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The Niutitang Formation of the lower Cambrian (Є1n) is a target reservoir of shale gas widely developed in China's Middle-Upper Yangtze region, with the characteristics of being widely distributed, having a big thickness and highly organic carbon abundance. However, the exploration and research degree are relatively low. Based on extensive core sample, experimental test results, drilling, and field outcrop surveying, the shale gas generation capacity, gas content, and gas composition are discussed. The preservation conditions of shale gas are then systematically analyzed from the aspects of tectonic movement, fault development, structural style, and thermal evolution degree. The results show that the organic-rich shale with a thickness ranging from 40 to 150 m developed in the mid-lower part of the Є1n Formation, with the TOC content values ranging from 0.4% to 14.64%. While it has unfavorable characteristics of a high thermal evolution, with Ro values ranging from 1.92% to 5.74%, a low gas content and a high nitrogen content (70% wells). The Є1n shale gas has complex preservation conditions. The Є1n Formation has good roof-to-floor conditions, but after the main gas generating peak of the Є1n shale during the Jurassic–Cretaceous, the most intensive tectonic activity of the Yanshan movement resulted in poor preservation conditions (faults developed and cap rock fractured). The huge faults extended to the surface are formed due to tectonic movement in an extensional environment, and the structural style and development are the main factors affecting the preservation conditions of the Є1n shale gas. Additionally, the high thermal evolution of the Є1n shales also has a certain impact on the preservation conditions. Therefore, the stable area far from large faults (>2.0 km), with weak local tectonic activity and tectonic deformation, is the favorable area for shale gas preservation in the Є1n Formation. [ABSTRACT FROM AUTHOR]

Taking the HD1 well as the research target, through intensive core sampling, experimental test analysis, comprehensive logging interpretation and other methods, the fracability of shale reservoirs was discussed in this paper. The results show that the Gufeng–Dalong Formation in the HD1 well has a high organic carbon content, and the organic-rich shale developed in the lower part of the Gufeng and Dalong Formations with thicknesses of 30 m and 15 m, respectively. For the low-porosity and ultralow-permeability shale reservoir type, the natural fractures are undeveloped in the lower part of the Dalong Formation, with a lower linear density, while they are well developed in the Xiayao and Longtan Formations and the lower part of the Gufeng Formation, and the interlayer bedding fractures are relatively developed. The Gufeng–Dalong Formation shale also has a high mineral brittleness index (average of 44.5%), high static Young's modulus (20–70 GPa), low static Poisson's ratio (0.10–0.31), and high horizontal pressure difference coefficient of two phases (0.17–0.56). It is concluded that the shale reservoir is favorable for fracture development in the lower part of the Dalong Formation, with depths of 1249–1289.5 m, and the lower part of the Xiayao, Longtan and Gufeng Formations, with depths of 1300–1335.3 m. Overall, the fracable shale section with high brittleness and rock strength is beneficial to fracturing. However, the existence of a large number of shale bedding fractures increases the complexity of the fractures, and at the same time, it has a certain negative impact on fracture generation. Double wing fractures are easily formed because of the high two-phase horizontal pressure difference. Therefore, the leakage caused by shale bedding fractures and the influence on the fracture height and extension length should be considered comprehensively in fracturing design. [ABSTRACT FROM AUTHOR]