FLUID MIGRATION ALONG GROWTH FAULTS IN COMPACTING SEDIMENTS

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Abstract
The movement of fluids up growth faults is proposed to be periodic; when faults are active they can concentrate fluid flow, but when inactive, flow is restricted. Higher flow rates are predicted to be caused by fault‐zone permeabilities and fluid potentials increasing at shear stresses close to the shear strength of the rock. In this way, significant flow is confined to the most active sections of moving faults. Since fault activity is related to sediment accumulation rates, the volume of fluid flowing up faults should be greatest when accumulation rates are high.Periodic flow explains the evidence which indicates that faults can be both barriers and avenues of fluid migration. Evidence for growth faults being avenues of fluid migration is shown by fault‐zone mineralization. lowered fluid potentials. thermal anomalies, salinity anomalies. and isotope studies. In contrast, fault‐zone porosity studies. measured flow rates, high juxtaposed fluid potential differences, differential subsidence caused by groundwater production, and hydrocarbon accumulations, all suggest that growth fault zones are of low permeability.The flow of‐oil from large pore spaces into smaller, water‐filled pore spaces requires a large potential gradient. Therefore., faults which cut water‐wet sections with variable vertical permeability are usually poor conduits for oil. In contrast, in hydrocarbon‐saturated homogeneous rocks, such as mature shales. the vertical flow of migrating hydrocarbons may be concentrated by faults. The degree of concentration will depend on the permeability of the fault zone.