RC. 885 til

GEOLOGICAL SURVEY CIRCULAR 885

Petroleum Geology and Resources of 

the Volga-Ural Province, U.S.S.R.
Petroleum Geology and Resources 

of the Volga-Ural Province, U.S.S.R

By James A. Peterson and James W. Clarke 

GEOLOGICAL SURVEY CIRCULAR 885

A resource assessment and a brief description 

of the petroleum geology of the largest 

petroleum-producing area of the European part 

of the U.S.S.R.

1983
United States Department of the Interior

JAMES G. WATT, Secretary

Geological Survey

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Peterson, James A.

Petroleum geology and resources of the Volga-Ural Province, U.S.S.R.

(Geological Survey circular; 885)

Bibliography: p. 26

1. Petroleum Russian S.F.S.R. Volga-Ural region.

2. Petroleum Geology Russian S.F.S.R. Volga-Ural region. I. Clarke, James W. (James Wood) II.

Title. Ill Series. 

QE75.C5 no. 885 557.3S [553.2'8'09478] 83-600028 [TN874.S652V64]

Free on application to Distribution Branch, Text Products Section, 

U. S. Geological Survey, 604 South Pickett Street, Alexandria, MA 22304
ASSESSMENT OF ENERGY RESOURCES

The World Energy Resources Program of the U.S. Geological Survey intends 

to develop reliable and credible estimates of undiscovered petroleum resources 

throughout the world. Initial program efforts have focused on the major producing areas of the world in order to gain a broad geological understanding of the 

characteristics of petroleum occurrence for resource assessment as well as for 

analysis of production potential. Investigations of production potential are carried out in cooperation with other U.S. Government agencies; specifically, studies 

of the main exporting nations of the free world, of which this study is a part, are 

carried out in cooperation with the Foreign Energy Supply Assessment Program 

of the Department of Energy.

The program seeks to investigate resource potential at the basin level, primarily through analogy with other petroleum regions, and thus does not necessarily 

require current exploration information commonly held to be proprietary. In conducting the investigations, we intend to build a support base of publicly available 

data and geologic synthesis against which to measure the progress of exploration 

and thereby validate the assessment. Most of these investigations will lead directly 

to quantitative resource assessments; to be effective, resource assessment, like 

exploration, must be an ongoing process that takes advantage of changing ideas 

and data availability- the results produced are but progress reports reflecting on 

a state of knowledge at a point in time. Because the program is coordinated with 

the Geological Survey's domestic assessment program and uses similar assessment techniques, the user can be assured that a thread of consistency will permit 

comparisons between the various petroleum basins of the world, including those 

in the United States, that have been assessed in the overall Survey program.

In addition to resource estimates, the program provides a regional base of understanding for in-country exploration analysis and for analysis of media reports 

regarding the exploratory success or failure of ventures in studied areas.

Geological Survey publications relating to the assessment of undiscovered conventionally recoverable petroleum resources include the following:

Open-File Report 81-986-Assessment of conventionally recoverable petroleum resources of Persian

Gulf basin and Zagros Fold Belt (Arabian-Iranian basin)

Open-File Report 81-1027-Assessment of conventionally recoverable petroleum resources, VolgaUrals basin, U.S.S.R.

Open-File Report 81-1142- Assessment of conventionally recoverable petroleum resources of Indonesia 

Open-File Report 81-1143 - Assessment of conventionally recoverable petroleum resources of northeastern Mexico

Open-File Report 81-1144-Assessment of conventionally recoverable petroleum resources of southeastern Mexico, northern Guatemala, and Belize

Open-File Report 81-1145-Assessment of conventionally recoverable petroleum resources of Trinidad 

Open-File Report 81-1146-Assessment of conventionally recoverable petroleum resources of Venezuela 

Open-File Report 81-1147-Assessment of conventionally recoverable petroleum resources of the West 

Siberian basin and Kara Sea basin, U.S.S.R.

These reports are available from Open-File Services Section, Branch of Distribution, U.S. Geological 

Survey, Box 25425, Federal Center, Denver, CO 80225.
CONTENTS

Abstract ________________ 1 

Acknowledgment ___________ 1 

Introduction _____________ 1 

Information sources ______ 1 

Geography __ __ 1 

Regional geology __________ 5 

Structure _____ ___ 5 

Archean crystalline basement structural stage __ 5 

Proterozoic Y and Z (Riphean-Vendian) structural stage ___________ ___ 7 

Middle to Upper Devonian (Eifelian-Frasnian)

structural stage _______________ 7 

Upper Devonian-Lower Carboniferous (Famennian-Tournaisian) structural stage ______ 7 

Carboniferous-Lower Permian structural

stage ______________________ 7 

Upper Permian and Mesozoic-Cenozoic structural stages _______ _ 7 

Stratigraphy ___________ _ 10 

Riphean (lower Bavly) cycle ___________ 10 

Vendian (upper Bavly) cycle _ _ 10 

Paleozoic cycles __________ ___ 10 

First (Middle Devonian-Tournaisian) ___ 10 

Second (Vise"an, Namurian, and Bashkirian) _ 13 

Third (lower Moscovian through Lower Permian) _________________ 13

Page

Stratigraphy - Continued

Paleozoic cycles -Continued

Fourth (upper Lower Permian and Upper

Permian) ____ > 13

Alpine cycle (Mesozoic) _ 14

Petroleum geology _____ 14

Introduction ___ 14

Productive sequences __ 14

Proterozoic Y and Z (Bavly beds) section 14

Devonian clastic section _ 14

Upper Devonian and lowermost Carboniferous

carbonate section _ 15 

Visean clastic section _ 17 

Lower and Middle Carboniferous carbonate

section _____ 17 

Moscovian clastic section 18 

Middle and Upper Carboniferous carbonate

section _____ 18

Lower Permian carbonate-evaporite section 18

Upper Permian clastic-carbonate section _ 20

Giant fields ___________ 20

Present exploration and future prospects 24

Assessment of undiscovered conventionally recoverable

petroleum resources ___ 25 

Comments _______ 25 

References _ _ 26

ILLUSTRATIONS

FIGURES 1-4. Maps showing:

1. Volga-Ural basin assessment region _________________________________ 2

2. Main structural features of Russian Platform region and locations of cross sections A-A' and B-B' 3

3. Positions of oilfields and gasfields with respect to main structural features of Volga-Ural province 4

4. Regional structure of Archean crystalline basement in Russian Platform region _________ 6 

5. Stratigraphic column showing general lithologic fades, productive intervals, and regional sedimentary

cycles of Volga-Ural province ________________________________________ 8 

6, 7. Generalized structural-stratigraphic cross sections showing:

6. North-south line A-A' from Sysola arch to Peri-Caspian depression ________________ 9

7. Northwest-southeast line B-B' from Soligalich depression to Ural Mountains ___________ 11 

8-11. Maps showing:

8. Post-Proterozoic sedimentary cover _________________________________ 12

9. Net sandstone content of Devonian clastic section and areas of Frasnian shale having an organic

carbon content greater than 1 percent _______________________________ 16

10. Marine facies patterns in Permian beds in Volga-Ural province __________________ 19

11. Structure on top of Pashiy horizon in Romashkino oilfield _____________________ 22 

12. Profile of Devonian clastic section of Romashkino oilfield __________________________ 23 

13, 14. Graphs showing probability distribution of:

13. Volga-Ural recoverable oil ______________________________________ 25

14. Volga-Ural recoverable gas _____________________________________ 25
TABLES

TABLE 1. Estimated ultimate recoveries from 11 giant oilfields and gasfields in the Volga-Ural petroleum province ___ 20

2. Stratigraphic section of the Romashkino oilfield in the Volga-Ural petroleum province _________ __ 21

3. Assessment of undiscovered conventionally recoverable petroleum resources in the Volga-Ural petroleum

province _______________________________________________________ 25

4. Supplementary and comparative data supporting the petroleum resource assessment of the Volga-Ural petroleum province ____________________________________________________ 25

VI
Petroleum Geology and Resources of the 

Volga-Ural Province, U.S.S.R.

By James A. Peterson and James W. Clarke

Abstract

The Volga-Ural petroleum province is, in general, coincident 

with the Volga-Ural regional high, a broad upwarp of the eastcentral part of the Russian (East European) Platform. The central part of the province is occupied by the Tatar arch, which 

contains the major share of the oilfields of the province. The 

Komi-Perm arch forms the northeastern part of the regional 

high, and the Zhigulevsko-Pugachev and Orenburg arches make 

up the southern part. These arches are separated from one 

another by elongate downwarps.

The platform cover overlies an Archean crystalline basement 

and consists of seven main sedimentation cycles. (1) Riphean 

(lower Bavly) continental sandstone, shale, and conglomerate 

beds, from 500 to 5,000 m thick, were deposited in aulacogens. 

