Abstract: Green Canyon Block 205: Geophysical Analysis of a Deepwater Gulf of Mexico Discovery
T. C. Bergeon, J. W. Rafalowski, B. W. Regel, W. L. Abriel
Chevron USA 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 turbidites 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 3-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 anistropic 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 and poor synthetic well ties. Remigration of the 3-D data at 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. AVO
seismic
data suggests that
modeling
results are valid. Preliminary
modeling
suggest 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.
AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994