3d Basin Simulation and Hydrocarbon System Analyses of the Northern West Siberia Basin
Scott A. Barboza3, Lev M. Burshtein2, Erik Fjellanger1, Martine J. Hardy1, Alexey E. Kontorovich2, and Valery R. Livshits2
1Russia Exploration, ExxonMobil Exploration Company, Ermyn Way, Leatherhead, Surrey KT22 8UX, United Kingdom
2Institute of Petroleum Geology and Geophysics, Russian Academy of Sciences Siberian Branch, Koptuga 3, Novosibirsk, 630090, Russia
3ExxonMobil Upstream Research Company, P.O. Box 2189, Houston, TX 77008, USA
The West Siberia Basin is the world’s largest intra-cratonic basin, with sediment fill of up to 10 km of Mesozoic and Cenozoic clastic rocks. Basement is composed of Paleozoic accretionary crust. Permian-Triassic rifting is accommodated along northward striking extensional grabens. The rifted basin was filled by fluvio-deltaic sediments prograding from the south and east, punctuated by periods of marine transgression from the north. Cenozoic basin inversion formed traps for petroleum [Kontorovich et al., 1975; Surkov and Zhero, 1981; Peterson and Clarke, 1991].
The combination of rifting, basin fill, abundant source and reservoir rocks, Cenozoic structuring and recent glaciation provide a natural laboratory to study control on thermal evolution of the basin and timing of hydrocarbon generation.
A regional high-resolution 3D basin simulation using structural grids from 2D seismic and well data was used to model the thermal evolution in the northern part of the West Siberia Basin. Twenty four intervals of the Mesozoic-Cenozoic sedimentary section were mapped in detail. Cenozoic erosion was assessed and incorporated in the modelling. Lithology was derived from Environment of Deposition (EOD) maps based on well logs and outcrops. Palaeo-bathymetry was also derived from the EOD maps, utilising available paleontological data. Basal heat flow was modelled by calibration to temperature and vitrinite reflectance measurements from numerous wells. Kinetic activation energies were derived from measurements performed on West Siberia rock samples.
Many giant gas fields have been discovered in the Cenomanian Pokur Fm reservoir in the northern West Siberia basin. The origin of the large amounts of very dry, isotopically light gas is still an enigma, albeit extensively addressed in literature [Kortsenshteyn, 1977; Vasilyev et al., 1979; Rice and Claypool, 1981; Grace and Hart, 1986; Galimov, 1988; Stroganov, 1990; Prasolov, 1990; Rovenskaya and Nemchenko, 1992; Surkov and Smirnov, 1994; Schoell et al., 1997; Nemchenko et al., 1999; Kontorovich et al., 1999; Littke et al., 1999; Cramer et al., 1999; Ulmishek, 2003]. The 3D basin simulation was used to quantify the thermal charge from key source intervals. These are 1) Middle Jurassic marine shales grading southward to deltaic coals and lacustrine shales; 2) Late Jurassic marine bituminous shales; 3) Cretaceous delta plain humic and coaly shales. In addition, the contribution from biogenic gas is discussed.
Following completion of the 3D basin model, the gas volumes expelled from the hydrocarbon kitchen drainage areas of the Leningradskoye, Bovanenkovskoye and Urengoy fields were compared with the gas volumes accumulated in these fields. Base case and high case scenarios were run, and Cretaceous and Jurassic sources were considered. The base case assumed best estimate for kinetic parameters, heat flow, source thickness and Eocene erosion, in addition to Early Paleocene trap timing. The high case assumed more optimistic parameters resulting in larger gas volume generation and entrapment.
The results show that the volumes of early mature gas generated from Cretaceous sources are not sufficient to fill the Cenomanian gas fields of Bovanenkovskoye and Urengoy with both base case and high case sensitivity parameters. However, the Leningradskoye field may have been fully charged by gas generated from Cretaceous sources in the high case scenario with early trap development and no Eocene erosion.
Analysis of the basin model in detail shows that trap timing and Cenozoic uplift/erosion are the key factors controlling the volumes of gas available for accumulation in the Cenomanian fields. Sensitivities performed on source thickness, kinetic parameters and heat flow showed less impact on the gas volumes.
While the Leningradskoye field can be charged by gas from Cretaceous source alone, additional contributions from Jurassic sources are required to charge the Bovanenkovskoye and Urengoy gas fields, and only if high case conditions apply.
This study only evaluates gas available from the drainage areas associated with each gas field. No account is made for gas migrating into the fields through fill and spill mechanisms from adjacent accumulations.
Biogenic gas is likely to have contributed to the Cenomanian gas accumulations. The modeling shows that the Pokur Fm has been exposed to temperatures in the range of 40 to 700C for the last 10 Million years. These conditions are favorable for biogenic methane generation derived from the organic material within the Cenomanian deltaic deposits. The mixing of biogenic gas and thermally produced gas from Jurassic and Cretaceous source rocks may produce an isotopic gas composition comparable to the recorded isotopic ratios of the methane in the Cenomanian gas fields.
In conclusion the study found that Cretaceous terrestrial sources can generate sufficient early thermal gas to charge the Leningradskoye field and potentially other accumulations in the South Kara Sea area. Cretaceous and Jurassic sources together may generate sufficient thermal gas to charge the Bovanenkovskoye and Urengoy fields if favorable conditions apply. Biogenic gas is likely to have contributed to the gas accumulations. Mixing of thermal and biogenic gas could explain the isotopic composition observed for the Cenomanian fields in northern West Siberia basin.
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