About This Item
- Full text of this item is not available.
- Abstract PDFAbstract PDF(no subscription required)
Share This Item
The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Green Canyon Block 205:
Geophysical Analysis
of a Deepwater Gulf of Mexico Discovery
By
Staff Geologist
Chevron USA
Chevron U.S.A. and Exxon are currently evaluating development plans for a large discovery in the Green Canyon Block 205 Unit. The field consists of Lower Pleistocene and Upper Pliocene turbidite reservoirs deposited along a fault-bounded salt-cored anticline. The reservoirs vary from massive, blocky sands to laminated sand-shale sequences. Seismic amplitude anomalies are associated with reservoirs containing hydrocarbons and 2-D seismic data has been used to define their areal extent.
Because deepwater operation costs are extremely high, detailed reservoir characterization is critical to making sound economic development decisions. Stratigraphic variations in the reservoir facies are complex and difficult to delineate seismically. In addition, vertical seismic resolution and tuning effects also complicate the seismic response. Delineation drilling within the field area indicates various facies have overlapping seismic amplitude characteristics. The primary problem is distinguishing between oil-filled laminated sands and massive wet sands which have similar stacked seismic amplitudes.
Checkshot, stacking and migration
velocities differ in the study area and
suggest that these deepwater, shale-dominated
sequences are anisotropic in
nature. The initial
3-D
seismic volume
was migrated with velocities similar to
the stacking velocities and 5% faster
than the checkshot functions which resulted
in under-
migration
of the seismic
data and poor synthetic well ties.
Remigration of the
3-D
data using the
prestack
3-D
migration
velocity analyses
with velocities 10% faster than the
checkshot surveys, enhanced the seismic
imaging and established strong well
ties necessary for reservoir characterization.
Amplitude versus offset modeling
suggests that certain facies not distinguishable
strictly from stacked seismic
amplitudes may be delineated by AVO
techniques. AVO modeling suggests that
massive oil sands can be distinguished
from laminated oil sands based on a distinct
increase in amplitude with offset.
Preliminary modeling suggests that gas
sands should not exhibit a strong AVO
increase. Chevron is currently evaluating
2-D versus 3-D
AVO imaging in this
area to help predict lateral reservoir
characteristics. Comparisons of 2-D and
3-D
AVO seismic data show distinct differences
which can be attributed to 2-D
fresnel zone effects where changes in
stratigraphy and seismic amplitude occur
perpendicular to the shooting direction.
Where this occurs,
3-D
imaging is
needed to correctly estimate stratigraphy
from AVO analysis.
End_of_Record - Last_Page 9---------------