Characteristics and prediction of fractures in large-scale metamorphic buried hills in southwestern Bozhong sag

Fractures are the important type of reservoir space in metamorphic buried hills.In this paper, the main development stages of fractures in large-scale metamorphic buried hills in southwestern Bozhong sag were determined based on the data of imaging logging, regional tectonic evolution and rock mechanics experiment.Then, the plane prediction method used for fractures was discussed on the basis of the simulated palaeo-stress field.It is revealed from the imaging logging data that the fractures in the study area have three predominant directions, i.e., NW, NE and EW, and they are the products of multiphase tectonic activity since the Yanshan epoch.There is a stage of stress conversion in late sedimentary phase of Dongying Formation, which controls two sets of faulting system in shallow and deep layers.And the faulting activity in the late stage is limited in the fracturing capacity.The analysis on fracture filling degree shows that the NW-trending fractures are the highest in filling degree, and the NE-trending and nearly EW-trending fractures are the main effective fractures, which are mainly formed in Indo-Chinese to Paleogene.The plane distribution rule of fractures is figured out based on the tectonic setting and the simulated equivalent palaeo-stress field.The development degree of fractures is controlled jointly by discordogenic faults and palaeo-structures, and the fractures are mainly distributed near the structural highs controlled by large-scale faults.