Influence Of Different Types Of Fractures On Shale Gas Preservation In The Southeastern Sichuan Basin

Preservation conditions are the key factors controlling the enrichment and high yield of shale gas in the Wufeng-Longmaxi formation in the southeastern Sichuan Basin.The southeastern Sichuan Basin has experienced multiple periods of complex structural deformation and developed various types of natural fractures,which greatly differ in the impact on shale gas preservation conditions.It is of great significance for shale gas exploration and development in the southeastern Sichuan Basin to clarify the influence of different types of fractures on shale gas preservation.Based on field,core,thin section data,fluid inclusions,trace elements,and isotope analysis,this paper clarified the development characteristics,formation mechanism,crack-seal history,and main controlling factors of different fractures.The paleo-temperature and pressure conditions,fluid source,and the sealing property of the shale system during the formation of different types of fractures are expounded,and the influence of different types of fractures on shale gas preservation is revealed.According to the relationship between fractures and bedding planes,the fractures of the Wufeng-Longmaxi formation in the southeastern Sichuan basin can be divided into strata-bounded fractures,non-stratabounded fractures,and bedding-parallel fractures.Strata-bounded fractures are developed in highly brittle shale and perpendicular to the rock interface,partly filled with blocky calcite and quartz.Non-strata-bounded fractures pass through multiple rock mechanics layers,intersects with the layer at a high angle,and is partly filled with blocky calcite.Bedding-parallel fractures are parallel to the layers,mainly filled with striped bedding veins and fibrous veins.The bedding-parallel fractures filled by striped bedding veins reflect the multistage crack-seal process.The crack-seal inclusion bands and inclusion trails developed in the veins can be used to indicate the opening times of fractures and track the opening trajectory of fractures,which are mainly formed under tectonic compression.The growth process of bedding-parallel fractures filled by fibrous veins usually shows two stages: the formation of bedding-parallel fractures under abnormal fluid pressure and the continuous growth of fibrous crystals,which are mainly formed under abnormal fluid pressure.The main controlling factors of different types of fractures are obviously various.The content of brittle minerals and organic matter is the basis of shale fracture development,and the high content of brittle minerals and organic matter is conducive to the formation and development of three types of fractures.Strata-bounded fractures are mainly controlled by rock mechanics layers and fold structures,while regional stresses and faults mainly control non-strata-bounded fractures.The development degree and scale of bedding-parallel fractures are controlled by the differential interface of rock mechanics,abnormal fluid pressure,and tectonic compressive stress.Different types of fractures have different formation periods,sources of vein-forming fluid,and influence on shale gas preservation.Strata-bounded fractures are formed in the oil generation stage,and the vein-forming fluid comes from the host rock.When the fractures are formed,the shale system has good sealing,providing reservoir space for free gas.Nonstrata-bounded fractures are formed from the oil generation stage to the gas generation stage of shale.The vein-forming fluid was mixed with external fluids.The development of fractures will connect the fluid inside and outside the shale layer and destroy the preservation conditions of shale gas.The formation of bedding-parallel fractures caused by tectonic compression is synchronized with the generation and migration of shale gas.The crack-seal speed of fractures controls the efficiency of hydrocarbon generation and expulsion of shale and affects the preservation of shale gas.The bedding-parallel fracture caused by abnormal high pressure is the symbol of shale high-pressure hydrocarbon expulsion.The initial fracture formation stage can improve the local seepage capacity of shale,and the subsequent vein growth has little impact on shale gas...