AAPG Bulletin, V. 83 (October 1999), No. 10, P. 1642-1665.

Gas Generation and Accumulation in the West Siberian Basin¹

R. Littke,² B. Cramer,³ P. Gerling,⁴ N. V. Lopatin,⁵ H. S. Poelchau,⁶ R. G. Schaefer,⁶ and D. H. Welte⁶

©Copyright 1999.  The American Association of Petroleum Geologists.  All Rights Reserved

¹Manuscript received December 1, 1997; revised manuscript received November 17, 1998; final acceptance March 31, 1999.
²Institut für Chemie und Dynamik der Geosphäre (ICG-4), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany. Present address: Lehrstuhl für Geologie, Geochemie und Lagerstätten des Erdöls und der Kohle, Rheinisch-Westfälische Technische Universität Aachen (RWTH), Lochnerstr. 4-20, 52056 Aachen, Germany; e-mail: [email protected]
³Institut für Chemie und Dynamik der Geosphäre (ICG-4), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany. Present address: Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, 30655 Hannover, Germany.
⁴Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, 30655 Hannover, Germany.
⁵Russian Federation National Research Institute for Geological, Geophysical and Geochemical Systems (VNIIGeosystem), Varshavskoe shosse 8, Moscow 117234, Russia.
⁶Institut für Chemie und Dynamik der Geosphäre (ICG-4), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany.
 

The study benefited considerably from substantial support by Ruhrgas AG, Essen, Veba Oel AG, Gelsenkirchen, and Wintershall AG, Kassel. Support by the German Ministry of Research and Development (Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie, BMBF, Kennziffer Gas 1000) is also gratefully acknowledged. We thank the exploration companies Urengoyneftegazgeologiya, Urengoy, and Purneftegazgeologiya, Tarko-Sale, for access to their well files and other relevant data and E. Galimov, Vernadsky Institute of Geochemistry and Analytical Chemistry, Moscow, for access to gas geochemical data and many scientific discussions. Reviews of an earlier version of this manuscript by K. E. Peters and F. D. Mango were helpful in clarifying parts of the text. 

ABSTRACT

The largest accumulations of natural gas occur in shallow, Cenomanian reservoir sands of the Pokur Formation in the northern part of the West Siberian basin. Various theories on the origin of this gas were developed in the past. To evaluate these theories, a large set of gas and rock samples was studied using organic geochemical methods. Our results reveal that early thermogenic methane generation can explain the isotopic composition of the natural gas in this area, but not the extreme dryness. In particular, the isotopic composition of early thermogenic methane generated upon laboratory pyrolysis  is in accordance with that of the methane in the large gas accumulations of northern west Siberia (d¹³C approximately equals -45 to -55?). The quantities of early thermogenic gas generated according to our calculations, however, are not sufficient to explain the largest accumulations. The huge amount of gas in the reservoirs is explained by degassing of methane-saturated pore water, mainly as a consequence of Neogene uplift of the basin. Since the late Eocene, the movement of water from the southern to the northern part of the basin enlarged the effective catchment area of the gas fields. 

INTRODUCTION

The West Siberian basin (Figure 1) covers an area of 3,400,000 km² and is the largest continental sedimentary basin worldwide. This basin is estimated to contain between 40 and 50 X 10¹² standard m³ of discovered natural gas (methane), i.e., one-third of world gas reserves (Grace and Hart, 1990; Khartukov et al., 1995). Much of this gas occurs in supergiant or even megagiant accumulations, of which Urengoy is the largest. In addition, significant resources of oil and condensate are present (Rovenskaya and Nemchenko, 1992). Thus, this area is one of the most prolific hydrocarbon provinces worldwide. In 1994, 607 X 10⁹ m³ of world gas production came from west Siberia (Avati, 1996), accounting for much of the gas consumed in Russia, eastern Europe, and western Europe. A summary of an estimate of the size of various gas fields is given in Table 1. Although the economic significance of west Siberian gas remains undisputed, much controversy exists with respect to the origin of the gas and the migration distances from source to reservoir.

The greatest proportion (about 65%) of west Siberian gas exists in Cenomanian reservoirs at shallow depth (usually 800-1200 m). The almost pure methane in these reservoirs is characterized by an isotopic composition (d¹³C approximately equals -45 to -55?) intermediate between the compositions of bacterial and deep, thermogenic methane. These two end-members are commonly cited as the most important sources of natural gas accumulations (Schoell, 1980; Tissot and Welte, 1984). Although several workers support the idea of a bacterial origin of the
 
 

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west Siberian gas (e.g., Ermakov et al. 1970; Nesterov et al., 1970; Vasil'ev et al., 1970), other workers favor a thermogenic origin. For example, Matusevich et al. (1984) and Prasolov (1990) proposed a deep origin in Lower Cretaceous, Jurassic, and Triassic source rocks and vertical migration of the gas via deep-reaching fault systems from the sources to the reservoirs. In contrast, Galimov (1988, 1989) and Galimov et al. (1990) proposed an early thermogenic gas generation within the upper Aptian-Cenomanian Pokur Formation. According to their hypothesis, vertical migration
 
 

Figure 1--Index of West Siberian basin with major rivers and cities, location of gas and oil provinces, and details of the study area including the gas fields studied and the location of the north-south cross section shown in Figure 2.
 
 
 
 

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from deeper and older rocks plays only a minor role in the west Siberian gas balance. Finally, Kruglikov (1967), Kortsenshtejn (1970, 1974), and Surkov and Smirnov (1994) proposed a hydrodynamic model where the northern parts of the basin were lifted, leading to a phase of Cenozoic erosion and gas release from the groundwater into the reservoirs. These workers presumed a groundwater flow from south to north and that the area of gas generation is in the southern part of the West Siberian basin.

