Seal failure prediction
A seal can fail when capillary pressure exceeds seal entry pressure(Intact membrane seal leakage), open fractures bypass matrix pore systems (Fractured membrane seal leakage), hydrofracturing occurs (Hydrofractured seal leakage), leakage takes place through micropermeable lithologies (Micropermeable seal leakage), or gas is diffused (Diffusive seal leakage). The most likely mechanism is controlled by the seal lithology and the geological history.
Seal failure timing
Seals are most likely to fail during trapping, so an accumulation does not form in the first place. Top-seal failure after charging is most likely caused by fracturing during deformation or by cumulative micropermeability or diffusive seal loss.
Leakage associated with faults
Leakage is commonly associated with faults. Fault leakage is a function of fault-fill lithology, lithology of surrounding rocks, and timing. Note these characteristics to evaluate fault-associated leakage:
- Calculate smear-gouge ratio or shale smear factor to estimate fault-fill lithology. Petroleum leakage up faults is a type of membrane seal failure. The higher the shale content of the fault fill, the less the chance of fault-plane leakage.
- Faults may localize fracturing through the top seal, so evaluate the potential for fractured membrane seal leakage in the top-seal lithology.
- Faults must connect to permeable beds higher in the section or to the surface to leak significant amounts of petroleum. If growth faults die upsection into a shale interval, leakage may be minimal except where natural hydrofracture ruptures seals.
- Fractures and fault fill may heal by cementation once fault movement stops. Leakage is less likely if trap charge significantly postdates fault movement.
- Conversely, fault movement during or after charge of the trap will always result in some leakage, probably by a form of natural hydraulic fracturing along the fault plane. If charge is sufficient, leakage may be slower than charge, so petroleum may accumulate and be preserved as long as charging continues.
Recognizing leaky traps
Traps with leaky seals or reduced seal capacity may still maintain an economic column of petroleum. Partially leaked traps are characterized by a zone of residual petroleum saturation thicker than the transition zone predicted by capillary pressure tests. Shows in traps that have leaked are similar to those in spilled traps. Paleofluid contacts are usually flat, not tilted like spilled traps.
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