Continental organic-rich shales are well developed in the Dameigou Formation within the Yuka Depression of the Qaidam Basin. Here, the Rock-Eval pyrolysis, biomarkers, organic petrology, and stable carbon isotope have...Continental organic-rich shales are well developed in the Dameigou Formation within the Yuka Depression of the Qaidam Basin. Here, the Rock-Eval pyrolysis, biomarkers, organic petrology, and stable carbon isotope have been carried out on the Middle Jurassic Dameigou Formation source rocks from the northwest part of Yuka Depression, Qaidam Basin in order to study their thermal maturity, source of organic matter(OM), and palaeoenvironment changes. The Rock-Eval pyrolysis data(e.g., T_(max)), vitrinite reflectance, and biomarker-derived thermal maturity parameters(e.g., carbon preference index, Ts/(Ts+Tm), C_(29)Ts/(C_(29)Ts+C_(29)αβ hopane), C_(30) αβ/(αβ+βα) hopanes, C_(29)ααα 20S/(20S+20R) steranes, and C_(29)αββ/(αββ+ααα) steranes) suggest all studied samples stay between immature and low mature stage. The maceral compositions, stable carbon isotope compositions, n-alkane distributions, and biomarker-derived source parameters(e. g., C_(27)/C_(29)ααα 20R sterane, ternary diagram of C_(27)-C28-C_(29)steranes, C24 tetracyclic terpane) indicate both aquatic organisms and higher plants are the source of OM in the shales, but land plants are dominant. Generally low gammacerane concentration and environment-related parameters(e. g., cross-plots of C_(27)/C_(29)ααα 20R sterane vs. Pr/Ph) indicate these source rocks may be derived from lacustrine and fluvial-deltaic environments with fresh water, which is also supported by the variations of stable carbon isotopes from OM in the source rocks. However, the stable carbon isotope compositions of OM in the source rocks were influenced by multiple factors(e.g., source types and depositional environment) in the Yuka Depression. Slightly brackish condition is recorded in the upper part of the ZK6-1 well favor the formation of lacustrine algae, as confirmed by high contents of C_(27) steranes and short-chain n-alkanes. The variation of reducing to oxidizing condition of study area is possibly associated with the periodical flooded river-influenced aquatic condition during the deposition of the Middle Jurassic Dameigou Formation.展开更多
基金financially supported by the National Natural Science Foundation of China (Nos.42102215,41702144)the Science and Technology Research Project for Higher Education Institutions in Hebei Province (No.QN2021033)。
文摘Continental organic-rich shales are well developed in the Dameigou Formation within the Yuka Depression of the Qaidam Basin. Here, the Rock-Eval pyrolysis, biomarkers, organic petrology, and stable carbon isotope have been carried out on the Middle Jurassic Dameigou Formation source rocks from the northwest part of Yuka Depression, Qaidam Basin in order to study their thermal maturity, source of organic matter(OM), and palaeoenvironment changes. The Rock-Eval pyrolysis data(e.g., T_(max)), vitrinite reflectance, and biomarker-derived thermal maturity parameters(e.g., carbon preference index, Ts/(Ts+Tm), C_(29)Ts/(C_(29)Ts+C_(29)αβ hopane), C_(30) αβ/(αβ+βα) hopanes, C_(29)ααα 20S/(20S+20R) steranes, and C_(29)αββ/(αββ+ααα) steranes) suggest all studied samples stay between immature and low mature stage. The maceral compositions, stable carbon isotope compositions, n-alkane distributions, and biomarker-derived source parameters(e. g., C_(27)/C_(29)ααα 20R sterane, ternary diagram of C_(27)-C28-C_(29)steranes, C24 tetracyclic terpane) indicate both aquatic organisms and higher plants are the source of OM in the shales, but land plants are dominant. Generally low gammacerane concentration and environment-related parameters(e. g., cross-plots of C_(27)/C_(29)ααα 20R sterane vs. Pr/Ph) indicate these source rocks may be derived from lacustrine and fluvial-deltaic environments with fresh water, which is also supported by the variations of stable carbon isotopes from OM in the source rocks. However, the stable carbon isotope compositions of OM in the source rocks were influenced by multiple factors(e.g., source types and depositional environment) in the Yuka Depression. Slightly brackish condition is recorded in the upper part of the ZK6-1 well favor the formation of lacustrine algae, as confirmed by high contents of C_(27) steranes and short-chain n-alkanes. The variation of reducing to oxidizing condition of study area is possibly associated with the periodical flooded river-influenced aquatic condition during the deposition of the Middle Jurassic Dameigou Formation.