One of our major objectives in this work was to find new or additional evidence to support or discard these theories on gas generation, migration, and accumulation. A large set of gas and rock samples was studied using organic geochemical methods. For the first time, pyrolysis experiments on source rocks from the West Siberian basin were done at different heating rates to quantify early thermogenic methane generation and the relevant isotope effects. The results of this kinetic study were brought into the context of basin evolution and temperature history to calculate volumes of methane generated under the megagiant gas accumulation at Urengoy.

In summary, the goal of this contribution is to present and discuss new data on source rocks and gases from the West Siberian basin in the light of the controversial discussion of the origin of the largest gas accumulations known on Earth.

GEOLOGIC BACKGROUND

The West Siberian basin extends between the Yenissey River and the Siberian highlands in the east and the Ural Mountains in the west, over a distance of 1500 km. In the south, the basin is bordered by the highlands of Kasakhstan, Altay, and Sayan, from which it extends 2500 km toward the north, where its submarine extension is found under the Kara Sea (Figure 1). The basin originated mainly from Triassic continental rifting, although earlier phases of sedimentary basin evolution and rifting have also been recognized (Zonenshain et al., 1990). Most Triassic structures and major faults are oriented in a north-south direction, such as the Koltogor-Urengoy graben. This graben is the central tectonic element of the basin and extends over a length of 1800 km and is 10-80 km wide. Late Mesozoic structures are large anticlines and synclines that still follow the Triassic north-south structural trend (James, 1995).

Triassic sediments are intercalated with rift-related volcanic rocks and are highly variable in thickness (Figure 2) (Peterson and Clarke, 1991). Deposits consist mainly of conglomerates and sandstones derived from the Taimyr highland and the northern part of the Siberian highland, as well as some coals. Based on crustal thinning and a b-factor of 1.09 (Yu. I. Galushkin, 1996, personal communication), a heat flow of 67 mW/m² was estimated for the Triassic synrift phase (Figure 3). Based on the results presented recently by Makhous et al. (1997) and Schaefer et al. (1999) maximum heat flows of 80 mW/m² may be reasonable for this early phase of basin evolution. During the Jurassic, crustal thinning ceased (b-factor 1.02) and heat flow decreased. Predominantly continental, clastic sedimentation continued. During the Early and Middle Jurassic, coals were deposited, as well. Marine transgressions from the north are characteristic of the Late Jurassic. The strongest transgression during the Tithonian led to deposition of the Bazhenov Formation, which is on average 30 m thick (Krylov and Korzh, 1984) and which is regarded as the principal source rock of the oil in the southern part of the basin (Peters et al., 1993). In the northern gas-rich part of the basin, the Bazhenov Formation is less organic carbon rich and less oil prone than in the south (Kontorovich, 1984).

During the Early Cretaceous, clastic sedimentation continued and deltaic progradation and high sedimentation rates led to a shift toward more continental conditions. Sandstones, siltstones, shales, and coals are the most widespread lithologies. Sandstones of the Achimov Formation, deposited during the Berriasian, are important reservoir rocks for condensate. During the early Aptian, clays of the Alym Formation bear witness of another marine transgression in the southern part of the basin, which was followed by regression of the sea and deposition of the thick, continental, coal-bearing Pokur Formation during the late Aptian, Albian, and Cenomanian. This formation is up to 1400 m thick and typically consists of 70% sands/sandstones and 30% intercalated siltstones, claystones, and coals. The coal seams reach thicknesses of up to 10 m (Rovenskaya and Nemchenko, 1992). Unconsolidated
 
 

Table 1. Size of Gas-Producing Reservoirs in West Siberia
 
 

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Figure 2--Line drawing of the regional seismic section SW 107 with major stratigraphic horizons, identified on the right margin, and wells in the West Tarko-Sale and Urengoy gas fields (see Figure 1 for location of profile). The main seismic reflector codes are shown near the left margin. Depth and horizontal scales are in meters.
 
 

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sands in the Cenomanian portion of the Pokur Formation are the principal gas reservoir rocks in western Siberia. During the Turonian, transgression of the sea led to deposition of the marine Kuznetsov and, until the Danian, Berezov and Gan'kin formations. These units consist of about 600 m of marine claystones
 
 

Figure 3--Basin evolution and calibration data. (A) Burial history, temperature, and heat flow evolution in the West Tarko-Sale and Urengoy fields. The gray shades represent temperature intervals. (B) Temperature and vitrinite reflectance data and simulated temperature and vitrinite reflectance for wells in the Urengoy and West Tarko-Sale fields. The simulations are based on the burial and heat flow history, heat conductivities of the respective lithologies and the Easy%R_o algorithm of Sweeney and Burnham (1990). Numerical simulations were performed with PetroMod software of IES, Juelich.
 
 

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with a few thin intercalated sands. This clay-rich sequence seals the Cenomanian gas reservoirs.

From the Paleocene to the middle Oligocene, another sequence of mainly fine-grained, marine clastic sediments was deposited, reaching a thickness of about 500 m (Peterson and Clarke, 1991); however, in the late Eocene initial uplift of the northern part of the West Siberian basin led to the development of the central Siberian swell at about 62°N latitude. Erosion of up to 700 m in the northern part of the basin was postulated by Surkov and Smirnov (1994) for the late Tertiary. Our own estimations of eroded thickness were based on sonic log data from claystones in several wells. Following the method of Magara (1976), eroded thicknesses were calculated to range from less than 200 m in the area of Tarko-Sale to several hundred or even more than 1000 m in the area of Urengoy (about 200 km north of Tarko-Sale; see Figure 3); however, possible error with respect to these numbers is large because few sonic log data were available for the upper 1000 m of the sedimentary column. Also, erosion calculations based on vitrinite reflectance-depth profiles (e.g., Littke et al., 1994) are unreliable because few vitrinite reflectance data were available for the upper 3500-4000 m of the sedimentary column (the post-Jurassic). No core or cutting samples at present are available from these shallower sedimentary units.

