Successful Reservoir Prediction through Integration of 3-D
Seismic Coherency and Sequence Stratigraphy, East Mayaro Giant Gas Field,
Offshore Eastern Trinidad
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