OIL, GAS PROSPECTIVITY OF WESTERN SIBERIA'S TOMSK OBLAST LISTED

Nikolai I. Karapuzov, Eduard V. Krivosheyev, Victor M. Tishchenko
Tomsk, U.S.S.,R.

Tomsk oblast is in the southeastern part of the West Siberian plain, midway along the course of the Ob river. It has an area of 316,900 sq km (Fig. 1).

The oblast is part of the major West Siberian oil and gas bearing province. Since prospecting began in the late 1940s in Tomsk oblast, 96 oil and gas fields have been discovered that include approximately 200 deposits.

Tomsk oblast is predominantly flat and swampy with elevations no more than 200 m above sea level. The wide Ob valley is in the center of the oblast, dividing it into two almost equal sectors.

The west sector includes an extensive boggy lowland, Vasyuganye, whereas the east sector is higher, less boggy, and extremely wooded.

In Tomsk oblast are oil and natural gas fields, iron ore deposits, and peat. There are also non-metallic minerals (clays, loams, construction stone, limestones, and various sands).

There are large reserves of underground water: they are bromide, iodide, hydrogen sulfide, and radonic waters.

The climate is continental. Winters are dry and long. The temperature in January is usually - 19- - 21 C., with a minimum of as low as -50 C. Summers are warm and short. The average temperature in July is 17-18 C.

The main river in the oblast is the Ob, with the tributaries Tom, Chulym, Ket, Tym, Chaya, Parabel, and Vasyugan. Water covers 2.5% of the oblast, bogs cover approximately 30%, and. woodland 56%.

The average population density is three people per square kilometer. As much as 70% of the population lives in the towns of Tomsk, Kolpashevo, Asino, and Strezhevoi, and 12 large towns.

The main industries are forestry, oil, machine engineering, and metal working.

The Alexsandrovskoye-Nizhnevartovsk and Aleksandrovskoye-Tomsk-Anzhero-Sudzhensk oil pipelines, and the Nizhnevartovsk-Tomsk-Kuzbass gas pipeline lie in the area. A large petrochemical combine has been put into operation in the Tomsk area.

The transportation of large freight is mainly by waterway and road. The rivers Ob, Tom, Ket, Tym, Chaya, Parabel, and Vasyugan are navigable by ship. There is well developed air transport.

PREVIOUS STUDIES

Systematic oil and gas prospecting that began in 1948 can be divided into three stages.

The first stage, 1948-1953, represents the beginning of largely regional scientific research, geophysical work, and reference (parametric) deep drilling. Direct evidence of the oil and gas potential of the West Siberian basin was obtained during this period in the Kolpashevo block.

The second stage, 19541961, was defined by an increase in the amount of drilling and geophysical research and a search for the optimum methods for oil and gas prospecting. The commercial fields Sovetsky, Sosninsky, and' others were discovered in this period.

The third stage, which began in 196 1, is distinguished by the discovery of oil, gas-condensate, and gas fields in various parts of the oblast.

To date three oil producing regions have been created in the oblast: Aleksandrovo, Vasyugan, and Pudino. Production from these regions totals approximately 15 million metric tons/year of oil.

Seismic surveys, mainly various versions of the reflected wave method, were the principal method used to study the tectonics of the platform deposits. Gravity prospecting and magnetic prospecting results were used successfully for regional tectonic constructions.

The entire oblast has been covered by a 5 milligal gravimetric survey and a considerable part of the highly prospective regions by a 2 milligal gravimetric survey.

Aeromagnetic surveying has been carried out everywhere at a scale of 1:200,000, and high precision aeromagnetic surveying at a scale of 1:50,000 in the west half of the oblast.

SEISMIC DATA BASE

A large number of seismic refraction surveys have been carried out. Some 140,000 km of seismic reflection profiles have been completed, including 62,000 km of single-channel profiles, 78,000 km of multiple profiles (common depth point); the average density of CDP surveying was 0.49 km/sq km in the western part of the oblast.

The main reflector, associated with the top of the Upper Jurassic and mapped throughout the entire oblast, is horizon pa (Bazhenovo suit e).

Horizon F2, associated with the basement surface, is mapped over a large part of the area.

Reference horizons of an almost regional nature are frequently identified in the section over extensive areas of the oblast. These include horizons of group I associated with the Lower-Middle Jurassic deposits, group III horizons near the top of the Lower Cretaceous, and group IV horizons associated with the Upper Cretaceous.

The extent to which oil and gas bearing horizons have been studied by deep drilling varies in the different parts of the oblast. The western area has been studied most and here the drilling density is 23 m/sq km.