(2) Vendian (upper Bavly) continental and marine shale and 

sandstone are up to 3,000 m thick. (3) Middle Devonian-Tournaisian transgressive deposits, which are sandstone, siltstone, 

and shale in the lower part and carbonates and abundant reefs 

in the upper part, range from 300 to 1,000 m in thickness. The 

upper carbonate part includes the Kamsko-Kinel trough system, 

which consists of narrow, interconnected, deepwater troughs. 

(4) The Vis6an-Namurian-Bashkirian cycle began with deposition of Vis6an clastic deposits, which draped over reefs of the 

previous cycle and filled in an erosional relief that had formed in 

some places on the sediments of the previous cycle. The Vis6an 

clastic deposits are overlain by marine carbonate beds. The cycle is from 50 to 800 m thick. (5) The lower Moscovian-Lower 

Permian cycle consists of 1,000 to 3,000 m of terrigenous clastic 

deposits and marine carbonate beds. (6) The upper Lower Permian-Upper Permian cycle reflects the maximum growth of the 

Ural Mountains and the associated Ural foredeep. Evaporite 

deposits were first laid down, followed by marine limestones 

and dolomites, which intertongue eastward with clastic 

sediments from the Ural Mountains. (7) Continental red beds of 

Triassic age and mixed continental and marine clastic beds of 

Jurassic and Cretaceous age were deposited on the western, 

southwestern, and northern margins of the Russian Platform; 

they are generally absent in the Volga-Ural province, however.

Approximately 600 oilfields and gasfields and 2,000 pools 

have been found in the Volga-Ural province. Nine productive sequences are recognized; these are, in general, the same as the 

sedimentation cycles, although some subdivisions have been 

added. The clastic section of Middle and early Late Devonian

age contains the major recoverable oil accumulations, including 

the supergiant Romashkino field.

Cumulative production to 1980 is estimated at 30 to 35 billion 

barrels of oil equivalent, identified reserves at about 10 billion 

barrels of oil equivalent, and undiscovered resources at about 7 

billion barrels of oil equivalent. Identified reserves of natural 

gas are estimated at 100 trillion cubic feet and undiscovered 

resources at 63 trillion cubic feet.

ACKNOWLEDGMENT

The resource assessment for this report was 

prepared in collaboration with the Resource Appraisal Group of the Branch of Oil and Gas 

Resources.

INTRODUCTION

INFORMATION SOURCES

Data and information used in preparing this 

report were compiled primarily from Soviet journals and other publications on the petroleum 

geology and the general geology of the area. Of 

particular value were publications by Maksimov 

and others (1970), the Atlas of Lithological-Paleogeographical Maps of the U.S.S.R. by the 

Academy of Sciences of the U.S.S.R. (1966), translations of the Soviet journals Geologiya Nefti i 

Gaza and Neftegazovaya Geologiya i Geofizika 

(published in the American journal Petroleum 

Geology), the Robertson Research International, 

Ltd. (1976) report on the Volga-Ural petroleum 

province, and the information files of Petroconsultants, S.A.

GEOGRAPHY

The Volga-Ural petroleum province is an area of 

about 500,000 km2 (200,000 mi2) located on the
FIGURE 1.-Volga-Ural basin assessment region (shaded).

Russian Plain between the Volga River on the The plain is a low erosional surface having elevawest, the Ural Mountains on the east, and the Peri- tions of 100 m (325 ft) or less along the Volga and

Caspian depression on the south (figs. 1, 2 and 3). Kama Rivers, but, as it approaches the Ural Moun-
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FIGURE 2.-Main structural features of the Russian Platform region and locations of cross sections A-A' (fig. 6) and B-B' (fig. 7).

Dashed outline encloses Volga-Ural province.
PERI-CASPIAN DEPRESSION 

EXPLANATION

Outline of major structures Dashed 

where uncertain

Kamsko-Kinel trough system 

Oilfield

Oilfield and gasfield

Gasfield

Axis of structural high

FIGURE 3. - Position of oilfields and gasfields with respect to the main structural features of the Volga-Ural province. The large field

in the middle of the map area is Romashkino.

tains, it becomes a low erosional plateau having the hot, dry desert of the Caspian on the south,

elevations of 200 m (650 ft) or more. The region ex- Climate is Atlantic continental, and precipitation

tends from the cold, humid taiga on the north to ranges from 30 to 40 cm (12 to 18 in) per year.
Average mean January temperature is -12° to 

-16°C (10° to 3°F); that for July is 20° to 24°C 

(66° to 76°F). The region lies at lat 50° to 60° N., 

about the same as the southern half of Canada. It 

lies in a forest-steppe belt of forest and grasslands 

that are extensively cultivated. The region was 

long neglected by Soviet planners and held to 

agricultural production. After World War II and 

the discovery of significant petroleum resources, 

this area (the "Second Baku") became the most 

rapidly developing industrial region in the Soviet 

Union.

REGIONAL GEOLOGY 

STRUCTURE

The Volga-Ural petroleum province is, in 

general, coincident with the Volga-Ural regional 

high, a broad upwarp of the east-central part of the 

East European (Russian) Platform. The province is 

bordered on the east by the Ural foredeep, on the 

west by the Kazansko-Kirov aulacogen, and on the 

south by the Peri-Caspian depression (figs. 2 and 

3). The central part of the province is occupied by 

the Tatar arch, an elongate basement uplift that includes the structurally highest part of the regional 

high and contains the major share of the petroleum 

deposits in the province. The Komi-Perm arch 

forms the northeastern part of the regional high, 

and the Zhigulevsko-Pugachev and Orenburg 

arches make up the southern part adjacent to the 

regional Peri-Caspian depression. Subsurface 

elevation of the Archean crystalline basement 

ranges from about -1,500 m (-4,900 ft) on the 

Tatar arch to more than -5,000 m (-16,000 ft) 

along the eastern border of the province (fig. 4) 

and more than -8,000 m (-26,000 ft) in the Ural 

foredeep. Maximum relief on the basement thus 

exceeds 6,000 m (19,500 ft) in the province. 

Thickness of the sedimentary cover ranges from 

1,500 m (4,900 ft) to more than 8,000 m (26,000 ft).

The relief on the basement surface differs significantly from the structure in the overlying 

sedimentary rocks because of deformation that occurred during deposition of successive stratigraphic units. These structural differences are the 

basis for recognizing seven structural stages: 

Archean crystalline basement, Proterozoic Y and 

Z, Middle to Upper Devonian, Upper DevonianLower Carboniferous, Carboniferous-Lower Permian, Upper Permian, and Mesozoic-Cenozoic. 

Many of the structural features, particularly the 

larger arches and downwarps, are inherited from

Proterozoic Y and Z-early Paleozoic structural 

trends. The arches are separated by relatively narrow, elongate basement-fault-bounded troughs or 

aulacogens (Maksimov and others, 1970; Nalivkin, 

1976; Valeyev and others, 1969). The arches form 

large (regional) uplifts on which more localized but 

relatively large "highs" are superposed. The internal structure of the major arches consists of 

polygonal block and interblock linear-zonal structures. The polygonal block structures are 

characterized by relatively mild deformation of the 

sedimentary cover, a relatively thinner sedimentary section, and primary development of carbonate facies. The interblock linear-zonal structures are marked by greater deformation and a 

thicker sedimentary cover.

Both the aulacogens and the interblock linearzonal structures formed through the action of deep 

faults in the basement. Movement along these 

basement faults was certainly associated with the 

contemporary subsidence in the Ural geosyncline 

to the east and in the Dnieper-Donets depression 

farther to the southwest, as well as with the 

downwarping of the Peri-Caspian depression to 

the south. The fault zones are characteristically 

associated with the steep gravity of magnetic gradients.

ARCHEAN CRYSTALLINE BASEMENT STRUCTURAL 

STAGE

In general, the Volga-Ural regional high is divided into three prominent ridge systems separated 

by elongate depressions or aulacogens (Maksimov 

and others, 1970). The Komi-Tatar ridge system is 

about 700 km (435 mi) long and 200 km (125 mi) 

wide and includes the Komi-Perm and Tatar arches 

and their local highs (fig. 3). The Lower Kama 

dislocation zone, which extends northeast across 

the central part of the Tatar arch, is a zone of extensive faults of the basement and sedimentary 

cover related to the Kama abyssal fault, which extends from the Voronezh arch to the Ural 

foredeep.

The Kama-Bashkir ridge system forms the 

northeastern part of the Volga-Ural regional high. 

It is separated from the Komi-Tatar ridge system 

by the Upper Kama depression and the Birsk saddle; this regional structural low (sometimes called 

the Birsko-Upper Kama aulacogen) is bordered by 

faults and is filled by thick Proterozoic Y and Z 

(Bavly) sedimentary and basic intrusive rocks. The 

Kama and Perm-Bashkir arches are the main
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FIGURE 4.-Regional structure of the Archean crystalline basement in the Russian Platform region. Structure contours in

kilometers. Dashed outline encloses the Volga-Ural province.

structural highs on the Kama-Bashkir ridge 

system, which is bounded on the east by the northern segment of the abyssally faulted Ural foredeep.

