--> Green Canyon Block 205: Geophysical Analysis of a Deepwater Gulf of Mexico Discovery, by T. C. Bergeon, J. W. Rafalowski, B. W. Regel, and W. L. Abriel; #90986 (1994).

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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