As a result of cold conditions on the continent during the Pleistocene a permafrost up to 500 m thick developed in northern west Siberia. Within and beneath the permafrost, gas hydrates are stable. Makogon et al. (1972) stated that considerable amounts of natural gas in some areas in northern west Siberia, for example in the Messoyakha field, occur as solid gas hydrates. Due to a lack of reliable data it is still debated whether the Messoyakha gas field contains any gas hydrates (see discussion in Collett and Ginsburg, 1997); however, the permafrost as a possible additional seal (Olovin, 1988) does not reach now and probably did never reach the main gas reservoirs at the top of the Pokur Formation.

ANALYTICAL METHODS

Total organic carbon (TOC) was measured after carbonate removal with HCl using a LECO IR-112? carbon determinator. Aliquots of powdered sediments (100 mg) were subjected to Rock-Eval pyrolysis using a GEOCOM? Rock-Eval II instrument (Espitalié et al., 1977; Peters, 1986). At least two measurements were made on each sample. Another aliquot of powdered sediments was treated with HCl to remove carbonates and combusted at 1100°C in a Carlo-Erba? elemental analyzer. After removal of water, the generated carbon dioxide was focused in a cold trap and stable carbon isotope measurements were performed using a MICROMASS OPTIMA? mass spectrometer. At least two measurements were done on each sample. Organic petrologic studies were made on polished sections with reflection microscopy under white light. Vitrinite reflectance measurements were made in oil immersion at a wavelength of 546 nm as described in Taylor et al. (1998).

Open-system pyrolysis measurements were done with at least three different constant heating rates between 0.1 and 5.0°C/min using a considerably modified version of the equipment described by Lillack (1992) and Krooss et al. (1995). Briefly, powdered aliquots (~100 mg) of eight coals and coaly sandstones from the Pokur Formation and one Holocene-age Siberian peat sample were heated in a quartz reactor from 200 to 800°C. Special care was taken with respect to the temperature control inside the oven. An accuracy of ±2°C for the sample temperature could be guaranteed over the significant part of the temperature range. About 40 ml/min helium was flushed through the reactor as carrier gas. Methane, ethane, ethene, and propane and propene were separated by gas chromatography and quantified using a flame ionization detector. These measurements were done every 3 min during each pyrolysis experiment. For details see Schaefer et al. (1999). The experimental data were evaluated to determine activation energy distributions assuming up to 50 first-order parallel reactions and an Arrhenius-type temperature relationship (Schaefer et al., 1990). The applicability of kinetic results from open-system pyrolysis to geological systems is still a matter of current research. For coals, in particular, one can assume that the secondary gas-generating potential during such pyrolysis experiments may be underestimated (Schenk et al., 1997), i.e., the obtained methane yields fall in the lower part of the expected range of values.

Stable carbon isotope measurements were made both on pyrolysis gas and on natural gas. From pyrolysis experiments with two coals from the Pokur Formation, gas samples were taken and carbon isotopes of light hydrocarbons were analyzed as described in Cramer et al. (1998). Isotope measurements on natural gas were performed in three different laboratories at the Vernadsky Institute, Moscow, the VNIIGeosystem Institute, Moscow, and the Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Hannover.

RESULTS

Source Rock Properties and Maturity

In the central part of northern west Siberia, the thick Mesozoic sedimentary sequence is rather rich
 
 

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in organic matter. The predominantly continental intervals, such as the Jurassic Tyumen and the upper Aptian-Cenomanian Pokur formations, contain some coals and dispersed coaly organic matter, whereas the marine Bazhenov Formation (and also intervals within the Berriasian-lower Aptian sequence above) contain algal-derived organic matter (Tissot and Welte, 1984; Peters et al., 1993). The 151 samples that we investigated for source rock properties are mostly among the more organic-carbon-rich sediments from the different intervals. Average TOC values for the individual formations of the basin will be much lower than the average values calculated for the different stratigraphic intervals from our samples. Also, HI (hydrogen index) values from Rock-Eval pyrolysis would be lower if more of the organic carbon-lean lithologies had been sampled; nevertheless, the data in Table 2 give some interesting information on source rock properties in the West Siberian basin.

The Triassic Tampey Formation contains some rocks with high TOC values (Table 2). These rocks are overmature (T_max greater than 500°C, vitrinite reflectance greater than 2%). Even with respect to thermal methane generation, these rocks have little remaining potential. Present-day temperatures are greater than 130°C and some 30-40°C lower than maximum burial temperatures.

Another potential source rock is the overlying thick Tyumen Formation, which contains abundant coal intercalations. Average TOC values of the samples are 10 wt. % and most samples are characterized by HI values between 50 and 200 mg HC/g TOC. Whereas these values are rather typical for coaly organic matter (type III kerogen) at mature stages, some samples show higher HI values (up to 400 mg HC/g TOC). These samples contain type II or type II-III kerogen and possess an elevated potential to generate hydrocarbons. The average TOC of thick, coal-bearing sequences exposed by mining activities usually ranges between about 1.5 and 5 wt. % (Scheidt and Littke, 1989). The Tyumen Formation is not extremely rich in coaly intervals, according to macroscopic observations, and its average TOC is estimated to be at about 1 wt. %. In view of this TOC estimate, the favorable maturity for thermal methane generation (Table 2), and the great thickness of this unit, the Tyumen Formation is regarded as a potential gas source rock in this area.

The Bazhenov Formation is only about 30 m thick, contains on average 6 wt. % TOC and is characterized by HI values that typically range between 200 and 400 mg HC/g TOC in the northern part of the basin. In view of the maturity (Table 2), the kerogen in these rocks can be classified as type II, making it rather oil prone; however, compared to the Bazhenov Formation in the central part of the basin, both TOC and HI values are low (cf. Peters et al., 1993). The Bazhenov Formation has reached about 0.9-1.3% vitrinite reflectance in the Urengoy area and is now at present at temperatures of about 110°C. Rocks that show less TOC and lower HI values include the Vasyugan Formation below and the Achimov Formation above the Bazhenov. In contrast, the Berriasian-lower Aptian, partly marine Megion, Vartov, and Alym formations contain many organic carbon-rich intercalations characterized by highly variable, usually high HI values.

