| Shale fracture is not only reservoir space,but also connected with other pore,forming fracture grid.Fractures,as the space for oil and gas accumulation,preservation and migration,have great influence on oil and gas exploration.In recent years,exploration has found that Chongqing is rich in oil and gas resources,Fuling,Wuxi and other places have made more breakthroughs in oil and gas exploration,but little research has been done on the shale of Lujiaping Formation in northeastern Chongqing,especially on fracture characteristics.This paper takes Chengkou area in northeastern Chongqing as the research area,Lujiaping formation strata as target strata,by means of profile observation,core observation,thin section identification,fracture stage matching,acoustic emission test,inclusion homogenization temperature measurement,carbon and oxygen isotope test,to study the development characteristics,main controlling factors,development stages,development mode,distribution rule and prediction of shale fractures,which provides basic data for future oil and gas exploration in the study area.The whole structure of Chengkou area is more and more intensive from north-east to south-west.The Lujiaping Formation of lower Cambrian in the study area can be divided into two sections:the siliceous rock section in the lower part include phosphorite nodule;the slate rock in the upper part.The Lujiaping Formation generally contains pyrite,which is a deep-water deposit in the outer shelf.The thickness of the rock gradually becomes thicker from bottom to top.Fractures are well developed in this area.The most common are plane X-type shear fracture and section shear fracture.Fracture length is mainly small and medium,width is mainly small,dip angle is mainly high-angle,and fracture strike is most developed in the North-South direction.The filling degree of cracks is high,and the filling materials are mainly calcite,quartz and pyrite.The average porosity and permeability of shale in Lujiaping Formation are 4.27%and 0.000451 mD,and the correlation coefficient is 0.712.The correlation is good,which indicates that the fracture connectivity of shale in Lujiaping Formation of Well Chengye 1 is relatively good.The main controlling factors affecting shale fracture development are mainly divided into two categories:internal and external factors.Internal causes include lithology and mineral composition,rock mechanical properties,rock thickness,TOC content,porosity,etc.External causes include tectonic factors,sedimentation,abnormal high pressure and so on.Combining with fissure stages in the field,rock acoustic emission experiments and temperature measurement of inclusions in fissure fillings,it is considered that the shale of Lujiaping Formation in Chengkou area of northeastern Chongqing were formed mainly under the tectonic movements of Yanshanian and Himalayan.The fractures in the middle and lower strata of Lujiaping Formation in Chengkou area are more developed than the upper strata,mainly due to the high silica content,thin layer thickness and high TOC content in the middle and lower strata.On the plane,the Chengkou area from NW to SE,the development of fractures is becoming more and more sparse.It is believed that the main causes are tectonic stress,NW-trending denser distribution folds and faults.It is predicted that "Grade I fracture development zone" in Chengkou area mainly distributes near Wuping fault zone,Chengba fault zone and its associated faults,and Xiuqiba-Gaoguansi area;"Grade I fracture development zone" distributes broadly,and generally distributes in NW direction around "Grade I fracture zone",mainly around the high point of structure in the study area. |
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