Investigation of Organic Matter Sources and Depositional Environment Changes for Terrestrial Shale Succession from the Yuka Depression: Implications from Organic Geochemistry and Petrological Analyses

Abstract

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), C₂₉ Ts/(C₂₉Ts+C₂₉ αβ hopane), C₃₀ αβ/(αβ+βα) hopanes, C₂₉ ααα 20S/(20S+20R) steranes, and C₂₉ 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., C₂₇/C₂₉ ααα 20R sterane, ternary diagram of C₂₇-C₂₈-C₂₉ 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 C₂₇/C₂₉ 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 C₂₇ 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.

Keywords

biomarker / stable carbon isotopes / dameigou formation / lacustrine sediments / paleoenvironment / petroleum geology

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