The Zhigulevsko-Pugachev and Orenburg arch 

system occupies the southern part of the VolgaUral regional high and is separated from the Tatar
arch by the Melekess depression and the abyssally 

faulted Sernovodsko-Abdulino aulacogen. The 

Zhigulevsko-Pugachev and Orenburg arch system 

is bounded on the south by the northern border of 

the deep Peri-Caspian depression and the Buzuluk 

depression. On the east, it is bounded by the UfaOrenburg monocline (fig. 2), which is separated 

from the southern Ural foredeep by an abyssal 

fault system. On the west, the deep-seated RyazanSaratov depression separates the arch system 

from the lower Volga monocline, which forms the 

western border of the Peri-Caspian depression.

PROTEROZOIC Y AND Z (RIPHEAN-VENDIAN) 

STRUCTURAL STAGE

Proterozoic Y and Z sedimentary rocks are as 

much as 10,000 m (32,500 ft) thick and, to a great 

extent, infill the deep aulacogen trough system 

within the basement (Kazan sko-Kirov and 

Sernovodsko-Abdulino aulacogens, Upper Kama, 

Buzuluk, and Ryazan-Saratov depressions, and the 

Birsk saddle) (figs. 5 and 6). A regional unconformity is present between the Riphean and Vendian 

sedimentary rocks; consequently, two independent 

structural substages are recognized. The dominant 

regional structures in the Proterozoic Y and Z beds 

are rounded anticlines, separated by basins or 

troughs; the outlines of these anticlines are less 

sharply defined than those of anticlines in the 

Archean basement.

MIDDLE TO UPPER DEVONIAN (EIFELIAN-FRASNIAN) 

STRUCTURAL STAGE

The general structure of this Devonian clastic interval does not differ substantially from that of the 

underlying basement surface. These clastic beds 

are thinner or absent altogether on some structural highs. Depositional thinning on the Kukmor 

high of the Tatar arch results in the Al'met'yev 

high's being structurally higher than the Kukmor 

high for the Devonian beds; the relationship is 

reversed for the basement surface. Deposition of a 

great thickness of Devonian clastic rocks in the 

Kazansko-Kirov aulacogen led to almost complete 

infilling of this trough. On the eastern edge of the 

trough, a lava flow present in the lower Frasnian 

rocks suggests possible renewed faulting along the 

borders of the aulacogen. The Tatar arch has a 

closure of 160 m (520 ft) on the top of the EifelianFrasnian structural stage; this closure is 50 to 80 m 

(163-260 ft) less than that on the surface of the 

basement. In this structural stage, the

Sernovodsko-Abdulino aulacogen and the Orenburg arch become buried structures beneath the 

monoclinal southeastern flank of the platform.

UPPER DEVONIAN-LOWER CARBONIFEROUS 

(FAMENNIAN-TOURNAISIAN) STRUCTURAL STAGE

Sedimentary processes, mainly carbonate mound 

or reef buildups, become significant in the development of the structural forms of the FamennianTournaisian. The Kamsko-Kinel trough system, 

which extends about 900 km (560 mi) and is from 

20 to 90 km (13-56 mi) wide, formed at this time 

(fig. 3). The pre-Mendym (Domanik and older) 

structure generally does not reflect either the 

troughs or the bordering highs of the KamskoKinel system. However, structural closure increases abruptly at the top of the Devonian owing 

to draping of the overlying beds, which is related to 

reef growth near the trough borders and thinner 

(uncompensated) deposition within the troughs. 

During the upper part of this structural stage 

(Tournaisian), draping of beds over the Devonian 

reef buildups caused a general lessening of relief; 

this lessening continued into the overlying structural stage and resulted in a tendency to return to 

structural elements more like those of the earlier 

Eifelian-Frasnian or Middle to Upper Devonian 

structural stage.

CARBONIFEROUS-LOWER PERMIAN 

STRUCTURAL STAGE

Draped structures continue into the lower VisSan clastic beds, but, during middle to late Vis6an 

time, surface evidence of the Kamsko-Kinel trough 

system and associated carbonate buildups disappeared owing to depositional processes. After this 

time, the main tectonic elements again became 

dominant but tended toward greater complexity. 

The main structural highs on the Tatar, PermBashkir, Zhigulevsko-Pugachev, and Orenburg 

arches and the Birsk saddle remained clearly defined. During Early Permian time, a thick, narrowly defined belt of reef growth developed along the 

eastern and southern borders of the Russian Platform, the result being a narrow band of reef structures adjacent to the Ural foredeep and the PeriCaspian depression.

UPPER PERMIAN AND MESOZOIC-CENOZOIC 

STRUCTURAL STAGES

During the Upper Permian and MesozoicCenozoic structural stages, a large zone of highs
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FIGURE 5. -General lithologic facies, productive intervals, and regional sedimentary cycles of the Volga-Ural province.
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FIGURE 6. -North-south generalized cross section A-A' from the Sysola arch on the southeastern flank of the Baltic shield to the Pe

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northwest across the ATmet'yev and Kukmor highs 

of the Tatar arch; within this arcuate system of 

highs is the Upper Kama depression. During the 

Mesozoic, the northwestern part of the Volga-Ural 

regional high subsided and became part of a 

depositional basin. The Buzuluk depression is particularly well defined in the Mesozoic systems. The 

Zhigulevsko-Pugachev arch became highly 

elevated and was eroded down to Middle Carboniferous beds.

STRATIGRAPHY

The sedimentary history of the Russian Platform 

is closely related to its structural stages. Soviet 

geologists subdivide the stratigraphic section into 

a correlation framework of series and stages 

similar but not identical to those used in North 

America (fig. 5). Sedimentary rock sequences of 

Proterozoic Y and Z, Devonian, Carboniferous, 

Permian, Mesozoic, and Cenozoic age make up the 

sedimentary cover on the platform (figs. 6, 7, and 

8). The total thickness of the post-Proterozoic 

sedimentary cover ranges from about 1,500 m 

(4,875 ft) on the Tatar arch to more than 8,000 m 

(25,000 ft) along the northern border of the PeriCaspian depression (fig. 8). Cambrian through 

Silurian rocks are not reported in the Volga-Ural 

province, although Ordovician and Silurian beds 

have been recognized in the complexly folded and 

faulted Ural foredeep a short distance to the east, 

and Lower Cambrian clastic rocks are present in 

the western part of the platform. Middle Devonian 

through Permian strata occur in more or less continuous depositional sequence, interrupted at 

several horizons by disconformities of variable extent. Upper Permian rocks make up the surface exposures over most of the province, and, except for 

the southern border of the region adjacent to the 

Peri-Caspian depression, Mesozoic rocks are absent. Tertiary rocks are present only in a narrow 

belt along the Volga River valley in the southwestern part of the province, where Pliocene continental beds of conglomerate, sandstone, and 

shale as thick as about 500 m (1,625 ft) are preserved. Most of the platform was emergent in early 

Paleozoic time and again during Mesozoic and 

Cenozoic time.

Maksimov and others (1970) recognize seven 

main sedimentation cycles in the Volga-Ural province: Riphean, Vendian, first Paleozoic, second 

Paleozoic, third Paleozoic, fourth Paleozoic, and 

Alpine (figs. 5, 6, and 7 show the elements of this 

subdivision). The general lithologic characteristics 

and thickness variations of the Paleozoic cycles are 

shown on regional cross sections (figs. 6 and 7) 

compiled from numerous Soviet literature sources, 

including regional thickness and facies maps, 

geologic analyses of local areas, and oilfield data. 

In general, the cycles are separated by unconformities of regional extent and are related to transgressive-regressive episodes of the Paleozoic 

seaway. The lower part of each cycle consists 

mainly of near-shore marine sandstone and shale, 

which grade upward to predominantly marine carbonate beds commonly containing reef or organic 

mound buildups. Evaporites in varying amounts 

are present in each cycle and are particularly 

prominent at the close of the third Paleozoic cycle 

(Kungur halite beds). Continental and marine red 

beds, evaporites, dolomite, and alluvial conglomerate beds dominate the uppermost cycle.

RIPHEAN (LOWER BAVLY) CYCLE

Riphean beds are from 500 to more than 5,000 m 

(1,625 to more than 16,250 ft) thick and consist 

primarily of terrigenous, coarse, clastic continental sandstone, shale, and conglomerate beds deposited directly on crystalline basement, mainly in 

elongate grabenlike depressions (aulacogens), and 

some marine deposits in the upper part.

VENDIAN (UPPER BAVLY) CYCLE

Vendian beds are as much as 3,000 m (9,750 ft) 

thick and are much more widespread than the 

underlying Riphean sedimentary rocks. Basal Vendian beds are coarse continental and marine clastic 

sediments that unconformably overlie Riphean 

beds. The lower Vendian grades upward to a sequence of marine shallow-water shale and sandstone that contains some carbonate and phosphorite beds.

