block 

faulting and a southeasterly dipping shelf developed. Late 

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Figure 4. Sylhet-Kopili/Barail-Tipam Composite Total Petroleum System (803401).

Sylhet-Kopili/Barail-Tipam Composite Petroleum System, Assam Geologic Province, India 5
INDIAN 

PLATE

AUSTRALIAN

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AFRICAN

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BURMA

PLATE

EURASIAN PLATES

INDIAN 

PLATE

Lowlands

Mountains

Shelf

Marine

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AFRICAN

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EURASIAN PLATES

BURMA

PLATE

Figure 5. Middle Jurassic (approximately 166 Ma). Perspective lat 

20°S., long 68°E. (modified from Scotese and others, 1988, and Scotese, 1997). 

Figure 6. Early Cretaceous (approximately 130 Ma). Perspective lat 

20°S., long 68°E. (modified from Scotese and others, 1988, and Scotese, 1997).

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Figure 7. Late Cretaceous (approximately 94 Ma). Perspective lat 

20°S., long 68°E. (modified from Scotese and others, 1988, and Scotese, 1997).

Figure 8. Latest Cretaceous (approximately 69 Ma). Perspective lat 

20°S., long 68°E. (modified from Scotese and others, 1988, and Scotese, 1997).

6 Petroleum Systems and Related Geologic Studies in Region 8, South Asia 
Cretaceous rifting between Madagascar and the western part 

of the Indian plate initiated formation of Mascarene Basin. 

Counterclockwise rotation of the Indian plate began, and the 

Seychelles portion of the Indian plate started to break away 

(Waples and Hegarty, 1999) (fig. 8). The Indian plate continued to move northward at an accelerated rate of 15–20 cm/yr. 

The east edge of the plate passed over the Kerguelen hot spot 

forming a line of islands near long 90°E. (fig. 9). The SinoBurman Ranges first emerged, and flysch was deposited in the 

area that is now the Central Burma Basin. The Assam Shelf 

began to tilt farther toward the southeast, and block faulting 

was reactivated. 

Continued northward movement and counterclockwise 

rotation of the Indian plate slowly closed the Tethyan Sea, and 

along the northwest edge of the plate, the Sulaiman-Kirthar 

fold belt (fig. 1) began to develop. Uplift on the Eurasian plate 

created a new primary sediment source, and the prevailing 

sediment transport direction of south to north was reversed. 

Carbonate platform build-up continued intermittently from 

the Eocene through middle Miocene time on the shelves surrounding much of the Indian plate. A trench formed along the 

subduction zone as the Indian plate began to slip beneath the 

Eurasian plates (fig. 10). 

As subduction continued, the Eurasian plates shed sediments into the trench. Terrestrial sediment influx from the 

rapidly rising Himalayan, Sino-Burman, and Indo-Burman 

Ranges exceeded carbonate accumulation on late Miocene 

platforms (Roychoudhury and Deshpande, 1982), and smothered the carbonate reefs in the shelf areas. The shelf areas 

along these collision zones were either subducted or became 

emergent fluvial-deltaic environments. 

The proto Indus, Narmada, Ganges, Brahmaputra, 

Megna, Chindwin, and Irrawaddy Rivers developed extensive 

deltas as the Himalayan and other ranges continued to shed 

sediments at a high rate. Today, uplift of the Himalayas and 

subduction of the Indian plate continue, and the growth rate 

of the Indus, Ganges-Brahmaputra (Megna), and Irrawaddy 

deltas remains high.

The present-day Assam geologic province, a cratonic 

margin, reflects three distinct tectonic phases. The earliest was 

Late Cretaceous to Eocene block faulting and development 

of a southeasterly dipping shelf. During the second phase, 

in Oligocene time, uplift and erosion occurred north of the 

Dukai fault (fig. 3); many basement faults were reactivated; 

and many basement-controlled structures became prominent 

(Naidu and Panda, 1997). Oligocene uplift and erosion were 

followed by late Miocene through Pliocene extensive alluvial 

deposition. The resultant sedimentary column is as much as 

7,000 m thick (fig. 11). The thickest section is along the Naga 

thrust fault and in the Dhansiri Valley where it is greater than 

4,500 m. The thinnest section lies along the axis of the central 

basement ridge, where it is less than 2,000 m thick.

