Title : Organic geochemical characteristics of carboniferous hybrid lacustrine–marine source rocks and their implications for unconventional shale oil potential
Abstract:
Carboniferous sedimentary basins contain extensive organic-rich shale successions deposited under alternating lacustrine and marine conditions, forming hybrid petroleum systems with considerable potential for unconventional shale oil exploration. Recurrent marine incursions into lacustrine environments generated mixed organic facies composed of terrestrial, lacustrine algal, and marine-derived organic matter. These hybrid depositional systems significantly influenced organic matter preservation, kerogen composition, retained hydrocarbons, and shale oil generation potential within fine-grained sedimentary sequences.
This study investigates the organic geochemical characteristics, depositional environment, and hydrocarbon extraction potential of Carboniferous lacustrine–marine hybrid source rocks using integrated sedimentological observations, solvent extraction analysis, Rock-Eval pyrolysis, and biomarker geochemistry. The research focuses on evaluating how mixed depositional conditions control retained hydrocarbons and unconventional shale oil prospectivity.
Geochemical analyses indicate the presence of predominantly mixed Type II–III kerogen with intervals enriched in oil-prone organic matter associated with lacustrine algal productivity and intermittent marine influence. Solvent extraction results reveal appreciable quantities of extractable organic matter (EOM), indicating effective hydrocarbon retention within low-permeability shale matrices. Saturated and aromatic hydrocarbon fractions obtained from extraction further demonstrate a combination of terrestrial and marine organic inputs.
Biomarker distributions, including sterane and hopane assemblages, indicate fluctuating depositional conditions influenced by episodic marine transgressions into lacustrine settings. Elevated C29 steranes reflect substantial terrestrial organic contribution, whereas increased C27 steranes and marine-associated biomarkers suggest periodic marine influence during deposition. Pristane/phytane ratios, gammacerane occurrence, and hopane distributions indicate deposition under variable redox conditions ranging from suboxic to anoxic environments, with episodes of water-column stratification and salinity fluctuation favoring organic matter preservation.
The integration of extraction data with biomarker geochemistry demonstrates that retained hydrocarbons are concentrated within organic-rich laminated shale intervals, highlighting the significance of hybrid depositional environments in shale oil accumulation. Marine influence appears to enhance preservation conditions and contribute to improved hydrocarbon generation characteristics relative to purely terrestrial Carboniferous source rocks.
The results suggest that Carboniferous lacustrine–marine hybrid shales represent promising yet underexplored unconventional shale oil targets in mature petroleum basins. Integrated geochemical and extraction-based evaluation provides an effective framework for identifying prospective shale oil intervals and improving unconventional resource assessment in mixed depositional systems.
