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

Abstract

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.

Linking Hydrothermal Activity With Organic Matter Accumulation in the Chang 7 Black Shale of Yanchang Formation, Ordos Basin, China

“…The former is closely related to primary productivity, while the latter is mainly related to redox conditions and sedimentation rate (Demaison and Moore, 1980;Pedersen and Calvert, 1990;Calvert and Pedersen, 1993;Curiale and Gibling, 1994;Jones and Manning, 1994;Hay, 1995;Parrish, 1995;Algeo et al, 2011;Ding et al, 2015;Wang et al, 2017;Yuan et al, 2020). Previous studies have found that the sedimentation rate of the Chang 7 Member in the Ordos Basin has little effect on the enrichment of organic matter (Yuan et al, 2016(Yuan et al, , 2020. Therefore, the enrichment of organic matter in the Chang 7 shale is mainly related to paleoproductivity and redox conditions.…”

“…The enrichment of organic matter is mainly controlled by the supply and preservation of organic matter (Arthur and Sageman, 1994;Sun, 2013;Ding, 2014;Wang et al, 2017). The former is closely related to primary productivity, while the latter is mainly related to redox conditions and sedimentation rate (Demaison and Moore, 1980;Pedersen and Calvert, 1990;Calvert and Pedersen, 1993;Curiale and Gibling, 1994;Jones and Manning, 1994;Hay, 1995;Parrish, 1995;Algeo et al, 2011;Ding et al, 2015;Wang et al, 2017;Yuan et al, 2020). Previous studies have found that the sedimentation rate of the Chang 7 Member in the Ordos Basin has little effect on the enrichment of organic matter (Yuan et al, 2016(Yuan et al, , 2020.…”

“…Therefore, the enrichment of organic matter in the Chang 7 shale is mainly related to paleoproductivity and redox conditions. P, Ni, Cu, and Zn, which have been successfully used to elevate the primary paleoproductivity, are critical nutrient elements (Algeo and Maynard, 2004;Tribovillard et al, 2006;Algeo et al, 2011;Wang et al, 2017;Yuan et al, 2020). To eliminate the influence of terrigenous clastics, palaeoproductivity indicators (P, Ni, Cu, and Zn) were normalized to Ti, which is largely supplied in association with terrigenous minerals (Arthur and Dean, 1991;Calvert and Pedersen, 2007;Qiu et al, 2015;Wang et al, 2017; Yuan 9).…”

Depositional environment of Middle Triassic organic‐rich shales in the Ordos Basin, Northwest China

“…The organic‐rich shales in the Yanchang Formation have been considered to be the main hydrocarbon source rock and provide the main reservoir of oil for the Mesozoic petroleum system (Qiu et al, 2014; Yang & Zhang, 2005; Zou et al, 2019). A high primary productivity level and oxic–suboxic bottom water conditions, occasionally interrupted by anoxic conditions, are considered to be the main factors that control the formation of the hydrocarbon resource potential of organic‐rich shales in the Yanchang Formation (Qiu, 2011; Qiu et al, 2015; Yang et al, 2010; Yang & Deng, 2013; Yang & Zhang, 2005; Yuan et al, 2017; Yuan, Liu, Zhou, Xu, & Li, 2019). Similarly, thick organic‐rich shales are also developed in the Middle Triassic Tongchuan Formation, which immediately underlies the Upper Triassic Yanchang Formation.…”

“…The earliest known appearance of a Mesozoic‐type of tropical, multi‐levelled lacustrine ecosystem after the end‐Permian mass extinction is reported from these shales (Zhao et al, 2020). Up till now, studies of the depositional environment of the Tongchuan shales have been lacking, although it is of significance for understanding the formation and preservation of organic‐rich shales and lacustrine fossils (Yuan et al, 2019; Zhao et al, 2020).…”