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Origin of H2 S in sour gas reservoirs in the Upper Permian Changxing Formation in northeastern Sichuan Basin
Pages: 183-192
Year: Issue:  2
Journal: Oil & Gas Geology

Keyword:  CASdifferential compactionTSRH2Ssour gas reservoirChangxing Formationnortheastern Sichuan Basin;
Abstract: High H2 S concentrations found in gas reservoirs within the Upper Permian Changxing Fm ( P3 ch) and Lower Triassic Feixianguan Fm ( T1 f) are generally thought to be formed via a process known as thermochemical sulfate reduction (TSR).However,unlike the T1f Fm,rare gypsum/anhydrite beds or anhydrite nodular were found in the P 3ch Fm.This cast doubts over the accepted model for the souring process of the P 3 ch reservoirs.This study provides an analysis of palaegeopressure and carrier system within this system using carbonate-associated sulfate(CAS),isotopes,trace and rare earth element data.Results shows that,the H2S of high concentrations within the P3ch reservoirs were not derived from downward migration of sour gas from the T 1 f Fm,but were generated in situ through TSR;reactive sulfates were mainly derived from cross-formational brines migration from the T 1 f Fm,but with insignificant contributions from the Late Permian P3 ch seawater and sulfate-rich brines within the Lower Triassic Jialingjiang Fm and Middle Carboniferous Honglong Fm formations,and the down-migration of T1 f brines may have resulted in the P 3 ch TSR calcites having positive Eu anomaly , high Sr contents(up to 7 767 ppm)and Ba contents(up to 1 279 ppm)close to the T1f TSR calcite,and 87Sr/86Sr ratios (0.707 24~0.707 55 ) more radiogenic than those of the late Permian seawater;the cross-formational brine flow mainly occurred during sedimentary stage or the early stages of diagenesis ,and seepage reflux during contemporaneous-penecon-temporaneous stage and differential compaction flux during early -mid burial stage might serve as the main migration mecha-nisms;④CAS released during dolomization was another possible sulfate source ,which brought the δ34 S values of H2 S and bitumen a little lower than those of the T 1 f Fm.
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