Main Controlling Factors for Lucaogou Formation β-carotane Enrichment in the Malang Sag of Santanghu Basin

Key words: 
• β-Carotane /
• Prokaryotes /
• Brackish water deposition /
• Hydrothermal activity /
• Lucaogou group /
• Malang sag

Abstract: [Objective] The second member of the Permian Lucaogou Formation (P2l2) is the main source rock in the Malang Sag of the Santanghu Basin, with high abundance of β-carotane, but its content shows great differences in the section. In order to further clarify the longitudinal variation of β-carotane in P2l2, and then to explore the main controlling factors of its enrichment and its distribution on the plane. [Methods] In this study, organic geochemical analysis was carried out and the source rocks investigated were divided into groups I, II and III from bottom to top according to the β-carotane index. The β-carotane index was relatively low in group I, and gradually increased from bottom to top in group II, while the β-carotane index in group III fluctuated greatly. [Results and Discussions] The results show that the relative content of β-carotane is an effective index to reflect the quality of source rocks in the study area. The organic matter of P2l2 source rocks in Malang Sag is mainly composed of lower aquatic organisms. However, compared with the group I, the group II and group III have a richer supply of prokaryote organic matter. The ratios of Steranes/Hopanes, C28 steranes/C29 steranes and (Pr+Ph)/(nC17+ nC18) indicated that the precursors of β-carotane in groups II and III might be algae with phytol side chains of chlorophyll a, b and c and bacteri. According to the parameters of Pr/Ph, C35 hopanes/C34 hopanes and ETR, it can be concluded that the water salinity of source rocks in groups II and III is high, mainly in a reducing environment. Compared with the fluctuating environment of weak oxidation-weak reduction during the deposition of group I and the relatively low salinity of water, it is more conducive to the preservation of β-carotane. A certain intensity of hydrothermal activity occurred during the deposition of group III source rocks, resulting in a large fluctuation of β-carotane content. [Conclusions] Therefore, the enrichment of P2l2 β-carotane in Malang Sag is mainly controlled by the source of organic matter and sedimentary environment.