Based on the theory of the whole petroleum system (WPS), we systematically reveal the differential characteristics of conventional and unconventional reservoirs in continental sequences within the western-central Sichuan Basin. Accordingly, the mechanisms underlying the control of these differential characteristics on hydrocarbon accumulation are explained. Through analyses and experiments, including core observations, thin section observations, high-pressure mercury injection (HPMI), nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM), we comprehensively analyze the reservoir characteristics and their variation patterns, as well as the genetic mechanisms of relatively high-quality reservoirs. Additionally, we determine parameters for the grading and classification evaluation of various types of reservoirs and establish the evaluation criteria. The results show that the conventional and unconventional reservoirs in the WPS of continental sequences in the western-central Sichuan Basin exhibit a spatially superimposed pattern in distribution. Specifically, the intermediate to shallow strata overlying the buoyancy-driven hydrocarbon accumulation depth (BHAD) exhibit strong free fluid activities, resulting in conventional high-quality reservoirs with low permeability under the joint control of sedimentary facies and fluids. The intermediate to deep tight sandstones exhibit multi-stage superimposed channel deposits and enhanced capillary resistance. Furthermore, fractures are developed in these sandstones under intense tectonic compression, resulting in multiple types of reservoirs, such as pore, fractured, and pore-fractured reservoirs, jointly governed by lithofacies, fluids, and faults. In contrast, deep lamellar shale reservoirs are generally dominated by nanoscale pores. Within hydrodynamic fields predominantly characterized by strong overpressure and diffusive forces, the occurrence of the shale reservoirs is significantly governed by the joint effects of lithofacies assemblages, organic matter abundance, and the overpressure system. For the intermediate to shallow conventional, low-permeability reservoirs, the grading and classification evaluation focuses on sedimentary and diagenetic facies, along with pore types. The evaluation results indicate that these reservoirs can be classified into four types, with types Ⅰ and Ⅱ serving as the high-quality ones. Both reservoir types primarily occur in the deltaic plain and front, with moderate to strong dissolution identified as their key genetic mechanism. The grading and classification evaluation of the intermediate to deep tight sandstone reservoirs principally considers pores, fractures, lithofacies, and pore structures. Types Ⅰ and Ⅱ of these reservoirs represent high-quality pore reservoirs, primarily occurring in medium-to coarse-grained sandstone in massive and parallel beddings. Integrating reservoir thickness, lithofacies assemblages, reservoir physical properties, degree of organic matter enrichment, brittle mineral content, and the volumetric proportion of movable oil, we classify the intermediate to deep shale reservoirs in the Da'anzhai Member of the Jurassic Ziliujing Formation into types Ⅰ, Ⅱ, Ⅲ, and Ⅳ. Types Ⅰ and Ⅱ, among others, are relatively high-quality reservoirs, consisting predominantly of pure shales and shales interbedded with shell layers.

Deep coarse-grained siliciclastic rocks in the eastern Jiyang Depression, Bohai Bay Basin hold great potential for oil and gas exploration. Given the presence of various types of deep coarse-grained siliciclastic reservoirs and the significant geological differences in hydrocarbon accumulation between the deep and the medium-to-shallow reservoirs in the depression, there is an urgent need to understand the formation mechanisms behind these differentiated geological features in order to advance the exploration and exploitation of deep oil and gas reserves. Employing techniques such as thin-section microscopy, reservoir fluid inclusion analysis, and basin simulation, we investigate pore types in the deep reservoirs and the origin of differentiated formation pressure in the Jiyang Depression. The accumulation models of various types of deep coarse-grained siliciclastic reservoirs in the depression are established thereby. The key findings are as follows: (1)The diagenetic evolution reveals the formation mechanisms of the various pore types of reservoirs. The preservation of primary intergranular pores in the deep coarse-grained siliciclastic reservoirs is attributed to the combined effects of early hydrocarbon charging and overpressured fluids. In contrast, the secondary dissolved pores in the reservoirs are developed largely due to dissolution reactions induced by acidic fluid charging during the hydrocarbon generation of source rocks. In addition, the overpressure in the reservoirs facilitates the development of microfractures. All these lead to the formation of pore-fracture reservoir spaces; (2)The evolution of fluid overpressure in the deep reservoirs is governed by hydrocarbon generation and depositional process. The formation of overpressure environment hinges on preservation conditions, and the pressure distribution in hydrocarbon reservoirs within structural-lithologic traps is regulated by the fault-sand body transport system. In deep normal pressured-weakly overpressured reservoirs, the residual pressure difference between source rocks and reservoirs serves as the primary driving force to hydrocarbon accumulation. While in overpressured reservoirs, this residual pressure difference gradually decreases over time. Furthermore, buoyancy provides the main driving force for the secondary migration and adjustment of hydrocarbons in areas with well-developed faults and in sand bodies with good vertical connectivity; (3)Based on the joint control of hydrocarbon generation-reservoir-pressure on hydrocarbon accumulation, we establish three hydrocarbon accumulation models for deep coarse-grained siliciclastic reservoirs in the eastern Jiyang Depression, including the model of vertically differentiated hydrocarbon accumulation in multi-phase superimposed nearshore subaqueous fans, the model of top-lateral joint hydrocarbon sealing by mudstone within nearshore subaqueous fans, and the model of near-source hydrocarbon accumulation within nearshore subaqueous fans-tubidite fans. The elucidation of these hydrocarbon accumulation mechanisms and models of multi-type deep coarsegrained siliciclastic reservoirs in the Jiyang Depression lays a solid foundation for subsequent in-depth exploration.

