Mineralogy and element geochemistry of salinized lacustrine organic-rich shale in the Middle Permian Santanghu Basin: Implications for paleoenvironment, provenance, tectonic setting and shale oil potential

• Published in: Marine and petroleum geology Vol. 120; p. 104569
• Main Authors: Liu, Bo, Song, Yu, Zhu, Kai, Su, Peng, Ye, Xiang, Zhao, Wanchun
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.10.2020
• Subjects: Middle permian; Northwest China; Provenance and tectonic setting; Salinized lacustrine organic-rich shale (SLORS); Shale oil potential; Middle permian; Shale oil potential; Provenance and tectonic setting; Salinized lacustrine organic-rich shale (SLORS); Northwest China
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2020.104569

The Middle Permian Lucaogou Formation in the Santanghu Basin (northwest China) develops one of the thickest salinized lacustrine organic-rich shale (SLORS) in the world. The SLORS has been proven as sources for crude oil and tight oil, as well as sources and reservoirs for shale oil in the basin. A total of 52 SLORS samples were collected from the second member of Lucaogou Formation (P2l2), and analyzed by mineralogy and element geochemistry, in order to reveal the paleoenvironment, provenance, tectonic setting and shale oil potential. The results imply an evolution of paleoenvironment from anoxic, saline to dysoxic-oxic, freshwater lake, which is caused by an increased freshwater influx. Early in the P2l2, volcanic ash and hydrothermal fluids have increased the nutrient level in the lake and therefore elevated the bioproductivity. The P2l2 SLORS is originated from intermediate volcanic rocks, accompanied by minor proportions of basic volcanic rocks. The parent rocks of P2l2 SLORS have experienced weak degree of chemical weathering, and probably derived from a continental island arc setting. Micro-, nano-pores (inter-grain pores, inter-crystalline pores, dissolution pores and organic matter pores) and microfractures are widely observed in the P2l2 SLORS, which provides sufficient storage space for hydrocarbons. The P2l2 SLORS is dominated by carbonate minerals, quartz and feldspar, which are all brittle minerals and favourable for artificial fracturing. Comprehensive study indicates that the upper P2l2 SLORS hosts a better shale oil potential, due to the relatively high TOC, abundant micropores and microfractures, and high brittle mineral contents. In addition, individual element geochemical parameters in the Santanghu Basin should be used with caution, for example, the paleoredox proxy U/Th and paleosalinity proxy Sr/Ba are also influenced by volcanic and hydrothermal activity. This implies that a multi-proxy analysis should be applied during the study of paleoenvironmental conditions. •Paleoenvironment of the P2l2 organic-rich shale was reconstructed.•Provenance and tectonic setting of the P2l2 organic-rich shale were indicated.•Shale oil potential of the P2l2 organic-rich shale was comprehensively evaluated.•Suggestions for further shale oil exploration and exploitation were proposed.