STRATIGRAPHY

The geologic section in Tomsk oblast is divided into Paleozoic formations of the pre-platform sequence and Mesozoic-Cenozoic sediments of the cover (Fig. 2).

At the surface of the preplatform sequence, there are different formations, ranging from almost normal sedimentary platform types to geosynclinal, metamorphic, and intrusive types.

The sedimentary formations of platform-type appearance are represented by a carbonate-clastic formation mainly of middle and late Paleozoic age (the Nyurolka trough and Pudino megas-well) and by a carbonate formation of early Paleozoic and Vendian-Riphean age (Vladimir arch).

The sequence is 3-4 km or more thick. The metamorphic formations are represented by rocks of various composition and identified in different blocks of the Koltogorsk-Urengoi graben, the Ust-Tym graben, and slopes of structures of the first order.

Granites (Aleksandrovo megaswell, Parabel megaswell and Nyurolka basin) and serpentinites (Nyurolka basin and Central Vasyugan megaswell) are found in the intrusive formations. Extrusive rocks of basic, intermediate, and acidic composition were established in many sections.

THREE ROCK GROUPS

The rocks of the pre-platform sequence are split into three groups: carbonates (carbonate-clastic), clastics, and those including metamorphic and magmatic rocks (intrusive and extrusive).

Types of weathering crusts have been identified at the top of these groups, their thickness being as great as 130 m (Ostaninka block). The weathering crusts in the upper part of the pre-platformal sequence are represented by zones of disintegration and leaching in which rocks with improved reservoir properties are developed and preserved.

The oldest Mesozoic deposits in the area are formed by a volcanic sedimentary unit of presumably Permo-Triassic age. It consists of dark grey, argillaceous and siliceous-argillaceous shales, coals, mudstones, tuffs, siltstones, and various extrusive rocks.

These deposits mainly occur in the deepest parts of the major basins (Nyurolka, Ust-Tym, and Koltogorsk megatrough).

JURASSIC SEQUENCE

Lower Jurassic deposits pinch out on the upper slopes and arches of large first order structures (Aleksandrovo, Central Vasyugan, Parabel, and Pudino megaswells) where they pinch out.

The undivided Triassic-Lower Jurassic deposits are combined into the Sherkalin horizon, well known for its productivity, and analogous horizons including sandy layers Yu18, Yu17, and Yu16.

Middle Jurassic deposits are represented by flood plain-fluvial and lacustrine sediments and by deposits from boggy depressions.

The deposition of a group of mudstones (Togur group) at the base of the Jurassic section is very important. This group is not only a good seal for the underlying sandstones; it is also oil producing.

Middle Jurassic deposits are 200-250 m thick. Sandy layers Yu5-14 forming shoestring and lens-type traps are characteristic of the entire unit.

The Upper Jurassic deposits include rocks of the Lower Callovian (top of the Tyumen suite), Middle Callovian-Oxfordian (Vasyugan suite), and Kimmeridgian-Volgian stage (Georgiyev and Bazhenovo suites). They occur practically throughout the oblast and are up to 100-120 m thick.

The Upper Jurassic deposits are represented mainly by shallow and coastal-marine deposits with small sections of continental deposits. The productive layers of the Upper Jurassic deposits are the sandy layers Yu1 and Yu2.

CRETACEOUS, LATER ROCKS

The Cretaceous sequence includes the Lower Cretaceous Kulomzin, Tarsk, Vartovsk, and Alym suites, and the Upper Cretaceous Pokursk, Kuznetsov, Berezov, and Gankin suites (Fig. 2).

The Lower Cretaceous deposits are represented by a sequential replacement of marine deposits of the Kulomzin suite by coastal marine and coastal continental deposits of the Vartovsk suite.

Permeable layer-reservoirs occur sporadically in these deposits and are identified as BV1-20 and AV2-12. The overall thickness of the Cretaceous sequence is 1,700-2,200 m.

Paleogene deposits are of marine origin and are represented mainly by clays with layers of sandstones, siltstones, and marls. Loams, sandy loams and sands of Neogene age complete the section.

MAIN TECTONIC FEATURES

The West Siberian platform is the northern part of the young Urals-Siberian platform.

It is located between ancient platforms, and its development occurred in four stages: geosynclinal, protoorogenic, deutero-orogenic, and platform.

The West Siberian platform basement is understood to include a complex combination of heterogeneous structural stages separated by a regional unconformity from the Mesozoic-Cenozoic deposits of the typically platform cover.

The structure of the platform cover in the Tomsk oblast is complex. This is because deep faults striking in a submeridional (nearly north-south) direction have been superimposed on the basement fold structures (Fig. 3).