PALEOZOIC CYCLES 

FIRST (MIDDLE DEVONIAN-TOURNAISIAN)

Rocks of the first Paleozoic cycle are 300 to more 

than 1,000 m (975 to more than 3,250 ft) thick and 

represent a major transgressive marine cycle that 

spread westward from the Ural eugeosyncline and,

10
B

TOKMOVO-KOTEL'NICH 

SOLIGALICH DEPRESSION ARCH

KAZANSKO-KIROV 

AULACOGEN

f- ^__7-"? ~~ *V*^ ~> i- -' " ~^~ ' t '^ * - ~~ ' "" T ''-

5*VAMSKO--'^S"~ 

-/"J(INEL- -?X^'

CAVIJR^AN 

PROTEROZOIC Z AND Y

Sandstone and gray or black 

shale (mostly marine)

Red beds (partly marine)

Limestone

Dolomite

Anhydrite

Halite

Fault Arrows show direction 

of relative movement

Contact Dashed where approximate 

Unconformity

FIGURE 7.-Northwest-southeast generalized cross section B-B' from the Soligalich depression to the Ural Mountains. Locatio
00

'l

 g
by the end of the cycle, had covered the entire Russian Platform. The lower part represents the Devonian clastic beds of Middle and earliest Late Devonian age. These beds, which contain the major 

petroleum deposits of the province, grade upward 

to a predominantly reef and mound carbonate section of latest Devonian (Famennian) and earliest 

Carboniferous (Tournaisian) age. The cycle closed 

with a marine regression and the emergence of the 

platform when erosion, karstification, and probably early dolomitization of the carbonate beds occurred prior to deposition of the lower VisSan 

clastic beds of the succeeding cycle.

The middle and upper parts of this cycle contain 

the highly bituminous Domanik facies, a restricted 

marine black shale, marl, and argillaceous limestone facies that was deposited primarily in the 

deeper water Kamsko-Kinel troughs of the shelf. 

The unique Kamsko-Kinel trough system comprises a grouping of narrow, interconnected, 

deeper water troughs that persisted between late 

Frasnian and Tournaisian time. The origin of the 

trough system is thought to be related to a combination of structural and sedimentary processes 

involving block faulting of the platform basement 

followed by deposition of an extensive shelf carbonate reef and mound facies that reached its 

greatest development along the trough borders.

SECOND (VISEAN, NAMURIAN, AND BASHKIRIAN)

The second Paleozoic cycle comprises a 50- to 

about 800-m (160 to 2,600 ft) sequence of terrigenous clastic deposits of early and middle 

VisSan Age, followed by fossiliferous marine carbonate rocks of late VisSan, Namurian, and 

Bashkirian Age. The basal VisSan beds are continental and near-shore marine sandstone and 

shale that represent the initial transgressive 

deposits of the depositional cycle. These deposits 

fill the Kamsko-Kinel troughs and the irregularities of the underlying channeled and 

karstified erosion surface on the Tournaisian carbonate beds. Vis6an clastic beds grade upward to 

marine carbonate and shale units and finally to 

widespread fossiliferous marine carbonate beds of 

Namurian and Bashkirian Age that covered the entire platform.

THIRD (LOWER MOSCOVIAN THROUGH LOWER PERMIAN)

The third Paleozoic cycle comprises a 1,000- to 

more than 3,000-m (3,250 to 9,750 ft) sequence of 

terrigenous clastic deposits of early Moscovian

Age followed by fossiliferous marine carbonate 

beds of late Moscovian, Late Carboniferous, and 

Early Permian age. Lower Moscovian sandstones 

are generally .fine grained and primarily of marine 

origin and grade to a relatively narrow band of 

deltaic and interdeltaic facies west and northwest 

of the Volga-Ural province. This sequence grades 

upward to upper Moscovian marine limestone, 

dolomite, and intertonguing shale beds, which contain porous carbonate mound buildups in places. 

Rocks of Late Carboniferous age form a 

blanketlike carbonate deposit 150 to 200 m (490 to 

650 ft) thick over much of the Volga-Ural province 

and mark the maximum transgressive stage of the 

cycle. These beds are overlain conformably by 

shallow-water carbonate rocks of earliest Permian 

(Asselian) age. Permian reef growth was initiated 

at this time in a relatively continuous belt along the 

eastern and southern borders of the platform, adjacent to the deepwater Ural foredeep and the PeriCaspian depression. Reef growth continued along 

this band during the remainder of the third Paleozoic cycle of deposition, and the main part of the 

Volga-Ural province was covered by back-reef carbonate, evaporite, and fine clastic beds. The reef 

bodies are primarily limestone, dolomitized to 

varying degrees and ranging up to several hundred 

meters thick. More than 2,000 m (6,500 ft) of deepwater clastic bituminous shale, sandstone, and 

siltstone are deposited in the Ural foredeep, which 

was actively subsiding at this time during the early 

growth stages of the Ural Mountains chain.

FOURTH (UPPER LOWER PERMIAN AND UPPER PERMIAN)

The fourth Paleozoic cycle reflects the maximum 

growth of the Ural Mountains and the associated 

Ural foredeep. At the close of the third Paleozoic 

cycle, evaporite deposits consisting of gypsum or 

anhydrite and some dolomite formed on the main 

platform, and thick deposits of halite accumulated 

in the Peri-Caspian depression and the southern 

part of the Ural foredeep. This event was followed 

by marine transgression and deposition on the 

platform of limestone and dolomite interbedded 

with some gypsum (Ufimian beds). These strata intertongue eastward with clastic sedimentary rocks 

from the Ural Mountains, which began to uplift 

vigorously near the close of the third Paleozoic cycle. During the remainder of the fourth Paleozoic 

cycle, continental coarse-grained clastic deposits 

from the Ural Mountain chain became increasingly 

dominant and continued until complete withdrawal

13
of marine conditions in latest Permian time. Parts 

of the Upper Permian beds are removed by postPermian erosion. The total thickness of the fourth 

Paleozoic cycle sequence ranges from less than 500 

m (1,625 ft) on the Tatar arch to more than 1,500 m 

(4,875 ft) in the southern part of the Ural foredeep 

and the Peri-Caspian depression.

ALPINE CYCLE (MESOZOIC)

The Alpine cycle was apparently a time of 

emergence of the platform area. Mesozoic rocks 

are not present in the main area of the Volga-Ural 

province and, according to most Soviet authors, 

were never deposited there. Continental red beds 

of Triassic age and mixed continental and marine 

clastic beds of Jurassic and Cretaceous age are 

present in the Peri-Caspian depression and in the 

southwestern, western, and northern parts of the 

Russian Platform. Rocks of this age, however, are 

generally absent in the Volga-Ural province (figs. 6 

and 7).

Lower and middle Tertiary rocks are not 

reported in the Volga-Ural province, and there is 

no evidence to show that they were ever deposited 

there. Continental clastic beds of Pliocene age 100 

to 400 m (325 to 1,300 ft) thick are present along 

the Volga River valley in the southwestern part of 

the province.

PETROLEUM GEOLOGY

INTRODUCTION

Maksimov and others (1970) list 525 fields in the 

Volga-Ural province (480 oil or oil and gas and 45 

gas) and 1,450 recognized oil and gas pools (425 in 

Devonian rocks, 890 in Carboniferous, and 135 in 

Permian). Since the publication of the Maksimov 

report, a large number of additional fields or pools 

have been discovered, notably in the Ural foredeep 

and on the northern border of the Peri-Caspian 

depression. Bol'shakov (1975) reports a total of 

1,900 oil and gas pools in the province. Many of the 

fields are small, but several are giants. Halbouty 

(1970) lists seven giant oilfields (more than 500 

million barrels of ultimately recoverable oil), totaling about 28 billion barrels of recoverable oil, and 

two giant gasfields, of which Orenburg (now 

reported as about 70 trillion cubic feet) is the 

largest. Most of the fields are located on anticlinal 

structures (either tectonic or draping over reef 

buildups), but, according to Shaykhutdinov (1975), 

almost half the pools on the Tatar arch are

stratigraphic traps. Many of the fields contain 

multiple pays, and some produce from all the main 

productive intervals of the province, particularly in 

the Kuybyshev-Orenburg area. The province is in a 

mature stage of exploration and production, and 

increasing emphasis is being placed on exploring 

for both clastic and carbonate stratigraphic trap 

prospects, which were not emphasized during the 

structural stage of exploration drilling. These efforts, aided by refined methods of seismic interpretation, have resulted in the discovery of many 

fields of both the stratigraphic and the structural 

type in recent years. A large percentage of the 

known reserves of the province is depleted, but 

Soviet geologists have identified a significant 

number of deeper, more complex prospects that 

promise to add continuing reserves for some time.

PRODUCTIVE SEQUENCES

Maksimov and others (1970) divide the 

petroleum-bearing and prospective strata into nine 

productive sequences (fig. 5) on the basis of 

stratigraphic characteristics, the nature of the oil 

and gas occurrence, hydrologic conditions, and the 

geochemical character of the oil and gas. These 

productive units fit well with natural stratigraphic 

facies divisions within the geologic column of the 

province and are considered separately in the 

following discussion.

