Organic geochemistry and elements distribution in Dahuangshan oil shale, southern Junggar Basin: Origin of organic matter and depositional environment

Abstract

The Dahuangshan oil shale, located in the northern Bogda Mountain, on the southern margin of the Junggar Basin, was deposited in a Late Permian lacustrine environment. A combined investigation of element and organic geochemistry was performed to define the source rock potential, the paleoenvironment, and source of the organic matter. Thick sequences of oil shales with an average thickness of 638m were deposited in Lucaogou Formation which mainly consists of oil shale, argillaceous dolomite, silty claystone, tuff, limestone, and dolomitic marl. A spot of plant stem fossils and abundance of pyrite crystals, fishtail and fish skeleton can also be found there.Analyzed oil shale samples from Dahuangshan area are characterized by high total organic carbon (TOC) contents (5.6–34.75%), S2 (22.65–199.25mgHC/g rock), hydrogen index (HI, 359–1068mgHC/g TOC), and oil yield (4.9–26.6%), indicating the oil shales have excellent source rock potential. Tmax values (433–453°C) show an early to medium maturation stage of organic matter, which is supported by organic geochemical maturation parameters. All of the obtained kerogen types are types II and I, with oil prone source rock potential.Dahuangshan oil shale samples are rich in SiO2 (68.59%), followed by Al2O3 (10.18%) and Fe2O3 (5.43%). Compared with average shale and North American Shale Composite (NASC), analyzed oil shale samples are obviously enriched in P (0.71%). There is a significant correlation between Al2O3 and Fe2O3, MgO, K2O, MnO, Cu, Ba, Co, and Ni for their association with clay minerals. Besides, the significant correlations between Fe2O3 and MnO, Co, and Ni are considered to result from their similarity on geochemical behavior. All selected oil shales are characterized by distinctly sloping light rare earth elements (LREE) trends (LaN/SmN=2.70–5.95) accompanied by flat heavy rare earth elements (HREE) trends, with distinct Eu negative anomalies (0.60–0.73). Two slightly different patterns of REEs in the oil shale samples are distinguished by the difference in Ce depletion and Nd anomaly.In addition, Dahuangshan oil shale samples are characterized by short- to middle-chain n-alkanes, low carbon preference index (CPI) values (0.93–1.24), single peak composed of nC20 or nC22, low Pr/Ph (0.41–0.91), relatively high Homohop index (0.061–0.99), and high concentrations of C27 sterane, indicating reducing, deep-water, and moderate saline environment with prevalent contribution of algae and microorganisms to organic matter accumulation.