The Characteristics Of Shale Fractures In Longmaxi Formation In Dingshan Area, Southeastern Sichuan And Their Relationship With Gas-bearing Properties

Fracture of shale deeply affect the quality of shale gas reservoirs and control the productivity of shale gas.Especially for the organic-rich shale which has experienced multi-stage tectonic evolution and high-over mature in southeast Sichuan,it is very important to study the development characteristics of fracture and the relationship between fracture and shale gas-bearing property.This paper takes the Longmaxi Formation shale in Dingshan area of southeast Sichuan as an example.On the basic of characterizing fractures in micro-macro scale such as outcrop,core and mirror,we have used rock mechanics experiments,logging,seismic interpretation and other techniques to study the main controlling factors of shale fracture development.Combined with pressure coefficient,gas content and other related data,the characteristics of shale fractures and its relationship with shale gas bearing property are analyzed in depth,and obtained the following conclusions:(1)The outcrop of Longmaxi Formation developed structural shear fractures associated with multi-stage tectonic movement and a small number of tensional and vertical fractures,these fractures are generally characterized by high angle,short extension distance and dense distribution,the shale of Longmaxi Formation has a high degree of fragmentation and a strong crushing capacity.The main types of fractures are shear fracture,pressure-solution fracture,tension fracture,slip fracture and composite fracture.Imaging logging has identified high-conductive fracture,high-resistant fracture,bedding fracture,micro fault and drilling induced fracture.The fractures in core are mainly high-angle oblique shear fracture and vertical shear fracture.The filling materials of micro-fractures include organic matter,pyrite,calcite,etc.It is divided into internal and external factors according to the control factors of micro-fractures.Micro-fractures formed by internal factors control include interbed lamellation fractures,interbedded clay fractures,abnormal pressure fractures and contraction fractures on the edge of organic matter.The microcracks controlled by external factors include,tiny diagenesis crack and artificial induced fractures.Under the microscope,there are several main fractures with a width of about several tens of microns and a few micrometers to ten microns of micro-fractures intertwined with each other.(2)The structural fractures of the Longmaxi Formation are divided into two sets,which are the result of the superposition and transformation of the third-stage tectonic movements such as the mid-late Yanshan movement,the end of the Yanshan movement-the middle Himalayan movement,and the late Himalayan movement.The farther away from the fault,the lower the degree of fracture development,and the transition from fault-controlled fracture zone to regional control fracture zone.The fracture pattern of organic-rich shale is more complicated than that of organic-poor shale,and the fracture development degree of organic-rich shale is higher than that of organic-poor shale.Quartz,feldspar and pyrite are the main brittle minerals of Longmaxi Formation in the study area.Compared with other regions,the level of brittle minerals is medium to upper,and natural fractures of shale are mainly composed of netted fractures.The Rickman brittleness index decreased from 53.7%～60.7%to 46.9%～52.7%while the buried depth increased from Om to about 4000m,and the fracture modes of rock transition from split type(complex)to shear type(single).The thinner the shale,the more fractures develop.The abnormal high pressure locally generated by organic hydrocarbons and hydrocarbons can promote the formation of micro-fractures.The micro-fractures of various genesis are intertwined to form a complex fracture system inside the shale.(3)Since the middle Yanshanian,the regional tectonic uplift near the Qiyueshan fault has been earlier(about 85Ma)and larger(about 2800m)than the area far away from the Qiyueshan fault.Near the Qiyueshan fault area,the fracture and fault sealing property are poor and the fracture development degree of roof and cap is high.The seismic interpretation in this area shows that the coherence attribute is strong and the tectonic curvature value is high.The comprehensive results show that the overall development of the fractures in this area is so high that the pressure coefficient of the shale gas layer of the Longmaxi Formation is lower than 1.2 and the gas-bearing property is poor.The degree of regional tectonic transformation far away from the Qiyueshan fault is low,the fracture and fault sealing property are good and the fracture development degree of roof and cap is low.The regional seismic interpretation far from the Qiyueshan fault shows that the coherence attribute is weak and the value of tectonic curvature is low,it means that the fracture development degree in this area is low,so the pressure coefficient is more than 1.2,and the gas-bearing property is good.Based on a systematic study of the shale fractures of the Longmaxi formation in the Dingshan area of southeast Sichuan,the development characteristics of shale fractures and their relationship with shale gas-bearing property have been preliminarily clarified.The research results not only enrich and deepen the comprehensive evaluation index system of shale gas preservation conditions,but also strengthen the theoretical understanding of finding favorable exploration target areas for shale gas in complex structural areas.It has extremely important research value and guiding significance for the exploration layout and efficient development of shale gas in southeastern Sichuan.