Origin of differential hydrocarbon accumulation in ultra-deep carbonate reservoirs along strike-slip fault zones in the Fuman area, northern Tarim Basin

In recent years, breakthroughs have been achieved in hydrocarbon exploration efforts in the ultra-deep marine carbonate rocks of strike-slip fault systems in the northern Tarim Basin. However, the Fuman oilfield in the basin exhibits pronounced differences in hydrocarbon distribution and enrichment, with the mechanisms driving the differential hydrocarbon accumulation in ultra-deep reservoirs governed by strike-slip faults remaining unclear. In this study, we investigate the geometric structures and evolution of strike-slip faults in the Fuman area, as well as their role in hydrocarbon migration and accumulation. By analyzing the hydrocarbon accumulation and enrichment mechanisms, we identify the dominant factors controlling hydrocarbon accumulation in ultra-deep carbonate reservoirs in the area. The results indicate that the strike-slip faults in the study area experienced a dynamic evolutionary process consisting of the early extension or weak compression, the middle-stage transpression, extension, or translation slip, and the late-stage stabilization, successive development, or tenso-shear inversion. The F_I5 and F_I17 fault zones underwent alternating compression, shear, and tensile stresses, resulting in significant evolutionary differences across their various parts. In contrast, the F_I7 and F_Il6 fault zones were primarily subjected to shear and tensile stresses, leading to relatively simple evolutionary processes. The faults with differential evolutionary processes exhibit distinct geometric structures, resulting in varying configurations of their connection to source rocks, hydrocarbon transport capacities, and reservoir properties. Consequently, three hydrocarbon charging models are formed: vertical charging as represented by F_I5 and F_I16, lateral migration for adjustment by F_I7, and a combination of the former two patterns by F_I17. The hydrocarbon charging process is governed by the differential evolution of fault zones. The late-stage strong activity of faults in the eastern part of the Fuman area, combined with the charging and accumulation of substantial highly mature pyrolysis gas during the Himalayan movement, results in the formation of a hydrocarbon distribution pattern characterized by “oil in the west and gas in the east”. Furthermore, the evolutionary differences across various parts of the fault zones cause more complex changes in hydrocarbon properties. For reservoirs dominated by vertical hydrocarbon charging, the degree of hydrocarbon enrichment is determined by the coupling of the connection to source rocks, hydrocarbon transport capacities, and reservoir properties of fault zones. Meanwhile, the hydrocarbon properties of the reservoirs are governed by the various hydrocarbon charging stages. For reservoirs dominated by lateral hydrocarbon migration, the degree of hydrocarbon enrichment and hydrocarbon property changes are controlled by their properties and the extent of lateral connections within.