川南地区中二叠统茅口组成岩流体来源及演化过程

四川盆地南部中二叠统茅口组碳酸盐岩经历了复杂的孔隙流体活动,目前从微观地球化学的角度精细分析其多期成岩流体与烃类流体之间耦合关系的研究尚不多见。为此,基于对该区茅口组的岩心观察与薄片观察,通过岩石学、稀土元素、碳氧同位素及流体包裹体分析,结合区域构造演化背景,研究了茅口组各成岩作用阶段的成岩环境、流体来源和流体演化过程。研究结果表明:①该区茅口组经历"(准)同生—早成岩期混合水环境下的方解石胶结作用→表生期大气淡水环境下的溶蚀作用→中—晚成岩期地层水环境下的方解石与白云石胶结、交代作用与酸性流体溶蚀作用"的成岩演化过程;②沉积成岩过程流体来源包括海水、大气淡水、烃类流体及深部(热)流体,其中偏氧化海水表现为左倾型石灰岩稀土元素配分型式、δ~(13)C特征与全球古海水特征类似,酸性热液流体具有铕值正异常、δ~(13)C值明显负偏特征,大气淡水参与成岩流体活动证据为裂缝与溶蚀孔洞碳酸盐岩胶结物δ~(18)O值明显负偏;③茅口组经历了多期油气充注,其中晚二叠世—早三叠世茅口组抬升遭受暴露淋滤,同时峨眉地幔柱异常热事件使地层发生白云石化和油气充注,该时期大气淡水与有机酸溶蚀、白云化作用以及裂缝化作用对茅口组储层起到了建设性作用,侏罗纪以来的压溶及粗亮晶方解石胶结作用对茅口组储层具有破坏作用,与构造裂缝相关的岩溶储层更有利于后期天然气的聚集成藏。

Source and evolution of diagenetic fluid in the Middle Permian Maokou Formation in the southern Sichuan Basin

Carbonate rocks in the Middle Permian Maokou Formation in the southern Sichuan Basin have experienced complex pore-fluid activities. So far, however, the coupling relationship between multi-stage diagenetic fluids and hydrocarbon fluids has been less researched from the perspective of microscopic geochemistry. For this reason, this paper firstly carried out core observation and thin section analysis on the Maokou Foramtion of this area. Then, petrology, rare earth elements, carbon and oxygen isotopes and fluid inclusions were analyzed. Finally, diagenetic environment, fluid source and fluid evolution of Maokou Formation in different diagenetic stages were studied in the regional structure evolution setting. And the following research results were obtained. First, the Maokou Formation in this area experiences the diagenetic evolution process of "(pene)contemporaneous–eogenetic calcite cementation in the mixed water→epigenetic dissolution in the atmospheric freshwater→phyllomorphic calcite and dolomite cementation, metasomatism and acidic-fluid dissolution in the formation water". Second, the fluid sources in the process of sedimentation and diagenetism include seawater, atmospheric freshwater, hydrocarbon fluids and deep (thermal) fluids. Among them, the oxidized seawater is characterized by left-lead limestone REE, similar δ13C features to those of the global paleo-seawater, Eu positive anomaly of acidic hydrothermal fluid and obvious negative δ13C. And the evidence for the participation of the atmospheric freshwater is that the δ18O of the carbonate cements in fractures and dissolved pores is obviously negative. Third, the Maokou Formation experiences multi-phase hydrocarbon charging. During the Late Permian–Early Triassic, the Maokou Formation was uplifted and exposed to leaching, and the anomalous thermal of the Emei mantle plume led to formation dolomitization and hydrocarbon charging. Dissolution of atmospheric freshwater and organic acid, dolomitization and fracturing in this period play a constructive role for the Maokou Formation reservoir. Pressure dissolution and coarse-grained calcite cementation since the Jurassic play a role in destructing the Maokou Formation reservoir. And karst reservoirs associated with structural fractures are more conducive to later gas accumulation.