Oil shale enrichment mechanisms in the Lower Cretaceous Jiufotang Formation from the Ludong sag, Songliao Basin, Northeast China

In order to investigate the oil shale enrichment mechanisms and the relationship among gravity flows, volcanism, and oil shale deposition, detailed petrological, sedimentological, and organic geochemical research have been carried out on the Lower Cretaceous Jiufotang Formation in the Ludong sag, Songliao Basin, Northeast China. Seven facies were recognized and were grouped into three facies associations, namely hyperpycnites and intrabasinal turbidites, delta, and lacustrine facies associations. During the deposition of the lower Jiufotang Formation, hyperpycnites were developed during catastrophic flooding events that caused an abrupt increase in the discharge of water and sediment into the deep lacustrine environments, and a delta persisted during normal river discharge intervals and prograded into lacustrine environments. During the deposition of the upper Jiufotang Formation, deep lacustrine facies further expanded, and the extent of hyperpycnites decreased. Four types of oil shales were recognized in the Ludong sag of which those in the lower and upper Jiufotang Formation were defined as medium -to good -quality and good -to best -quality source rocks, respectively, exerting an overall thermal maturity of early oil generation. The lacustrine redox conditions show oxic to dysoxic conditions in the lower Jiufotang Formation, and anoxic to dysoxic conditions in the upper Jiufotang Formation. Oil shales were poorly -developed in the lower Jiufotang Formation and were well -developed in the upper Jiufotang Formation. During the active rift stage of the lower Jiufotang Formation, frequent occurrences of gravity flows and watercarried re -sedimentation of tuffaceous materials were unfavorable for oil shale accumulation. During the waning rift phases of the upper Jiufotang Formation, minor wind -transport of volcanic ash input into the deep lacustrine environments fertilized the lake water and supported algae blooms, leading to reduced oxygen concentrations in the bottom lacustrine waters along with high bioproductivity that were favorable for organic matter preservation and oil shale formation.