THE GENERATION AND EXPULSION OF GASES IN YA131 GAS-FIELD, SOUTH CHINASEA - IMPLICATION OF LABORATORY PYROLYSIS RESULTS

Laboratory pyrolysis methods were used to determine the source and the rmal maturity of the gas accumulation in the Ya131 gas field, South Ch ina Sea. The mudstone source rock samples used in the pyrolysis experi ments were selected from the Yacheng and Meishan Formations in the Yin g-Qiong Basin. The total amount of gases generated and expelled from t he Yacheng Formation mudstone is larger than that from the Meishan For mation mudstone, suggesting that the gas generation potential of the Y acheng Formation is higher than that of the Meishan Formation. The sim ilarity between the gases from the Ya131 field and the gases produced at the higher pyrolysis temperatures used in the maturation study, in terms of wetness, the amounts of C-1, C-2, C-3, and the percentages of C-1, C-2-C-4 and C-5-C-7, implies that the gases of the Ya131 field a re high maturity gases. However, there are distinct differences betwee n the normalised molecular compositions of the C-6 and C-7 species of the gases from the Ya131 field and those of the pyrolysis gases produc ed in the laboratory. Comparison of the stable carbon isotope composit ion of the n-alkanes, pristane (Pr) and phytane (Ph) in the saturated hydrocarbon fraction of the condensate from the Ya131 field with that of rock extracts from the Yacheng, Meishan, Ying-Huang Formations indi cates no definite correlation between the condensate and the extracts. These observations suggest that there may be additional source rocks for the Ya131 field, existing within the deeper part of the basin. (C) 1998 Elsevier Science Ltd. All rights reserved.