INTEGRATING SEDIMENTOLOGY AND GEOCHEMISTRY TO UNDERSTAND DEPOSITIONAL PROCESSES IN LACUSTRINE SHALE: A CASE STUDY FROM THE EOCENE SHAHEJIE FORMATION IN THE DONGYING DEPRESSION, EASTERN CHINA

Recent studies have shown that integration of sedimentologic and geochemical studies to marine shale is a powerful approach for identifying subtle differences in marine depositional process and environment; however, such practice to lacustrine shale is relatively rare. The lacustrine shale succession of the upper fourth Member (Es4U) and lower third Member (Es3L) of the Eocene Shahejie Formation (Es4U–Es3L) in the Dongying Depression, eastern China is the most important hydrocarbon source rock and an unconventional reservoir. Here we conduct detailed lithofacies characterization to the Es4U–Es3L shale succession in the NY1 drilling core by combining core descriptions, microscopic observations, mineralogy and organic geochemistry analyses. We identify four major lithofacies, including the laminated calcareous shale deposited in deep, stratified, anoxic lake with high salinity bottom water similar to marine water mass with intermittent episodes of dust storms and storm events; the deformed calcareous shale deposited in deep, stratified, saline lake with frequent earthquake activities and seasonal floods; silt-bearing clay-rich shale deposited in a distal pro delta developed in a shallow and hydrologically open lake; and the interbedded evaporite and shale deposited in small salt lake. The lithofacies changes of the Es4U–Es3L shale succession through time suggest that the depositional environment changed from a salt lake to a shallow, hydrologically open lake, then to deep, stratified lake. We suggest that the sudden change from a shallow lake to a deep lake was controlled by the tectonic subsidence associated with the development of depression-bounding faults. The paleoclimate during the studied period was generally arid with strong evaporation given the majority of the shale succession was deposited in saline lacustrine environments. Episodes of floods associated with relatively humid climate freshen the saline lake water and brought terrigenous clastic such as detrital quartz grains and plant fragments into the shale succession. This study highlights that integration of sedimentologic observations and rigorous geochemical analyses is a powerful approach to interpret the depositional processes and guide future hydrocarbon exploration in lacustrine shale successions.