Development characteristics and classification of shale laminae in the Chang 7₃ sub-member of the Triassic Yanchang Formation in the Ordos Basin

In the organic-rich shale deposited in the Chang 7₃ sub-member of the Triassic Yanchang Formation in the Ordos Basin, the frequent laminae lead to strong heterogeneity of shale series, which in turn affects the reservoir characteristics and oil-bearing property of the shale. Based on typical laminated shale samples, this paper quantitatively measured the laminae in the core with vernier calipers, and studied the development characteristics, laminae types and vertical distribution of laminae in the Chang 73 sub-member shale by combining microscopic thin section identification, total organic carbon determination, X-ray diffraction mineral analysis and rock pyrolysis experiments. The results show that the thickness of single layers of the laminae developed on the macro core is generally between 0.4 and 5.0 mm, and the ratio of the laminae to the core (the total thickness of the laminae/the core length of the statistical section) is between 5.7% and 16.8%, and the development frequency of the laminae is highly heterogeneous. Microscopically, it is divided into six types according to the morphology of laminae sedimentary structures, which are three kinds of straight laminae, corrugated laminae, granular laminae with continuous clay morphological sedimentary characteristics and three kinds of lenticular laminae, mottled laminae and weak laminae with discontinuous morphological sedimentary characteristics. The mineral composition and organic matter abundance of shale with different morphologies are obviously different. According to the microscopic material composition inside the laminae, they are mainly divided into organic laminae, felsic laminae and tuffaceous laminae, and according to the vertical superposition of different laminae, the laminated combinations are divided into homogeneous mudstone, felsic laminated shale and tuffaceous laminated shale. In a single well, the distribution of laminae is heterogeneous, and the development frequency of different laminae changes regularly with the changes of clay mineral content, total organic carbon(TOC) and oil saturation index(OSI).