Characteristics of Black Shale Reservoirs and Controlling Factors of Gas Adsorption in the Lower Cambrian Niutitang Formation in the Southern Yangtze Basin Margin, China

The pore structure and shale adsorption capacity have a great impact on the formation of shale gas field of yield industrial gas flow. This investigation focuses on the characteristics of the reservoir and adsorbed gas of the Lower Cambrian Niutitang Formation in the southern margin of the Yangtze basin. On the basis of geochemical analysis, low pressure nitrogen gas adsorption, X-ray diffraction, and isothermal adsorption experiments on core samples, the Niutitang Formation shows following characteristics: (1) The pore of the Niutitang shale can be divided into four categories: interparticles pores, intraparticles pores, organic matter pores, and microfractures. (2) The pores structure of shale shows three characteristics: the micropores frequency peaks are higher than that of mesopores in the d(v)(d) curve, and specific surface area frequency is greatest in micropores; these pore characteristics primarily appear in siliceous shale. Pore volume frequency is primarily dominant in mesopores, which mainly appear in carbonaceous shales. The pore volume peaks of micropores and mesopores are extremely low, and specific surface frequency is commonly dominant in mesopores, which mainly appear in silty shale. (3) Organic matter and quartz are beneficial to the growth of shale porosity, and organic matter in shale is the main control factor in the development of micropores, while quartz is a primary control factor in mesopore and macropore development. High clay mineral content is not conducive to shale porosity development and is particularly detrimental to the development of micropores. (4) The function relationship between buried depth and the adsorbed gas capacity was establish based on the relationship of temperature and pressure with gas adsorbed. The results suggest that the maximum gas adsorption capacity occurs at depths of around 1800 m, and pressure is the dominant control factor at depths below 1800 m, where gas adsorption capacity increases with depth, while temperature is the primary control factor at depths greater than 1800 m, where gas adsorption capacity decreases with depth. (5) Black shale of the Lower Cambrian under high evolution, Good pore structure (pore volume and specific surface area) and high total organic carbon are advantageous to the gas adsorption capacity of the highly mature Lower Cambrian black shales, while high maturity and clay content may inhibit gas adsorption capacity.