Pressures, Seals and Traps
Pressure, seals and traps are the basis for the Petroleum System to work efficiently. The compaction of sediments is related to fluid expulsion, mainly water, but also hydrocarbons, i.e. oil, gas, and mineral in solutions. Sediments loose mass and volume during the compaction and the diagenesis process, through a reduction of porosity. Water (mostly) is expelled and finds its way towards the surface. Most of the fluid loss occurs during the first 2000 m of burial. Pressure is defined as the ratio between a force and an area. The analyses of pressure through tests are fundamental for the petroleum geologist. Solids transmit forces and fluids transmit pressures, and rocks can be deformed, and when tight have no measurable pressure. Pressure is normal when hydrostatic, and abnormal when overpressure is developed in undrained systems. Sealing of hydrocarbons is related to impermeable rocks and ruled mainly by two processes: capacity and integrity. The capacity of a seal is linked to its entry pressure and wettability, the more of this the more the hydrocarbon column can be. The integrity of a seal is related to its resistance to hydraulic fracturing, and conversely the closer the pressure is to the minimum stress the weaker is the sealing and the hydrocarbon column. There is another but much more marginal phenomenon - the gas molecular diffusion, which occurs longer over the geological time scale. Hydrocarbon trapping is the result of a contrast of a reservoir having a low entry pressure and a seal having a high entry pressure. If there is an important overpressure or if the hydrocarbon column is high, then the pressure can be close to the hydraulic fracture point of the seal and there is possible leakage.
AAPG Datapages/Search and Discovery Article #90334©2018 AAPG Middle East Region, Second EAGE/AAPG Hydrocarbon Seals of the Middle East Workshop, Abu Dhabi, UAE, April 16-19, 2018