Mineralogy and geochemistry of the Middle Permian Pingdiquan Formation black shales on the eastern margin of the Junggar Basin, north‐west China: Implications for palaeoenvironmental and organic matter accumulation analyses

Abstract

The Middle Permian black shales from the eastern Junggar Basin of northern Xinjiang, north‐west China are one of the richest and thickest lacustrine hydrocarbon source rocks in China. The palaeoenvironmental settings and controlling factors of organic matter accumulation along the margins of the Middle Permian lacustrine basin remain unclear, although the same works have been undertaken in the centre of the Middle Permian lacustrine basins. Here, we performed a combination of mineralogical and geochemical analyses on the black shales of the Middle Permian Pingdiquan Formation from the eastern margin areas of the Junggar Basin. The results show that the Pingdiquan Formation black shales in the study areas are organic‐rich, with total organic carbon (TOC) contents of 0.74–17.61 wt% (avg. 5.81 wt%). Salinity proxies including Sr/Ba and Rb/K ratios, combined with traces of gypsum and halite in a few samples, suggest brackish to saline water conditions. Palaeoclimate proxies indicate a warm‐humid environment during the deposition of the Pingdiquan Formation black shales. The redox proxies V/(V + Ni), V/Cr and U/Th ratios, and Eu and Ce anomalies together indicate dysoxic‐anoxic conditions of lake water. Strong positive correlations between palaeoproductivity proxies (P/Al, Ni/Al, Cu/Al, and Zn/Al) and TOC contents imply that palaeoproductivity may have played an essential role in organic matter accumulation during the deposition of the black shales. In contrast, negative correlations between terrestrial detrital input proxies (Al and Ti) and TOC contents indicate that terrestrial detrital input may have acted as a diluent to organic matter accumulation. Additionally, although the correlation between redox proxies [V/(V + Ni) and U/Th] and TOC contents is not straightforward, dysoxic‐anoxic conditions of lake water are an essential prerequisite for organic matter accumulation in the Pingdiquan Formation black shales.

Paleoenvironmental Conditions and Shale Oil Potential of the Carboniferous Ha’erjiawu Formation in the Santanghu Basin, NW China

“…The paleoenvironmental conditions of shale can be discerned through its elemental geochemical composition [29][30][31][32][33]. In general, the B content and certain geochemical indicators, such as Sr/Ba, Rb/K, and B/Ga ratios, may serve as proxies for paleosalinity [22,[34][35][36].…”

“…The concentration of the element B in the black shale samples from the Ha'erjiawu Formation ranges from 1.54 to 3.77 ppm, which is significantly lower compared to that found in saline and brackish water environments. The Rb/K and B/Ga ratios have been widely established as excellent paleosalinity proxies [22,29,40]. In general, Rb/K ratios greater than 0.006, between 0.004 and 0.006, and less than or equal to 0.0028 correspond to saline, brackish, and freshwater environments, respectively [41].…”

“…Processes 2023, 10, x FOR PEER REVIEW 9 of 18 excellent paleosalinity proxies [22,29,40]. In general, Rb/K ratios greater than 0.006, between 0.004 and 0.006, and less than or equal to 0.0028 correspond to saline, brackish, and freshwater environments, respectively [41].…”

Study on Sedimentary Environment and Organic Matter Enrichment Model of Carboniferous–Permian Marine–Continental Transitional Shale in Northern Margin of North China Basin

“…The abundance of shale organic matter affects the generation and accumulation of shale oil and gas, but the enrichment and preservation of organic matter are complex physical and chemical processes [17][18][19][20][21]. Previous studies have shown that organic matter enrichment and preservation are jointly controlled by the depositional environment (terrigenous input, sedimentation rate, paleoclimate, paleoredox, paleosalinity, and primary productivity) and the source of organic matter [19][20][21][22][23][24]. The controlling factors of the depositional environment and organic matter enrichment of marine-terrestrial transitional shale are more complex [21,25].…”