Sealing Mechanism for Cap Beds of Shallow Biogenic Gas Pools in Late Quaternary Deposits of the Qiantang River Incised Valley, Eastern China
Ming, Lin C.; Xia, Zhang; Wei, Qu C.; Jun, Wang S.
Late Quaternary shallow biogenic gas pools are found in the Qiantang River incised valley area, eastern China. The main gas reservoirs are sand lenses in the floodplain-estuarine facies. Silty clay beds of floodplain-estuarine facies and mud beds of estuarine-shallow marine facies serve as respectively the direct and indirect cap beds of shallow gas. Physical and mechanical properties and the variations in properties of sediments indicate that silty clay and mud beds can seal the gas pools, and the direct cap beds have much better sealing ability.
The difference of permeability between cap beds and the reservoirs result in the formation of capillary sealing mechanism. Pore-water pressure of silty clay and mud beds can exceed the total of pore-water pressure and original gas pressure of underlying sand reservoirs, so the pore-water pressure sealing ability is formed. Disequilibrium compaction, swelling of clay minerals, and gas generation are responsible for the generation of high pore-water pressure. The downward diffusion of gas in the cap beds can restrain the vertical flow of gas in the reservoirs, thus a seal is formed by the gradient of hydrocarbon concentration. Capillary sealing, pore-water pressure sealing, and hydrocarbon concentration sealing all contribute to the conservation of shallow biogenic gas pools. Pore-water pressure sealing mechanism may be the most important factor for the formation of sealing ability of cap beds.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013