Paleoenvironmental conditions, organic matter accumulation, and unconventional hydrocarbon potential for the Permian Lucaogou Formation organic-rich rocks in Santanghu Basin, NW China

Abstract

Organic-rich rocks have received increasing attention for they are essential for evaluating hydrocarbon potential and unravelling the interaction between Earth surface environments and the biosphere. In this paper, we performed a combination of mineralogical, inorganic and organic geochemical analyses on a 200-m core of the Permian Lucaogou Formation organic-rich rocks obtained from Santanghu Basin, NW China, to investigate paleoenvironmental conditions, organic matter accumulation, and hydrocarbon potential. The redox- and salinity-sensitive proxies (e.g. V/Cr, V/(V+Ni) and Sr/Ba, Rb/K, B/Ga, and B content), together with the wide distribution of pyrite and carbonate minerals in these dark, thinly laminated organic-rich rocks, suggest that dysoxic-anoxic, brackish-saline, alkaline conditions prevailed during the deposition of the Lucaogou Formation in the Santanghu Basin. The occurrence of hydrothermal minerals, including strontianite and barite, and enrichments of B (up to 158.0ppm), Mn (up to 1291ppm), and certain incompatible elements (Li (up to 923ppm), Rb (up to 848ppm), Sr (up to 1760ppm), Hf (up to 12.7ppm)) as well as Ni vs. Co vs. Zn and Fe vs. Mn vs. (Cu+Co+Ni)×10 ternary diagrams demonstrate that the Lucaogou Formation received hydrothermal input during deposition, which suggests hydrothermalism was the source of brine that has caused the salinization of the water column.The positive correlation of HI value and TOC content, together with redox-sensitive proxies, illustrates primary productivity in surface water and preservation in anoxic bottom water collectively contributed to the enrichment of organic matter. Moreover, the positive correlation between silicate mineral content and TOC content and the negative correlation between carbonate content and TOC content suggest that terrigenous input as the main source of nutrients promoted organic matter accumulation, while carbonate constituents acted as a diluent to organic matter accumulation during deposition of the Lucaogou Formation. The faint positive correlation between TOC content and hydrothermal intensity indicator (Cr/Zr) seems to indicate that hydrothermal fluids may provide an additional source of nutrients for organic matter accumulation.In addition, the oil generation capacity of Lucaogou Formation, with abundant oil-prone organic matter (Type I/II1 kerogen with HI values ranging from 421.18–918.41mgHC/g TOC) in the early mature to mature stage (Tmax values ranging from 435 to 451°C), is considerable. The Rock-Eval oil saturation index (OSIS1/TOC×100) values range from 6.04–164.07mgHC/g TOC, suggesting the Lucaogou Formation contains producible oil. The high brittle mineral (including quartz and carbonates) contents (>50%wt%) and traces of clay minerals (<5wt%) imply that the Lucaogou Formation is likely to respond well to hydraulic stimulation. Consequently, the Lucaogou Formation in Santanghu Basin is expected to be a potential lacustrine shale oil play.