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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Modern Analytical Techniques for
Fault
Surface Seal Analysis:
A Gulf Coast Case History


By
1Marathon Oil Company, Lafayette
2Department of Geology, University of Southwestern Louisiana, Lafayette
Today's exploration and, particularly,
exploitation methods, with a major reliance
placed on mapping with 3-D seismic data,
generate a great deal of potential information
about prospective reservoirs. Effective
prospect evaluation requires consideration
of the sealing characteristics of faults, and
techniques have been developed to improve
fault
surface analysis. "Allen"
fault
surface
profiles permit assessment of sand juxtaposition
across the
fault
, and can be prepared
by manual mapping methods if
adequate structural maps are available
from 3-D seismic interpretation and well
control. Commercial software is available
to perform similar analyses directly from
the 3-D seismic interpretation.
Vermilion Block 331 Field, operated by
Marathon, was selected for a pilot study.
The field consists of a low-relief anticline,
downthrown to a regional growth fault
.
Numerous small faults, with limited vertical
separation, cross the crest of the anticline
and compartmentalize reservoir sands
of Trimosina A (Pleistocene), Angulogerina
B (Pleistocene), and Lenticulina (Miocene)
age. Faulted reservoirs with multiple,
stacked sands are particularly prone to loss
of hydrocarbons by leakage across
fault
surfaces, so that this field was considered
ideal for testing the effectiveness of
fault
surface analysis. Both lateral and top seal
risk were evaluated by means of
fault
surface
profiles along five of the crestal faults
to determine the limits of trapping potential
and paths for vertical migration. A
detailed review of actual hydrocarbon distribution
was then compared with the predictions
made from
fault
surface analysis.
70% of a total of 83 predicted hydrocarbon/water
contacts were found to be correct
within 10 meters (30 feet).
The role of faults in permitting up-fault
migration along the fracture surface, or in
providing shale smear barriers to cross-
fault
migration from sand to sand, may confound
interpretations based only on
fault
surface
profile geometries. For this reason, shale
smear factors were also determined and
used in assessing trapping potentials. A
critical value of shale smear factor appropriate
for this field was found empirically
to be between 1.85 and 2.0. Capillary-limited
cross-
fault
migration was blocked in
all cases where the value was lower than
critical, while spill point-limited traps
occur where values are above critical. This
analysis explained all the remaining discrepancies
between predicted and actual
hydrocarbon/water contacts mentioned in
the preceding paragraph.
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