In the Bonan area, the lower fourth member of the Shahejie Formation (Es4x) is buried beneath a sedimentary pile ranging from 2500 to 5000 m. Understanding the impact of diagenetic alterations on these deeply buried reservoirs is crucial for effective hydrocarbon exploration and production. This study employs a terminal fan sedimentation model, encompassing depositional environments such as feeder channels, distributary channels, floodplains, and basinal zones, to provide insights into the spatial distribution of reservoir properties and their influence on the localization of optimal reservoirs within the sag. The analysis integrates diagenetic facies with well log responses, subsurface porosity trends, and permeability variations across the formation. The petrographic analysis indicates that the sandstone is composed primarily of litharenite, feldspathic litharenite, lithic arkose, and minor amounts of arkose. The dominant clay cement is illite, accompanied by mixed-layer smectite/illite, chlorite, and kaolinite. Thin section observations reveal secondary porosity formed through the dissolution of quartz grains, volcanic rock fragments, and feldspar, along with their associated cements. These sandstones exhibit relatively good sorting, with average porosity and air permeability values of 14.01% and 12.73 mD, respectively. Diagenetic alterations are categorized into three processes: porosity destruction, preservation, and generation. Key diagenetic mechanisms include compaction, cementation, replacement, and dissolution, with compaction exerting the most significant control on reservoir porosity reduction. Statistical analysis indicates that the average porosity loss due to compaction is approximately 13.3%, accounting for about 38% of the original porosity. The detrital rock cement predominantly comprises quartz (42%), feldspar (32%), clay minerals (14%), and carbonate (12%). Under the prevailing depositional conditions, porosity is enhanced by dissolution and fracturing, while late-stage diagenetic cementation by clay and carbonate minerals—excluding chlorite—adversely affects reservoir quality. Consequently, the distributary zone is identified as the primary target for exploration.

The lower fourth member of the Paleogene Shahejie Formation (Es4L) in Bonan sag, Bohai Bay Basin, East China was taken as an important target for oil and gas exploration in Shengli oilfield. Bonan sag is one of several sag in the Bohai Bay Basin, located in north-east China. Paleogene deposits are characterized by red rock layers, formed under a hot-warm and arid climate. Four depositional systems were developed in the Bonan sag. Among them, the fan delta developed in the central part of the study area, consisting of gray-white fine glutenite with poor sorting and rounding. The braided river delta is developed in the southern part with large lithofacies. The lithology shows some light gray conglomerate, pebbly sandstone, sandstone and small amount of mudstone. Floodplain facies is mainly developed in the inundated plain that in some weather conditions have the characteristics of shallow lake facies. However, a number of questions remain unanswered about the disparate presence of evaporites, the diverse colors of the mudstones and the varying levels of total organic carbon (TOC) in the area. These disparities can be seen in the many oscillations in TOC content, from the source of the sediments in the upper relief to the flood lacustrine plain. Consequently, given to this conditions, the depositional system which occurs in the area remains unclear from previous works. However, looking at the distribution of the sediments in an apparently short distance of less than 50km from the south to the North of sag, suggests that they are more likely to be terminal fan deposits. Terminal fans are an architectural system for draining and depositional process from high relief to flood plain, with gradual discharge of water through infiltration and evaporation. Although some researchers have conducted studies on terminal fans around the world, the concept remains unfamiliar and deserves to be more elucidated. This approach postulates to be useful to better understanding the sedimentary deposits in the Bonan sag and must be reviewed for similar hydrocarbon explorations elsewhere.

The fourth member of the Shahejie Formation (Es4x) represents a critical unit within the Bohai Bay Basin (BBB), not only as a significant oil reservoir but also for understanding depositional environments and evolution during the late Paleocene to early Eocene, particularly in the Bonan Sag. This study emphasizes the influence of paleotopography and paleoclimate on the spatial and temporal distributions of sedimentary facies within Es4x, focusing on reconstructing and analysing the evolution of the fan system through paleogeographic modelling. A multidisciplinary approach integrates seismic data, well logs, core samples, petrographic analyses, and geochemical data to provide a comprehensive understanding of depositional processes influenced by tectonic activity, climate, and sedimentary systems. These findings underscore the profound impact of geomorphological and paleotopographic variations on the distributions of terminal fans and their sandstone reservoirs within Es4x. Proximal, middle, and distal fan systems were identified as products of regional topography and sediment transport pathways. Additionally, paleoenvironmental and paleoclimatic reconstructions based on sedimentary features and climate proxies reveal that arid to semiarid conditions prevailed in the region, significantly influencing the sediment supply. A key discovery is the evolution of the spatial and temporal framework of terminal fan systems, marked by progradation indices during arid to hot humid intervals, followed by evaporite deposition, total organic carbon (TOC) accumulation, and mudstone color variations linked to the relative expansion of water bodies. In addition to providing a comprehensive understanding of the interplay among paleoclimate, paleotopography, tectonics, and terminal fan sedimentation during this critical period of complex evolution in the Bohai Bay Basin, this study also offers valuable perspectives for petroleum exploration. By elucidating the evolution of terminal fan depositional systems and their responses to environmental changes, the findings can enhance predictions of reservoir distribution and quality within similar deposits.