New eustatic concepts developed primarily at Exxon Production Research complement the many technical improvements in seismic data and allow us to better predict reservoir and seal in Gulf of Mexico sediments. We can now more successfully integrate biostratigraphic and environmental interpretation of well data with sequence and seismic facies interpretation by using a succession of environmentally related chronostratigraphic units, called systems tracts.
Of particular interest, because of their economic potential, are deposits formed during eustatic lowstands, the lowstand systems tract.
Three principal phases of lowstand deposition are recognized: basin floor fan, slope fan and prograding complex. Basin floor fans in the Gulf of Mexico typically occur in topographic lows and consist of sands of high porosity and permeability in multilayered beds with gross thickness of 5 to 50 meters. Slope fans contain discrete sands in submarine channels, typically 5 to 40 meters thick with good porosity and permeability. Overbank sands associated with the slope fan are thin (1–30 cm), but may have excellent porosity and permeability. Rapid sedimentation during late Pliocene and Pleistocene time has also resulted in major accumulation of slump deposits, primarily associated with the slope fan. Prograding complex sands occur as stacked sands in shallow marine to fluvial environments deposited at or below the previous shelf edge, and in turbidite fans at the toes of the prograding clinoforms. Stratigraphic entrapment by internal shale seals is characteristic of many of the deep-water lowstand deposits. Sands of the shallow water portion of the prograding complex generally require structural entrapment.
Interpretation of Gulf of Mexico shelf, slope and basin sediments is moving into a new era. The combination of improved seismic technology with the new "sequence stratigraphy" (Vail, 1988) significantly enhances our ability to predict reservoir and seal in both stratigraphic and structural prospects.
New seismic technology applied to stratigraphic interpretation centers on the routine use of 3-D surveys, with their enhanced resolution and on improved migration of stratigraphic as well as structural events. Emphasis on direct detection and prediction of hydrocarbons from seismic data has produced a bonus for stratigraphic analysis through improved control over phase and amplitude during processing. Finally, interactive interpretation of both seismic sections and of quantitative seismic attributes, integrated with well data via improved synthetic seismograms, opens marvelous new doors for the modern Gulf Coast explorationist.
The new eustatic concepts of sedimentary response to cycles of sea level change complement this technical progress. These concepts have particular application in the Gulf of Mexico because:
High rates of sedimentation of prograding siliciclastic sediments into a deep basin provide a wide variety of seismically resolvable sedimentary responses to cyclic sea level changes.
Structural deformation contemporaneous with sedimentation enhances the thickness and the seismic expression of the resulting deposits.
The relationship of cyclical alternation of continental glaciation and of warm relatively ice-free times during the Pleistocene is well established and provides a laboratory for the study of the associated effects of eustatic sea level cycles on sedimentation.
