An Integrated Stratigraphy Study of a Paralic Lacustrine Basin: Insights into the Organic Matter Accumulation

Abstract

Several shale oil intervals, including those in offshore China, were deposited in paralic lacustrine basins that experienced marine incursions. Marine incursions could be either favorable or unfavorable for the accumulation of organic matter (OM) and shale oil. However, the influence and specific mechanisms of seawater on OM accumulation require further in-depth investigation. During the deposition of the Triassic Chang 7 Member in the Ordos Basin, seawater from the Paleo-Tethys Ocean intruded into the basin. Taking this interval as a case study, this paper employs comprehensive analyses to reveal the influence of marine incursion on water column conditions and OM accumulation. Under humid climatic conditions, the water body was fresh to brackish, characterized by high productivity and oxic–dysoxic conditions. The OM is primarily derived from algae, and its accumulation was jointly controlled by primary productivity, redox conditions, and terrigenous input. OM accumulation is controlled by fluctuations in the relative water level (RWL) associated with third-order sequences. During the period of high RWL, seawater incursions enhanced water column productivity and reduced conditions by increasing nutrient supply and salinity, resulting in the highest OM content. During the early and late periods of the RWL, as seawater receded, OM production declined while consumption and dilution increased, resulting in a gradual decrease in its content. The RWL fluctuations at the fourth-order scale also significantly influence OM accumulation. These results can enhance the understanding of OM accumulation in paralic lacustrine basins with a history of seawater incursion. While promoting shale oil exploration in the Ordos Basin, they can also serve as a research analog for shale oil exploration in basins with similar geological backgrounds, such as the Bohai Bay Basin.

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