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The Character and Origin of Lacustrine Source Rocks in the Paleogene Synrift Section, Bohai Bay Basin, Northwest, China


The Langgu depression, located at the northwest Bohai Bay Basin, is characterized by a thick paleogene organic rift sequence. The integration of bulk geochemical, biomarker, major and trace elements, and isotopic data with geological and paleontological information provided the basis for a better assessment of rift lake evolution and its impact on source rock development. The lacustrine source rocks under different tectonic and climatic conditions, including a lower initial rifting section(Es4), deposited during inactive faulting, slow subsidence and arid climate, overlain by an upper intense rifting section, deposited during active faulting, rapid subsidence and wet climate (Es3). Total organic carbon (TOC) averages 0.5∼1% throughout the lower initial rifting section, and 1∼5% in deep lake shales in the upper part of the active rift section. Organic matter consists of mixed types and III kerogen in the source rock. Redox proxies (Pr/Ph, C35 22S/C3422S hopane, gammacerane index and V/(V+Ni)) indicate that the lake was relatively reducing throughout deposition of the inactive rift and active rift sections. Therefore, enhanced anoxia did not trigger deposition of the richest source rocks. The major nutrients elements, d13 C isotopic and 4-methylsterane indicate that the primary productivity of Es4 was lower than that of Es3. Moderate hydrogen indices andd13C-depleted carbon isotopic composition of the organic matter might reflect enhanced organic preservation during Es4 deposition. On the other hand, high contents of phosphate, 4-methylsterane, and d13 C enrichment of the organic matter are interpreted to have resulted from enhanced primary productivity. The richest source rocks are associated with high rates of organic productivity triggered by the change of climate. High rates of weathering and a high flux of nutrients to the basin during humid and semihumid periods reflected by a positive correlation between high Al/K and TOC. We propose that the major nutrients that controlled the organic carbon richness of the shales impacted by the climate accounted for the deposition and distribution of high quality lacustrine source rocks.