Lowlands

Mountains

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Figure 9. Middle Eocene (approximately 50 Ma). Perspective lat 

20°S., long 68°E. (modified from Scotese and others, 1988, and Scotese, 1997).

Figure 10. Late Oligocene (approximately 27 Ma). Perspective lat 

20°S., long 68°E. (modified from Scotese and others, 1988, and Scotese, 1997). 

Sylhet-Kopili/Barail-Tipam Composite Petroleum System, Assam Geologic Province, India 7
Stratigraphy

The oldest sedimentary rocks reported near the Assam 

geologic province are thin Cretaceous limestones to the 

south, in eastern Manipur (fig. 3). Within the Assam geologic 

province, the oldest sedimentary rocks are the continental to 

lagoonal sandstones and interbedded shales of the Upper Cretaceous and Paleocene Dergaon and Disang Formations (fig. 

12). More than 5,300 m of shales and sandstones of the Upper 

Cretaceous and Paleocene Disang Formation were deposited 

in the Manipur and Mizoram areas (fig. 3), and on the Assam 

Shelf more than 500 m of sandstones and shales of Upper Cretaceous Dergaon Formation were deposited. The top of the Dergaon and Disang is marked by an unconformity and overlain 

by the medium-grained massive sandstones of the Paleocene 

and Eocene Jaintia Group Tura and Langpar Formations. More 

than 250 m of the Tura and Langpar were deposited in a fluvial 

to marginal marine environment. The overlying Eocene Sylhet 

Formation was deposited in a range of environments and is 

subdivided into members generally representing these different 

depositional environments. More than 350 m of thin sandstones 

and interbedded shales and coals of the basal part of the lower 

Lakadong member were deposited in a lagoonal environment. 

The middle part of the Lakadong member typically consists of 

the thick sands of strand plain or barrier-bar environments. The 

upper part of the Lakadong member is a calcareous sandstone 

of a restricted shallow-water platform (Mathur and others, 

2001). The overlying Narputh member consists of claystones 

and siltstones of a shelf environment. The upper member of the 

Sylhet, the Prang member, is a shelf carbonate with interbedded siltstones and clay. Within the Assam geologic province, 

the Sylhet Formation is depositionally thicker from northwest 

to southeast due to contemporaneous platform tilting and basement-sourced block faulting. The top of the Sylhet is marked 

by a regional unconformity on which as much as 500 m of the 

shallow marine to lagoonal shales and interbedded limestones 

of the Eocene Kopili Formation accumulated. 

On the partly unconformable top of the Kopili more than 

3,000 m of the upper Eocene and Oligocene Barail Group rocks 

were deposited. The Barail Group is divided into the Barail 

Lower Arenaceous Unit, also referred to as the Tinali Formation in some places and the Upper Barail Group Coal-Shale 

Unit (also called the Rudrasagar Formation). The primarily 

delta front Barail Lower Arenaceous Unit consists of the Main 

Pay Sand, Main Lower Sand, and minor shales with a combined 

thickness of as much as 900 m. The Upper Barail Group CoalShale Unit consists of as much as 1,200 m of interbedded coals, 

shales, and discontinuous sandstone reservoirs deposited in a 

delta-plain environment. The Barail Group strata are truncated 

by an unconformity north of the Dukai fault. This unconformity 

is probably associated with uplift caused by crustal shortening. 

In the Dhansiri Valley (fig. 3), erosion completely removed the 

Barail Group and Kopili Formation from the southeastern portion of the Mirkir Hills at the end of the Oligocene. 

The Miocene-Pliocene Surma Group is missing on much 

of the Assam Shelf (fig. 12). However, as much as 1,400 m 

was deposited on an unconformable surface in the southwestFigure 11. Generalized cross section showing development of Assam Shelf (modified from Murty, 1983; and Naidu and Panda, 1997).

