Effect of Sedimentary Environment on Mudrock Lithofacies and Organic Matter Enrichment in a Freshwater Lacustrine Basin: Insight from the Triassic Chang 7 Member in the Ordos Basin, China

Gradually replacing fossil fuels with renewable energy constitutes a long-term strategy for achieving sustainable development. In the short term, it is necessary to explore unconventional oil and gas resources to support current economic sustainability and to secure essential time for the energy transition. With the continuous growth in global energy demand, unconventional resources such as shale oil and shale gas have become important alternative energy sources. Lacustrine mudrock successions demonstrate significant potential for unconventional oil and gas resources. However, the unclear understanding of how paleoenvironmental evolution influences lithofacies and organic matter enrichment restricts the optimization of shale oil reservoirs and evaluation of shale oil resources, thereby hindering the progress of lacustrine shale oil exploration and development. The mudrocks in the Chang 7 Member of the Triassic Yanchang Formation, Ordos Basin, were deposited in a pro-delta to a deep lacustrine environment and are rich in shale oil resources. Through petrographic, sedimentological, sequence stratigraphic, and geochemical analyses, this study reveals how the evolution of the paleoenvironment controlled the development of mudrocks and the enrichment of organic matter, and establishes a sedimentary model for freshwater lacustrine systems. Six lithofacies have been identified within the mudrock interval of the Chang 7 Member. According to the T-R (transgressive–regressive) sequence model, the Chang 7 Member can be subdivided into three fourth-order sequences, termed Parasequence Set 1–3 (PPS1–3). Mudrock is predominantly developed in the fourth-order sequences PSS1 and PSS2. The PSS1 and the lower part of PSS2 consist of lithofacies 1–4, representing semi-deep to deep lacustrine deposits. The upper part of PSS2 develops lithofacies 5, representing shallow lacustrine to pro-delta deposits. Fluctuations of the lake level controlled the vertical stacking of lithofacies and the transition in depositional mechanisms. During lake-level rise, bottom currents shifted to suspension settling, whereas the opposite occurred during lake-level fall. The organic matter is derived from algae, and its enrichment is jointly controlled by productivity and the redox conditions. Volcanic–hydrothermal activity and a humid climate promoted high productivity in the water body. This high productivity promotes dyoxic conditions in the bottom water. Fourth-order relative lake-level fluctuations also influence organic matter enrichment. During lake-level rise, increased productivity coupled with reduced consumption and dilution favors organic matter enrichment. Conversely, organic matter accumulation is inhibited during lake-level fall. Ultimately, a depositional model for a freshwater lacustrine basin under a humid to semi-humid climatic background was established. This paper elucidates the influence of sedimentary environment on mudrock lithofacies and organic matter enrichment, providing a theoretical basis for optimizing shale oil reservoir selection and resource assessment, thereby promoting efficient exploration and low-carbon development of shale oil in lacustrine basins.