Natural fractures in deep continental shale oil reservoirs: A case study from the Permian Lucaogou formation in the Eastern Junggar Basin, Northwest China

Abstract

The deep continental shale of the Permian Lucaogou Formation in the eastern Junggar Basin is abundant in oil resources, characterized by extensive development and various types of natural fractures. Based on core observations, image log interpretation, and thin section analyses, this study identified fracture types and analyzed the development characteristics of different types in deep continental shales. Moreover, the main controlling factors affecting fracture development and distribution were discussed by integrating statistical and experimental analysis. Results show that natural fractures in deep continental shale oil reservoirs can be grouped as non-bed-parallel and bed-parallel fractures based on their relationship with bedding. Non-bed-parallel fractures are subdivided into subvertical fractures (≥80°) (translayer and intralayer fractures) and moderately dipping fractures (<80°), and bed-parallel fractures are further divided into bedding fractures and stylolites. Since silty and dolomitic rocks appear widely as interlayers and increase the rock brittleness, the deep continental shales are more prone to developing non-bed-parallel fractures. Moreover, intralayer fractures with high dip angles are more developed within these thinner mechanical units and commonly terminate at the mechanical interface, while the translayer fractures are usually less developed. Deep continental shales with higher TOC and argillaceous content are more conducive to forming bed-parallel fractures because of their stronger plasticity, more densely developed laminar structure, and easier formation of abnormally high pressure during the hydrocarbon generation and evolution. In addition, due to the frequent interaction of different types of laminae and the stronger compaction and pressure solution produced by the larger burial depth, the bed-parallel fractures, especially the stylolites, are more developed as a prominent feature in deep continental shales.