Overpressure is significant to the exploration and exploitation of petroleum due to its influence on hydrocarbon accumulation and drilling strategies. The deep-burial hydrocarbon reservoirs of Jurassic strata in the central Junggar Basin are characterized by intensive overpressure, whose origins are complex and still unclear. In this study, Bowers’ method and sonic velocity-density crossplot method based on well logging data were used as a combination for overpressure judgements in geophysics. Furthermore, the corresponding geological processes were analysed in quality and quantity to provide a rational comprehension of the overpressure origins and the model of overpressure evolution and hydrocarbon accumulation processes. The results showed that hydrocarbon generation in the Jurassic source rocks led to overpressure in the mudstones, while hydrocarbon generation in Permian source rocks led to overpressure in the sandstone reservoirs in Jurassic strata by vertical pressure transfer. The burial and thermal history indicated that the aquathermal effect of pore fluids by temperature increase in deep strata is also an important origin of overpressure, while disequilibrium compaction may not be the dominant cause for the overpressure in deep-buried strata. Furthermore, the continuous tectonic compression in both the north–south and west-east trends from the Jurassic period to the present may also have enhanced the overpressure in deep strata. Meanwhile, the developed faults formed by intensive tectonic compression led to pressure transfer from source rocks to the Jurassic reservoirs. Overpressured geofluids with hydrocarbons migrated to sandstone reservoirs and aggravated the overpressure in the Jurassic strata. To conclude, the intensive overpressure in the central Junggar Basin is attributed to the combination of multiple mechanisms, including hydrocarbon generation, the aquathermal effect, tectonic compression and pressure transfer. Furthermore, the developed overpressure indicated hydrocarbon migration and accumulation processes and the potential of oil and gas reservoirs in deeply buried strata. We hope this study will provide a systematic research concept for overpressure origin analysis and provide guidance for petroleum exploration and exploitation in deep-buried strata.

Paleogene depositional systems in the south of Lufeng sag have complex spatial distribution, which are influenced by pre-depositional paleogeomorphology and multi-period tectonic activities. In this paper, to clarify the controlling effect of tectonic-paleogeomorphology on sedimentary facies distribution and effectively guide oil and gas exploration, the Paleogene paleogeomorphic pattern in the south of Lufeng sag is reconstructed by the impression method, and the temporal and spatial evolution laws of the main faults are clarified. The results show that braided river deltas developed stably in the long-axis gentle slope belt of the lake basin, while the short-axis sedimentary system changed from fan deltas to braided river deltas in response to the change of active strength of dominant faults from strong to weak. It is found that the scale of the sedimentary fan is closely related to the activity of the main fault, the area of the catchment, and the vertical elevation difference. The steep cliff is controlled by the boundary fault with large fault throw and steep section, and there are wedge-shaped sand bodies near the steep cliff. The multi-level fault-step zone provides the driving force for the advancement of the sedimentary system, and the sand body extends for a long distance. It is established that the supply capacity of the source area and the accommodated space of the lake basin are coupled to control the deposition scale. Moreover, the slope controlled by the combination of paleogeomorphic assemblage and the activity of the main fault determines the sedimentary type, and the structural slope-break zone defines the spreading pattern of the sands.