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Dihing Group

Duptilia Group and upper part of the Tipam Group

Lower part of Tipam Group and Surma Group

Barail Group and Upper Jaintia Group Kopili Formation

Jaintia Group Sylhet Formation

Basement

EXPLANATION

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8 Petroleum Systems and Related Geologic Studies in Region 8, South Asia 
ern part of the Assam geologic province. The Surma Group is 

divided into the Bhuban Formation (>500 m thick), overlain 

by the Bokabil Formation (>900 m), and is typified by a series 

of thin siltstones, sandstones, and shales deposited in fluvial 

deltaic to estuarine environments. 

Following further southward tilting of the Assam Shelf, 

an early Miocene transgression occurred, depositing tidal 

mudflat and minor tidal sand-flat facies (Dhar and Debashis, 

1997). Several minor transgressions followed, and by middle 

Miocene there was widespread deposition of the fluvial Tipam 

Formation sandstones. The Surma, or Barail, and Jaintia 

Groups (where the Surma is missing) are overlain by as much 

as 2,200 m of massive sandstones and subordinate shales and 

clays of the (Nahorkatiya or Tipam Group) Tipam Sandstone 

Formation. The upper Nahorkatiya or Tipam Group Girujan 

Formation, found over most of the Assam Shelf, consists of ����� �����

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Figure 12. Generalized stratigraphy of Assam Shelf (modified from Mathur and others 2001; Murty, 

1983; and Petroconsultants, 1996).

Sylhet-Kopili/Barail-Tipam Composite Petroleum System, Assam Geologic Province, India 9
more than 1,300 m of mottled clays containing minor sandstone lenses. 

By late Miocene time the course of the Brahmaputra 

River east and south of the Shillong Plateau was altered, possibly by uplift along the edge of the Assam Shelf, to its present-day course around the north side of the Shillong Plateau 

(fig. 3) and then south to the Bay of Bengal. In the Manipur 

and Mizoram areas, the Nahorkatiya or Tipam Group thins to 

900 m, possibly due to the uplift. During the Pliocene fluvial 

shales, sands, and clays were deposited in the Indus, Bengal, 

and Central Burma Basins and onto the Assam Shelf, where 

the Nahorkatiya or Tipam Group is overlain by the sandy clay 

and mudstones of the Duptilia and Dihing Groups.

Exploration and Production History

Oil seeps were reported in the Assam geologic province 

as early as 1825. In 1867 the first well was completed in the 

Namdang area by “mechanical” means and encountered oil 

at a depth of 35 m. The first reported commercial discovery 

in India occurred near Digboi in about 1889, in the thrust 

and fold belt of the Assam Shelf (fig. 13). By 1901 a refinery 

had been established in Digboi and 500 barrels of oil a day 

were being produced (Indian Ministry of Petroleum, 1999). 

Between 1922 and 1932 Burma Oil Company (BOC) mapped 

and drilled 10 structures in the Schuppen Belt (fig. 3), with 

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0 10 20 KILOMETERS

Figure 13. Oil and gas fields and identified prospects in Assam geologic province (modified from Naidu and Panda, 1997; and Mallick and 

others, 1997).

10 Petroleum Systems and Related Geologic Studies in Region 8, South Asia 
no economic successes reported (Rangaroo, 1983). BOC also 

conducted seismic surveys in 1937. Though further exploration was postponed until after World War II, the seismic surveys conducted in 1937 resulted in the next major discovery, 

the Nahorkatiya field, in 1953. According to the Petroconsultants International Oil and Gas Field Database (Petroconsultants, 1996), 38 oil fields and 1 gas field have been discovered within the Assam geologic province (fig. 13). These 

fields each have more than 1 million barrels of oil equivalent 

(MMBOE) of cumulative production and (or) proved reserves. 

These 39 fields contain at least 89 reservoirs that have 

produced oil, gas, or condensate. The largest fields are Nahorkatiya, Moran, and Lakwa discovered in 1953, 1956, and 

1964, respectively (figs. 13, 14). These three fields each contain more than 500 MMBOE, discovered recoverable reserves. 

