Identification and Dynamic Activation of Gas Capillary Sealing in Caprocks: A Case Study from the Junggar Basin, China

Caprock sealing mechanisms critically influence hydrocarbon accumulation. Gas capillary sealing, a specialized type, occurs in seals of interbedded coarse-fine sediments. Free gas accumulating in coarse-grained pore throats forms a microscopic "gas cap", preventing upward migration of hydrocarbons from the underlying strata. Though recognized historically, prior studies overlooked the impact of formation pore gas. Experimental evidence confirms gas significantly enhances sealing efficiency. Sediment size contrast is essential: gas accumulation at coarse-fine interfaces strengthens sealing. This mechanism is documented globally, including the Anadarko Basin, ODP Site 975, and Junggar Basin. In the Penyijingxi sag periphery in Junggar Basin, conventional seals are suboptimal. The lithology is predominantly characterized by sand-mud interbeds. The direct cap rocks are widely distributed across multiple stratigraphic horizons, yet overpressure exists with reduced hydrocarbon dissipation postgas charging. Thus, gas capillary sealing is identified as the primary caprock mechanism here. It accounts for gas accumulation despite poor seals, advancing the understanding of sealing mechanisms in Junggar Basin.