Factors impacting on oil retention in lacustrine shale: Permian Lucaogou Formation in Jimusaer Depression, Junggar Basin

Luocaogou formation developed a set of good quality hydrocarbon source rock in the Jimusaer Depression. Plenty of hydrocarbon shows in shale have been discovered, indicating a good prospect for shale oil exploration. In this paper, the data including Rock-Eval pyrolysis, X-ray diffraction (XRD), lithology, and porosity were used to analyze the impact of oil retention in Lucaogou shale. The results showed that the TOC contents of lacustrine shale have a maximum of 15.51%, averaging 3.49%; the maturity is in the range 0.5%-1.1%; and the organic matters are of type I; the largest net thickness of source rock can reach 194 m, mainly distributed in the centre area rounding J35-J174 well block. Impacts of oil retention in lacustrine shale include TOC, Ro and reservoir physical property. S1 contents increase with growing TOC, and when TOC content is more than 3%, hydrocarbon show thickness ratios (thickness ratio of hydrocarbon show to strata) are over 90%. When Ro reaches 0.8%, OSI (S1/TOC✕100) become largest, and hydrocarbon show thickness ratios are over 80%. Good oil saturation relates to high porosity. Argillaceous limestone and argillaceous dolomite primarily develop matrix pores, while mudstone mainly develops organic pores. Since lower content of clay and higher content of brittle minerals in argillaceous limestone and argillaceous dolomite than in mudstone, both of the porosity and oil saturation of argillaceous limestone and argillaceous dolomite are better than that of mudstone. Generally, the areas deeper than hydrocarbon expulsion threshold (3200-4300 m of depth or 0.8%-1.3% of Ro) are most favourable for shale oil accumulation, which matches the actual oil production data. The depths of high production wells are no less than hydrocarbon expulsion threshold, since S1 contents are high enough to satisfy critical residual amount and pores are developed. Meanwhile, good quality of reservoir developed when organic pores and overpressure were formed during hydrocarbon generation and expulsion.