Accumulation Mechanism of Organic Matters in Paleogene Qaidam Basin, Northwestern China

Abstract

The high-quality laminated source rock organic matter (OM) originated from planktonic algae, and its sedimentation was affected by global climate change significantly in the upper Xiaganchaigou Formation of the western Qaidam Basin. However, coupling research on the paleoenvironment change and OM enrichment during the sedimentation period of the source rock is still lacking. This study from the aspects of sedimentary petrology, geochemistry and paleontology palynology, the paleoenvironment of source rock is restored and the OM enrichment model is established in the study area. Firstly, kerogen maceral identification indicates that the kerogen maceral is mainly composed of Botryococcus, accompanied with amorphous organic matter and plant debris. Secondly, arid climate and relatively active tectonic were observed during the deposition of the source rock. The water column was received felsic source from the continental island arc tectonic background, and has the environmental characteristics of relatively saline, shallow depth, medium low productivity, fast sedimentation rate and anoxic reduction and so on. Lastly, the first-order controlling factors for the OM enrichment are anoxic water conditions and suitable sedimentation rate, and the secondary controlling factor is paleoproductivity. Through the coupling study of paleoclimate, paleoenvironment and OM enrichment, the paleoclimate high frequency alternating evolution was the root cause of sedimentary environment change and OM enrichment of the laminated shale in the Upper Xiaganchaigou Formation. The study on the OM enrichment mechanism of algae in Qaidam provides a good model for understanding the coupling relationship between the algae bloom in the saline lake basins and the environments, and provides important theoretical basis for predicting shale oil “sweet spot” and production well sites arrangement for the continental saline lacustrine basins.

Keywords

Botryococcus / paleoclimate / control factors / enrichment model / organic matter / Upper XGCG Formation / Qaidam Basin / petroleum geology

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