Characterization and Genetic Mechanism of Deep Clastic Reservoirs: A Case Study of Yangwuzhai Block in Raoyang Sag

This study aims to reveal the development characteristics and controlling factors of deep favorable reservoirs in the Lower Third Member of the Shahejie Formation (Es₃^L) in the Yangwuzhai Block of the Raoyang Sag, Jizhong Depression, Bohai Bay Basin. The reservoir heterogeneous properties and the genetic mechanisms are investigated by an integrated approach combining sedimentological and diagenetic analysis, thin section petrography, scanning electron microscopy (SEM), cathodoluminescence (CL), and porosity-permeability measurements. The results indicate that the Es₃^L reservoirs are dominated by lithic arkose, with average porosity and permeability values of 10.8% and 8.79 mD, respectively, which are generally classified as low-porosity and ultra-low-permeability reservoirs. The reservoirs are at the middle diagenetic stage A₂, with complex diagenetic processes including compaction, cementation, and dissolution. Three main reservoir types were identified: (1) tightly cemented reservoirs with strong carbonate cementation, (2) dissolution-enhanced reservoirs with secondary porosity, and (3) primary pore-preserved reservoir. The tightly cemented reservoirs are mainly developed in thin, sheet-like sand bodies of lobe microfacies, characterized by frequent interbedding of sand and shale, with intense carbonate cementation. The dissolution-enhanced reservoirs occur in thin-bedded sand bodies associated with distributary channels of gravity flows, showing a diagenetic sequence of moderate compaction followed by intense dissolution. These reservoirs exhibit high porosity but poor permeability, owing to weak late-stage hydrocarbon charging and limited carbonate dissolution. In contrast, the primary pore-preserved reservoirs are developed in the middle parts of thick sand bodies within gravity flow channel microfacies, and exhibit diagenetic features of chlorite grain-coating, moderate compaction and weak carbonate cementation. Strong late-stage hydrocarbon charging inhibited further carbonate cementation, resulting in relatively favorable reservoir quality. These findings provide valuable insights for deep hydrocarbon exploration in the Yangwuzhai area and other regions with similar geological settings.