Structural evaluation of column-height controls at a toe-thrust discovery, deep-water Niger Delta

Abstract

Industry discoveries to date in the deep-water toe-thrust play have limited column heights and lack any unequivocal thrust-fault-dependent hydrocarbon columns. The causes of the limited success are controversial, the main issue being whether the encountered columns are caused by leaking thrust faults or other causes. The controversy is in large part caused by the presence of no-seismic-image zones that, even on three-dimensional (3-D) prestack-migrated data, obscure the possible thrust cutoffs. We use dipmeter, seismic, stratigraphic, and fluid pressure data to construct an integrated geometrically and kinematically balanced cross section through a recent toe-thrust discovery in which a prominent no-seismic-image zone on the southwest flank of the Alpha structure appeared to be a thrust fault zone holding about 100 m (328 ft) of thrust-fault-dependent oil column. To constrain the structure within the unimaged forelimb, dip panels and fold axial surfaces were constructed from dipmeter data recorded in both a vertical hole and a sidetrack well through the no-seismic-image zone. Stratigraphic tops from both wells were projected through the no-seismic-image zone using the dip panels and axial surfaces, maintaining the observed stratigraphic thicknesses. These data and analyses tightly constrain potential fault locations and offsets within the no-data zone. The resulting structural model shows that the no-seismic-image zone in the forelimb is not a thrust but instead an overturned limb without any fault offset of the pay section. The results show that the trapped hydrocarbons are confined completely within the four-way dip closure, and that the trap's potential is not limited by the thrust faults' seal capacities. In addition, top-seal analyses show that top-seal integrity is not a limiting factor for hydrocarbon column heights at the present time and has not been a limiting factor throughout the geological history of the structure. Taken together, these observations indicate that the column heights are most likely controlled by access to charge instead of trap integrity, consistent with an observed lack of thermogenic hydrocarbons in nearby four-way dip closures. These conclusions imply that where sufficient access to thermogenic charge is present, there may be additional thrust-fault-dependent hydrocarbon columns.