Evaporite seals

Typical bedded evaporites, especially beds composed of monomineralic assemblages such as massive nodular anhydrite or massive halite, have measured entry pressures in excess of 3000 psi. Most impure evaporite beds have entry pressures greater than 1000 psi, as do many evaporite-plugged reflux dolomites. Contrast this with most shales, which tend to be water-bearing (mostly bound and structural water) and have typical entry pressures between 900 and 1500 psi . Although such shales are respectable seals, over time shale allows substantial diffusive leakage of methane and even liquid hydrocarbons via inherent microporosity, less so if the shales are organic-rich. Even ignoring salts ability to reanneal under stress, an evaporite seal has much lower intrinsic permeability that helps maintains its integrity. With permeabilities of 10-7 md, a hydraulic gradient of 0.01, and a porosity of 0.01, a brine would take somewhere between 3 and 30 million years to flow 1 metre into an unfractured halite seal, while anhydrite, which has permeability some 100 times higher, a brine would take between 30,000 and 300,000 years to flow 1 metre into the seal. Evaporites are excellent longterm seals to substantial hydrocarbon columns and tend to focus the flow of rising basinal fluids along their undersides or edges (Warren, 2017)





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