Experimental investigation of the characteristics of organic matter pores in Chang 7 member lacustrine shale from the Ordos Basin due to organic matter evolution induced by hydrous pyrolysis

Abstract

Organic shale deposited in lacustrine environments in China contains large amounts of shale gas, and organic matter (OM) pores are the main storage units for this gas. The pore characteristics of samples from hydrous pyrolysis experiments of lacustrine shale were analysed via a combination of mercury intrusion, low-pressure nitrogen, and carbon dioxide adsorption. An original lacustrine shale sample was collected from the upper Triassic Chang 7 member in the Ordos Basin. Four aliquots were separated from the original shale and artificially heated to a higher maturity using hydrous pyrolysis. The results suggest the following: (1) The conversion of OM to hydrocarbon compounds (HC) improves the mesopore (2–50 nm) size distribution (meso-PSD) and micropore (<2 nm) size distribution (micro-PSD) as OM maturity increases. Micro-PSD promotion increases when the total organic carbon is low. (2) The total pore volumes (PV) of the heated samples increase and are nearly identical, indicating that most new pore formation occurs in catagenesis during the conversion of OM to oil and cracked oil to gas. Macropores (>50 nm) and mesopores primarily contribute to the total PV, and the positive and negative balance controls their contribution throughout the evolution toward OM maturity. (3) The conversion of OM to HC results in an initial increase in the total pore specific surface area (SSA), followed by a decrease. Mesopores primarily contribute to the total pore SSA at the stages of oil and wet gas-catagenesis, but are gradually replaced by micropores. (4) A well-developed micro-PSD increases pore connectivity and shortens flow paths; however, having both well-developed micro- and meso-PSD deteriorates pore connectivity and has an uncertain effect on the flow paths.