A Coupled Petrophysical-Geophysical Model Demonstrates Velocity Dispersion in a Thin-Bedded, Fluvial Reservoir
The Stratton Field 3-D seismic
survey has been publically
available and widely distributed and studied for many years. The primary
reservoir in this prolific gas play is the Frio formation, a thin-bedded
reservoir of fluvial origin. The two-mile by one-mile survey area has good well
control with twenty one wells with well-logs, however, the correlation of
fluvial reservoirs found in the wells to time-slices of the 3-D
seismic
has
been enigmatic.
Using an interval of the upper-middle Frio formation
that has good well coverage, good stratigraphic control and minimal tectonic deformation,
a 3-dimensional statistical model was developed to search the 3-D seismic
for
correlation. The correlation of the well-logs to the VSP time-depth chart was
very good; however, the positions of the targeted fluvial objects in the
3-dimensional search showed that the VSP time-depth chart was much faster than
the time-depth positions in the 3-D
seismic
. The noted shifts between VSP and
3-D
seismic
were on the order of 18mS (TWT) shift in 500 feet; this translates
to approximately a 90-foot targeting error. This is a very significant error
when targeting the thin fluvial reservoir sands that are typically 30 to 60
feet thick.
In the petrophysical model, a classifier was
developed to identify beds based on three predominate facies: reservoir sands,
tight sands and floodplain mudstones. This information was used to generate bed
thickness distributions for the wells. The scales of the seismic
velocities,
wavelengths and bed distributions appear scalable to forward
modeling
and
experimental results on velocity dispersion. Based on this finding, an improved
time-depth chart is published for this section of the survey based on a linear
relation to the target positions.
These results have significant impacts in the proper
handling and targeting of seismic
data in reservoirs such as the Frio
formation: 1. Thin-bed velocity-dispersion is a potent effect in these types of
reservoirs; 2. Lateral heterogeneity in dispersive parameters may require
special consideration when targeting thin fluvial members. 3. Synthetic
seismogram techniques such as Weiner-Levinson wavelets rely on linear
statistical models and do not account for dispersion.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California