川东北地区长兴组酸性气藏中硫化氢来源及成因

Origin of H_2S in sour gas reservoirs in the Upper Permian Changxing Formation in northeastern Sichuan Basin

川东北地区长兴组（ P3 ch）和飞仙关组（ T1 f）天然气藏中检测出的高浓度H2 S气体目前被认为是硫酸盐热化学还原作用（ TSR）成因。然而，长兴组内并无类似于飞仙关组的膏岩层或硬石膏结核发育，因此长兴组气藏的酸化过程成疑。通过针对研究区地层古压力以及输导体系方面的探讨，并结合碳酸盐岩晶格硫酸盐（ CAS）含量、同位素以及稀土微量元素分析，研究认为：①长兴组气藏中高浓度H2 S气体不太可能由飞仙关组酸性气藏“倒灌”形成，而是由本地地层中发生的TSR作用所导致；②嘉陵江组（ T1 j）和黄龙组（ C2 h）沉积时期蒸发性卤水以及长兴期海水并非主要来源，而飞仙关期蒸发性卤水的倒灌为长兴组TSR反应的发生提供了主要的SO2－4来源，并导致其内的TSR成因方解石具有与飞仙关组方解石相似的正Eu异常、高Sr（高达7767×10－6）和Ba含量（高达1279×10－6）以及相对同时期海水较高的87Sr／86Sr比值（0.70724～0.70755）；③卤水穿层流动主要发生于沉积期或成岩早期，沉积－准同生期渗透回流作用和早－中埋藏阶段差异压实作用为卤水运移的主要机制；④长兴组白云岩化过程中释放的CAS也可能是重要的SO2－4来源，并导致该层位气藏中H2 S及储层沥青δ34 S值较飞仙关组略有偏负。

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.