China is rich in shale oil resources. By the end of 2022, the predicted reserves of continental shale oil in China have reached 3 billion tonnes, but only recoverable shale oil has economic value under such reserves. The shale oil reservoirs of the Lucaogou Formation in the Jimusar Sag can be divided into three types: interlayer type, lamina type and block type according to mineral composition and source-reservoir ratio. However, due to the large difference in pore structure characteristics and fluid occurrence state of the three types of reservoirs, the productivity difference is high using the same fracturing method. In order to clarify the pore structure characteristics of the Lucaogou interlayer and laminated reservoirs in the Jimusar sag and the difference of fluid mobility under their constraints, this paper studies the pore structure characteristics by means of XRD, casting thin sections, scanning electron microscopy and nitrogen adsorption. Nuclear magnetic resonance centrifugation technology was used to quantitatively evaluate the mobility of shale oil in laminated and laminated reservoir samples. The T₁-T₂ spectrum method was used to clarify the occurrence state of shale oil in different reservoir types. Finally, the main controlling factors of fluid mobility in shale oil reservoirs were analyzed by combining characteristic pore structure parameters. The results show that the carbonate content of laminated reservoirs is high, and the reservoir space is dominated by carbonate intergranular pores, clay mineral interlayer fractures and organic matter pores. The fluid component is dominated by kerogen, and the free oil component content is extremely low. The average value of movable fluid saturation is only 7.97%. The felsic content of the interlayer type is higher, the reservoir space is mainly composed of intercrystalline pores and dissolved pores in feldspar grains, the fluid composition is mainly movable oil, followed by bound oil and kerogen, and there is no movable water. The average saturation of movable fluid is 29.3%. The pore throat radius in the characteristic pore structure parameters is the main factor controlling the movable fluid saturation of shale oil reservoirs. The two are exponentially correlated, and the correlation coefficient can reach 0.95. Through the study, the main reservoir space types of the intergranular pores and intra - granular pores in the Lucaogou Formation and the laminar shale oil reservoirs in the Jimsal Depression are determined. The mobile fluid saturation decreases gradually from unimodal interlayer reservoirs to bimodal laminated reservoirs, but increases exponentially with the increase of the maximum pore throat radius. The results show that the maximum pore throat radius has a great influence on the mobile fluid saturation of shale oil reservoir.

The whole petroleum system in the Permian Fengcheng Formation of the Mahu Sag within the Junggar Basin comprises conventional, tight, and shale hydrocarbon reservoirs, whose formation and distribution are governed by the coupling effects of multiple dynamic fields. Using statistical and geological analyses, we systematically investigate the dynamic boundaries among the three reservoir types and the differences in the contributions of migration dynamics to these reservoirs. Based on data from physical property tests of 1024 conventional and unconventional reservoir samples, 1235 high-pressure mercury injection (MICP) experiments, and pyrolysis of 1630 samples, we define the quantitative relationships of porosity, permeability, and maximum pore-throat radius with burial depth. Accordingly, the critical parameters are determined for the buoyancy-driven hydrocarbon accumulation depth (BHAD), hydrocarbon accumulation depth limit (HADL), and active source-rock depth limit (ASDL). The results indicate that the BHAD corresponds to a burial depth of 4290.86 m (porosity: 8%, permeability: 1 × 10^-3 μm², pore-throat radius: 0.800 μm). The HADL is approximately 8000.00 m (porosity: 2%, pore-throat radius: 0.025 μm), while the ASDL corresponds to a critical burial depth of approximately 10000.00 m. Using the quadripartite method, we quantitatively assess the contributions of buoyancy, capillary pressure difference, tectonic stress, and fluid dissolution. The results reveal that the conventional hydrocarbon reservoirs (above the BHAD) are dominated by buoyancy-driven migration, primarily found in the deltaic plain facies along the margin of the Mahu Sag. In contrast, the hydrocarbon migration of unconventional reservoirs (below the BHAD) is controlled by capillary pressure difference, as well as hydrocarbon generation and expulsion dynamics. These reservoirs are principally distributed in the slope transition zone and the sag center, characterized by delta front and shallow to semi-deep lacustrine subfacies. By determining the quantitative relationships of porosity, permeability, and maximum pore-throat radius with burial depth and by assessing the dynamic contributions using the quadripartite method, this study serves to advance the theoretical framework of the hydrocarbon accumulation dynamics in the whole petroleum system, providing scientific support for the collaborative exploration and efficient exploitation of conventional and unconventional hydrocarbon resources in the Junggar Basin and comparable geological settings.

Volcanic rocks in the Permian Jiamuhe Formation of the Jinlong oilfield in the Junggar Basin exhibit unclear lithofacies types and dominant reservoir distribution, which hinder the exploitation of the oil and gas resources therein. To address these challenges, we systematically analyze the volcanic facies types, reservoir physical properties, and storage space characteristics in the study area using data from core observations, log analysis, and laboratory tests. Accordingly, we establish lithofacies assemblage models, while clarifying their controlling effects on productivity. The results indicate that the volcanic rocks in the Jinlong oilfield in the Junggar Basin can be categorized into three facies:explosive, overflow, and volcanic sedimentary facies. The explosive facies, among others, consists primarily of welded volcaniclastic rocks and andesitic volcanic breccias, and exhibits an average porosity exceeding 10 %, forming dominant reservoirs. While the overflow facies is dominated by lavas, featuring an average porosity of below 6 %, and the volcanic sedimentary facies exhibits poor physical properties. In the volcanic reservoirs in the study area, vesicles, intraamygdale pores, and dissolution pores predominate, with a minor presence of primary intergranular pores. Additionally,dissolution vugs are most developed at the top of the explosive and overflow facies. Fractures in the volcanic reservoirs are dominated by structural and dissolution fractures, with developmental degrees closely related to their distance from faults. The intermediate-acidic overflow facies mainly exhibits oblique and reticulate fractures, while the mafic overflow facies is dominated by high-angle, straight-split fractures. The study area exhibits four lithofacies assemblages: interbedded intermediate-acidic pyroclastic flow subfacies and intermediate-acidic overflow subfacies, frequently interbedded mafic pyroclastic flow subfacies and mafic overflow subfacies, interbedded neutral air-fall subfacies and neutral overflow subfacies, and interbedded neutral air-fall subfacies and intermediate-acidic pyroclastic flow subfacies. The interbedded neutral air-fall subfacies and intermediate-acidic pyroclastic flow subfacies exhibits the highest productivity, followed by interbedded intermediate-acidic pyroclastic flow subfacies and intermediate-acidic overflow subfacies and interbedded neutral air-fall subfacies and neutral overflow subfacies, and with the frequently interbedded mafic pyroclastic flow subfacies and mafic overflow subfacies coming at last in daily production. The productivity of the volcanic reservoirs is governed most significantly by the effective reservoir thickness and oil saturation, followed by porosity and formation pressure.