PROTEROZOIC Y AND Z (BAVLY BEDS) SECTION

Soviet geologists report that the Bavly beds are 

not highly metamorphosed and contain a significant thickness of porous marine and continental 

sandstone beds that have good reservoir qualities 

(Stankevich and others, 1977; Suleymanov and 

Bazev, 1974). The Bavly beds are also highly fractured in some areas. Carbonaceous and bituminous 

beds, as well as algal carbonate bodies, are also 

reported. These factors indicate that accumulations of indigenous gas and nonindigenous oil and 

gas may be found, particularly as deep drilling progresses.

DEVONIAN CLASTIC SECTION

The Devonian clastic section of Middle and 

earliest Late Devonian age has been the main oil 

objective in the Volga-Ural province. These beds 

contain the major recoverable oil accumulations on 

the Tatar arch, which includes the Romashkino, 

Novoyelkhov-Aktash, Tuymazy, Shkapovo,

14
Mukhanovo, and Yarino-Kenennyy oilfields, 

among the largest in the province. Significant production from these beds (mainly oil) is also present 

in all other parts of the province as well. More than 

200 oilfields produce from the Devonian clastic 

reservoirs, and 9 main productive sandstone or 

siltstone horizons are reported. The main pays occur in the uppermost Givetian and lower Frasnian 

sequence; almost half of the Devonian pools of the 

Volga-Ural province are in the lower Frasnian 

clastic section. The Devonian clastic sandstone, 

shale, and minor limestone interval occurs in a 

north-south belt extending across the central part 

of the province. The greatest thickness is in the 

northern and southern parts of the region (fig. 9)

The reservoirs are marine near-shore sandstone 

and siltstone derived from a west-to-northwest 

source area. Net sandstone content is highest to 

the north and northwest-175 to 300 m (570 to 975 

ft). A belt of sandstone facies also extends from 

north to south across the southern crest of the 

Tatar arch. Sandstone intervals pinch out to the 

west along the eastern flank of the northern crest 

of the Tatar arch and the Komi-Perm arch and also 

to the east along the border of the platform. 

Porosity and permeability are highly variable in 

the clastic Devonian reservoirs, ranging from very 

low in siltstone reservoirs to as much as 25 to 30 

percent and 3 to 4 darcies in the cleaner and better 

sorted quartz sandstone reservoirs. Fracturing is 

important in increasing reservoir quality in many 

fields, particularly those controlled partly by fault 

traps.

The sandstone beds of the Devonian clastic section are interbedded with dark near-shore marine 

and lagoonal dark shale beds, which have variable 

contents of organic carbon. The organic carbon 

content increases upward in the section, reaching 

an overall maximum source-rock quality in the uppermost Givetian and lower Frasnian when the 

Domanik facies began to be deposited in the more 

depressed areas of the shelf (Ronov, 1958). Intertonguing and interbedding of the dark organic 

shale beds with individual sandstone reservoir 

bodies, many of which are lensing in nature, have 

provided an efficient interrelationship for the 

generation and preservation of petroleum accumulations.

Most of the important Devonian clastic oilfields 

are located on structural closures, mainly anticlinal, but some are also partly fault controlled. 

Numerous "lithologic" (stratigraphic) accumulations have been found during field development 

drilling, mainly on the flanks of the larger structures (BoPshakov, 1975). More than 300 pools of 

this type are reported. The lower Frasnian shale 

and lensing sandstone interval provides a regional 

impermeable seal for most structural accumulations, and individual dark shale units of lesser extent provide local cap rock beds for stratigraphic 

and structural pools.

Possible analogs to the Devonian clastic section 

of the Volga-Ural province include the productive 

Chesterian (Upper Mississippian) sandstone and 

shale sequence of the northern Midwestern United 

States (Illinois basin and adjacent area), the Upper 

Devonian clastic section of the northern Appalachians ("Bradford" and other sands), and beds 

of the Simpson Group (Middle Ordovician) of the 

midcontinent region. Of these three, the 

Chesterian sequence is probably the best analog 

with respect to paleogeography, the nature of the 

sandstone facies and its source, and the interrelation with indigenous source-rock facies.

UPPER DEVONIAN AND LOWERMOST CARBONIFEROUS 

CARBONATE SECTION

The Upper Devonian and lowermost Carboniferous carbonate interval contains the VolgaUral province's most widespread and fully 

developed reef and organic carbonate facies, which 

attains a thickness of more than 500 m (1,625 ft) in 

all the paleostructurally high areas of the province. 

At least 150 fields and 10 or more reservoir intervals from these beds are productive, although the 

total amount of oil discovered is considerably less 

than that found in the Devonian clastic section. 

Most of the oil pools of this interval are in the Tournaisian reefal and organic mound carbonate section, especially its upper part, which, in most 

places, is considerably thicker than the underlying 

Upper Devonian carbonate beds (Ovanesov and 

others, 1972). Most pools found thusfar are in 

structural traps in reservoirs where primary intercrystalline and skeletal porosity is accompanied by 

fracture and vuggy porosity. Porosity, generally 

not high, averages about 8 to 10 percent. Excellent 

source-rock beds of the Domanik facies are present 

in the Kamsko-Kinel trough system that surrounds 

all of the main carbonate buildup belts (figs. 3, 6, 

and 7). These beds are highly bituminous and have 

an organic carbon content of as much as 5 percent 

or more. The Domanik facies beds, however, 

generally do not intertongue widely with the car15
52° -

EXPLANATION

* Greater than 100 m sandstone in 

clastic Devonian section

-V Greater than 1 percent organic carbon,

-^ Frasnian shale facies

50 

i

100 MILES

_i

r i i

0 50 100 KILOMETERS

FIGURE 9.-Net sandstone content of the Devonian clastic section and areas of Frasnian shale having an organic carbon content 

greater than 1 percent. Areas of major oil accumulation (see fig. 3) tend to occur where a belt of good source-rock facies combines with a higher sandstone content. A western area having a sandstone content of more than 100 m is outside the main belt of 

good source rocks and contains no oilfields. The structurally highest area of the northwestern side of the Tatar arch has low 

sandstone and source-rock contents and contains no oilfields. Contours in kilometers.

16
bonate buildup facies and thus may not be as effective a source-rock facies for these carbonate reservoirs as they are for the underlying clastic beds.

In general, Upper Devonian-Tournaisian carbonate beds have not yet been of primary importance as petroleum reservoirs in the province. 

However, in the poststructural phase of exploration, greater attention is being paid to the isolated 

nature of some reefal porosity; some success is apparent in infill drilling on larger structures 

(Shikhov and others, 1976; Yarullin and Yunusov, 

1976.)

VISEAN CLASTIC SECTION

At least 10 or 12 regionally productive sandstone 

intervals are present in the lower VisSan section 

(Malinovka and Yasna Polyana beds) of the VolgaUral province. More than 250 fields produce from 

Vise"an sandstone and siltstone reservoirs distributed across all of the region; according to 

Maksimov and others (1970), this interval contains 

the greatest number of economically extractable 

oil and gas deposits in the province. The Vis6an 

clastic section ranges from 100 to more than 400 m 

(325 to more than 1,300 ft) thick; the thickest section is present within the Kamsko-Kinel trough 

system, which, in its final phase of development, 

was filled with Vise"an clastic deposits.

Vis6an clastic reservoirs, in general, are highly 

lensing, discontinuous, elongate quartzose sandstone bodies of highly variable thickness and extent. Many are sinuous channel sandstones, particularly well developed within the Kamsko-Kinel 

trough areas, which contain many of the oilfields 

producing from these beds (Tsotsur, 1974). Many 

of the reservoirs are classed as "stray sands," and 

well density is important for adequate exploration 

(Baymukhametov, 1976). Porosity and permeability characteristics are highly variable, ranging from 

very low in siltstone reservoirs to as much as 35 to 

40 percent and 4 to 5 darcies, respectively, in 

coarser sandstone reservoirs. Lower Vis6an sandstone reservoirs in the Kuybyshev region have 

average porosities of about 22 percent and average 

permeabilities of about 650 millidarcies (Aleksin 

and others, 1974).

Dark bituminous shales of good source-rock 

quality are present in the Vise"an clastic section; 

total thickness generally increases from west to 

east across the Volga-Ural province. The organic 

carbon content of these beds, however, is highest 

within a northeast-southwest belt that passes approximately across the central part of the VolgaUral province. The Vis6an oilfields are concentrated in this northeast-southwest belt.

Many of thfc oilfields producing from the Vis6an 

clastic section were discovered on structural 

closures, mostly anticlines or domes, mapped on 

Permian strata at the surface or on shallow seismic 

horizons. Traps formed by draping over Upper 

Devonian-Tournaisian reefs are widespread. The 

Arlan field, the second largest field in the province, 

is of this type. Lithologic or stratigraphic trapping 

is involved in most accumulations, and, because of 

the consistent lensing nature of almost all the sand 

bodies, many primarily stratigraphic trap fields 

have been found. Many are relatively small, but, 

according to Soviet geologists, significant reserves 

are now being added by the current emphasis on 

delineating isolated sandstone reservoir bodies on 

the flanks of large structures or along sand trends 

within the Kamsko-Kinel troughs. The clay and 

dense carbonate beds of the uppermost Vise"an 

clastic section form a regional impermeable seal 

that extends across all of the province.

