Influence of extractable organic matters on pore structure and its evolution of Chang 7 member shales in the Ordos Basin, China: Implications from extractions using various solvents

Abstract To explore the effect of extractable organic matters on pore structure and its evolution of shales, a suite of lacustrine Chang 7 shales with various maturities from the Ordos Basin, China were extracted using acetone, tetrahydrofuran (THF), carbon disulfide (CS2), and benzene, respectively. Fractionations of extracted organic matters were employed to examine group components. Low-pressure nitrogen and carbon dioxide adsorptions were used to characterize the pore structure before and after extraction. The results showed that the asphaltene comes to be significantly extractable in the presence of THF, which may be attributed to that THF has remarkable ability of decreasing the interaction between asphaltene and shale matrix. The micropores with a diameter ranging from 0.3 nm to 0.86 nm in the samples with high TOC content (>15.6%) clearly increased after extractions, thus soluble organic matters of shales may be mainly stored in the micropores with the diameter range above. On the contrary, mesopores have not obvious changes after extractions. Comparably, CS2 has greater access to release micropores and mesopores than other solvents, which may be related to that CS2 is easier to enter the most inner spots due to its smallest dynamic diameter and strip molecular structure. THF has greater access to release micropores due to its ability of reducing molecular size of asphaltene fraction and dissolving abundant asphaltene with small molecular diameter. TOC exerts a stronger control on mesopores and micropores than minerals, mesopores and micropores mainly exist in organic matters of the studied samples. Effect of soluble organic matters on pore structure was much less than maturity, suggesting the generation and cracking of liquid hydrocarbons aren't the major influence factor of evolution of porosity in studied samples.