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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Seal Controls on
Trap
Capacity and Migration
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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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