Genesis of Botryococcus-rich laminae of the Eocene Upper Xiaganchaigou Formation in the western Qaidam Basin, NW China

Abstract

Laminated source rocks exhibit high hydrocarbon potential and good reservoir characteristics. Botryococcus fossils with high hydrocarbon potential have been discovered in the laminated source rocks of the Upper Xiaganchaigou Formation in the Qaidam saline basin in earlier research. However, planktonic algae typically thrive in freshwater or slightly brackish water environments. Subsequently, the seemingly contradictory phenomenon of Botryococcus blooms in saline lakes may be the key to revealing the formation mechanism of these high-quality algae-rich laminated source rocks. This study investigates the rich Botryococcus laminated source rocks in saline lacustrine basins using petrology and elemental geochemistry. First, this study indicates that the detrital laminae are rich in Botryococcus and pyrite framboids and form a couplet with clay or carbonate laminae. Comparatively, the clay laminae in the couplet lack Botryococcus but are rich in halite and amorphous microcrystalline pyrite particles, whereas the carbonate laminae lack Botryococcus and contain very little pyrite. Second, the nutrient element phosphorus exhibits minor changes among these laminae, suggesting that the key factor controlling the Botryococcus bloom may not be the nutrient element but water salinity and temperature conditions related to growth habits. Finally, the particle size and formation characteristics of pyrite framboids indicate water column redox stratification, suggesting that in warm and humid summer, surface runoff resulting from heavy rainfall brings abundant terrigenous minerals and elements into the lake water, the lake deepens and stratifies under the joint influence of temperature, salinity, and algae; the detrital laminae contain debris, Botryococcus, and pyrite framboids. Conversely, in cold and dry winters, the shrinking salinized lake water inhibits Botryococcus growth and pyrite framboid formation. Clay laminae rich in halite or carbonate laminae are deposited. The regional distributions of these sediment types also exhibit obvious differences that the Yingxiongling sag is rich in clay–detrital couplet laminae, whereas the Zhahaquan sag exhibits a high abundance of carbonate–detrital couplet laminae. Overall, the investigation of the genesis of laminated source rocks in saline lake basins may be of great significance to hydrocarbon understanding and exploration.