川东北地区长兴组酸性气藏中硫化氢来源及成因

High H₂S concentrations found in gas reservoirs within the Upper Permian Changxing Fm(P₃ch)and Lower Triassic Feixianguan Fm (T₁f) 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 P₃ch Fm. This cast doubts over the accepted model for the souring process of the P₃ch 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 T₁f Fm, but were generated in situ through TSR; reactive sulfates were mainly derived from cross-formational brines migration from the T₁f Fm, but with insignificant contributions from the Late Permian P₃ch seawater and sulfate-rich brines within the Lower Triassic Jialingjiang Fm and Middle Carboniferous Honglong Fm formations, and the down-migration of T₁f brines may have resulted in the P₃ch TSR calcites having positive Eu anomaly, high Sr contents (up to 7767 ppm) and Ba contents (up to 1279 ppm) close to the T₁f TSR calcite, and ⁸⁷Sr/⁸⁶Sr ratios (0.70724~0.70755) 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-penecontemporaneous stage and differential compaction flux during early-mid burial stage might serve as the main migration mechanisms; (4) CAS released during dolomization was another possible sulfate source, which brought the δ³⁴S values of H₂S and bitumen a little lower than those of the T₁f Fm.