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Nikolai Lopatin1
(1) Geosystem Institute, Geochemistry, Moscow, Russia

Abstract: Supergiant gas accumulations and coal-type kerogen: Geological and geochemical relationship

The largest gas accumulations occur in shallow Cenomanian reservoir of sands or slightly-consolated sandstone of the Pokur Formation in the northern part of the West Siberian Basin. Different hypotheses about this gas genesis were developed: biogenic mechanism, low maturity thermogenic or both, and also vertical-migrated gases from the deepest part of the basin. For testing these models more than 250 gas samples and 300 rock samples were studied by organic geochemical and isotope methods, which, also, include open and close pyrolysis experiments. The results of chemical-kinetics modeling were brought into the context of the basin geological and paleothermal evolution using GALO-basin modeling system in order to reconstruct volumes of methane generation and accumulation on the Urengoi, Yamburg and other supergiant gas fields.

Our results confirm that low maturity coal-type formed gas is favorable model, which can explain many specific gas-compositional characteristics of the biggest gas commercial accumulations in a world. Principal sources of gas are Albian and Upper Aptian coaly particles widely distributed into Pokur Formation. The petrographic composition of Pokur coals includes predominantly vitrinite maceral group (ca. 80 to 85 %), liptinite (1 to 15 %) and inertinite (1 to 22 %). On the base of Pokur Fm. R0 values range between 0.55 to 0.64 %. About 20 % of initial gas-generation potential of Pokur kerogen were realized in geological histories of the Northern West Siberian sub-basin. The isotopic composition of low maturity thermogenic gas generated upon laboratory pyrolysis experiments is in accordance with the isotope composition data set for the Cenomanian supergiant gas pools (^dgr13    C  =  -47 to  -54 0/00). The isotope composition of methane is very indicative. Low maturity of early thermogenic gas-generation from coal-type of kerogen is the most important process for commercial gas accumulation.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana