川东南二三叠系气藏硫化氢成因探讨

通过天然气组分、碳氢同位素组成与薄片、包裹体等资料分析，研究了川东南二-三叠系的长兴组、飞仙关组与茅口组气藏H₂S成因。结果表明，天然气组分以烷烃气为主，非烃含量平均24.68%。其中，烷烃气均偏干，部分样乙烷低于检测下限，可测样乙烷含量0.03%~0.39%，均值0.15%；δ¹³C₁值均偏重，为–28.3‰~–35.2‰，平均–31.1‰，部分样出现δ¹³C₁ > δ¹³C₂倒转。非烃以CO₂和H₂S为主，H₂S含量1.0%~21.7%，平均5.3%。储层中代表原油充注与古油藏裂解的各类沥青较少见，方解石脉中烃类包裹体以含气态烃包裹体为主，含油包裹体少见。结合前人研究成果认为，川东南茅口组几乎不含H₂S的岩溶缝洞型气藏（分布于涪陵北部与綦江地区）未见遭TSR改造迹象；飞仙关组气藏（分布于涪陵北部）、长兴组气藏（主要分布于涪陵北部与中部）与茅口组热液白云岩气藏（分布于涪陵中南部）的H₂S均为TSR成因，部分遭受TSR改造气藏现今H₂S含量较低或为后期被消耗所致；其中，飞仙关组参与TSR反应的烃类以液态烃为主，而其他层位气藏以气态烃（甲烷）为主，这可能与TSR发生时储层中是否存在一定量液态烃有关。TSR反应于各层内独立发生，并非飞仙关组发生后向下“倒灌”。长兴组TSR反应所需硫源或来自飞仙关沉积期卤水的侧向排泄，茅口组硫源或来自相邻层位富含SO₄^2-流体。

An Investigation of the Genesis of Hydrogen Sulfide in Permian and Triassic Gas Reservoirs in Southeast of Sichuan Basin

In this study, by analyzing natural gas components, hydrocarbon isotopic compositions, flakes, inclusions, and other geochemical features, we investigated the genesis of hydrogen sulfide (H₂S) in the gas reservoirs of the Changxing, Feixianguang, and Maokou Formations of the Permian and the Triassic in the southeast of the Sichuan Basin. The results indicated that the natural gas in these formations is mainly constituted by alkane gas, with an average ratio of non-hydrocarbon of 24.68%. The alkane gas of the analyzed samples was consistently dry. The ethane content of some samples was too low to be detected. Among the samples with a detectable content of ethane, the concentration of ethane ranged from 0.03% to 0.39% (0.15% in average). In contrast, the content of δ¹³C₁ was high across the samples, ranged from -28.3‰ to -35.2‰ (-31.1‰ in average); some samples showed a δ¹³C₁ > δ¹³C₂ inversion. The non-hydrocarbon component was mainly constituted by CO₂ and H₂S, with the concentration of H₂S ranging from 1.0% to 21.7% (5.3% in average). Bitumen of various types, which would suggest histories of oil filling, and cracking of paleo-reservoirs was uncommon in the reservoirs. The hydrocarbon inclusions in calcite veins were predominantly in gaseous state, whereas oil inclusions were rare. By combining our results with published ones, we concluded that the fractured vuggy gas reservoirs of the Maokou Formation in the southeast of the Sichuan Basin (occurring in north Fulin and Qijiang), which barely contained any H₂S, showed no signs of thermal sulfate reduction (TSR) alteration. The geneses of the H₂S contained in the reservoirs of the Feixianguang Formation (in north Fulin), the reservoirs of the Changxing Formation (mainly in north and central Fulin), and the hydrothermal dolomite reservoirs of the Maokou Formation (mainly in central and south Fulin) all showed TSR. Some of the gas reservoirs altered by TSR showed a low H₂S content, which is likely due to recent reduction.In the reservoirs of the Feixianguan Formation, the hydrocarbons participating in the TSR reaction were predominantly in liquid state. However, in reservoirs at other horizons, hydrocarbons were mainly gaseous (methane). This probably depends on the amount of liquid hydrocarbons present in the reservoir when the TSR occurred. In addition, it was found that the TSR reactions took place independently in each reservoir other than being affected by those occurring in the Feixianguan Formation. The source of sulfur for the TSR reaction in the Changxing Formation could have come from the lateral brine discharge of the Feixianguang Formation during the deposition period, while that required in the Maokou Formation could have come from the fluid of the adjacent horizon rich in SO₄^2-.