Gas occurrence characteristics in marine-continental transitional shale from Shan23 sub-member shale in the Ordos Basin: Implications for shale gas production

Abstract

Pore structure characteristics, gas content, and micro-scale gas occurrence mechanisms were investigated in the Shan₂³ sub-member marine-continental transitional shale of the southeastern margin of the Ordos Basin using scanning electron microscope images, low-temperature N₂/CO₂ adsorption and high-pressure mercury intrusion, methane isothermal adsorption experiments, and CH₄-saturated nuclear magnetic resonance (NMR). Two distinct shale types were identified: organic pore-rich shale (Type OP) and microfracture-rich shale (Type M). The Type OP shale exhibited relatively well-developed organic matter pores, while the Type M shale was primarily characterized by a high degree of microfracture development. An experimental method combining methane isothermal adsorption on crushed samples and CH₄-saturated NMR of plug samples was proposed to determine the adsorbed gas, free gas, and total gas content under high temperature and pressure conditions. There were four main research findings. (1) Marine-continental transitional shale exhibited substantial total gas content in situ, ranging from 2.58 to 5.73 cm³/g, with an average of 4.35 cm³/g. The adsorbed gas primarily resided in organic matter pores through micropore filling and multilayer adsorption, followed by multilayer adsorption in clay pores. (2) The changes in adsorbed and free pore volumes can be divided into four stages. Pores of <5 nm exclusively contain adsorbed gas, while those of 5–20 nm have a high proportion of adsorbed gas alongside free gas. Pores ranging from 20 to 100 nm have a high proportion of free gas and few adsorbed gas, while pores of >100 nm and microfractures are almost predominantly free gas. (3) The proportion of adsorbed gas in Type OP shale exceeds that in Type M, reaching 66 %. (4) Methane adsorbed in Type OP shale demonstrates greater desorption capability, suggesting a potential for enhanced stable production, which finds support in existing production well data. However, it must be emphasized that high-gas-bearing intervals in both types present valuable opportunities for exploration and development. These data may support future model validations and enhance confidence in exploring and developing marine-continental transitional shale gas.