Deep Shale of Lower Cambrian: Implications for Prospective Shale Gas Resources

Abstract

A large set of source rock has been developed in the lower Cambrian Qiongzhusi Formation, which holds significant potential for the economic exploration of deep shale gas resources. However, due to the lack of well-established evaluation criteria, accurately assessing prospective deep shale gas resources in the Yangtze region remains challenging. Therefore, it is critical to establish a comprehensive evaluation framework and identify the key controlling factors. This study novelly established an integrated regional-scale framework, which combines geochemical, petrological, and petrophysical analyses, along with multiparameter investigation to identify favorable exploration zones within the Qiongzhusi Formation of the lower Cambrian shales. The results show that the Qiongzhusi Formation deep shales in the Yangtze region are characterized by substantial thickness and broad distribution, particularly in the center of Deyang–Anyue rift, western Hubei–Hunan, and northern Yunnan–Guizhou areas, with thicknesses ranging from 90 to 220 m, 35 to 75 m, and 22 to 110 m, respectively. High TOC values are observed in the center of the Deyang-Anyue rift, western Hubei–Hunan, and relatively lower in the northern Yunnan–Guizhou. The organic matter is predominantly kerogen type I, with a minor presence of Type II₁ kerogen. Thermal maturity is generally high. The mineral composition is mainly detrital and clay minerals, with a slightly higher clay content in northern Yunnan–Guizhou compared with the other subregions. Overall porosity is relatively low, with northern Yunnan–Guizhou showing the lowest compared to the Deyang–Anyue Rift and western Hubei–Hunan. High gas content is present in the center of the Deyang–Anyue rift, western Hunan–Hubei, and northern Yunnan–Guizhou, indicating that spatial variations in geological characteristics significantly influence shale gas distribution and exploration potential. By establishing clear spatial and stratigraphic trends and revealing the interplay of key shale attributes, this work provides a new basis for targeted exploration and resource assessment in deep shale gas systems in the Yangtze region.