Distribution Characteristics Of Fractures In Volcanic Reservoirs Of Yingcheng Formation In Xujiaweizi Fault Depression And Their Relationship With Hydrocarbon Accumulation

The volcanic strata in the Yingcheng Formation of Xujiaweizi fault are fractured-porosity reservoirs.The existence of fractures has an important effect on improving reservoir,natural gas migration and accumulation.The development of fractures has a great influence on the enrichment of gas reservoirs and the production wells.Therefore,the study of volcanic reservoir fractures has great significance to the exploration and development of volcanic reservoirs.The causes of formation and characteristics of the fractures of volcanic reservoirs,the controlling factors,the distribution regularity and the formation period of the volcanic reservoirs were studied by means of the core,the interpretation of the cracks in the imaging logs and the experimental analysis.The role of tectonic fractures in the process of natural gas accumulation is analyzed by combining the evolution process of hydrocarbon source rocks and the closure capacity of cap rocks.Based on the internal and external causes of fracture development,the geological model is established.Based on the numerical simulation of the tectonic stress field during the formation of fractures,the development and distribution of cracks are predicted.The results show that the main strata and secondary cracks are mainly developed in Yingcheng Formation,which is mainly composed of secondary structural fractures.The development of fractures is influenced by lithology,facies,tectonics and rock heterogeneity.The fractures are mainly controlled by the facies and faults on the plane.The degree of fracture development is relatively large in the overflow phase and the invasion phase.Near the fault fractures developed best,especially at the end of the fault,the intersection,the junction,and the overlay area.In the vertical,unconformity below the surface,the fracture was a cyclical distribution.There are three stages in the formation of fractures in Yingcheng Formation according to the regional tectonic stress field evolution,the intercrossing relationship of fractures,the fault growth profile profile and the same sedimentary fault activity rate and tectonic evolution profile analysis.The first stage tectonic fracture is formed at the end of Yingcheng Formation with most fractures filled with mud or calcite.They belong to invalid crack.The second tectonic fractures are formed at the end of the sedimentary period of the Quantou Formation to the depositional period of the Qingshankou Formation.The number of fractures is large and the effectiveness is good in this period,which matched with the hydrocarbon generation peak period and the gas source fracture activity period.The effective cover of the period has been formed.So it is the main channel of natural gas migration and storage space.The third tectonic fracture is formed at the end of the Nenjiang Formation.In this period,the tectonic fractures cut the early frctures,so that the reservoir connectivity was further improved.At the same time,the pyrolysis gas in the Shahezi Formation began to migrate along the faults and fractures to the Yingcheng Formation volcanic reservoir.Based on the internal and external geological factors influencing the structural fractures of Yingcheng Formation in Xujiaweizi fault depression,a prediction method of volcanic reservoir fractures based on geologic and mechanics theory model is proposed.The finite element software of ANSYS is used to simulate the stress field of a reservoir in the selected area.Based on the rock fracture criterion,the degree and distribution of fractures in the area are predicted.Through the prediction results,it is considered that the development of fractures in the east of Xushen 5 is a favorable area for further exploration and development.