Quantitative characterisation of storage space and oil occurrence in low-maturity organic-rich shales: a case study of 3rd member of Chang 7 formation on the Southern Margin of Ordos Basin, China

The fluid vacuum imbibition experiments were performed to quantitatively evaluate the storage space and oil occurrence in low-maturity organic-rich shales. Results showed that: 1) the imbibition volume of water was significantly lower than that of oil, reflecting the important influence of organic pores on the storage space and occurrence state in shale reservoirs; 2) the average inorganic porosity is 3.0% and average organic porosity is 0.6%, the storage space of low mature shales is mainly composed of inorganic pores. In contrast to high-maturity shales, the porosity of low-maturity shales decreases with increasing TOC; 3) the average volume proportion of free-state oil is 32.5%, and the average volume proportion of dissolved-state oil is 67.5%. In contrast to high-maturity shales, low-maturity shale oil mainly exists as dissolved-state, and as TOC increases, the volume proportion of dissolved-state oil increases, which is unfavourable for oil mobility. [Received: September 12, 2024; Accepted: March 20, 2025]