Even more enriched in organic matter is the coal-bearing Pokur Formation, which contains a
 
 

Table 2. Source Rock Properties, Maturity, and Present-Day Temperature Data for Different Stratigraphic Intervals in Northern West Siberia
 
 

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typical type III kerogen (Table 2). The average TOC of this unit is difficult to estimate because little core and cutting material is available. At present, we assume that TOC is at the lower limit typical of coal-bearing sequences (Scheidt and Littke, 1989) at 1.5 to 2 wt. %. In contrast to the widespread theory of thermal methane generation at high levels of maturation (e.g., Tissot and Welte, 1984; Hunt, 1995), Galimov et al. (1990) proposed methane generation from the immature (R_r = 0.4-0.7%) Pokur Formation to explain the giant gas accumulations in western Siberia. Present-day temperatures in the Pokur Formation are between 30 and 60°C (see Figure 4) and maximum paleotemperatures were about 35°C higher during the Oligocene-Miocene.

The organic matter in all stratigraphic intervals of the northern West Siberian basin is characterized by variable d¹³C values. Whereas most values for the Tyumen Formation and the Upper Jurassic and Lower Cretaceous sequences scatter around -25?, values for the lower part of the Pokur Formation are slightly greater, at about -23?. Within the Pokur Formation there is a depth trend toward isotopically lighter organic matter at the top (d¹³C of about -26?). This value also was established for Holocene-age peat from the area. Overall, the differences within each of the stratigraphic intervals are much greater than differences between the stratigraphic intervals with respect to carbon isotope values (Figure 5).

Geochemistry of Natural Gas

About 62% of the west Siberian gas reserves are in sands and sandstones of the Cenomanian part of the Pokur Formation (Peterson and Clarke, 1991), whereas 36% of the gas reserves rest in Lower Cretaceous reservoirs and the remaining 2% are in the Jurassic (Ermakov and Skorobogatov, 1986). The upper Aptian-Cenomanian Pokur Formation contains about 95% of the gas (Rovenskaya and Nemchenko, 1992). We would like to emphasize that the gas data discussed here are from 18 gas fields distributed over an area of almost 1,000,000 km². Thus, variations in gas geochemistry must be expected and there are regional trends and local depth trends; nevertheless, many common features exist for the different gas fields that strongly support a common origin of the gas accumulated in the Pokur Formation.
 
 

Figure 4--(A) Depth plots of temperature and (B) pore pressure in northern west Siberia. Depth trends are slightly different for different areas.
 
 
 
 

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The Cenomanian natural gas consists of almost pure methane characterized by d¹³C values between -45 and -55? (Table 3; Figure 6B). The variability in d¹³C values of methane follows distinct depth-related trends. In addition, there are systematic differences between the different gas pools. For example, methane in the Bovanenko field, in which the Cenomanian reservoirs are at a shallow depth of only about 700 m, is characterized by heavier methane (-46?) than in the Urengoy (-50?) and in the Geologicheskaya (-54?) gas accumulations, where the Cenomanian reservoirs are at a depth of about 1200 m (Figure 6B). By comparing 16 different Cenomanian gas accumulations in northern west Siberia, this inverse trend between reservoir depth and isotope composition of methane was verified, although with some scatter.

Gas pools below the Cenomanian reservoirs are characterized by higher concentrations of C₂₊ components and isotopically heavier methane (Table 3; Figure 6A, B). In the Achimov Formation (Berriasian), C₂₊ concentrations reach up to 15% and d¹³C values of methane increase to -40 to -33?. Although the data show significant scatter, a clearer picture
 
 

Figure 5--Depth plot of d¹³C values of organic matter (kerogen) according to our measurements and data of Galimov et al. (1990) for rocks from the Urengoy field (triangles).
 
 

Table 3. Mean Composition of Gases in Different Stratigraphic Intervals from Northern West Siberia
 
 

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Figure 6--Depth plots (A) of C₂₊ percentages of total gas, (B) d¹³C₁ values of methane, and (C) d¹³C₂ values of ethane in northern west Siberia. Depth trends are slightly different for different areas (e.g., Bovanenko, Urengoy, Geologicheskaya), especially with respect to carbon isotope data of methane.
 
 

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evolves for the d¹³C of methane if regional differences are taken into consideration. Distinct depth trends can be deduced for the rocks below the Bovanenko, Urengoy, and Geologicheskaya gas fields, for which the largest data sets are available. The increase in d¹³C of methane for these fields is about 7-8?/1000 m in the stratigraphic interval between the Berriasian and the Cenomanian. Jurassic gases show similar methane isotope ratios and C₂₊ concentrations to those of gas from the lowermost Cretaceous and clearly differ from the Cenomanian gases (Table 3; Figure 6A, B).

In contrast to the carbon isotope composition of methane, that of ethane shows no depth trend within the thick Cretaceous sequence where values scatter between -32 and -25?. Only in Jurassic strata is there a trend toward higher d¹³C values of ethane (Figure 6C).

Methane from reservoirs within Mesozoic rocks is characterized by dD values ranging between -150 and -270? (Table 3; Figure 7). Methane isotope data were used for a first genetic interpretation of west Siberian gas following previously published schemes (Schoell, 1980; Whiticar, 1990) (Figure 7). Gas data from the Jurassic and Lower Cretaceous (Neocomian) plot in the field of thermal origin. Methane from reservoirs in the Pokur Formation plots between the empirical fields of thermal and bacterial gas generation or at the edge of the thermogenic field (Figure 7); it cannot be easily attributed to one of these. Within the Pokur Formation, the differences in d¹³C values of methane, as has been mentioned, are related to the depth of reservoirs.