The Vise"an clastic section is comparable to the 

productive Chesterian sandstone section of the 

north-central United States, which contains 

numerous petroleum accumulations in the deltaic 

and near-shore marine lensing and sinuate sandstone bodies interbedded with dark marine shale 

facies. Source areas for the Vise"an clastic and the 

Chesterian quartzose sandstones are comparable; 

both cases involve reworking and redeposition of 

previously deposited sands by streams flowing off 

the margins of a cratonic continental mass into an 

advancing marine seaway.

LOWER AND MIDDLE CARBONIFEROUS 

CARBONATE SECTION

The carbonate Lower and Middle Carboniferous 

section is about 100 m (325 ft) thick in the western 

part of the Volga-Ural province and thickens 

uniformly eastward to about 500 m (1,625 ft) on the 

borders of the Ural foredeep and the Peri-Caspian 

depression.

The upper Vis6an and Namurian carbonate beds 

are mainly low-porosity limestones; few significant 

productive horizons have been delineated. The 

Bashkirian carbonate beds, however, are more 

porous and are characterized by a much more 

variable facies or by organic carbonate buildups 

and dolomitized sectors. Potential source rocks are 

not reported in this section, however, and, thusfar, 

the section has been of secondary importance in

17
number of oilfields (about 100) and volume of oil 

reserves found.

MOSCOVIAN CLASTIC SECTION

The Moscovian clastic section is thickest (200 to 

400 m or 650 to 1,300 ft) in the western and 

southwestern parts of the Volga-Ural province, 

generally outside the area of petroleum occurrence. These beds thin uniformly eastward to less 

than 50 m (160 ft) along the eastern border of the 

province where the section grades into a dark 

marine shale facies. About 100 oilfields produce 

from these sandstone and siltstone reservoirs, 

which are scattered throughout the province. Most 

of the oil accumulations occur in the eastern part of 

the clastic facies belt, where the sandstone beds 

are generally thin, fine grained, and low in porosity. For this reason, the Moscovian clastic section is 

of secondary importance as an exploratory objective.

The dark-gray marine shale beds, which enclose 

the Moscovian quartzose sandstone reservoirs, are 

the most likely petroleum source beds for this section, if burial depth is sufficient. Specific sourcerock data, however, are lacking.

Most of the fields are on structures, but the lensing and spotty distribution of the sandstone reservoir bodies necessitate a strong stratigraphic trap 

approach in exploration procedures. The relatively 

thick, impermeable shale section enclosing the 

sandstone bodies forms a natural seal for hydrocarbons.

MIDDLE AND UPPER CARBONIFEROUS CARBONATE 

SECTION

Only a few scattered fields produce from Middle 

and Upper Carboniferous carbonate beds. Upper 

Moscovian reservoirs are porous (in places cavernous) fossiliferous limestones and some dolomite intervals. Except for the underlying gray marine 

shale and argillaceous carbonate beds of the 

Moscovian clastic section, rocks of suitable sourcerock quality apparently are not present in association with these beds and may not have been buried 

deeply enough for organic matter to have matured. 

These factors, plus the absence of suitable cap 

rocks within or above the section, may account for 

the lack of significant hydrocarbon production 

from these beds.

LOWER PERMIAN CARBONATE-EVAPORITE SECTION

Oil and gas production from the Lower Permian 

carbonate-evaporite unit is entirely from reefal 

and other carbonate reservoirs that are developed 

along the southeastern and southern margins of 

the Volga-Ural province. More than 100 gasfields 

or gas-condensate fields and a few oilfields have 

been discovered, mainly in reefs along the eastern 

edge of the platform and in dolomite reservoirs of 

the Zhigulevsko-Pugachev and Orenburg arches.

Porosity and permeability in Lower Permian carbonate reservoirs are highly variable, ranging 

from very low to as much as 30 to 35 percent and 1 

to 2 darcies, respectively. Some of the reef reservoirs along the platform edge are dolomite beds 

characterized by very low porosity and permeability, but the net productive section is often high (as 

much as 600 m or 2,000 ft), and fracture 

permeability is very important. Reservoirs in the 

Zhigulevsko-Pugachev arch area are mainly fine-to 

medium-grained crystalline dolomites in which 

porosity and permeability are variable. The lower 

part of the Melekess tar deposits occurs in similar 

dolomite reservoirs and appears to have accumulated in pinchout or permeability traps over a 

paleostructural thinning trend that passes 

southwest-northeast across the area (figs. 6, 7, and 

10).

Permian beds of good source-rock quality apparently are not present on the platform, but a 

thick section (several hundred meters) of 

bituminous marine shale is present in the Ural 

foredeep east of the Lower Permian reef belt (fig. 

10). These beds, which also may be present at 

depth in the Peri-Caspian depression, appear to be 

the most likely indigenous source-rock facies for 

Permian hydrocarbons. According to Svetlakova 

and Kopytchenko (1978), however, most hydrocarbon accumulations in Permian reservoirs probably 

originated by upward migration from Devonian or 

Carboniferous source-rock beds.

Most of the Permian oilfields and gasfields on the 

Orenburg arch are on local structures mapped in 

shallow Permian beds. Those along the platform 

edge are mainly in reef structures, some of which 

are associated with post-reef vertical faulting and 

some of which are thrust faulted. The Orenburg 

gasfield, the largest in the province (70 trillion 

cubic feet) and clearly one of the world's 

supergiants, is located on a large faulted anticlinal 

fold. The Kungurian halite and gypsum-anhydrite 

facies regionally overlies the productive carbonate

18
54°

52° hLower Permian salt

03

Lower Permian 

carbonate belt

^EPn-Q^i Y/LOWer rerr 

ESSSSfeX' *belt

EXPLANATION 

Halite 

Limestone 

Conglomerate 

Reefs

100 MILES

_l

50 100 KILOMETERS

FIGURE lO.-Isopach map showing marine facies patterns in Permian beds in the Volga-Ural province. Thickness contours in

meters.

and carbonate reef section to form an impermeable West Texas-New Mexico Permian basin. Both

cap rock facies over most of the province. areas contain thick, narrowly defined shelf-margin

The Lower Permian productive carbonate sec- reef buildups of similar organic content as much as

tion of the Volga-Ural province is closely similar to several hundred meters thick. An important difthe productive Upper Permian reef section of the ference is that the West Texas-New Mexico Per19
mian basin reefs are closely associated with a 

marine source-rock facies that probably contains 

much more organic carbon than the corresponding 

facies in the Volga-Ural province. Probably for this 

reason and because the Lower Permian beds have 

been buried to greater depths in much of the 

Volga-Ural province, gas and gas-condensate accumulations dominate there, whereas oil accumulations dominate in the West Texas-New Mexico 

Permian basin.

UPPER PERMIAN CLASTIC-CARBONATE SECTION

Production (mainly of gas and minor oil or condensate) from this interval is confined to the 

southern part of the province, mainly the 

Zhigulevsko-Pugachev and Orenburg arches and 

the Sernovodsko-Abdulino aulacogen. According 

to Maksimov and others (1970), three productive 

intervals are present in the Upper Permian section. The two lower intervals contain sandstone 

and siltstone (and minor dolomite) reservoirs of 

Ufimian Age, and the upper, more widespread interval contains dolomite reservoirs of early Kazanian Age. The upper part of the Melekess tar 

deposits is present a short distance west of the productive area, mainly in clastic reservoirs of this age 

(Akishev and others, 1974). These accumulations, 

which make up the greater part of the Melekess 

deposits, appear to be related to an updip pinchout 

of porous sandstones, siltstones, and some 

dolomite beds along the southwestern flank of the 

Tatar arch. The reservoirs are on the margin of a 

major Upper Permian deltaic complex (figs. 6, 7, 

and 10). Comments on the origin of these deposits 

are also given by Demaison (1977).

Evidence for the presence of petroleum source 

rocks is lacking in this interval, which is dominated 

by evaporite deposits and red beds. Some marine 

shales of possible source-rock quality may be present in the Peri-Caspian depression or in the 

southern Ural foredeep. The presence of petroleum in Upper Permian beds also may be related to 

vertical migration of deeper accumulations along 

fault or fracture systems, as suggested by 

Svetlakova and Kopytchenko (1978).

GIANT FIELDS

Halbouty (1970) lists seven giant oilfields (more 

than 0.5 billion barrels ultimate recovery) and two 

giant gasfields (more than 3.1 trillion cubic feet 

ultimate recovery) in the Volga-Ural petroleum

province (table 1). St. John (1980) lists nine giant 

oilfields and one giant gasfield (table 1).

