Controls on organic matter accumulation on the Late-Ordovician shales in Awati Sag in northwestern Tarim Basin, NW China

Abstract

The black shales of the Upper Ordovician Yingan Formation (Fm.) in the northwestern margin of the Tarim Basin was previously considered to be an important source rock. However, the total organic carbon (TOC) values of Yingan black shales was low, averaging 0.51%. Therefore, it remains debated whether they can be served as high-quality source rock. Here, we investigated the sedimentary environment (including terrigenous input, chemical weathering intensity, paleoproductivity, paleoredox conditions) of the Yingan shales and the factors affecting organic matter (OM) enrichment through organic and inorganic geochemical methods. The proxies of terrigenous flux (e.g., Ti/Al and Zr/Al) and sedimentation rate flux [e.g., (La/Yb)P] shows that the terrigenous inputs has no effect on OM enrichment. The low primary productivity (e.g., low values of P/Al, Ba/Al and Ni/Al) of the Yingan shales also exhibits a little effect on OM enrichment. However, the paleoredox proxies (e.g., U/Al, V/Al, U/Th and V/Cr) show that the black shale was deposited in a partial oxidation environment of bottom waters, which is unfavorable for the preservation and enrichment of OM evidenced. The oxidation environment was caused by the enhanced thermohaline circulation and ventilation of bottom waters due to the increasing cooling global climate during Late Ordovician. Therefore, the Yingan shales cannot be served as a source rock or shale gas reservoir. Additionally, the poor OM enrichment model provides new insights for the evaluation of shales linked to cold climatic conditions.