Figure 8 plots all available carbon isotope data of ethane and propane from northern west Siberia. d¹³C values of ethane and propane from all gases are typical of thermal gas generation. All gas data plot along the maturity trend line defined for marine source rocks by Berner and Faber (1996). The maturity range of source rocks derived from Figure 8 extends from 0.8 to 2% vitrinite reflectance, which generally corresponds to the stratigraphic interval from the base of the Aptian down to the Lower Jurassic. Isotope data of ethane and propane from the Jurassic generally indicate slightly higher maturities than those from the Neocomian and correspond to an in-situ generation in Jurassic rocks. Most carbon isotope data of ethane and propane from the Neocomian
 
 

Figure 7--d¹³C and dD values of methane from northern west Siberia. Genetic classification after Schoell (1980) and Whiticar (1990).
 
 
 
 

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suggest a lower source rock maturity of about 1% vitrinite reflectance. This maturity is typical of the Jurassic-Cretaceous boundary interval and could imply that ethane and propane in the Neocomian might have been generated in-situ in the Lower Cretaceous marine sequences of the Megion Formation. In principle, a contribution of Jurassic gas cannot be ruled out, but the existence of a strong overpressure barrier at the Jurassic-Cretaceous boundary interval (Figure 4B) probably inhibited an upward migration of hydrocarbons and argues against an admixture from Jurassic sources. The gas accumulations in the Pokur Formation contain only small concentrations of ethane and propane (Figure 6). Isotopically, this gas is similar to the Neocomian gas (Figures 6C, 8). This fact can be explained by an upward migration of Lower Cretaceous gas into reservoirs of the Pokur Formation. Methane from the Pokur Formation, however, isotopically differs significantly from thermogenic gas from a mature source rock.

The fact that most of the gas from northern west Siberia does not simply fit into the current theories on thermal or bacterial gas generation also is obvious from the Bernard plot (Figure 9). In this diagram, the Jurassic and Neocomian gas data plot in the field of thermogenic gas. Gas from Jurassic strata describes a typical maturity trend. In contrast, many of the Aptian, the Albian, and especially the Cenomanian gas data plot outside of the established fields of thermal and bacterial gas generation.

A hypothetical model of a continuous mixing of two different gases could explain the isotopic features encountered in the study area. If Cenomanian gas with isotopically light methane and high methane concentrations were regarded as one end member of this mixture and thermal Neocomian gas would be regarded as the other end member, relative percentages of each of the end members could be calculated for each reservoir. In Figure 9 this is visualized with a mixing grid that takes into account the range of d¹³C values of methane in different fields. According to this interpretation, less than 10% of the Cenomanian gas is derived from the thermogenic mature source rock represented by the Neocomian gas end member. The Aptian and Albian gases that are second in economic importance are calculated to contain a more significant portion of the thermogenic end member, between 10 and 50%.

The stratigraphic trend toward isotopically lighter methane and toward smaller concentrations of other hydrocarbon gases within the Cretaceous cannot be interpreted as caused by migrational fractionation. In Figure 9 we added the three most important trends in gas alteration: mixing, source rock maturation, and migrational fractionation due to diffusion (as the most important migrational effect) (Prinzhofer and Pernaton, 1997). The principal curvature of the gas data can be interpreted as being caused only by mixing of two different gas sources (Figure 9).

In summary, the Jurassic gas was generated thermally from a pre-Cretaceous source rock, probably within the thick Jurassic Tyumen Formation. Gas from Neocomian strata also is characterized by a typical thermogenic pattern. A possible source of
 
 

Figure 8--d¹³C values of ethane plotted vs. d¹³C values of propane for west Siberian gases.
 
 
 
 

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this gas is the lower, marine part of the Megion Formation. Gas from the Cretaceous interval shows a clear depth trend. More and more isotopically light methane is admixed with decreasing depth of the reservoir. In the main reservoirs at the top of the Pokur Formation the contribution of the thermal gas from deep sources (Neocomian or deeper) does not exceed 10%. The mechanism of generation of the isotopically light methane, whether early thermogenic or bacterial, is discussed in a following section.

Early Thermogenic Methane Generation: Experimental Results

Galimov (1988) and Galimov et al. (1990) advocated the idea of early thermogenic methane generation as a source for the Cenomanian gas; however, it was never tested whether the coaly organic matter from the Pokur Formation could generate early methane in sufficiently large quantities and whether the isotopic composition of such methane would correspond to the carbon isotope data of the natural gas. Open-system pyrolyses were done to test whether methane generation from coaly organic matter of the Pokur Formation at moderate temperatures is sufficiently effective to explain the large gas accumulations of northern west Siberia. In this paper, we report the geologic consequences of our experiments and the most important background data. Schaefer et al. (1999) treated the entire set of experimental data and kinetic interpretation of gas generation and Cramer et al. (1998) treated the isotopic composition of the methane generated upon pyrolysis.

Among the nine rock samples investigated, eight are from the Pokur Formation and one is a
 
 

Figure 9--Bernard-diagram (d¹³C of methane vs. methane over the sum of ethane and propane) with gas data from northern west Siberia and fields for thermogenic and bacterial gas; after Bernard (1978) and Whiticar (1990). Mixing lines are also shown (see text for explanation).
 
 
 
 

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Holocene-age peat (see Table 4 for details). Total masses of methane generated during the experiments range from 21 to 37 mg/g TOC. The generated methane was regularly accompanied by smaller, yet significant, amounts of C₂ and C₃ hydrocarbons. In terms of molecular composition, therefore, the pyrolysis gas exhibited a typical wet gas rather than a dry gas signature. The relevant kinetic data for the generation of methane in relation to its heavier homologs, including ethene and propene, are presented and discussed in detail elsewhere (Schaefer et al., 1999). We emphasize in this respect that the extreme dryness of the naturally occurring gas accumulations in the West Siberian basin was never reached by any of our laboratory experiments (see following sections).

