Fault-cavity bodies: probable spaces for large-scale hydrocarbon migration and accumulation in non-carbonate rock areas

In carbonate rocks, it is easy to understand that fault-cavity bodies form because they dissolve easily when fluids flow along faults, creating caves. However, non-carbonate rocks are not as easily dissolved by fluids. This raises the question of whether caves still develop along faults in such areas, and if so, whether they can facilitate the migration, accumulation, and enrichment of hydrocarbons. The study aims to demonstrate the existence of large caves along faults in non-carbonate rock areas, which are key spaces for hydrocarbon migration and accumulation. Taking the Chepaizi Uplift in the western Junggar Basin as an example, the internal structures in fault rocks, the distribution and development characteristics of caves, and differential hydrocarbon migration and accumulation were studied through field investigation, core observation, drilling analysis, seismic interpretation, and physical simulation experiments. This study suggests that large cave-like geological bodies, such as fault-cavity bodies, can form along fault cores in non-carbonate rock areas. The void space, formed by the relative displacement of two fault blocks during fault activity, is defined as a fault-cavity body. It can either form as the two fault blocks move along an undulating fault plane (zone), or they separate along the dip direction of the fault. The formation of fault-cavity bodies is not related to dissolution but is instead controlled by fault surface (zone) undulations, strike or dip separation, fault occurrence, and the movement direction of two fault blocks. fault-cavity bodies can be massive in scale and are often found where strike-slip fault occurrence changes, at the intersections of different faults, or in areas of concentrated extensional or transtensional stress with significant dip separation. These fault- cavity bodies can trap hydrocarbons, forming fault-cavity type hydrocarbon reservoirs. The concept of fault-cavity bodies challenges traditional perspectives, suggesting that in extensive non-carbonate rock areas, such as clastic, volcanic or metamorphic rock areas where dissolution effect is minimal, hydrocarbon-rich, large-scale fault-related hydrocarbon reservoirs, similar to those in carbonate rock regions of the Tahe and Shunbei (Fuman) oilfields can exist.