Formation and evolution of nanopores in shales and its impact on retained oil during oil generation and expulsion based on pyrolysis experiments

Abstract

There have been few systematic studies on the relationship between shale nano-pore formation and evolution and its oil generation and expulsion since geological samples are confined by both maturities and compositions. A low maturity shale sample with type II kerogen was used for artificial pyrolysis experiments (spanning a maturity range of 0.72%–1.46% Ro) to simulate its oil generation and expulsion, the nano-pore structure evolution and its impact on the occurrence of retained oil of the shale with increasing maturity were investigated. The results show that the pore structure is controlled by maturity, with a close relationship with oil and gas generation and expulsion. In the low-middle maturity stage (from 0.72% to 0.83% Ro), the organic matter nano-pores are not developed, and the pore structure parameters (denoted by the BET specific surface area and pore volume of the extracted sample) increase slightly with increasing Ro, and the retained oil occurs mainly in macropores and mesopores with a size > 10 nm. In the high maturity stage (from 0.83% to 1.05% Ro), the pore structure parameters increase obviously owing to the formation of organic matter nano-pores resulted from strong oil generation and expulsion, and the retained oil can also occur and enrich in mesopores with size <5 nm as well as micropores. As matured to the early stage of wet gas (from 1.05% to 1.26% Ro), the retained oil is cracked into light oil and wet gas, associating with an enhanced oil expulsion efficiency, and the pore structure parameters increase slightly, which leads to a significant decrease of the retained oil occurring in macropores and mesopores with size > 30 nm. With further increasing Ro from 1.26% to 1.46%, solid bitumen derived from the cracking of the retained oil blocks a portion of the pores, leading to a decrease of pore structure parameters, and the retained oil occurring in different size pores decreases obviously due to its further cracking. These results provide some new information for understanding the influence of maturity on the occurrence and enrichment of shale oil under geological conditions and could give a guide to the evaluation and exploration of shale oil.