Methane released from groundwater: the source of natural gas accumulations in northern West Siberia

The West Siberian Basin is the largest continental sedimentary basin on earth and contains about one third of the world's conventional reserves of natural gas. From gas compositional and isotope data this gas can be described as a mixture of thermogenic gas from lower Cretaceous rocks (delta(13)C(1) approximate to -36 parts per thousand, C2+ approximate to 10 vol%) and almost pure methane (98 vol%) with the relatively light stable carbon isotope signature of -51 parts per thousand (delta(13)C(1)) on average. The proportion of the isotopically light methane increases from the lower Cretaceous to the top of the Cretaceous hydrocarbon system (top of the Pokur Formation), where the main reservoirs are situated. The composition and the huge volume of gas is difficult to explain with the established models of gas generation in sediments. The hydrogeology of the basin is determined by thick aquifer systems with an almost basin wide lateral continuity. The largest hydrodynamic system is the Cretaceous aquifer, in which 98% of the natural gas is trapped. The groundwater within the Cretaceous aquifer is saturated with the same isotopically light methane as that found in the reservoirs of the Pokur Formation. Based on petrophysical data a detailed hydrogeological model of the Cretaceous aquifer beneath the largest gas accumulation, the Urengoy gas field, has been developed and the contribution of gas release from groundwater was compared with the volume of gas in the reservoirs. The potential methane release from groundwater has been evaluated as a result of: (1) cooling of sediments in Quaternary limes, (2) the lateral gradient of the hydraulic pressure under the gas field and (3) the pressure and temperature decrease due to Cenozoic uplift. The role of Quaternary cooling is insignificant and, according to our calculations, the contribution of the effect by groundwater flow is small. The volume of isotopically light methane released from groundwater due to a Neogene uplift is very large and can explain the entire gas in place. The source of this gas must be located south of the gas field province. (C) 1999 Elsevier Science Ltd. All rights reserved.