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 (delta(13)C approximate to -45 to -55 parts per thousand). 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.