Palaeoproductivity and organic matter accumulation during the deposition of the Chang 7 organic-rich shale of the Upper Triassic Yanchang Formation, Ordos Basin, China

The lacustrine Chang 7 organic-rich shale of the Upper Triassic Yanchang Formation is a very important hydrocarbon source rock and contains high total organic carbon content (average = 5.54 wt.%). Previous studies suggest that deposits of the Chang 7 accumulated slowly under dominantly oxic-suboxic bottom-water conditions occasionally interrupted by the establishment of anoxic conditions. However, questions remain regarding the role of palaeoproductivity in accumulation of the Chang 7 organic matter. Palaeoproductivity indicators, including P/Al, Ni/Al, Cu/Al, and Zn/Al, suggest that the deposition of the Chang 7 period was accompanied by high primary productivity level. Furthermore, productivity appears to have been highest in the middle and south-east region of the lake, diminishing to the north-east, the north-west, and the south-west. Elevated productivity resulted in the flux of large amounts of organic matter to the lake floor. Intermittent anoxic bottom-water conditions caused by oxygen consumption during organic matter degradation favoured enhanced preservation of organic matter even under oxic-suboxic water column. Low sedimentation rate made the organic matter in sediments avoid dilution of terrigenous clastics, which made it play a positive role in the accumulation of organic matter.