Influence of high-frequency lake level fluctuations on organic matter accumulation in the northern Qaidam Basin, NW China: Insights from spectral attribute analysis and geochemistry

Abstract

Assemblages of multiple organic matter-rich sedimentary energy carriers are frequently found in Jurassic nonmarine basins of China. These basins are strongly influenced by high-frequency fluctuations in lake levels. A series of coal- and oil shale-bearing successions reflecting periodic lake level variations is also present in the Shimengou Formation, northern Qaidam Basin, NW China. These Middle Jurassic coal and oil shale seams provide an excellent basis for studying the effects of high-frequency lake level fluctuations on organic matter accumulation. Moreover, additional factors influencing the primary organic matter input (e.g., wildfires) and its preservation and later diagenetic transformation (e.g., processes during coalification) must be considered. In the present study, organic matter maturity, source potential, and paleoenvironmental conditions during source rock formation are investigated based on macro- and micropetrographic data, proximate analyses, bulk geochemical parameters, biomarker analyses, stable isotope geochemistry, and spectral attribute analysis to provide a sequence stratigraphic framework for the accumulation of organic-rich intervals. According to the established depositional model, the Shimengou Formation can be subdivided into four depositional sequences, including three organic matter-rich intervals. These include the thick mixed gas- and oil-prone Sq1 coal seam with a relatively high ash yield, which represents a highstand systems tract, the gas-prone Sq3 coal seam corresponding to a transgressive systems tract, and the Sq4 oil shale layers likely corresponding to a subsequent highstand systems tract, indicating a drowned mire. The results provide insights for the optimization of high-quality sedimentary mineral resource production in similar basins worldwide.