Gas Generation and Accumulation in the West Siberian Basin

Abstract

We appreciate the discussion by Murris (2001) and take the opportunity to clarify some of his objections and discuss some of the still open questions in more detail. Murris's main objections to the conclusions of Littke et al. (1999) concern, first, the influence of overpressure in the Lower Cretaceous and Upper Jurassic on hydrocarbon distribution and, second, the model of gas release by exsolution (degassing) from deep ground water as the main mechanism for accumulating the huge gas fields of northern West Siberia. The first objective of Murris's discussion is how the overpressure in Jurassic strata might have influenced the distribution of hydrocarbons in the overlying Cretaceous sediments. Two crucial points regarding this question are the reason and timing of overpressuring and its influence on hydrocarbon movement. We agree with Murris that the effect of compaction disequilibrium is not sufficient to explain the extent of overpressure in the Urengoy area and that hydrocarbon generation in the Bazhenov Formation has contributed to the buildup of overpressure. Two-dimensional basin modeling along a seismic line through the Urengoy field, however, showed that hydrocarbon generation in Jurassic source rocks (Bazhenov and the Tyumen formations) did only generate sufficient overpressure to explain the recent situation if the models were laterally completely sealed. This sealing appears to be highly unlikely. Furthermore, hydrocarbon generation ceased during the Miocene, when the tectonic movement started to lift the source rocks into shallower depths, where lower temperatures were reached. Thus, any overpressure caused by petroleum generation must have been preserved over a period of about 5-20 m.y. In addition, the overpressure is not restricted to the Jurassic Bazhenov Formation but extends into the Cretaceous, including the Achimov sands. Uplift might have contributed to the overpressure, however. Maximum pore pressure from the stage of deepest burial might have …