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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Seal Controls on Trap Capacity and
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The capillary properties of sealing
rocks can control hydrocarbon column
heights and influence the geometry and
position of migration
pathways. We have
used mercury-injection capillary pressure
(MICP) data to investigate the sealing
properties of both cap rock and fault
seals, and apply the results to predict
trap seal capacities and model
migration
processes.
Most mudrock seals, irrespective of
depositional environment, have extremely
fine pore-throat systems that are capable
of trapping large (>1000 ft.) hydrocarbon
columns. These seal capacities
are typically greater than trap closure
heights, and only limited vertical leakage
is expected through matrix pores.
Results to date suggest that seal quality
does not degrade significantly until total
clay contents fall to <30 wt.%. Flow simulations
show that in these silty "waste"
zones, long-distance lateral migration
can occur at geologically-rapid rates.
The capillary-pressure response of several North Seal fault seals is highly variable, due to differences in lithology and deformation conditions. Grain-scale deformation and cementation dramatically increase capillary entry pressures relative to the undeformed reservoir. However, entry pressures are not generally as high as those measured on common top seals. These results show that fault zone material can provide an effective hydrocarbon seal, but may trap only limited fault-dependent column heights.
The rock property data can be integrated
into a "fill-and-spill" type migration
model that assumes: ( 1) impermeable
top seals, (2)rapid
migration
rates,
and (3) bottlenecking at faults. In places
where vertical
migration
is critical (e.g.,
Gulf of Mexico), we propose that the
same fault can seal an accumulation and
provide an effective
migration
pathway.
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