雪峰隆起西缘页岩气构造保存条件的古流体评价

Paleofluid as Indicator of Shale Gas Tectonic Preservation in the Western Margin of Xuefeng Uplift

古隆起周缘页岩生烃时间晚、热演化程度低,已成为中扬子地区油气勘探的重要领域。本文对雪峰古隆起西缘典型井JD1井寒武系页岩脉体系统采样并开展流体地球化学与包裹体分析,从构造保存的角度探讨了影响研究区页岩气富集的主控因素。研究结果认为(1)牛蹄塘组页岩方解石脉FeO、MnO组分自下而上增高,方解石脉δEu也整体具有向上增加的趋势,与牛蹄塘组页岩Fe、Mn元素含量、δEu值自下而上呈降低的趋势刚好相反,表明下部古流体的还原性弱于上部。(2)包裹体群组分中CH_(4)含量、包裹体类型比例中甲烷包裹体比例都指示牛蹄塘组上部页岩的含气饱和度高于下部,显示下部裂缝系统是页岩与外界气水交换的通道。(3)以包裹体类型、均一温度和纯CH_(4)包裹体压力计算为基础,揭示了牛蹄塘组在中侏罗世处于超压状态,压力系数最大为1.4;中‒晚侏罗世牛蹄塘组下部水溶液沿破碎带侵入页岩时的埋深为4~4.8 km,致使页岩含气饱和度明显降低、含水饱和度明显上升。雪峰古隆起形成时间早,但是在缺少刚性基底保护的条件下,印支期以来构造变形强度大,大型断裂发育使之碎片化,大型断裂与牛蹄塘组底部破碎带形成了连通的疏导体系并持续开启,成为页岩气散失的直接通道,导致该区构造保存条件差,不利于页岩气的富集成藏。

The shales around paleo-uplifts have become the targets of petroleum exploration in the middle Yangtze region because of the late hydrocarbon generation and relatively low maturity of the source rocks.In this paper,geochemical characteristics and fluid inclusions from the hydrothermal veins in the Niutitang Formation from the Well JD1 in the Xuefeng paleouplift are reported,and the tectonic preservation and main control factors of shale gas enrichment are discussed.The following results were obtained:(1)FeO,MnO contents andδEu values of calcite veins in the Niutitang Formation increase from bottom to top,which is quite contrary to the trend of the shales,indicating the paleo-fluid in the upper strata is more reducing.(2)Both the CH_(4) contents and the proportions of methane inclusions indicate that the gas saturation in the upper part of the Niutitang Formation is high,and the fractures in the lower part are the exchanging pathway for gas and water.(3)Based on the inclusion types,homogenization temperatures and the trapping pressures calculated with the state equation of CH_(4) system,several important geological events are rebuilt.The shale reservoirs were over pressured in the Middle Jurassic,with the max pressure coefficient of 1.4.During the Middle-Late Jurassic,water from the lower part of the Niutitang Formation(with depth of 4 to 4.8 km)penetrated the shales along the fractures,which lowered the gas saturation and elevated the water saturation.We conclude that without the protection of a rigid basement,the early-formed Xuefeng uplift was fragmented by numerous faults and intense tectonic deformation during Indosinian,thus the shale gas in the lower part of the Niutitang Formation dispersed in an open and permeable zone.Therefore,the poor tectonic preservation in the western margin of the Xuefeng uplift makes it an unfavorable area for Cambrian shale gas reservoirs.