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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: A Case History of Velocity Problems
in the Shadow of a Large Growth Fault in
the Frio Formation, Texas Gulf Coast
By
False structure can appear on seismic
data
in the
shadow of normal growth faults, with the upthrown time
structure appearing to roll into the fault where no such roll
exists in depth. This phenomenon was encountered in a
prospect in the Frio formation of the Texas Gulf Coast,
where a large growth fault expanded a thick, low-velocity
shale above the Frio. Two check-shot velocity surveys, one
at either end of the
seismic
control, demonstrated a severe
average velocity gradient across the prospect at the level of
the objective geologic formation.
Assuming first one and then the other velocity survey
was valid over the entire area for conversion of well data
to
time produced a drastic difference in the structural interpretations
of the
seismic
data
. The more obvious
interpretation
required a down-to-the-northwest fault with a downthrown
rollover anticline at the objective formation. This
interpretation
had been previously tested with a dry hole on
the crest of the time structure. This
interpretation
also
contradicted a dipmeter in this "crestal" well.
Using a model derived from interval velocities from the
nearby check-shot surveys, we were able to demonstrate
that over 200 ms of apparent seismic
dip in the shadow of
the growth fault was false. When we used stacking velocities
to perform a depth conversion of the
seismic
time section,
both the apparent dip direction of the formation and
interpreted throw direction of the fault were reversed.
Based on these results, a well was drilled which demonstrated
the qualitative success of our model. We also discovered
that minor buried faulting mapped behind the major fault
was a false effect caused by velocity contrasts across the
growth fault.
This case history proves that the recognition and proper use of velocity gradients can be extremely important in the Gulf Coast.
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