Shale Gas Preservation Conditions Of Wufeng-Longmaxi Formation In Sangzhi Block,Northwestern Hunan Province

The Sangzhi block in the northwest of hunan province has undergone multi-phase tectonic movements,with more developed folds and faults,and shale gas preservation conditions have been affected to a certain extent.Through reservoir material foundation,tectonic characteristics and evolution history,hydrogeological conditions,capping property of Wufeng-Longmaxi Formation shale gas preservation conditions for comprehensive analysis,to establish suitable Sangzhi block shale gas preservation conditions comprehensive evaluation index system,and optimizing preservation conditions favorable area.The specific understanding is as follows:The shale of the Wufeng-Longmaxi Formation in the Sangzhi block is widely developed in the area,with a thickness between 50m and 150m.The type of kerogen is mainly type?sapropel type.The lower Longmaxi Formation and Wufeng Formation have higher organic matter abundance?0.36%⁴.05%?,organic matter maturity?1.84%³.78%?and considerable gas content.Generally speaking,it has a good material foundation.Based on the analysis of structural features and evolution,it is concluded that the strong compressive stress during the Yanshan period resulted in the syncline and anticline of the north-eastern axis,as well as most of the high-angle thrust faults of the NE strike,and the large-scale faults are mainly distributed in the southeastern part of the region.The buried depth of the target layer in the northwest direction was gradually shallower and exposed due to the great uplift of the Xishan period.Through comprehensive analysis of fault distribution,uplift and erosion,and structural styles,the preservation conditions in the wide and gentle western area of Yanwukou syncline are relatively favorable.Observation of the macro and micro cracks in the target layer reveals that the cracks have a relatively high degree of development,which is mainly affected by the mineral composition and structure.The comprehensive fracture coefficient is introduced to evaluate the fracture development degree,and it is considered that the fracture development areas of grade?and?are favorable areas for shale gas enrichment and preservation.The hydrogeological conditions were studied from two aspects:ancient fluid and current fluid.According to the analysis of uniform temperature,salinity and carbon and oxygen isotopes of fluid inclusions,the intrusion period of ancient fluid was Yanshan-Xishan period,and the infiltration depth of atmospheric water was up to4560m.According to the calculation of hot spring water temperature,the current depth of atmospheric water circulation is 250m¹410m.The water type is mainly Na₂SO₄.The limestone of Permian,Ordovician formed karst caves,which caused certain damage to the capping property.According to the gas composition analysis of the target layer,the nitrogen content is basically less than 5%,indicating that the destruction of atmospheric water can be reduced with good capping conditions.The shale thickness of Wufeng-Longmaxi Formation in Sangzhi block is basically greater than 70m,the burial depth is generally 1000m³000m,the permeability is low,and it has a good ability of original self-sealing.The top and bottom rock layers have weak water content and poor permeability.They are in integrated contact with the target layer,and no weathered crust is found,which means they have good sealing properties.The Silurian Luoreping Formation is widely and continuously distributed in the area,with large thickness and dense lithology,which plays a good regional sealing role.Based on the comprehensive material foundation,sealing conditions,tectonic action and hydrogeological conditions,an evaluation parameter system for preservation conditions is established,and the wide and gentle slopes in the north and west of the Yanwukou syncline were selected as the favorable areas for preservation conditions.