TABLE 1. Estimated ultimate recoveries from 11 giant oilfields 

and gasfields in the Volga-Ural petroleum province

[?, date unknown; -, no data]

Field

Discovery 

date

Estimated ultimate recovery

Halbouty (1970) St John (1980)

Oil recovery, in billion barrels

Romashkino

Arlan

Aktash-Novoyelkov ____ 

Tuvmazv

Shkapovo

Kuleshovka

Yarino-Kamennyy _____ 

Mukhanovo

Kotur-Tepe

1948 

1955 

1955 

1937 

1944 

1958 

1954 

? 

?

14.3 

4.1 

3.2 

2.2 

1.25 

.76 

.5

12.4 

2.8 

1.3 

1.9 

1.25 

.76 

.8 

1.55 

1.46

Gas recovery, in trillion cubic feet

Orenburg

Korobki

1966 

1949

26.5 

3.1

26.5

Romashkino is a supergiant oilfield almost in the 

middle of the oily belt that extends along the 

eastern part of the Russian Platform (fig. 2). The 

domal structure was first detected by geologic 

mapping in 1934, and oil was discovered in 1948. 

For 20 years, between the mid-1950's and the 

mid-1970's, Romashkino, the largest producing 

field in the Soviet Union, provided a significant 

portion of total Soviet production. The field covers 

an area of 3,600 km2 (1,400 mi2), and more than 

10,000 wells have been drilled. Cumulative production as of January 1, 1979, was 11.25 billion barrels. Although the field is now in decline, production during 1979 was still 500 million barrels (International Petroleum Encyclopedia, 1980, p. 248).

The stratigraphic section at the Romashkino 

oilfield is about 2,000 m (6,500 ft) thick. Except for 

minor unconformities, the section is continuous 

from Middle Devonian through Permian age (table

2).

The crystalline basement in the area of 

Romashkino oilfield is a broad, hummocky structural plateau 80 to 90 km (49-56 mi) in diameter 

and outlined by the - 1,640-m (- 5,330 ft) structure 

contour on the basement surface. The highest 

points on this surface are at about -1,535 m 

(-4,990 ft), and the lowest are at -1,650 m 

(- 5,360 ft). Closure does not exceed 110 m (360 ft). 

The Al'mefyev basement high (fig. 3) is the core of 

the structure. The structure on the top of the 

Pashiy horizon, which is the main pay zone, has one 

very broad high, which conforms in position and 

size to the ATmet'yev basement high. The surface

20
TABLE 2.-Stratigraphic section of the Romashkino oilfield in the Volga-Ural petroleum province

Unit

Kazan-Tatar _ ________

Ufu __

Kungur __________________ 

Sakmar-Artinsk _ _____

Thickness, 

in m (ft)

170 (550) 

40 90 (130 290)

Absent over field 

area. 

90-120 (290-390)

Lithology

Permian

Marine and continental sandstone, clay, marl, and 

limestone. 

Marine and continental red 

beds.

Marine dolomite, gypsum, 

anhydrite, and limestone.

Pay zones

Melekess tar 

deposits.

Melekess tar 

deposits.

Porosity and 

permeability Remarks

Middle and Upper Carboniferous

350-560 

(1,100-1,800)

Marine dolomite and 

limestone. 

Marine sandstone, siltstone, 

and shale.

Carbonate beds 0.902 g/cmffl; 2.11 

percent sulfur.

Lower Carboniferous

Namurian

Visean ___ __________

Tournaisian _ _ _ _

80-100 (260-325) 

175-240 (570-780)

60-75 (95-245)

Marine dolomite and limestone

Upper part, marine limestone 

and dolomite. 

Lower part, marine sandstone, 

siltstone, dark-gray shale; 

coal bearing at base.

Marine limestone, dolomite, 

and dark-gray shale.

BashWrian-Namurian 

limestone (one 

large pool).

Tula B-l and 

Bobrikov B-2 

(70 pools).

Carbonate beds (72 

small pools).

0.876 g/cmffl; gasoil ratio 11-19.3; 

formation temperature 20 °C.

0.870 g/cmffl; formation temperature 20°-25°C.

Upper Devonian

Famennian _______________

Frasnian: 

Upper

Middle _____ _ _

Lower ________________

240-285 (780-930)

75-140 (245-460) 

55-110 (180-360)

45-135 (145-490)

Marine limestone, dolomite, 

and black shale.

Marine dark-gray limestone 

Marine dark-gray to black 

bituminous limestone, marl, 

and shale. 

Marine sandstone, siltstone, 

dark-gray shale, and bituminous limestone.

Upper Famennian 

carbonate beds 

(oil shows).

Domanik carbonate 

beds.

D-0 (many 

pools).

D-I (many pools). 

(10-15 m net 

pay); (as many as 

14 reservoir 

sands).

10-22 percent; 

100-800 millidarcies. 

18-24 percent; 

40-2,000 miffidarcies.

0.805 g/cmffl 

Gas-oil ratio 41-54 

mffl/mffl; formation temperature 

48°C.

Middle Devonian

Givetian 55-125 (180-400) Marine sandstone, gray shale, 

and minerals.

D-II (8 pools) 

D-IH (7 pools)

D-IV (3 pools)

20 percent

10-24 percent; 

400-1,100 miffidarcies. 

21 percent average

Precambrian

Gneisses and migmatites

of this broad high is differentiated into several 

smaller highs and lows, which were used as a basis 

for dividing the field into producing sectors. 

Closure on the top of the Pashiy is about 60 m (197 

ft); the highest point is at -1,435 m (-4,660 ft). 

Although the structure on the top of the Pashiy 

corresponds with that on the top of the crystalline 

basement in form and dimensions, the smaller 

structures on these surfaces do not seem to correspond. Most of the basement closures are only 

structural noses on the Pashiy horizon (fig. 11).

Also, the basement highs on the cross section (fig. 

12) are not directly beneath the highs on the top of 

the Pashiy horizon. The thicker net pays beneath 

highs on the top of the Pashiy, as shown on the 

cross section, suggest that sandstone is thicker at 

these sites. Consequently, the Pashiy highs may be 

due in part to differential compaction within the 

sandstone and shale section.

On the base of the coal-bearing (Bobrikov) formation (lower Visean), the Romashkino structure is a 

broad, gentle high complicated by warps having

21
52° 53°

54° 

30'

54C

0 10 20 MILES 

I

I 

20

300

30 KILOMETERS 

EXPLANATION 

Well location

 Margin of oil pool in pay zone D-I

 Line of cross section in figure 12

FIGURE 11. - Structural contour map on top of the Pashiy horizon (pay zone D-I) in the Romashkino oilfield. Numbers correspond to

those used on the cross section in figure 12.

long dimensions of 1 to 15 km (0.6-8 mi). Although 

the trends of these small warps are diverse,

southeast-northwest trends are predominant. The 

structures within the coal-bearing formation here

22
140 48 11-88 518 519 14-91 27 

1400-

627 19-553 1

Clayey limestone l ++ 1650-

Shale 

Sandstone

FIGURE 12.-Profile of the Devonian dastic section of Romashkino oilfield (after Maksimov and others, 1970, 
are due largely to deposition on an irregular erosional surface of the underlying upper Tournaisian 

beds.

The Romashkino oilfield is bounded on the west 

by the long, narrow Altunino-Shunak downwarp, 

which separates the field from the anticlinal structure of the Aktash-Novoyelkov oilfields. Maximum 

amplitude of this downwarp is more than 100 m 

(325 ft) on the basement surface. It is less well expressed on the Carboniferous and is barely 

noticeable on the Permian beds.

In the Devonian clastic section at Romashkino 

are five pay zones: D-IV, D-III, and D-II in the 

Givetian Stage, D-I in the Pashiy, and D-0 in the 

Kynov beds, the last two units being in the Frasnian Stage. The D-I zone produces about 90 percent of the oil at Romashkino. In addition, flows of 

oil have been recovered from limestones of the 

Domanik beds, upper Famennian carbonate beds, 

Tournaisian carbonate beds, clastic rocks of the 

Tula and Bobrikov horizons of the Lower Carboniferous, and carbonate beds at the base of the 

Middle Carboniferous.

The oil-water contact of the D-I zone is inclined 

from -1,480 m (-4,810 ft) on the north to -1,490 

m (-4,840 ft) on the south. This inclination has 

been attributed generally to hydrodynamic 

pressure of southward-moving formation water. 

There is no abrupt separation of oil and water in 

the oil-bearing beds in the majority of the wells. 

The oil-water transition as seen in well logs is 

generally a zone in which the resistivity gradually 

decreases until formation water appears. The transition zone is probably caused by rapid alternation 

of sandy-silty and clayey beds.

The field was placed on water flood early in its 

history and divided into 23 sectors separated by 

water injection wells generally located in structural lows between domal highs. Water flooding 

results have been highly variable. Facies changes 

result in abrupt termination of permeable beds 

throughout the field, yet there is a remarkable 

hydrodynamic interconnection within individual 

reservoir bodies over great distances. Gerasimov 

(1964) reports variations in the water levels of 

hydrogeologic observation wells in response to 

water injection at distances of as much as 60 km 

(37 mi). However, some wells only a short distance 

apart show no hydrodynamic interconnection. As a 

result, the more permeable beds have been flushed, 

whereas less permeable beds have remained virtually undisturbed by the water flooding operations. To induce flow from low-permeability intervals, high pressures (as much as 500 atm or 7,350 

psi) are maintained in the injection wells.