The amounts of generated methane are in accordance with, yet slightly smaller than, those amounts detected in earlier studies with similar analytical equipment for Carboniferous humic coals but are smaller than the masses expected for methane generation under natural conditions (Ungerer and Pelet, 1987; Krooss et al., 1995; Littke et al., 1995). This difference is mainly due to the generation and removal of molecular hydrogen and bitumen in open pyrolysis systems that cannot further react to form methane (and pyrobitumen) at higher temperatures; however, these secondary reactions influence mainly the later stages of methane generation at high temperatures, but are believed to be of minor importance with respect to the early phases of generation. In our study, methane, higher molecular weight hydrocarbon gases, and total bitumen generated were monitored during the experiments at different heating rates. Bitumen measurements revealed a distinct, yet small, generation at very low temperatures, i.e., in a similar range when first traces of methane generation were observed. This first bitumen commonly is rich in heterocompounds, such as sulfur- and oxygen-bearing molecules, and does not undergo expulsion (Welte, 1987; Burrus et al., 1993).

As a result of laboratory open-system pyrolysis, based on the kinetic evaluation of experimental "methane generation curves," we made some predictions in regard to natural gas generation in west Siberia. The results revealed differences with respect to the onset of methane generation and temperatures of maximum methane generation. For a constant geological heating rate of about 5.3°C/m.y., the onset of methane generation is predicted to occur between 80 and 130°C (Table 4). Temperatures well below 100°C are significant only for rocks that are presently at depths of less than 1200 m. These samples are predicted to produce greater (yet still small) amounts of methane at temperatures below 100°C. As expected, samples from greater depth have already realized their "early"
 
 

Table 4. Methane Generation Temperatures for a Geologic Heating Rate of 5.3°C/10⁶ yr a (= 10^-11°C/min) as Calculated from Kinetic Parameters of Open-System Pyrolysis Experiments at Different Heating Rates
 
 

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methane generation potential and therefore do not produce additional methane at such low temperatures; however, this possible loss of the early methane generation potential should not affect the temperatures of maximum methane generation, which do range between 159 and 201°C for the heating rate of 5.3°C/m.y. and correlate positively with the temperatures of early methane generation (Table 4). This coincidence hints toward some differences with respect to structure and composition of the coaly organic matter in the Pokur Formation; nevertheless, the data indicate that some of the organic matter in the Pokur Formation starts to generate methane at temperatures below 100°C if a heating rate of 5.3°C/m.y. is assumed. For a smaller heating rate of 0.53°C/m.y., an even earlier start of methane generation (by about 15-20°C) is predicted from the kinetic data.

Application of these data to the temperature history of the Pokur Formation is shown in Figure 10A, in which the activation energy distributions for two of the samples are shown. Both samples (2 and 4) are from the Cenomanian part of the Pokur Formation at about the same depth (1170 and 1250 m, respectively); nevertheless, they differ strongly with respect to methane generation (Figure 10). Based on the temperature history of well Urengoy 411, sample 2 would have generated up to the present day a cumulative amount of 1.4 mg methane/g TOC, whereas sample 4 would only generate 230 µg methane/g TOC. This variability was also found for the other samples. Sample 3 and peat sample 1 would produce 800-900 µg methane/g TOC, and samples 5-9 would produce between 80 and 350 µg methane/g TOC. In conclusion, the least mature samples (1-3) seem to have a greater potential to generate methane at low temperatures than the other samples. Another important result of these pyrolysis experiments concerns the generation of higher molecular weight hydrocarbon gases. The ratio of total methane over C₂ and C₃ gases is in
 
 

Figure 10--(A) Activation energy distribution for samples 2 and 4 (frequency factors 2.1 X 10¹¹/s and 9.1 X 10¹³/s, respectively) and (B) temperature history as calculated for well 411 in the area of the Urengoy gas field (temperatures for base and top of Pokur Formation, respectively, as a function of geologic time) with calculated cumulative generation curves for the base (dashed line) and the top (dotted line) of the Pokur Formation. Results are shown for sample 2 (with maximum yields of early generated methane) and sample 4 (with low yields of early generated methane).
 
 

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the range of 2 (between 1 and 3, by weight), i.e., a rather wet gas is observed. This wet gas production also occurs in the initial phase of the gas generation process according to the pyrolysis data. This observation does not fit the observed dryness of the natural gases in the Cenomanian reservoirs of northern west Siberia. The amount of generated early thermogenic methane, however, could account for a significant contribution to the reservoirs. The problem of whether laboratory kinetic experiments, such as open-system programmed temperature pyrolysis, can simulate the geological processes with sufficient accuracy is still a matter of discussion in the scientific community. Coal scientists found, for instance, that high-molecular-weight gases may be overrepresented in the pyrolysate of coals. Jüntgen and Klein (1975) demonstrated the dependence of tar and methane generation from coals as a function of heating rate with the tendency to higher dryness of the gas with decreasing heating rate. This means that, particularly for coaly organic matter, the secondary gas generating potential during open-system pyrolysis may be underestimated (Schenk et al., 1997). From the chemical point of view, thermal cracking proceeds through free radical intermediates with methane coming from the methyl radical. Because the methane radical is the least stable compared to its homologs, formation pathways leading to methane encounter higher activation energies, which may explain that thermal cracking of oil or source rocks gives predominantly wet gas, not dry gas. Recently, Mango (1996, 1997) and Mango et al. (1994) presented another hypothesis that could explain the dryness of the west Siberian gases; they argued that the observed predominance of methane could be caused by catalytic rather than uncatalyzed thermogenic processes, although a number of related questions remained unanswered.

The reaction kinetic extrapolation of isotope data of methane from laboratory pyrolysis to geologically relevant heating rates indicates for the first time that thermogenic methane generated at low temperatures from coaly organic matter can be isotopically as light as methane within gas
 
 

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reservoirs of the Pokur Formation (Cramer et al. 1998). Isotope data were measured on gases produced by open-system pyrolysis at a heating rate of 2°C/min from samples 4 and 5 (Table 4). The respective carbon isotope data of methane are plotted vs. temperature in Figure 11. Measured carbon isotope data from both rock samples display similar trends and do not show a steady increase of d¹³C values with increasing temperature as predicted from empirical relationships (e.g., Faber, 1987; Shen Ping et al., 1988; Berner, 1989). In particular, the methane generated at lowest temperatures is isotopically characterized by very low d¹³C values (compare fields of thermal methane generation in Figures 5 and 6). The conversion of these measured isotope values from laboratory pyrolysis to geological heating rates leads to isotopically unusual light methane (d¹³C of -43 to -53?).