Until the 1974 discovery and subsequent development of the 

Bombay High field on the western Indian Shelf, the Assam 

geologic province was the largest producer of oil in India.

Exploration history in the Assam geologic province 

indicates that it is a mature basin where the largest fields were 

found early in the exploration history. The known oil fields 

ranked by size and grouped by discovery date in thirds (fig. 

15) show a marked decrease in field size in the second and 

third thirds. The number of gas fields was not sufficient to 

produce a meaningful plot. The decrease in field size through 

time indicates that the province is relatively mature in terms 

of exploration, at least for the type of play that the discovered 

fields represent on the plot. An assessment unit that is mature 

in terms of exploration should show a large decrease in field 

sizes between the first and third thirds. Note that the second 

and the third third show similar distributions. Figure 16 shows 

plots of cumulative grown oil volume versus cumulative newfield wildcat wells in Assam province and cumulative grown 

oil volume versus field discovery year. Comparison of these 

plots shows that the number of new-field wildcat wells (measure of effort) required to find the same volume of oil has 

increased, but the effort required to find more, though smaller, 

fields has remained consistent. 

The reservoir depth versus field discovery year plot (fig. 

17) indicates that, while generally deeper, some reservoirs 

found later were very shallow. This may indicate that substantial resources remain to be found in known hydrocarbonproducing zones by use of recently improved borehole and 

seismic technology (Mathur and others, 2001). 

Composite Petroleum System

Source Rock

The USGS recognizes that several petroleum systems 

are present within the Assam geologic province (Chandra and 

others, 1995; Naidu and Panda, 1997; Kent and others, 2002). 

For assessment purposes, however, they were combined into 

Figure 14. Cumulative number of new-field wildcat wells versus well completion year plot, an indication 

of exploration effort. Based on Petroconsultants well and field data (Petroconsultants, 1996).

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Sylhet-Kopili/Barail-Tipam Composite Petroleum System, Assam Geologic Province, India 11
exceeds 400 m. The overlying Eocene Kopili Formation 

consists of alternating fossiliferous fluvial shales and finegrained sandstones more than 500 m thick; it contains numerous potential source rock intervals (Naidu and Panda, 1997). 

Naidu and Panda (1997) calculated source rock richness for 

the Kopili as a product of the average TOC values and thickness of the source rock. The point values derived were used to 

create an isopach map showing source rock richness (fig. 18). 

The product of the source rock richness value and a scaled 

maturity value (Ro) provides a source-potential value (Naidu 

and Panda, 1997). Source rocks within these formations contain types II and III kerogens and have TOC values commonly 

ranging from less than 0.5 percent to 1.5 percent. TOC values 

are higher in the Kopili Formation shales, where the maximum 

TOC values exceed 1.8 percent. 

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Figure 15. Maturity of exploration indicator plot, showing grown 

(expected final size when production is ended) field sizes grouped in 

thirds by age of discovery and ranked by size. Based on Petroconsultants field data (Petroconsultants, 1996).

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Figure 16. Cumulative grown oil volume versus cumulative new-field 

wildcat wells and cumulative grown oil volume versus field discovery 

year in Assam geologic province.

a composite petroleum system designated the Sylhet-Kopili/

Barail-Tipam TPS (803401). A composite TPS was used 

because few correlations of source to reservoir hydrocarbons 

were available at the time of the assessment. Efforts continue 

to better define the source-rock reservoir-rock relationships 

(Mathur and others, 2001). Multiple stacked source and reservoir rock sequences, and extensive fault systems allowing 

mixing of hydrocarbons from multiple sources (Chandra and 

others, 1995) also make distinguishing individual systems difficult. When more geochemical data become available, it may 

be possible to identify three or more distinct petroleum systems and one or more composite systems in this area. The distinct systems might include the Sylhet-Kopili, Barail-Tipam, 

and in the southwest a Buban-Bokabil system. The petroleum 

systems of the Assam Shelf most likely are also genetically 

and temporally related to some of the gas-prone petroleum 

systems in the Bengal Basin (Wandrey and others, 2000).

