Control of Lake-Level Changes on the Distribution of the Organic-Rich Shales of the Upper Sha-IV Sub-member in the Leijia Area in the Liaohe Depression

Multiple sets of organic-rich shales developed in the fourth Member of the Shahejie Formation (Sha-IV Member) in the Leijia area. These shales have received considerable attention owing to their accumulated thickness, wide distribution range, and high organic matter (OM) content. To better explore the spatial and temporal distribution features of organic-rich shales in the Sha-IV Member, the favorable development stages of organic-rich shales, and the favorable development environments, this study aims to investigate the corresponding depositional system and depositional facies of the upper part of the Sha-IV Member (upper Sha-IV sub-member) in the Leijia area using core data and thin section information from several wells in the Leijia area. Additionally, the stratigraphic cyclicity and evolution of the sedimentary facies, sequence stratigraphic framework, and paleogeographic patterns of the upper Sha-IV sub-member were established based on well-logging data. The favorable stages and sedimentary facies for the formation of organic-rich shales are further discussed under the sequence stratigraphic framework. The results show that the upper Sha-IV sub-member constitutes a third-order sequence (SQ₁), the organic-rich shales are mainly developed in the transgressive systems tract (TST₁) and the forced regressive systems tract (FRST₁) in SQ₁ and are developed in both the shallow-lake and lacustrine subfacies. High primary productivity during the deposition period was the main factor controlling OM accumulation in the upper Sha-IV sub-member shale. This study provides the basic geological materials and foundations for unconventional Eocene oil and gas exploration in the Leijia area.