Abstract
The exploration of lacustrine shale oil systems has become a critical frontier in global unconventional energy. Current research primarily focuses on deepwater lacustrine shales in large-sized basins, with limited attention to lacustrine shale oil in oil-coal coexisting strata of small- to medium-sized basins. The Fuxin Basin, a small-sized sedimentary basin in northeast China, is known for its cooccurrence of coal and petroleum resources. The third (K(1) sh (3)) and fourth (K(1) sh (4)) members of its Lower Cretaceous Shahai Formation host archetypal coal-measure and lacustrine source rocks, respectively. Therefore, K(1) sh (4) is an excellent research object for studying this category of lacustrine shale oil resource. To date, exploration of the Shahai Formation and related geological studies in this basin have focused primarily on conventional clastic reservoirs, coalbed methane, and shale gas, leaving its shale oil potential largely overlooked. Focusing on 23 K(1) sh (4) mudstone samples in Well LFD1, this study integrated inorganic-organic geochemical and petrological data to evaluate oil-generation potential, reconstruct paleoclimate and paleodepositional environments, determine the biological origin of organic matter (OM) in these samples, and reveal shale oil enrichment and the controls on oil mobility in the K(1) sh (4) lacustrine mudstones. The mudstones are characterized by high OM abundance, mainly Type II kerogen dominated by oil-prone sapropelinite, and midmaturity OM, collectively indicating substantial oil-generation potential. Trace element and molecular geochemical evidence indicates that deposition of these mudstones occurred in a warm, semihumid to semiarid paleoclimate, within a stable, highly restricted, predominantly freshwater (partly brackish) lacustrine environment characterized by weakly oxidizing to weakly reducing, dysoxic conditions. This depositional regime facilitated OM accumulation and preservation, providing a material basis for shale oil enrichment. High ΣnC(21-)/ΣnC(22+) and low terrigenous/aquatic ratio (TAR) indicate that OM is predominantly sourced from algal and prokaryotic bacteria, while a low regular sterane/17α-hopane ratio underscores the significant contribution of prokaryotic bacteria. Samples display favorable shale oil enrichment (mainly medium-high oil content), yet oil mobility is restricted by strong oil adsorption onto OM. The oil saturation index is negatively correlated with ΣnC(21-)/ΣnC(22+) but positively correlated with TAR. These relationships demonstrate that OM biological origin exerts an intrinsic control on oil mobility: First, although algal/bacterial organisms serve as the primary biological sources, a higher terrigenous contribution (relative to algal/bacterial OM) lowers TOC and adsorbed-oil volumes, thereby increasing the movable oil. Second, at equivalent thermal maturity, low ΣnC(21-)/ΣnC(22+) and high TAR values can be associated with larger pore sizes and enhanced reservoir connectivity, facilitating fluid mobility and more complete fractionation of n-alkanes within nano- to microscale confinement. This ultimately leads to a more uniform distribution of n-alkanes across different carbon numbers in terms of content. This study confirms that the K(1) sh (4) lacustrine mudstones in the Fuxin Basin constitute promising targets for future shale oil exploration and provides a pivotal reference for evaluating the lacustrine shale oil potential of the widely distributed Shahai Formation.