Quantitative influence of soluble organic matter on pore structure in transitional shale

Some representative samples of the Taiyuan Formation from the eastern uplift of the Liaohe Depression were selected in order to quantitatively evaluate the influence of organic matter on pore structure in transitional shale. Methods include quantitative mineral analysis by X-ray diffraction (XRD), Rock-Eval pyrolysis, and low pressure nitrogen adsorption. Analyses were performed on aliquot samples in the natural state and after soluble organic matter (SOM) removal by treatment with hydrogen peroxide (H₂O₂). The results indicated that the crystal structure of inorganic minerals is stable and the SOM removal process oxidizes and decreases or removes pyrite and siderite from the samples. All other inorganic phases remain unaffected. Compared with marine shale, OM pores are undeveloped, and the SOM filled in inorganic pores and matrix in the form of discrete particles or as laminations in the transitional shale samples. The cumulative fraction of total SOM actually held in inorganic pores ranges from 53.06% to 90.38%, and it mainly filled in mesopores and macropores. After SOM removal, the pore volume and pore surface area of samples both increase. The cumulative increase fraction of pore volume ranges from 72.14% to 121.83% and the cumulative increase fraction of pore area ranges from 32.8% to 52.74%. The original TOC content and the SOM proportions control pore volume and pore area increase.