Characteristics and discovery significance of the Upper Triassic–Lower Jurassic marine shale oil in Qiangtang Basin, NW China

Mesozoic marine shale oil was found in the Qiangtang Basin by a large number of hydrocarbon geological surveys and shallow drilling sampling. Based on systematic observation and experimental analysis of outcrop and core samples, the deposition and development conditions and characteristics of marine shale are revealed, the geochemical and reservoir characteristics of marine shale are evaluated, and the layers of marine shale oil in the Mesozoic are determined. The following geological understandings are obtained. First, there are two sets of marine organic-rich shales, the Lower Jurassic Quse Formation and the Upper Triassic Bagong Formation, in the Qiangtang Basin. They are mainly composed of laminated shale with massive mudstone. The laminated organic-rich shale of the Quse Formation is located in the lower part of the stratum, with a thickness of 50–75 m, and mainly distributed in southern Qiangtang Basin and the central-west of northern Qiangtang Basin. The laminated organic-rich shale of the Bagong Formation is located in the middle of the stratum, with a thickness of 250–350 m, and distributed in both northern and southern Qiangtang Basin. Second, the two sets of laminated organic-rich shales develop foliation, and various types of micropores and microfractures. The average content of brittle minerals is 70%, implying a high fracturability. The average porosity is 5.89%, indicating good reservoir physical properties to the level of moderate–good shale oil reservoirs. Third, the organic-rich shale of the Quse Formation contains organic matters of types II1 and II2, with the average TOC of 8.34%, the average content of chloroform bitumen 'A' of 0.66%, the average residual hydrocarbon generation potential (S1+S2) of 29.93 mg/g, and the Ro value of 0.9%–1.3%, meeting the standard of high-quality source rock. The organic-rich shale of the Bagong Formation contains mixed organic matters, with the TOC of 0.65%–3.10% and the Ro value of 1.17%–1.59%, meeting the standard of moderate source rock. Fourth, four shallow wells (depth of 50–250 m) with oil shows have been found in the organic shales at 50–90 m in the lower part of the Bagong Formation and 30–75 m in the middle part of the Quse Formation. The crude oil contains a high content of saturated hydrocarbon. Analysis and testing of outcrop and shallow well samples confirm the presence of marine shale oil in the Bagong Formation and the Quse Formation. Good shale oil intervals in the Bagong Formation are observed in layers 18–20 in the lower part of the section, where the shales with (S0+S1) higher than 1 mg/g are 206.7 m thick, with the maximum and average (S0+S1) of 1.92 mg/g and 1.81 mg/g, respectively. Good shale oil intervals in the Quse Formation are found in layers 4–8 in the lower part of the section, where the shales with (S0+S1) higher than 1 mg/g are 58.8 m thick, with the maximum and average (S0+S1) of 6.46 mg/g and 2.23 mg/g, respectively. © 2023 Science Press. All rights reserved.