In summary, the pyrolysis measurements revealed early methane generation from the least mature samples but cannot explain the extreme dryness of the natural gases. The early methane generated during pyrolysis bears a very light carbon isotope signature similar to that of the natural gases in northern west Siberia. Whether the mass of this early thermogenic, isotopically light methane is sufficient to explain the huge gas fields such as Urengoy remains to be answered (see discussion).

DISCUSSION

Gas Generation and Accumulation of Methane

About one-third of the known conventional natural gas reserves have been found in the northern part of the West Siberian basin. More than 90% of this gas accumulated in different layers of almost unconsolidated sands in the Aptian-Cenomanian Pokur Formation (Rovenskaya and Nemchenko, 1992). The accumulation of these giant gas pools was favored by large volumes of source rocks, migration pathways, large-scale tectonic structures, the presence of laterally continuous cap rocks of low permeability, and a rather young age of the gas accumulations (Ermakov and Skorobogatov, 1986). As has been discussed, the gas in the Pokur Formation consists of almost pure methane characterized by d¹³C values of between -45 and -55?. The origin of this methane was a matter of intense scientific debate since the first discoveries were made in the late 1960s.
 
 

Figure 11--Measured d¹³C values of methane generated during open-system pyrolysis experiments from sedimentary organic matter of the Pokur Formation as a function of temperature.
 
 

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The geochemical gas data presented here allow the identification of three gas families that occur in distinct stratigraphic units: in the Jurassic, in the Neocomian, and in the Pokur Formation. The geochemical characteristics of these gas families strongly argue for different generation processes within the respective stratigraphic units. The occurrence of the deeper gas from the Jurassic and Neocomian is caused by thermal gas generation. In contrast, a deep origin of the gas from Triassic or Jurassic rocks with subsequent vertical migration into the Pokur Formation is highly unlikely.

Most of the gas within the Pokur Formation was generated within the Cretaceous interval, but established models of gas generation fail to explain the gas geochemistry and the huge volume of gas trapped in the reservoirs. The uppermost Jurassic and Cretaceous sediment in west Siberia is characterized by two clay sequences that act as migrational barriers. The upper barrier is represented by the Upper Cretaceous marine clays with the Turonian Kuznetsov Formation at the base. This several-hundred-meter-thick sequence is an effective seal for the huge gas pools in the Cenomanian. The lower barrier consists of marine rocks of Late Jurassic-Early Cretaceous age. This barrier also restricts fluid movement, resulting in the development of very high pore pressure in Jurassic rocks (Figure 4); consequently, gas migration through these rocks also is inhibited. Indeed, gas composition data (Table 2; Figures 6, 9) indicate different sources above and below the Jurassic-Cretaceous boundary. Between both barriers, in Neocomian rocks and in the Pokur Formation, the lithology is quite complicated with intercalated sand, silts, clays, and coal seams that are laterally discontinuous. The gas data indicate for this sequence mixing of genetically different gases consisting of quantitatively minor, thermally generated gas from the Neocomian and predominant, isotopically light methane from another source.

In the light of these new findings, two models on the generation of the gas in the Pokur Formation remain--bacterial generation and thermal generation at moderate temperatures within the Pokur Formation. Arguments in favor of a bacterial origin of most of the Cenomanian gas include the high porosity and permeability of the rocks of the Pokur Formation, low temperatures, and the high concentrations of organic matter within the Pokur Formation (Rice and Claypool, 1981; Rice, 1993).

One problem in arguing for a bacterial origin of the Cenomanian gas is related to the timing of methane generation. Without doubt, bacterial methane generation must have been an active process during the Cretaceous when the Pokur Formation was deposited; however, much of this 90 m.y. old gas was not preserved, partly because of the lack of an efficient cap rock prior to the deposition of the Kuznetsov Formation and partly because gas accumulations were destroyed by diffusion (see Krooss and Leythaeuser, 1997; Nelson and Simmons, 1997). Several workers recently described a deep biotic assemblage that includes methanogenic bacteria (Murphy et al., 1992; Parkes et al., 1994; Stevens and McKinley, 1995; Tseng et al., 1998); however, favorable temperatures for bacterial activity (<70°C) were exceeded within the Pokur Formation during the middle Tertiary when maximum burial occurred (Figure 3). In the Holocene, temperatures for bacterial activity are more favorable (Figure 3). Thus, gas generation and accumulation by bacterial processes are rather difficult to envisage for the Neogene because some kind of reactivation of bacteria has to be assumed. Topographically driven groundwater flow provides a possible mechanism for the introduction of ancient surface and subsurface bacteria into deep environments (Morley et al. 1998; Tseng et al. 1998). As stated by Surkov and Smirnov (1994) and Cramer (1997), rapid flow of groundwater from the southern parts of the basin to the north exists; however, neither carbon isotope data nor hydrogen isotope data of methane fall into the empirical field of bacterial gas generation (Figures 5, 6), but plot between typical bacterial and thermogenic gas or even in the thermogenic field. In summary, the assumption of biogenic gas generation as a source of the Cenomanian gas cannot be ruled out, but it is not supported by the isotope data.

According to our experiments, early thermogenic gas generation, as suggested by Galimov (1988) for example, can yield isotopically light methane (d¹³C about -50?) similar to that found in the Cenomanian reservoirs. This light methane, however, is generated in the earliest stage of methane generation, when only a very small fraction of the total methane generation potential is realized. Clearly, early thermogenic gas generation in the marginally mature Pokur Formation directly below the giant Urengoy accumulation is insufficient to supply all the gas in the reservoir.

