Reservoir Characteristics and Architectural Elements Associated with Production from a Localized Slump Deposit, Deepwater Gulf of Mexico

John B. Wagner¹, Richard L. Blythe¹, and Patrick F. Rush^2
1 Nexen Petroleum, Dallas, TX
² Integrated Reservoirs Solutions, Core Laboratories, Houston, TX

The Pleistocene to Pliocene age Gunnison Field, Garden Banks Area, Gulf of Mexico, was analyzed using an integration of well log, seismic, paleontologic, pressure, and lithologic information from both whole core and sidewall core data. Local variations in reservoir architecture and performance are related to evolving mini-basin margin paleotopography and associated localized slumping that intercalates remobilized sediment into more laterally continuous time-equivalent strata.

Sedimentological characteristics include high-angle ripple cross-stratified beds, contorted mudstones, and turbiditic sands that become completely overturned while preserving the original sedimentary fabric. Paleoecology data within the slumped interval was indeterminate and suggested possible missing section with bounding shale intervals showing good correlation in mid-bathyal environments. Well logs from time-equivalent strata away from the slump facies have motifs that are quite dissimilar even though they are separated by only several hundred feet. Seismic and well data indicates the producing sand occupy the lower portion of the slump complex with seismic showing a rotated “stair stepping” character with underlying parallel bedded seismic reflectors abruptly truncated by the overlying slump complex. Seismic reflectors within the slump complex are discordant in dip angle with respect to units above and below the complex. Production from a nearby well that is stratigraphically equivalent to the slumped interval showed a rapid pressure decline and pressure depleted after producing only about 6 BCFGE.

Producibility of remobilized sediment within locally derived slumped deposits can be dependant upon hydrocarbon type, overall net-sand percent, degree of primary sediment fabric preservation, and development of top seal.