Occurrence characteristics and controlling factors of transitional shale gas: a case study from Shanxi Formation, Eastern Ordos Basin, China

Abstract

Accurate evaluation of gas content is significant for evaluation of unconventional reservoirs and determination of unconventional hydrocarbon reserves. Based on nuclear magnetic resonance (NMR) experiment on plug samples saturated methane and methane isothermal adsorption experiment of crushed samples, the free gas content and adsorbed gas content of Shanxi Formation transitional shale under high-temperature and high-pressure were determined. The experimental results show that the maximum theoretical gas content of the Shanxi Formation transitional shale in the study area is 2.58 cm3 /g-5.44 cm3 /g, mainly composed of adsorbed gas, and the proportion of free gas is generally lower than 50%. The adsorption capacity of the transitional shale is mainly controlled by organic matter, followed by kaolinite and I/S mixed layer. The free gas content of the transitional shale is mainly controlled by pore structure and is less affected by mineral composition. The specific surface area controls the adsorption capacity of transitional shale gas, and the effect of specific surface area on total gas content is more significant than that of pore volume. Adsorbed gas mainly exists in micropores (< 2nm), while free gas mainly exists in macropores (> 50nm). This study proposes an experimental method for the simultaneous measurement of shale free gas adsorption gas using nuclear magnetic resonance of plug sample saturated methane and methane isothermal adsorption of crushed sample, clarifying the gas-bearing characteristics of transitional shale, and providing theoretical support for the optimization and efficient development of sweet spots in transitional shale gas in the next step.