Development Characteristics of Fractures in the Upper Palaeozoic Along the Northeastern Edge of the Ordos Basin and Their Controlling Effects on Hydrocarbon Accumulation

Abstract

The northeastern edge of the Ordos Basin is characterised by low-amplitude structures and the presence of intrusive rock bodies in the Upper Palaeozoic strata. Tectonic movements and stratal denudation during the Mesozoic and Cenozoic have resulted in the development of well-defined and effective fractures in the area. However, the controlling role of fractures on the Upper Palaeozoic gas reservoirs is not yet clear. This paper uses the Linxing Block as a case study to conduct a comprehensive analysis of the role of fractures in controlling gas distribution. The study shows that the Upper Palaeozoic in this area is primarily characterised by the development of vertical fractures, which are of a tensile-shear nature and have a low degree of filling. The formation and evolution of fractures are controlled by the generation of hydrocarbons, denudation, and tensile strain during the sedimentation-erosion process. Tensile fractures are more likely to occur when the horizontal minimum principal stress is lower than the formation pressure during hydrocarbon generation. During the denudation process, the horizontal minimum principal stress in tight sandstone decreased, facilitating the formation of near-vertical tensile fractures. At the same time, the strata also produce a stress tensor, with the reduction in sandstone strata being greater than that in mudstone strata. Ultimately, fractures are more developed in reservoirs with high brittle mineral (quartz and feldspar) content in the uplifted and sloped areas of low-amplitude structures, which is favourable for the accumulation of natural gas. In contrast, fractures in the trough zones are usually underdeveloped, and fractures are extremely developed near the Zijinshan Pluton, which is detrimental to the preservation of natural gas.