Lacustrine shale oil systems in China: Advances in characterization methods and resource accumulation models

Abstract

Lacustrine shale oil is an important replacement for conventional oil and gas in China, yet it differs markedly from North American marine shales, exhibiting stronger heterogeneity both between stratigraphic intervals and within beds. Such variability demands high-density sampling, but conventional reservoir testing and sampling methods may be inefficient and yield biased results. The applicability of marine shale exploration models to lacustrine systems also remains uncertain, and a unified enrichment framework is lacking. This study: 1) investigates the lithofacies and organic facies of different lacustrine shale systems; 2) reviews advances in characterizing the four key properties of shale (storage capacity, oil content and mobility, and fracability); 3) synthesizes recent insights into shale oil occurrence, flow, and enrichment; 4) classifies lacustrine shale plays by source–reservoir configuration; 5) identifies geological sweet spots under different tectonic–sedimentary conditions; and 6) establishes corresponding enrichment models. Results show that freshwater lacustrine basins, with substantial terrigenous input, mainly develop siliceous–argillaceous shales or fine sandstone–siltstone lithofacies, whereas saline systems dominated by endogenous deposition favor carbonate-rich lithofacies. Organic matter enrichment is primarily controlled by high primary productivity in freshwater lakes and by selective preservation in saline lakes. Tailored techniques such as magnetic fluid volume measurement, portable X-ray fluorescence, and multistep pyrolysis improve the accuracy of key geological parameters. Organic matter content and kerogen–hydrocarbon interactions are the main controls on oil retention, while oil mobility is influenced by organic–inorganic coupling, pore structure, pressure gradients, and fluid properties. Fundamentally, enrichment mechanisms are similar in marine and lacustrine shales, differing mainly in their macroscopic expressions. Based on the source–reservoir configuration, lacustrine shale plays can be categorized into three types: (1) source-storage separation on meter-scale, (2) source-storage coexistence on centimeter- to millimeter-scale, and (3) source-storage integration on sub-millimeter-scale. Low–medium maturity integration plays in saline settings show the greatest exploration potential, while analogous plays in freshwater systems require high thermal maturity and favorable preservation conditions.