The Sylhet-Kopili/Barail-Tipam TPS is composed of 

the rocks of the Eocene-Oligocene Jaintia Group Sylhet and 

Kopili Formations, the Oligocene Barail Group, and the Oligocene-Miocene Surma and Tipam Groups. These rocks include 

platform carbonates, shallow marine shales and sandstones, 

and the sandstones, siltstones, shales, and coals of deltaic, 

alluvial, and lagoonal facies. 

With the exception of the Barail Coal Shale (BCS), 

most rocks in the Assam geologic province do not have high 

total organic carbon (TOC) contents today. Many formations, 

though, have low to moderate TOC values. The Eocene Jaintia Group contains the oldest source rocks in this TPS and 

includes interbedded shales of the Langpar and Lakadong 

members of the Sylhet Formation (see fig. 23) (Mathur and 

others, 2001). The Sylhet Formation is a shallow marine limestone interbedded with thin sandstones and shales deposited 

in marine to lagoonal environments. In places its thickness 

12 Petroleum Systems and Related Geologic Studies in Region 8, South Asia 
API gravities range from 13° for heavy oils to 45° for 

“normal” oils (fig. 19). The “normal oils” from the Kopili 

Formation range from 29° to 45°. Oils derived from the Sylhet 

range from 24° to 37°, and wax content ranges from 11 to 15.6 

percent.

The upper Eocene and Oligocene Barail Coal-Shale Unit 

reaches a maximum thickness of 660 m between the Naga 

and Disang thrusts (Balan and others, 1997); TOC values 

range from less than 0.8 percent to more than 12 percent in 

the Barail Coal-Shale Unit. The Barail is considered to be the 

primary source rock throughout much of the Assam geologic 

province. Although younger rocks with sufficient organic content occur, they are immature where sampled. To the southwest 

in Bangladesh, the Miocene Surma Group Bubhan Formation 

is hypothesized to be the most likely source rock. 

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Figure 17. Reservoir depth versus oil field discovery year, showing 

that depth range of discoveries has not increased significantly with 

time. Based on Petroconsultants field data (Petroconsultants, 1996).

0 5 10 KILOMETERS

B R A H M A P U T R A

R I V E R

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EXPLANATION

Kopili Formation

 richness contour

NAGA THRUST FAULT

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Figure 18. Kopili Formation richness map (percent TOC × thickness of source rock) (modified from 

Naidu and Panda, 1997).

Sylhet-Kopili/Barail-Tipam Composite Petroleum System, Assam Geologic Province, India 13
Maturation 

Thermal maturity values range from Ro 0.5 to 0.7 percent 

in both the Sylhet and Kopili Formations (fig. 20). Maturity 

is expected to increase with depth to the southeast toward 

the Naga thrust fault and be even higher in the subthrust. In 

younger rocks, such as the Barail Group, source rock maturities are also generally low, from Ro 0.45 to 0.7 percent where 

sampled. The vitrinite reflectance values may be suppressed 

where the sediments were deposited in reducing marine or 

lacustrine environments. Surma Group source rocks are only 

mature in the southwestern part of the Assam geologic province where burial depths are greater.

Generation and Migration

Oil generation began by early to middle Miocene for the 

Sylhet and Kopili Formations near and below the Naga thrust 

fault. Mathur and others (2001) placed the onset of generation 

for the Langpar and Lakadong members of the Sylhet at about 

1,750 million years ago (Ma), and generation is continuing 

today in the deeper portions of the Assam geologic province. 

Although volumetric estimates of suitable mature organic 

material in place for the Sylhet are not available, estimates of 

source rock potential by Naidu and Panda (1997) provide a 

basis for estimating source rock volumes for the Kopili. For 

this study, sufficient organic material is assumed to remain 

for continuing generation. Migration is primarily updip, along 

the northeast-trending slope of the Assam Shelf. Migration 

paths may extend to adjacent reservoirs but more often as far 

as 15 km. The source rocks in many fields are reported to be 

only marginally mature; therefore, significant volumes of the 

oil found there must have migrated from deeper areas along 

the Naga thrust fault. Oil generated beneath the thrust sheet 

has probably migrated to the leading edge of the thrust sheet 

and contributes materially to the total oil volume in reservoirs 

along the leading edge of the thrust sheet. Vertical migration 

through reactivated basement-rooted faults associated with 

plate collision may predominate in some areas, particularly 

near the edge of the Naga thrust fault.

