Research On Multi-scale Fractures And Their Influences On Gas-bearing Properties In Marine Shales In The Southeastern Sichuan Basin

Wufeng-Longmaxi Formation of Lower Paleozoic in southeastern Sichuan Basin is the key area of marine shale gas development in China.There develop natural fractures with multi-scale characteristics in marine shale.Different scale fractures have different effects on shale gas enrichment and preservation.Studying the effects of development characteristics of shale multi-scale fractures on gas bearing property plays an important guiding role in shale gas exploration and development.In this paper,based on outcrop data,core data,thin section and well logging data,combined with the test and analysis of mineral rocks,geochemistry and physical properties and discrete element numerical simulation,the classification scheme of multi-scale fractures in marine shale is established and the formation mechanism,main control factors and development law of multi-scale fractures are expounded.Furthermore the influence of fractures with different scales on shale gas-bearing property is discussed.According to the distribution characteristics,scale and controlled interface of natural fractures,marine shale fractures can be divided into four levels: large-scale fractures,meso-scale fractures,small-scale fractures and micro-scale fractures.Large-scale fractures develop in the shale strata,cutting the lithofacies interface and being controlled by the boundary of the strata;Meso-scale fractures developed in a single shale facies,cutting the interface of striated interval,controlled by lithofacies interface;Small-scale fractures develop in shale interval and are controlled by shale interval boundary.Microscale fractures are controlled by interlaminar interface.The distribution of multi-scale structural fractures is controlled comprehensively by rock mechanics layer and its thickness as well as tectonic stress.The striated section is an important rock mechanics layer of shale,and the brittle striated section is beneficial to the formation of fractures whereas the plastic laminae limit fractures expansion;The more plastic layers exist,the stronger effects the fractures receive,and the greater stress is needed for fractures expansion.Lamellation fractures is the main type of micro-scale fracture in shale,and its development degree is controlled by TOC content,brittle minerals,pyrite and laminae.In the longitudinal direction,the development degree of lamellation fractures in the lower part of Wufeng-Longmaxi Formation is at a high level while the density decreases on the upper page.The influence of fractures of different scales on shale gas-bearing property is quite different.Large-scale fractures play a significant role in communication and seepage within shale series,which mainly affects the preservation conditions of shale gas.Microscale fractures are small in scale but highly developed,which is beneficial to shale gas enrichment.The micro-scale fracture pore volume accounts for 22.6% of shale reservoir pores on average.Among them,the lamellation fractures can provide 6%～10% pore volume for shale reservoirs.The horizontal permeability of shale reservoir is increased by 2～3 orders of magnitude by the development of lamellation fractures.The stress sensitivity of shale permeability is extremely strong,with the increase of effective pressure,the opening of lamellation fractures decreases,and shale permeability shows three stages: rapid decline,slow decline and constant.The corresponding effective pressures are 5～20MPa,20～40MPa and 40～50MPa respectively.