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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Loma Vieja Field:
Structural Geology and Related Velocity
Fault
Shadow in the Upper Wilcox (Fandango) in
South Texas
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By
Samedan Oil Corporation
A new Upper Wilcox (Fandango) field discovery was made at Loma Vieja field in 1989 in Zapata County, Texas. Production from the field to date is 44.1 BCF from numerous Fandango sands that trend from Bob West field northward to E. Seven Sisters field.
The principal trapping mechanism for Lama Vieja field is a
high side closure against a very large down-to-the-coast fault
.
The
fault
trends across Zapata County through Escobas field,
which is immediately updip to Fandango field, northward to
NE Thompsonville field in northwest Jim Hogg and southeast
Webb counties.
The large down-to-the-coast fault
that creates the trap for
the field has a definitive velocity shadow resulting in a significant
time sag on conventional migrated seismic data. Expansion of
Weches, Queen City, and Reklaw shales on the downthrown side
of the
fault
causes major changes in the average and interval
velocities across the trapping
fault
at Lama Vieja field. Seismic
data and time structure maps over the field have southeast dip
toward the
fault
(Fig. 1) where actual dip as measured from well
log correlations and dipmeter data is northwest away from the
fault
. Figure 2 is a depth structure map that is significantly
different structurally from the time structure map.
Although the field was discovered with 2D seismic data, 3D seismic data confirmed the presence of a velocity gradient across the field and reduced structural risk during continued drilling in the field. However, structural complexities in the geologic section above the Fandango sands have a major impact in the velocity regime across the field. To reduce risk in drilling wells, it is crucial for the explorationist to understand the effects of velocity
Figure 1. Time structure map at the top of the Fandango series (top R sand), Loma Vieja field.
End_Page 8---------------
in a 3-dimensional domain by considering all structural features, as well as stratigraphic bodies, that may have any effect on the ray paths and velocity of seismic energy.
Figure 2. Depth structure map at the top of the Fandango series (top R sand), Loma Vieja field.
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