Identification and contribution assessment of hydrocarbon accumulation dynamics in the whole petroleum system: A case study of the Permian Fengcheng Formation, Mahu Sag, Junggar Basin

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.