Abstract

Lacustrine shales are globally important both as archives of paleoenvironmental change and as unconventional hydrocarbon reservoirs. The first Member of the Upper Cretaceous Qingshankou Formation in the Songliao Basin, NE China, represents a prominent interval of high-quality petroleum source rocks. Despite their significance, the factors governing organic matter accumulation and pore evolution in these lacustrine deposits remain inadequately constrained, especially with respect to the interplay between paleoenvironmental conditions and porosity development. This research explores the geochemical, mineralogical, and pore structure features of lacustrine shales and mudstones from this formation. The samples analyzed display TOC contents between 0.62 and 3.13 wt%, with Rock-Eval pyrolysis results (Tmax avg. 439 °C) reflecting thermal maturity spanning the early to peak oil window. Mineralogically, the samples are dominated by quartz (avg. 28 wt%) and clay minerals (avg. 51 wt%), with feldspar as a minor component (avg. 14 wt%). Geochemical proxies suggest deposition under arid to semi-arid climatic conditions, characterized by minimal chemical weathering, elevated paleo-salinity (Sr/Ba avg. 0.89; 100 × Mg/Al avg. 14.94), and predominantly suboxic to oxic bottom-water conditions (U/Th avg. 0.38; Ni/Co avg. 1.77). Organic matter enrichment is primarily driven by high paleoproductivity (Cu/Al avg. 3.77 × 10−4) and stratified water columns, while detrital input (Zr/Al ratio) is unfavorable for organic matter accumulation. Pore structure analysis reveals micropore volumes averaging 0.0053 cm3/g and meso- and macropore volumes averaging 0.0233 cm3/g. The contents of quartz and clay minerals exhibit no substantial correlation with pore volume, likely due to secondary quartz overgrowth and mechanical compaction. Similarly, the weak negative correlation between TOC and pore volume is attributed to the poorly developed pore networks within kerogen. This study provides new insights into the depositional and diagenetic controls on organic matter accumulation and pore development in the lacustrine Qingshankou Formation, with implications for paleoenvironmental reconstruction and unconventional hydrocarbon exploration in analogous lacustrine basins worldwide.

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