Quantitative assessment of hydrocarbon transport effectiveness along faults in the Middle Jurassic Shaximiao Formation, central Sichuan Basin

The hydrocarbon transport effectiveness along faults is a key factor influencing hydrocarbon accumulation and enrichment in moderately shallow reservoirs. Assessing this factor holds great significance for the emplacement of hydrocarbon exploratory wells in medium-shallow reservoirs in the central Sichuan Basin. This study focuses on the Shaximiao Formation in the central Sichuan Basin. Following the analysis of fault distributions and origins, we assess the gas transport capacity of source rock-rooted faults, investigate the fault-sand configuration and its evolution, and reveal the controlling effects of fault-sand configuration on the migration and differential enrichment of gas, based on fault activity rates, compressive stress directions, and normal stress on fault plane. The results indicate that the reverse and normal faults cutting through the Upper Triassic Xujiahe Formation and the Lower Jurassic source rocks in the central Sichuan Basin serve as the source rock-rooted faults of the Shaximiao Formation. The reverse faults, among others, are governed by the tectonic compression in different tectonic movement periods, whereas the normal faults are formed due to the differential subsidence with uplifting and sagging in a weak extensional setting during the early to middle sedimentary stages of the Shaximiao Formation. The reverse faults, Longquanshan, Jiao-1, and Jianyang-1, as well as normal faults in the central Sichuan Basin exhibit high transport capacity during the Late Yanshanian and the Late Himalayan. Considering factors such as fault size and the hydrocarbon generation intensity of underlying gas sources, it can be inferred that the Longquanshan and Jiao-1 faults play the most significant role in hydrocarbon transport. The source rock-rooted faults in the central Sichuan Basin are mostly of the inherited type, conducive to the continuous gas charge into favorable sand bodies in different charging periods. In contrast, the Jiao-1 fault is of the inverted type, exhibiting different gas charge directions in the early and late stages and allowing for natural gas charge on both sides. Major factors governing the differences in natural gas enrichment include the size of source rock-rooted faults, fault-sand configuration, and the hydrocarbon generation intensity of source rocks.