Gas generation and accumulation in the West Siberian basin

The largest accumulations of natural gas occur in shallow Cenomanian reserv oir 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 pas t, To evaluate these theories, a large set of gas and rock samples was stud ied using organic geochemical methods. Our results reveal that early thermo genic methane generation can explain the isotopic composition of the natura l gas in this area, but not the extreme dryness. In particular, the isotopi c composition of early thermogenic methane generated upon laboratory pyroly sis is in accordance with that of the methane in the large gas accumulation s of northern west Siberia (delta(13)C approximate to -45 to -55 parts per thousand). The quantities of early thermogenic gas generated according to o ur calculations, however, are not sufficient to explain the largest accumul ations. 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 souther n to the northern part of the basin enlarged the effective catchment area o f the gas fields.