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Geological Society, London, Special Publications; 1998; v. 141; p. 157-167;
DOI: 10.1144/GSL.SP.1998.141.01.10
© 1998 Geological Society of London

Geochemical modelling in an organic-rich source rock: the Bazhenov Formation

J. A. Hegre1, J. L. Pittion1, J. P. Herbin2 & N. V. Lopatin3

1 TOTAL, TEP/DE/DPN, Tour Total, 24 cours Michelet, Cedex 47, 92069 Paris La Défense, France
2 IFP, 1-4 av. de Bois-Préau, BP 311, 92506 Rueil Malmaison Cedex, France
3 The Russian Federation National Geosystems Institute, Varshavskoye shosse 8, 113105 Moscow, Russia

Three wells located in the West Siberian basin have a high density of geochemical information (closely spaced samples) from the Upper Jurassic Bazhenov Formation. The nature of the organic matter is homogeneous and is classified as a marine type II (Hydrogen Indices up to 700 mg of HC g–1 of TOC). The organic content is very high with TOC values up to 20 %. The source rock potential is also very good (S2 up to 100 kg HC tonne–1 of rock). The maturity level in the wells varies from immature (Tmax around 435 °C) to near the end of the oil window (Tmax around 455 °C). This situation is somewhat puzzling since the source rock is at the same depth in the three wells (between 2800–2900 m). The IFP 1-D software GENEX, which is a maturity model that integrates subsidence, thermal reconstruction, hydrocarbon generation and expulsion, was used in order to:

(a) test a geological hypothesis, which could reasonably explain this situation;
(b) investigate the accuracy of the model’s ability to calibrate with geochemical data;
(c) test the applicability of calibrating the expulsion saturation threshold from S1 and PI data. The very high S1 values with respect to the immaturity are also discussed. Model results indicate that different expulsion saturation thresholds are required in order to calibrate S1 and PI data. This is probably related to an inadequate definition of the formation’s porosity.