Influence of internal textures in fracture development in dolostones: A case study in the Mesoproterozoic Wumishan Formation in the Jizhong Depression, Bohai Bay Basin, North China

Abstract Fractures provide pore spaces for the preservation and migration of hydrocarbons and control the quality of dolomitic reservoirs. Textures caused by sedimentary and tectonic processes control the mechanical properties of dolostones and the development and distribution of fractures in them. An understanding of the influence of textures present in dolostones in the development of fractures is crucial. Therefore, massive micritic, brecciated, columnar stromatolitic, and siliceous dolostones and laminites present in the early Mesoproterozoic Wumishan Formation (between ca. 1.5 Ga and ca. 1.45 Ga) in the Jizhong Depression with varying internal textures were analyzed based on observations of outcrops, core, and microstructures to determine their influence on reservoir quality. The development characteristics of fractures in the dolostones were investigated, and their petrophysical properties were measured. Uniaxial and triaxial compression tests were utilized to explore the impacts of textures on the distribution patterns of fracture in the dolostones. The fracture density, porosity, and permeability are 5.4/m, 1.55%, and 17.10 mD in the homogeneous and massive micritic dolostone, with fracture oriented parallel or at an acute angle to the maximum principal stress direction. The fracture density, porosity, and permeability, respectively, are characterized by averages of 8.9/m, 6.39%, and 106.68 mD, and are significantly better in the brecciated dolostones, and the fractures develop along the preexisting weakly cemented planes between the brecciated textures. The fracture density of the siliceous dolostone is 9.8/m, and its porosity and permeability are 2.74% and 29.68 mD. The extension of high-angle fractures is usually restrained by the low-angle bedding fractures, and intensive microcracks are developed in the siliceous textures. The occurrence of algal laminae in dolostones increases the proportion of low-angle fractures; these fractures can extend along the maximum principal stress direction and rupture along the weak algal laminae. The fracture density and petrophysical properties of columnar stromatolitic dolostones (7.4/m, 4.39%, and 87.75 mD) are better than those of laminites (4.3/m, 1.21%, and 14.57 mD). Therefore, the formative environment of textures in dolostones controls the quality of the reservoirs in the Wumishan Formation. The reservoirs in brecciated dolostones that are located in structural highs and fracture zones and columnar stromatolitic dolostones deposited in high-energy intertidal to upper subtidal zones have the highest reservoir quality. The quality of the reservoirs in siliceous banded dolostones deposited in mid-energy intertidal zones is average, and that of massive micritic dolostones and laminites deposited in low-energy lower-subtidal zones is poor.