Evolution of pore structure and methane adsorption in thermally matured shales: A case study of low-mature Dalong shale in Sichuan Basin, China

Abstract

Adsorbed gas makes a great contribution to shale gas reserve and shale gas development, and therefore it is very important to clarify the dynamic evolution mechanism of adsorbed gas. However, the co-evolution modes of pore structure and methane adsorption capacity are still unclear. Accordingly, a series of experiments, including total organic carbon (TOC), XRD, low-pressure N₂ adsorption (N₂GA), scanning electron microscopy (SEM), field-emission scanning electron microscopy (FE-SEM) and methane adsorption capacity, were performed on a series of artificially simulated samples from a low-mature Dalong shale in the Sichuan Basin. The results demonstrate that the evolution of shale pores particularly OM-related pores, coupled with a decrease in OM content and the alternation of clay minerals, is significantly controlled by thermal maturity. At mature and early high-mature stages, kerogen evolves into liquid hydrocarbons which subsequently fill shale pores, and with further increased thermal maturity, liquid hydrocarbons are cracked and OM pores reappear. As thermal maturity increases, OM pores are developed extensively in solid bitumen and pyrobitumen. Methane adsorption capacities of the simulated samples range from 2.29 to 3.80 cm³/g and from 1.85 to 3.18 cm³/g under dry and moisture-equilibrated conditions, respectively. Methane adsorption capacity is predominately contributed by OMs, and the contribution of clay minerals accounts only for 15.15-22.85% across the simulated samples. Clay mineral content exerts a positive influence on methane adsorption capacity at the mature stage, while it shifts to negligible effect at the overmature stage. The rise in methane adsorption capacity can be attributed to: the decomposition of kerogen, the progression of thermal maturity, the cracking of liquid hydrocarbon, and the expansion of micro- to mesopore networks. In terms of dynamic evolution of shale pores and methane adsorption behavior, four distinct stages of methane adsorption can be identified. At the early mature stage (eqvRo < 1.30%), methane is adsorbed on the surface of OMs and clay minerals. At the high mature stage (1.30%<eqvRo < 2.0%), methane is adsorbed in single-layer form within pores related to original OM, solid bitumen and clay minerals. At the over-mature stage (2.0%<eqvRo < 3.0%), methane exists in pores related to original OM in a single-layer form, and in pyrobitumen pores and clay mineral pores in a multi-layer form, and 4) at extremely over-mature stage (eqvRo > 3.0%), methane is predominantly adsorbed firstly in a multi-layer form in OM pores of pyrobitumen and secondly in pores related to original OM, and the adsorption capacity of clay minerals is enhanced due to the discharge of water. These findings revealed the mechanism of adsorbed gas in different thermal mature stages, and also provided guidance for shale gas evaluation and exploration.