Reservoir Rocks 

Reservoir rocks are present throughout most of the 

stratigraphic section in the Assam geologic province (fig. 

21). Reservoir rocks include the Eocene-Oligocene Jaintia 

Group Sylhet Formation limestones and Kopili Formation 

interbedded sandstones; Tura and Langpar (basal) marine 

sandstones also have reservoir potential, and Surma 

Group alluvial sandstone reservoirs are productive in the 

southwestern part of the Assam geologic province. The most 

productive reservoirs are the Barail main pay sands and the 

Tipam Group massive sandstones. Permeability ranges from 

less than 8 millidarcies to as much as 800 millidarcies in the 

Tipam Group sandstones with porosities ranging from less 

than 7 percent to 30 percent.

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Figure 19. Gas chromatograms and a composition plot for two Eocene crude types (heavy and normal), found at Dikom, Kathalani, and Tengahat fields. Samples were analyzed from 18 wells (modified from Mallick and others, 1997).

14 Petroleum Systems and Related Geologic Studies in Region 8, South Asia 
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Figure 20. Temperature, maturity, and subsidence history for Geleki-174 (modified from Balan and others, 1997).

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Figure 21. Temporal and lithologic distribution of producing reservoirs by youngest age and predominant lithology. Percentages represent number of reported occurrences (IHS Energy, 2001).

Sylhet-Kopili/Barail-Tipam Composite Petroleum System, Assam Geologic Province, India 15
Traps and Seals

Anticlines and faulted anticlinal structures, subparallel to and associated with the northeast-trending Naga thrust 

fault, are the primary traps (figs. 3, 11). Subthrust traps are 

probably present below the Naga thrust sheet. There have also 

been stratigraphic trap discoveries, such as Dholiya gas field, 

described as an Oligocene Barail clastic depositional lens, and 

Hapjan and Sarojani oil fields identified as Barail depositional 

sandstone lenses (Petroconsultants, 1996) (figs. 22, 23).

Seals include interbedded Oligocene and Miocene shales 

and clays, and the thick clays of the Pliocene Gurjan Group. In 

the southwestern part of the Assam geologic province, the upper 

marine shale at the top of the Tipam Sandstone is a regional seal 

that extends into and throughout much of Bangladesh.

Assessment Unit

The Sylhet-Kopili/Barail-Tipam Composite AU 

(80340101) consists of the same rock intervals described 

for the TPS, and its boundaries correspond with those of the 

TPS (fig. 4). As more data are accumulated, the AU could be 

divided into several distinct units.

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Figure 22. Distribution of trap types. Percentages represent number 

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Figure 23. Events chart summarizing stratigraphy, source rocks, reservoirs, seals, traps, and petroleum information for the Syllhet-Kopili/

Barail-Tipam Composite Total Petroleum System. 

16 Petroleum Systems and Related Geologic Studies in Region 8, South Asia 
Assessment of 

Undiscovered Oil and Gas

Based on Petroconsultants International Data Corp. data 

current to 1996, Assam geologic province (8034) was ranked 

83rd in the world, including United States provinces. This 

categorized it as a priority province for the USGS Assessment 

of World Oil and Gas Resources (Klett and others, 1997; U.S. 

Geological Survey World Energy Assessment Team, 2000). 

Known petroleum volumes are 2.5 BBO and 6.5 TCFG for 

a total of 3.6 BBOE including natural gas liquids (Petroconsultants, 1996). This volume is approximately 0.1 percent of 

world volume excluding the United States.

Previous USGS estimates of undiscovered oil and gas 

in this region include those by Kingston (1986) and Masters 

and others (1998). Kingston (1986) estimated the mode for 

remaining undiscovered oil at 0.1 billion barrels (BBL) and 

mode for gas at 1 trillion cubic feet (TCF). The 1998 assessment estimate (Masters and others, 1998) for North Assam 

mean undiscovered oil was 0.1 BBL and for mean undiscovered gas was 2.0 TCF. Both previous assessments considered 

a smaller area than was considered in this report. The methodology utilized in those earlier assessments employed analogs 

from well-known productive regions of the world and also 

relied heavily on volumetric considerations. 

