Origin and Geological Significance of the Cross Fractures in the Upper Triassic Yanchang Formation, Ordos Basin, China

Cross fractures are well developed in the Upper Triassic Yanchang Formation in the Ordos Basin. According to the data on outcrops and cores and to our mechanical experiments, the origin of these cross fractures is discussed and their influence on petroleum exploration and development is analyzed. Four assemblages of fractures were formed in the Upper Triassic Yanchang Formation within the Ordos Basin. But in a certain place, there are generally distributed two assemblages of cross fractures, i.e. E-W and S-N fractures, at the center of the basin, and NE-SW and SW-SE fractures in the southwest of the basin. Fractures here are mainly high-angle shear fractures that are nearly vertical to the beds that were formed during the Yanshan (135Ma) and Himalayan (65-23.3Ma) periods due to horizontal tectonic compression, deep burial and intense uplift or denudation. The rock anisotropy is the prime reason for impacts wrought on the development of different assemblages of cross fractures in some geological periods, which usually restricts the development of one assemblage of conjugate shear fracture. Theoretically, there should have been formed two assemblages of shear fractures in either tectonic period, and there should have developed four assemblages of shear fractures here. But, due to the effect of a strong rock anisotropy, in each period developed only one assemblage of fracture. Thus two assemblages of cross fractures are usually developed in each part of the Ordos Basin at present. The cross fractures are important pathway system for oil migration in the low-permeability sandstone reservoirs of the Upper Triassic Yanchang Formation. At the same time, they also serve as main channels for fluid flow during the water-injection development process and impact the development of such reservoirs.