The continental shale in the Permian Fengcheng Formation of the Mahu Sag in the Junggar Basin has a maximum burial depth exceeding 5000 m,rich in oil and gas resources. The widely developed natural fracture plays a crucial role in the accumulation and exploration of hydrocarbons in these reservoirs. A classification scheme is established for natural fractures in deep continental shale based on their geological genesis and occurrence by observing cores,image logs,thin sections,and SEM samples. Moreover,the development characteristics and effectiveness of different types of natural fractures Were analyzed,and the natural fracture heterogeneity and their contributions to reservoirs are discussed. Accordingly,natural fractures in deep continental shale are divided into tectonic,diagenetic,and abnormal high-pressure related fractures based on their geological genesis. Tectonic fractures can be subdivided into translayer shear,bed-parallel shear,and intralayer open fractures based on their occurrence,while diagenetic fractures are divided into bedding,stylolite,and shrinkage fractures. Tectonic fractures have relatively large scales,obvious groups,and high dipping and nearly vertical angles. Diagenetic fractures mainly develop horizontally,with curved surfaces and easily branched extensions. Intralayer open,bedding,and stylolite fractures are the dominant types of fractures in deep continental shale reservoirs. Natural fractures can be filled in varying degrees by minerals such as calcite and fine-grained mixtures containing organic matter,among which tectonic and bedding fractures are less filled,and stylolites are easier to be filled. Microscopic tectonic fractures have smaller apertures,while diagenetic fractures usually have larger apertures. This study speculates that tectonic fractures mainly provide effective pathways for fluid flow in reservoirs,while bedding fractures are more developed,which are not only seepage channels for reservoir fluid but also an important part of the effective storage space. The research results provide an important reference for improving the classification scheme of natural fractures in deep continental shale and for better understanding of the natural fracture distribution in such reservoirs.
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