Classification and oil system of continental shale: Es3L sub-member of Bonan sag, Jiyang depression, Eastern China

Abstract

On the basis of shale textures, carbonate contents, and total organic carbon (TOC) contents, five types of continental shale are identified: organic-rich laminated argillaceous limestone, organic-rich laminated lime mudstone, organic-lean laminated argillaceous limestone, non-laminated massive argillaceous limestone, and non-laminated massive lime mudstone. Criteria for shale oil system is also proposed, characterized by Ro value higher than 0.5 %, TOC content higher than 0.5 wt%, or S1 (free hydrocarbon) value higher than 0.5 mg/g, with more than 2-m-thick shale top and bottom layers, and the whole system thickness is no less than 30 m. The Es3L (lower sub-member of the third member of Paleogene Shahejie formation) continental shale has total organic carbon (TOC) values ranging from 0.71 to 9.32 wt%, with an average of 3.1 wt%. The organic matter (OM) is predominantly type I kerogen, with minor amounts of type II kerogen. Vitrinite reflectance ranges from 0.7 to 0.9 %, indicating that most of the shales are in the oil generation window. The temperature of maximum yield of pyrolysate (Tmax) ranges from 424 to 447 °C, with an average of 440 °C. Free hydrocarbon (S1) values range from 0.03 to 13.12 mg HC/g rock, with an average of 2.14 mg HC/g rock, the hydrocarbons cracked from kerogen (S2) yield values in the range of 1.19–78.59 mg HC/g rock, with an average of 17.07 mg HC/g rock, and hydrogen index (HI) values range from 160.58 to 1041.82 mg/g TOC, and with an average of 495.87 mg/g TOC. The dominant minerals of the Es3L shale are carbonates (including calcite and dolomite), ranging from 38 to 92 wt%, with an average of 65 wt%, along with secondary clay and quartz, both at an average of 17.5 wt%. The continental shale oil resource potential of Es3L in the Bonan sag was evaluated through further research on TOC content, and free oil content (S1). The evaluation results show that the shale oil accumulation intensity ranges from 0.3 to 4 × 106 ton/km2. The oil content of the Es3L continental shale is similar to that of Monterey shale and Antelope shale of the USA.