Graben-rifts (Ust-Tym, Koltogorsk-Urengoi) formed along these major faults in the basement at the beginning of the Mesozoic era between and above these trench-type narrow trough zones formed in the platform cover.

The Lower Vartovsk and kaimysovo arches and Aleksandrovo, Middle Vasyugan, Pudino, Parabel, and Pyl-Karaminsk megaswells are identified amongst the positive structural elements. The arch uplifts are nearly isometric in form, and their areas range from 5,000-23,000 sq km.

The amplitude of the major positive structures at the basement surface is 4001,000 m, and the amplitude is reduced by approximately a factor of 10 at the top of the section (in the Upper Cretaceous and Paleogene deposits).

Discontinuous dislocations are usually noted in the basement surface. The amplitude of these dislocations is low, and in most cases the dislocations become attenuated in the Jurassic deposits. The basement usually occurs at depths of 3-3.5 km in basins and troughs but it sometimes reaches 4 km (Koltogorsk megatrough). At the crests of arches and megaswells it is 2-2.5 km.

All the known major structures are inherited and were formed during the initial stages of deposition of the cover.

OIL, GAS POTENTIAL

Seven stratigraphic levels of concentrated oil and gas reserves are identified in the Paleozoic and Mesozoic deposits.

In the first stratigraphic level, shallow water marine deposits of the Upper Jurassic (horizon Yu1), are the main target of oil and gas prospecting. The deposits in horizon Yu1 are layer and arch type and are lithologically limited.

Recently, the high prospectivity of non-anticlinal type traps and associated deposits in this stratigraphic sequence has been shown.

The second stratigraphic level consists of continental, alluvial, lacustrine-alluvial, and deltaic deposits of Middle Jurassic age (horizons Yu3-Yu14).

The Middle Jurassic deposits are mainly oil bearing in depression zones, primarily in the Nyurolka basin and on the slopes of the first order uplifts surrounding it. The majority of the deposits are associated with non-anticlinal type traps.

The third stratigraphic level consists of Triassic-Lower Jurassic alluvial and proluvial deposits of the base of the platform cover.

Mainly non-anticlinal deposits, associated with traps on the slopes of granite massifs, fluvial and deltaic facies in the depression zones above rifts, and in incisions in the basement, are predicted in the sequence.

The fourth stratigraphic level is the zone of weathering of Paleozoic deposits, the so-called oil and gas bearing horizon of the zone of contact between Paleozoic and Mesozoic deposits (Oghzc).

The reservoir rocks in the Oghzc are represented by hypergenetically and hydrothermally altered organic limestones and tuffs. Fifteen oil deposits and three gas deposits have been discovered in this stratigraphic sequence.

ADDITIONAL POTENTIAL

The fifth stratigraphic level is formed by the deep-lying Paleozoic horizons.

The main prospectivity in the sequence is associated with carbonates and reef carbonates of Middle Paleozoic age in a number of zones in the Nyurrolka basin.

Here, oil shows and carbonate or carbonate-clastic deposits in the Lower Paleozoic and Vendian-Riphean have been recorded in three blocks in the east of the oblast at the junction between the West Siberian platform and the ancient Siberian platform.

Progress in the study of these sequences is being held up by the need for a technique to "prepare" traps and by the great depth of the prospective deposits.

The sixth stratigraphic level is associated with the Cretaceous deposits. Sovetsky field, the largest in the oblast, has been discovered and developed in this sequence.

Individual deposits have been discovered in rocks of this age in five blocks in the oblast.

The seventh stratigraphic level with oil prospectivity is the Bazhenov suite, the commercial oil and gas prospectivity of which has been proven in a large number of blocks (East Moiseyev, Olenya, Festival, etc.).

The vast majority of Tomsk oil and gas deposits are associated with sandy and sandy-silty layers, in the Jurassic and Cretaceous sequences, with some discovered in the carbonate reservoirs of the upper part of the Paleozoic deposits.

In these conditions two dominant factors, the structure and the lithological variability of the deposits, predetermine the geometry of the deposits.

Evaluations of the initial geological reserves of oil, natural gas, and condensate in the oblast are 4.84 billion metric tons of conventional hydrocarbons, including 3.69 billion metric tons of oil.

The initial reserves of oil and gas are unevenly distributed in the stratigraphic sequences. Most of the initial geological reserves of hydrocarbons in Tomsk oblast are concentrated in Jurassic deposits (68%), with 19% in Cretaceous deposits and approximately 3% in the Oghzc.

However these evaluations are approximate since the Lower-Middle Jurassic and Paleozoic sequences have not undergone full study.