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Two Episodes of Structural Fractures: Numerical Simulation of Yanchang Oilfield in the Ordos Basin, Northern China



The tight oil sand reservoirs of the Yanchang Oilfield in the Upper Triassic Yanchang 6, 7, and 8 Formations (T3Yc6,7,8 Fms.) in the Ordos Basin are in the exploration stage. Two episodes of fractures during the Yanshanian and Himalayan periods, observed in outcrops and cores of the T3Yc6,7,8 Fms., are thought to be the controlling factors for the tight oil ‘sweet spot’. Finite element numerical simulation (FES) technology is used to perform a three-dimensional (3D) fracture simulation for a large region during the exploration stage, to serve in tight oil reconnaissance for the Yanchang Oilfield. The innovations of our work are mainly shown in the following simulation process: (1) three controlling factors, including the paleotopography, layer thickness and rock mechanical faces, are used to construct the 3D geologic model in the simulation; (2) the rock mechanical environment, characterized by the An’gou, Qiliha-Zhangjiatan and Dongjiahe outcrops, the Y1011 well cores and the corresponding mechanical experimental parameters, is divided into three types as follows: the delta front - tractive current - sand faces (DF-Tr-S), the lacustrine - turbidity current - sand faces (L-Tu-S) and the mud faces (M); (3) the spatial boundary stress conditions are calculated based on the Newberry formula for tensional and compressional stress fields, considering the synthetical effects of the overburden pressure, the pore-fluid pressure and the tectonic stress; (4) the modified Griffith’s criterion is used to fit the fracture index by simultaneously taking into consideration the tension and shear fracture mechanisms. For the simulation results, the predicted fracture index distribution map agrees well with the measured fracture density. The Yanshanian fractures mainly propagated in the Dingbian and An’bian areas adjacent to the western thrust belt, while the Himalayan fractures mainly propagated in the Zhidan, Fuxian, and Huangling areas adjacent to the Weihe Graben. The simulated fracture index has a positive correlation with the stratigraphic dip and a negative correlation with the layer thickness. Additionally, structural fractures are prone to propagate near the mechanical interface in either the upward or downward penetration direction and are assumed to produce a fracture-type sweet spot section in the vertical direction. The reservoir oiliness also increases with the simulation fracture index, indicating the fracture-type tight oil sweet spot of the Yanchang Oilfield. Moreover, the critical components used in the 3D fracture FES, including the 3D geologic model, the spatial boundary stress conditions and the fracture index fitted by the modified Griffith criterion, can be of great significance to produce more accurate numerical simulations during the oil and gas exploration stage in large regions.