Production goals for the individual sectors at 

Romashkino were seldom met during the first 

decade of operation. Goals for the field as a whole 

were attained, however, by bringing on stream 

new parts of the field having high-flowing wells 

(Mirchink, 1964).

PRESENT EXPLORATION AND 

FUTURE PROSPECTS

Exploration activity in the Volga-Ural province 

has declined significantly in recent years, partly 

because of the greatly increased Soviet emphasis 

on the West Siberian basin, which has monopolized 

a major share of available drilling equipment. 

However, reports in Soviet geological journals and 

other publications indicate that exploration interest remains high in several parts of the VolgaUral province, particularly the Ural foredeep, the 

Perm-Bashkir arch, the Upper Kama area, and the 

northern border of the Peri-Caspian depression. At 

least 125 discoveries have been made in these 

areas in the past 10 years. Soviet exploration 

geologists also express considerable interest in infill drilling and stratigraphic exploration for Carboniferous and Upper Devonian reefs and other 

carbonate buildups and for Devonian and Vis6an 

discontinuous sandstone reservoirs in the areas of 

the main oilfields (Muslimov and others, 1976).

The thrust belt that makes up the western flank 

of the Ural Mountains is also of major interest, as 

are the graben and other fault structures in the 

Ural foredeep (Klyuchnikov and Kazantsev, 1974). 

Most of these structures are gas or gas-condensate 

prospects; however, infill drilling is being done on 

known structures and in the Kamsko-Kinel trough 

areas, which are primarily oil prospects. Interest 

continues in the Permian reef belt and Carboniferous carbonate mound belt along the 

western border of the Ural foredeep and the northern border of the Peri-Caspian depression, where 

gasfields have been discovered in recent years 

(Kuznetsov and Proshlyakov, 1976). These prospects are deep, and their potential is primarily for 

gas and gas condensate, but the reserves may be 

significant. The pre-Devonian Bavly beds are of interest, primarily because of their good reservoir

24
quality and thickness. These reservoirs appear to 

be promising for oil in faulted areas of major 

uplifts, particularly the Perm-Bashkir arch, and for 

gas in the western Urals thrust belt.

ASSESSMENT OF UNDISCOVERED

CONVENTIONALLY RECOVERABLE

PETROLEUM RESOURCES

The location of the Volga-Ural basin and assessment region is shown in figure 1. USGS estimates 

of oil and gas resources in this basin are given in 

table 3 and figures 13 and 14. Data supplementary 

to these estimates are shown in table 4.

Mean 

Median 

95 percent 

75 percent 

50 percent 

25 percent

5 percent 

Mode 

Standard

deviation

7.23

6.48

3.00

4.73

6.48

8.89

14.00

5.20

3.58

0 3 6 9 12 15 18 21 

RECOVERABLE OIL, IN BILLIONS OF BARRELS

FIGURE 13.-Probability distribution of Volga-Ural recoverable 

oil as of April 2, 1981. The number of significant figures 

reflects the precision of the statistical process rather than the 

accuracy of an estimate.

Mean 

Median 

95 percent 

75 percent 

50 percent 

25 percent

5 percent 

Mode 

Standard

deviation

62.62

51.94

19.00

34.39

51.94

78.45

142.00

35.74

42.15

"0 35 70 105 140 175 210 245 

RECOVERABLE GAS, IN TRILLION CUBIC FEET

FIGURE 14.-Probability distribution of Volga-Ural recoverable 

gas as of April 2, 1981. The number of significant figures 

reflects the precision of the statistical process rather than the 

accuracy of an estimate.

TABLE 3. -Assessment of undiscovered conventionally

recoverable petroleum resources in the Volga-Ural petroleum

province

[Resource assessment by USGS as of April 2,1981. Fn denotes the nth fractile; the 

probability that more than the amount Fn is present is n percent]

Crude oil, in billions of barrels

Natural gas, in trillions of cubic

feet (billions of barrels of oil 

_____equivalent)1_____

Low (Fas) High (F6) Mean Low (Fas) High(FB) Mean

14 19(3) 142(24) 63(10)

>1 barrel of oil equivalent to 6,000 ft9 of gas.

TABLE 4. -Supplementary and comparative data supporting the

petroleum resource assessment of the Volga-Ural petroleum

province

[Average depth of undiscovered resources: 9,500 ft for oil and 14,000 ft for gas. 

Analogs: Midcontinent, United States; Permian basin, United States; Paleozoic, 

western Canada basin; Arctic coastal plain, Canada. Cumulative production and 

reserves are composited estimates based on various sources. +, quantity positive but 

no data available]

Cumulative production to Jan. 

1, 1980

Identified reserves1 to Jan. 1. 

1980: 

Demonstrated

Inferred

Total

Original recoverable 

resources (ultimate): 

Cumulative

Identified reserves _____ 

Undiscovered resources 

(mean)

Total _________

Crude oil, in 

billions of 

barrels of oil 

equivalent

30-35

5

5

10

35

10

7

52

Natural gas, in 

trillions of 

cubic feet

+

75

25

1002

+

100 

63

163+ 8

'Follows terminology outlined by U.S. Bureau of Mines and U.S. Geological Survey (1980). "Demonstrated" is equivalent to American Petroleum Institute's 

"Proved and Indicated Additional." "Inferred" represents anticipated field growth 

in existing fields.

* 17 billion barrels of oil equivalent.

 27+ billion barrels of oil equivalent.

COMMENTS

Assessment does not include Permian Melekess 

tar sands, which have approximately 100 billion 

barrels of oil in place.

Assessment does not include the Peri-Caspian 

depression or the lower Volga area but does include the Orenburg area.

Cumulative production and reserve estimates 

are approximate.

Significant new plays in the area are not likely. 

Additional reserve delineation probably will involve small fields proximal to existing production 

and from the same part of the stratigraphic section.

25
Exact areas and amounts of exploration are not 

well known, but indirect evidence based on the 

location of subsurface data points suggests a 

highly mature exploration province.

Extension of production to the northwest is 

limited by the lack of maturation of the Domanik 

and Domanik-type source-rock facies.

Major oil occurs in the Devonian and Lower Carboniferous. The Upper Carboniferous and Permian 

have many pools, but reserves are small.

Permian reef play along the Ural foredeep may 

have been well tested already.

The Orenburg gasfield in the southeastern part 

of the area is included in the assessment and contains estimated current reserves of about 70 

trillion cubic feet.

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Troyepol'skiy, V. I., 1974, Bitumen deposits of the Permian 

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Aleksin, A. G., Alekseyev, G. I., Mityakhina, L. I., and Khanin, 

I. L., 1974, Exploration for oil and gas of the non-dome 

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Baymukhametov, K. S., 1976, Systematic variations in the 

geology of clastic reservoirs of the Devonian and Lower 

Carboniferous and procedure for selection of production objectives in multi-strata pools in the early stages of planning 

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Mirchink, M. F., 1964, Status of geology and practice of 

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(English translation in Petroleum Geology, v. 8, no. 6, p. 

307-316.)

Muslimov, R. K., Abdullin, N. G., Kirshfel'dt, Y. E., and 

Suyetenkov, V. S., 1976, Final exploration of carbonate 

sediments of the Devonian and Carboniferous of the 

Romashkino and Novo-Yelkhov fields of the Tatar ASSR: 

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Nalivkin, V. D., 1976, Dynamics of the development of the 

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Ovanesov, G. P., Abrikosov, I. K., and Khachatryan, R. 0., 

1972, Reefs of the Kamsk-Kinelsk system of troughs and 

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Neftegazovaya Geologiya i Geofizika, no. 12, p. 13-16. 

(English translation in Petroleum Geology, v. 13, no. 5, p. 

232-233.)

Shikhov, S. A., Kuznetsov, Y. I., Provorov, V. M., Vetchinkin, 

V. E., and Sosland, S. A., 1976, Systematic variations in the 

distribution of Upper Devonian-Lower Carboniferous reefs 

in the north part of the Ural-Volga area and, geophysical 

methods for distinguishing them: Geologiya Nefti i Gaza, no. 

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14, no. 1, p. 38-40.)

St. John, B., 1980, Sedimentary basins of the world and 

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V. V., and Doronkin, K. N., 1977, Hydrogeological bases of 

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translation in Petroleum Geology, v. 14, no. 12, p. 526-527.)

Suleymanov, R. G., and Bazev, N. S., 1974, Petroleum potential 

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26
Svetlakova, E. A., and Kopytchenko, V. N., 1978, "Formation Valeyev, R. N., Klubov, V. A., and Ostrovskiy, M. I., 1969, 

and distribution of gas-oil accumulation in the Paleozoic sub- A comparative analysis of the conditions of formation and 

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27