K. A. Ortmann and L. J. Wood
Stratigraphy in the Columbus Basin, as in most clastic basins, is characterized by episodes of shelf progradation and retrogradation in response to relative shoreline translations. Tectonic activity strongly influences accommodation space at the shelfbreak causing very rapid facies changes in a depositional dip direction. Using sparse well-data alone makes it difficult to accurately predict the shelfbreak separating sand-prone shelfal versus shale-prone deeper marine facies at any one tune in the basin s history. Regional sequence stratigraphic analysis was integrated with a new 3-D discontinuity algorithm that was used for delineating faults and stratigraphic features (Bahorich and Farmer, 1994). Density-driven impedance differences between sand and shale, in the Columbu Basin, generate 3-D coherency variations that can be horizon-slice mapped to illustrate shelfbreak migration through time.
Regional sequence stratigraphic analysis in the Columbus Basin indicate a diachronous eastward facies transition from shallow to deep marine facies throughout the Plio-Pleistocene, with a shoreline consistently oriented NW-SE. Horizon-based slices of 3D seismic data were taken in the East Mayaro area, which showed a boundary between high and low 3-D coherence areas that occurred progressively more westward with increasing depth in the section. Well data within the study area reflected deep marine, laminated sands and shales associated with the more eastward low coherency value areas. These observations were combined with regional sequence stratigraphic analysis results to predict the presence to time-equivalent, higher quality sands located on a paleo-shelf to the west. Predictions we e accurate with subsequent exploration wells encountering high quality reservoir sands >450 feet (150 meters) at deeper horizons previously unseen across the area. Reservoir presence, and favorable structure and migration history, combined to result in the discovery of a giant gas/condensate field. Future wells are planned to further delineate this shelfbreak reservoir transition across the East Mayaro Field.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California