Characteristics of continental shale reservoirs in the Xuanhua Basin of Yanshan area and their influence on gas-bearing properties

Abstract

Abstract The continental shale of the Jurassic Xiahuayuan Formation in the Xuanhua Basin of the Yanshan area displays widespread occurrence and notable thickness, indicating good potential for industrial-scale shale gas production. However, there are still many uncertainties regarding the characteristics of shale reservoirs, their gas-bearing properties, and the main controlling factors. To characterize the Xiahuayuan Formation shale, this study employed total organic carbon (TOC) analysis, X-ray diffraction (XRD), low-temperature N 2 adsorption (LTN 2 A), and in-situ gas content measurement to systematically investigate organic matter (OM) composition, reservoir properties, and gas occurrence characteristics. Moreover, the principal influencing factors of the gas-bearing characteristics were discussed. The results indicate that the Xiahuayuan Formation shale exhibits relatively high TOC content and is in the mature to high-mature stage. The OM type is mainly Type Ⅲ kerogen, providing a favorable foundation for the formation and storage of shale gas. The in-situ desorption gas content is predominantly composed of free gas. Both the adsorbed and free gas contents gradually increase as burial depth increases. Shale gas content is influenced by multiple geological factors. Among them, the TOC is conducive to hydrocarbon generation in shale. It promotes the development of OM pores, increases pore volume (PV) and specific surface area (SSA), and thus shows a positive correlation with various gas contents. As R o increases, both the hydrocarbon adsorption capacity and the ability to store free gas are enhanced. In addition, quartz has a remarkable resistance to compaction and can effectively protect the pore structure. When its content is relatively high, it tends to have a slight positive relationship with various gas contents. Conversely, carbonate minerals, by filling the original pores and microfractures, show a negative association with various gas contents. In contrast, clay minerals contribute to the formation of numerous secondary pores and show a positive correlation with various gas contents. The pore structure of shale primarily consists of mesopores and micropores. An increase in micropore volume leads to a reduction in the average pore size (APS), promoting an increase in SSA and PV, and further enhancing the adsorption capacity and storage space of shale. This research are intended to offer a robust geological foundation for future exploration and development efforts related to continental shale gas resources.