West Siberian gas accumulations - the origin of methane and losses to the atmosphere: ABSTRACT

Abstract

West Siberia is the most prolific gas producing sedimentary basin of the world. Nevertheless the problem of the origin of these large gas accumulations has not yet been solved. Late thermogenic gas generation from Jurassic strata (usually 3500 m and deeper) and early thermogenic or bacterial gas generation from Cenomanian to Aptian strata (usually at 600 to 2500 m depth) are the processes currently in discussion. Organic-geochemical analyses of 150 core samples as well as compositional and isotopic data of about 120 gases provided the database of this study. The Cenomanian reservoirs contain almost pure methane (average 98.6% CH[sub 4]) with [delta][sup 13]C of methane as light as -55[per thousand], while the gases from Neocomian and Jurassic reservoirs show more positive values, which are typical for high maturity source rocks. A model of linear mixing between Cenomanian and Jurassic gases fits fairly well with the observed compositional trends. This model indicates that up to 90% of the Cenomanian methane was probably generated in the Cenomanian-Aptian sequence. The question of bacterial versus early thermogenic generation will be discussed. Wireline log interpretation allowed the detection of up to three layers of permafrost in this region, which might be effective seals to preventmore » upward gas migration. Surface geochemical measurements show no detectable amounts of methane diffusing from the reservoirs into the atmosphere.« less