Free Gas Accumulation System in the Makran Accretionary Prism

The Makran Accretionary Prism is one of the largest accretionary prisms in the world and hosts substantial natural gas hydrate resources. However, research on the distribution characteristics and accumulation mechanisms of free gas remains limited. This study identifies structural elements associated with free gas, such as thrust faults, piggyback basins, unconformities, and décollements, through detailed interpretation of newly acquired seismic data. Free gas reservoirs within piggyback basins are located in the folded zone of the accretionary prism, whereas unconformity-type and horizontal sandstone-type free gas reservoirs are identified in undeformed areas. In the folded zone, décollement, thrust faults, and permeable sand layers act as primary migration pathways for free gas, which accumulates in turbidite sands beneath bottom simulating reflectors (BSRs) in piggyback basins. In the undeformed zone, free gas migrates along décollements and thrust faults into horizontal sandstones, where substantial accumulations are found near unconformities below BSRs. The distribution of free gas reservoirs across the study area is extensive and diverse. This study is the first to document unconformity-type and horizontal sandstone-type free gas reservoirs in the undeformed zone, highlighting their considerable resource potential. The findings are of substantial value for oil and gas exploration at the front of the accretionary prism and provide important theoretical and practical insights into natural gas accumulation systems along active continental margins.