Pressure sealing: An important agent for hydrocarbon entrapment

Abstract

Undercompacted shales, overlying a hydrocarbon reservoir act as an effective pressure seal, primarily because of their excess fluid pressure, high ductility and high capillary pressure. These characteristics restrict vertical flow of the reservoir fluids. Although it can be helpful in any trapping situation, pressure seal is particularly effective for deep and tight gas reservoirs in areas near a tectonic belt. In such areas, accumulated gas could have been lost to the surface through fractures and faults as a result of tectonism, unless the reservoir was properly sealed by pressure seal. The ductile nature of the seal would close the fracture openings almost instantly (geologically speaking). Such a self-sealing mechanism would have limited the upward loss of natural gas. A pressure seal can also prevent the downward movement of near-surface fresh water, after a tectonic event, which could deteriorate the quality of the accumulated hydrocarbons by water-washing and biodegradation. A pressure seal is formed at an intermediate stage of compaction when the permeability of the shale was reduced to the level beyond which normal compaction and normal fluid expulsion can no longer take place. Geologic time and burial depth of the pressure-seal formation may be interpreted in conjunction with the period of hydrocarbon generation, maturation and migration as well as of trap formation for a better assessment of petroleum prospect. Pressure seals are known to be important in many regions, such as Lousiana-Texas Gulf Coast, U.S. Rocky Mountains, California Coast Ranges, various Canadian basins, North Sea, Niger Delta, Venezuela and Argentina.