川南宁西地区五峰组-龙马溪组多期流体活动

Multi-stage fluid activity characteristics ofWufeng-Longmaxi Formation in Ningxi area,southern Sichuan Basin

富有机质页岩中方解石脉体普遍发育,其形成过程和机制对成烃储层和成藏具有重要指示意义,为了弄清川南宁西地区五峰组—龙马溪组的多期流体活动特征与区域构造运动过程以及油气成藏的相关性,基于含油气系统成藏动力学过程的基本原理,通过岩心-薄片观察、碳氧同位素分析、流体包裹体测温等方法,并结合四川盆地及周缘沉积、构造演化特征,分析宁西地区多期次的流体活动特征、恢复埋藏史及热演化史。结果表明:研究区碳氧同位素在深度上具有差异,并受流体来源、沉积环境、温度等因素控制,依据同位素特征,可将目的层系分为2期流体:第一期主要活动于龙一1亚段,为液态烃类流体,深度为3870~3950 m,脉体δ^(13) C为-6.56‰~-1.58‰,平均值为-3.45‰,δ^(18)O为-13.72‰~-11.06‰,平均值为-13.32‰,碳同位素负偏,脉体中流体包裹体均一温度为120~140℃,对应早三叠世的生油阶段;第二期流体主要活动于龙一2亚段,为高温富含甲烷的有机流体,深度为3830~3890 m,δ^(13 )C为-13.27‰~-12.16‰,平均值为-12.72‰,δ^(18)O为-9.06‰~-6.61‰,平均值为-7.84‰,碳同位素严重负偏,脉体中流体包裹体均一温度为160~180℃和200~220℃,对应晚侏罗世油高温裂解生气阶段。2期流体活动时页岩气藏处于高压—超压状态,且沉积环境可影响保存条件。该研究结果可为川南地区页岩气富集成藏及保存条件的研究提供一定理论支持。

Calcite veins are commonly developed in organic-rich shale,and their formation process and mechanism have important indications for hydrocarbon-generating reservoirs and accumulations.In order to clarify the corre‐lation between multi-stage fluid activity characteristics of the target strata and the regional tectonic movement process,oil and gas accumulation,based on the hydrocarbon-bearing system accumulation dynamics,through core-slice observation,carbon and oxygen isotope analysis,fluid inclusion temperature measurement,combined with the sedimentary and structural evolution characteristics of Sichuan Basin and the surrounding areas,multi-phase fluid activity characteristics,recovery burial history and thermal history in Ningxi area were analyzed.The results show the carbon and oxygen isotopes are different in depth,they are controlled by fluid sources,depositional en‐vironment,temperature,etc.,and they can be divided into two stages of fluids based on isotope characteristics.The first stage of fluid is mainly active in the Long-11 sub-member,which is a liquid hydrocarbon fluid with a depth of 3870-3950 m,the range ofδ^(13)C in the vein is-6.56‰--1.58‰,with an average value of-3.45‰,and the range ofδ^(18)O is-13.72‰--11.06‰,with an average value of-13.32‰.The carbon isotope is depleted,and the homogenization temperature of the fluid inclusion in the veins is concentrated at 120-140℃,corresponding to the oil-generating stage of the Early Triassic.The second stage of fluid is mainly active in the Long-12 submember,which is a high-temperature methane-rich organic fluid,with a depth of 3830-3890 m.Theδ^(13) C ranges from-13.27‰to-12.16‰,with an average value of-12.72‰,the range ofδ^(18)O is-9.06‰--6.61‰,with an average value of-7.84‰.The carbon isotope is severely depleted,and the homogenization temperature of the fluid inclu‐sions in the veins is concentrated at 160-180℃,200-220℃,corresponding to the oil pyrolysis gas generation stage in Late Jurassic.The shale gas reservoir was in a high-overpressure state during the two stages of fluid activ‐ity,and the depositional environment affected the preservation conditions.The research results can provide certain theoretical support for the study of shale gas accumulation and preservation conditions in southern Sichuan Bain.