This mass balance question can be tested. If we estimate the average organic carbon content of the Pokur Formation to be 2% and if we find that in the entire 1400-m-thick Pokur Formation, 1 mg methane/g organic carbon was generated as early thermogenic methane, a total methane generation of 3-3.5 X10¹⁴ g results. This value is less than 10% of the mass of methane found in the reservoir, although the assumptions about organic carbon content and methane generation are regarded as
 
 

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optimistic (see previous sections); therefore, a much larger drainage area and lateral migration have to be considered.

Methane generated from dispersed organic matter will more readily dissolve in water and be adsorbed on organic matter than migrate in gas phase toward the reservoirs. If no additional factors, such as lateral migration, are considered, this approach would lead to even smaller quantities of early thermogenic methane available to fill the Cenomanian reservoirs. Based on recent pressure and temperature conditions characterizing the Pokur Formation in the Urengoy area, about 1.5-2 g of methane can be dissolved in 1 L of water, whereas the generation potential for early thermogenic methane from sedimentary organic matter is only about 0.25 g calculated for a rock volume containing 1 L of pore water. Thus, much or most of the early thermogenic gas would be expected to be dissolved in water, provided that the water was not already saturated. The recently observed saturation of the groundwater in the Pokur Formation by isotopically light (d¹³C = -50?) methane can be explained only if the water became saturated during lateral migration. This methane originated from either early thermogenic or bacterial gas generation within the Cretaceous aquifer south of the present gas accumulations. For this area, bacterial gas generation seems to be more likely than for the Urengoy area, because no deep burial (and subsequent uplift) has to be assumed.

Reservoir Filling

In contrast to the common concept of migration of gas (or oil) as separate phase (e.g., Tissot and Welte, 1984), we suggest that the gas in northern west Siberia was transported in aqueous solution to the north toward the present-day
 
 

Figure 12--Cenozoic burial history in the West Siberian basin, after Surkov and Smirnov (1994).
 
 
 
 

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accumulations. As already stated by Kruglikov (1967), Kortsenshtejn (1970, 1974), and Surkov and Smirnov (1994), long-distance lateral migration through the West Siberian basin is strongly favored by the hydrodynamic situation that leads to flow of water through the Pokur Formation and Neocomian rocks (see Figure 2). A detailed hydrogeological model concerning the hydrodynamics and its impact on gas accumulation is described in Cramer et al. (1999). Here, we present the main hydrogeological features of the West Siberian basin and the principal mechanisms of gas entrapment.

The hydrogeology of west Siberia is governed by one large hydrodynamic system, the Cretaceous aquifer, which is sandwiched between the two aquitards represented by the clays and claystones of the Kuznetsov Formation and the fine-grained Upper Jurassic and lowermost Cretaceous rocks. As has been previously mentioned, the lack of hydraulic communication between the hydrodynamic regimes of Cretaceous and Jurassic strata is reflected by the large differences in pore pressure (Figure 4).

Fluid flow within the Cretaceous aquifer was initiated in the Eocene when the depocenter moved from the northern to the southern part of the basin (Figure 12). From this time on, a pressure gradient developed within the Cretaceous aquifer; this gradient forced the pore water to flow from south to north. Present-day fluid flow velocities of up to 25 km/m.y. have been calculated (Cramer et al., 1999). Water flowing through the basin continually took methane into solution until saturation was reached. In the northern part of the basin, hydrocarbon generation slowed with uplift and erosion in the late Tertiary; however, in the southern and central part of the basin, thermogenic gas generation is active today and there may be a significant contribution from bacterial activity.

The water in the Cretaceous aquifer in the central northern part of the basin is almost saturated with gas having a composition similar to the gas in the main reservoirs at the top of the aquifer (Zor'kin and Stadnik, 1975). Additionally, the carbon isotope signature of the dissolved methane is similar to that of methane in the reservoirs (Zor'kin et al., 1984). These findings support the genetic association of the large gas accumulations and the groundwater flow in the Cretaceous aquifer (Figure 13).

The main mechanism for degassing of the groundwater within the Cretaceous aquifer is the decrease of pressure and temperature due to the Neogene uplift. With an estimated value of 1000 m for the uplift (a smaller value was assumed in
 
 

Figure 13--Schematic representation of the hydrogeology of the West Siberian basin and possible effects on the accumulation of gas in the northern part of the basin; after Surkov and Smirnov (1994).
 
 

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Figure 3), this effect can explain even the largest gas accumulation on earth, the Urengoy gas field (Cramer et al., 1999). Up to 150 g of methane degassed from 1 m³ of rock within the Cretaceous aquifer as a consequence of the pressure decrease. Another important mechanism is the difference in the hydraulic pressure between the southern and the northern margins of the gas traps. This pressure gradient forces the water to flow and also helps in the continuous degassing of the water.

This unconventional model of reservoir filling by exsolution of gas from groundwater is the only process that can explain the large gas fields of northern west Siberia. The amount of gas exsolved from groundwater decreases with increasing depth due to buoyancy-driven upward migration into the reservoirs after release from aqueous solution. The fact that, according to the mixing calculations based on the Bernard diagram (see Figure 6), the released, isotopically light methane is present even in the Neocomian (although in smaller quantities than in the Cenomanian) indicates that indeed most of the Lower Cretaceous acts as one large aquifer.

CONCLUSIONS

The giant gas accumulations in northern west Siberia originate from methane generation inside the Cenomanian and Lower Cretaceous formations. Deeper source rocks contributed to gas accumulations in the Jurassic and Neocomian but had only a marginal impact on the filling of the Cenomanian gas pools. Although the gas-generating mechanism (early thermogenic, bacterial, even catalytic) is still a matter of controversy, the importance of the large-scale hydrodynamic system in the Cretaceous rocks upon the filling of the reservoir is evident. Gas generation and solution in water south of the recent gas fields, combined with long-distance lateral migration of gas dissolved in water and gas release due to pressure drop in the northern part of the basin, are the factors responsible for the accumulation of a third of the world gas reserves in northern west Siberia. 

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