The 2000 assessment incorporates the petroleum system 

concept as defined by Magoon and Dow (1994). The TPS 

used for the geologic basis of the 2000 assessment in the 

Assam geologic province is the Sylhet-Kopili/Barail-Tipam 

Composite TPS (803401). Table 1 shows the estimated ranges 

of assessed undiscovered oil and gas volumes allocated by 

assessment unit and the totals for the TPS.

Summary

The Sylhet-Kopili/Barail-Tipam Total Petroleum System 

has produced the majority of the hydrocarbons on the Assam 

geologic province (fig. 23). It is likely that portions of this 

system buried near or beneath the Naga thrust fault are more 

mature than the rocks northwest of the thrust fault. The rocks 

beneath the thrust fault contribute a significant portion of the 

oil and gas produced from reservoirs adjacent to and northwest of the thrust fault. Dense, crosshatched fault systems 

may enhance migration of generated hydrocarbons to adjacent 

reservoirs and updip over longer distances. Although structural traps are predominant, stratigraphic traps such as updip 

pinchouts are likely.

Favorable elements of this TPS are suitable organic material, sufficient maturation, ongoing charging of reservoirs, 

short migration paths, and early trap formation relative to generation. There are probably untested structures in the subthrust 

yet to be discovered. The subthrust area, although difficult to 

explore, may contain significant resources. Source rocks are 

Code Undiscovered Resources

and Field MFS Prob. Oil (MMBO)

Type (0-1) F95 F50 F5 Mean F95 F50 F5 Mean F95 F50 F5 Mean

Gas (BCFG) NGL (MMBNGL)

803401 Sylhet-Kopili/Barail-Tipam Composite Total Petroleum System80340101

Oil Fields 1 167 374 710 399 371 896 1,865 980 20 52 119 59

Gas Fields 6 28 157 515 198 1 7 23 9

Total 1.00 167 374 710 399 398 1,053 2,379 1,178 22 59 142 67

Sylhet-Kopili/Barail-Tipam Composite Assessment Unit (100% of undiscovered oil fields and 100% of undiscovered gas fields allocated to ONSHORE province 

8034)

1.00

Table 1. USGS estimates of undiscovered oil and gas resources (Wandrey and others, 2000), Assam, province 8034; assessment results summary—allocated resources. 

[MMBO, million barrels of oil. BCFG, billion cubic feet of gas. MMBNGL, million barrels of natural gas liquids. MFS, minimum field size assessed (MMBO or BCFG). Prob., probability (including both geologic and accessibility probabilities) of at least one field equal to or greater than the MFS. Results shown are fully risked estimates. For gas fields, all liquids are included under the NGL (natural gas liquids) category. F95 represents a 95 percent chance of at least the amount tabulated. Other fractiles are defined similarly. Fractiles are additive under the assumption of perfect positive correlation. Shading indicates not applicable] 

Sylhet-Kopili/Barail-Tipam Composite Petroleum System, Assam Geologic Province, India 17
likely to be more mature in or near the subthrust. Heavier oils 

found in bypassed Eocene and Oligocene reservoirs, in the 

future, may be produced with the use of secondary recovery 

techniques such as steam injection.

Although the prospective stratigraphic section to the 

north of the Brahmaputra River is thinner and probably contains leaner, less mature source rocks, it has some potential, 

particularly for gas.

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Manuscript approved for publication February 6, 2004

Published in the Central Region, Denver, Colorado

Edited by L.M. Carter

Graphics by the author and Gayle M. Dumonceaux

Page layout, photocomposition by Richard W. Scott, Jr.

Section 508 compliance, David G. Walters

Webmaster, Tracy A. Pinto

Sylhet-Kopili/Barail-Tipam Composite Petroleum System, Assam Geologic Province, India 19