Reservoir porosity evaluation is crucial for successful prediction of reservoir quality in deeply buried heterogeneous strata. Recent studies have demonstrated that high-quality reservoirs occur in deeply buried strata. However, little is known about the details of pores related to the good reservoirs. The northern Dongying steep slope sandy conglomerate was investigated to understand the porosity related to high-quality reservoir formation in sandy conglomerate using seismic data, well-logs, SEM-EDS, cathodoluminescence, and optical microscopy. The result reveals three genetic pores: primary, secondary, and mixed pores. The dissolution porosity consists of intergranular pores, intragranular pores, intercrystalline pores, and moldic pores. The intergranular dissolution and enlarged pores are the main contributors to good reservoir quality among the different pores. The index of compaction indicates that compaction is the crucial factor diminishing the reservoir quality in the progradation sequences, while cementation stands as the critical factor for porosity reduction in the retrogradation sequences. Comparing the reservoir properties reveals that the dissolution porosity is more significant in the Es4s than the Es4x, which renders the Es4s reservoir relatively good compared to the Es4x. This study demonstrated that for oil exploration in the fourth member of the Shahejie Formation, the Es4s should be prioritized.

The genetic mechanism of high-quality reservoirs in the tight sandstone of the Shaximiao Formation, Sichuan Basin, remains poorly understood, hindering effective exploration. This study integrates petrographic analysis and fluid inclusion homogenization temperature measurement to reconstruct the diagenetic sequence and restore porosity evolution. This approach reveals the controlling factors of reservoir densification and their interplay with hydrocarbon charging. We find that reservoir heterogeneity results from the competing effects of densifying processes (compaction and cementation) and porosity-enhancing dissolution. The critical control is identified as the timing of hydrocarbon charging relative to densification, leading to a new three-type reservoir classification. Based on the temporal relationship between hydrocarbon charging and densification, reservoirs are classified into three types: Type I (“hydrocarbon charging before densification”), Type II (“synchronous charging and densification”), and Type III (“densification before charging”). Type I (high-quality) reservoirs are primarily controlled by the synergistic effects of high-energy depositional facies and early chlorite coatings. The ‘deposition-diagenesis-hydrocarbon charging’ model elucidates high-quality reservoir mechanisms. These findings offer a genetic model and a predictive tool for pinpointing sweet spots, with significant implications for reducing exploration risk in this and similar tight gas basins worldwide.

In order to clarify the sedimentary system evolution and sandbody development characteristics of Shaximiao Formation, this paper takes the Shaximiao Formation in the central Sichuan Basin as the research object to systematically study the sedimentary microfacies and sandbody genetic types of Shaximiao Formation by using core data, thin section analysis, logging and seismic data comprehensively. And the following research results are obtained.First, due to the gradual uplifting of the Daba Mountain and the Micang Mountain on the northeast, the Sichuan Basin was a NE-SW trending foreland basin during the sedimentation of Shaximiao Formation, the topography in the basin was wide and gentle and the ancient water system was mainly in the NE-SW direction, so the sedimentary pattern of Shaximiao Formation in the central Sichuan Basin is under the joint control of the Daba Mountain on the north and the provenance on the south.Second, glauconite minerals are common in the sandstone of the central Sichuan Basin, indicating that the depocenter of Shaximiao Formation is located in the center of the basin. Shaximiao Formation is characterized by frequent alternation of dry and wet climate, shallow and turbulent lake basin and wide and gentle terrain, and has the sedimentary background to form shallow water deltas.Third, the shallow water deltas developed in the first Member of Shahejie Formation under the condition of semi-arid climate are mostly in the shape of lobate, with a wide distribution of sedimentary facies. In the plane, they are presented as a complex of multiple lobates, sandbodies are overlapped and developed continuously, single-phase channel sandbodies are wide and river mouth bars are common.Fourth, branched distributary channel-type shallow water deltas are mainly developed in the second Member of Shahejie Formation under the condition of arid climate, and they are mostly in the shape of bird’s foot or branch. Sandbodies are distributed in strips along channels, which are narrow and cut each other. River mouth bars are only distributed at the channel end, with a small single scale.In conclusion, this paper systematically analyzes the evolution and distribution laws of sedimentary facies of Shaximiao Formation in the central Sichuan Basin and summarizes the sedimentary characteristics and sandbody development models of Shaximiao Formation.