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Tobago: Implications of 4-D Seismic in an Ultra-Deep Water Setting

Abstract

The ultra-deep water Gulf of Mexico Tobago field (~9600ft water depth) is located on a major northeast-southwest trending anticline structure in the Perdido fold belt about 6 miles to the east of the main Shell operated Perdido Great White field. The Lower Eocene age WM12 Sand is the producing reservoir at Tobago. This turbidite sand package is the same extensive “sheet” sand that is the primary producing reservoir at the Great White field. Tobago rock properties are similar to Great White with thick high net to gross sands exhibiting outstanding porosity and permeability. It has been on production since October 2011 with the start-up of the TA001-ST2 well. A series of faults across the field had been identified and mapped interpreting the possibility of multiple reservoir compartments within the field (Southern and Main compartments). Along with the faults a “Minor” stratigraphic channel overlying the WM12 sand had been also identified as a possible barrier which could have potentially separated the Main part of the field from a Northern accumulation (North compartment). A range of scenarios had been considered including a totally compartmentalized field with three separated “tanks”; a semi compartmentalized field with two bigger “tanks” separated by the “Minor” channel; and a more “baffled” field with partial communication between “tanks”. Multiple seismic surveys, including OBN (Ocean Bottom Nodes), have been acquired over the Tobago field. Two surveys were co-processed to obtain 4D difference results. Recent interpretation showed “seismic hardening” validating aquifer encroachment across many segments indicating drainage past the previous recognized sealing/barrier features as well as “seismic softening” along the crest of the structure indicating connectivity. The 4D analysis also indicated a tilted deeper OWC to the southwest of the structure. This explains the differences between the seismic amplitude shut-off and the WM12 structure. Great finds with 4D Seismic have allowed the Subsurface team to gain very important insights and steer further development in the field, and 4D seismic continues to be an essential tool for Tobago development.