Characteristics and quantitative prediction of distribution laws of tectonic-fractures of low-permeability reservoirs in Yanhewan area

The Chang 61 layer is a low-permeability reservoir of Yanchang Formation in the Upper Triassic System and is also the main oil layer in Yanhewan area of Ordos Basin. The distribution and development degree of tectonic fractures restrict the petroleum exploration and valid development. The observation of outcrop and drilled core, thin-section analysis and paleo-magnetis core orientation were applied to the quantitative statistics of tectonic fractures, including the basic characteristics, mechanics and valitidy of tectonic fractures of Chang 61 low-permeability reservoir in the Yanhewan area. Chang 61 reservoir developed the regional conjugate high-angle tectonic fracture system with NEE and NS directions. The density of tectonic fractures is from 0.12 to 3.56 ribbons per meter. The most tectonic fractures are tensional and filled seriously, which belong to the invalid tectonic fractures in Chang 61 reservoirs of Yanhewan area. According to the density of tectonic fractures in cores obtained from different drilling wells in Yanhewan area, a mathematical model for quantitatively predicting and evaluating the distribution and development degree of tectonic fractures was developed by using the finite element simulation method, the rock burst method and the strain energy method. The model was used to predict the areas of developed, sub-developed and un-developed fractures and to determine the strikes of tensile-shear fractures. These results can provide the important geological references for establishment of fracture network model and further petroleum exploration in Yanhewan area.