Sequence stratigraphic analysis is a discipline which focuses on the extent and quality of both reservoir and seal strata. In many fields, particularly those over which high-quality 3-D data are available, traps are precisely imaged. Therefore, the greatest component of risk is reservoir and seal quality. Accurate imaging of these strata before drilling and completion can have a large impact to the bottom line of any company. Much of the work done in sequence stratigraphy to date has focused on use of 2D seismic data, properly calibrated with well-logs and biostratigraphic data, to divide strata into depositional sequences and in turn, systems tracts. A depositional sequence is a relatively conformable succession of genetically related strata, bounded by unconformities and their correlative conformities. Systems tracts are linked, contemporaneous sets of depositional systems. Different systems tracts developed when sea-level was at a certain point relative to the shelf edge and rising or falling at a certain rate. Highstand, transgressive, upper lowstand (prograding wedge) and lower lowstand (slope fan and basin floor fan) can generally be identified.

Systems tracts are commonly identified on 2-D seismic data on the basis of discontinuity surfaces which separate them. Where these boundaries are, concordant, regional changes in reflection character are used to differentiate various systems tracts. Seismic facies within each system tract can then be identified on the basis of specific reflection configurations and calibrated to specific character with available well-log data. Interpretations regarding lithology are then made in areas in which well-logs are not present. The workstation allows us to take this analysis to an order of magnitude of higher resolution. We can begin to look at various facies within the systems tracts at an attribute level along specific chronostratigraphic surfaces. This allows us to use seismic data to effectively map depositional environments and interpret both reservoir morphology and continuity.

This approach is illustrated in the following example. We recently completed a detailed synergistic study of an offshore Gulf of Mexico producing property, in order to identify additional potential. Part of this study involved determining reservoir morphology of pay intervals in both the lower lowstand (slope fan - no basin floor fan was present) and upper lowstand (lowstand prograding wedge). Systems tract boundaries in the depositional sequence which included the pay sands were interpreted and tied throughout the 3-D survey over the field. Amplitude maps were then made on top of the lower and upper lowstand systems tract. Isochron maps were also made for each systems tract. The isochron maps were overlain on the horizon amplitude map to determine the relationship between thickness of the individual systems tracts and variations in amplitude.

Amplitude patterns in the lower lowstand (slope fan) exhibit an orientation along depositional dip (Figure 1). Since reservoir sands in this field exhibit amplitude anomalies, the high amplitudes probably image hydrocarbon charged sands: The isochron map shows dip oriented depositional thicks '' which coincide with the high amplitude trends (shaded areas). The production in this systems tract probably occurs in channel complexes. Sandstone continuity is very good in the central channel complex and sands within this complex tend to be blocky in nature. These sands are probably comprised of amalgamated channels which exhibit very good ommunication between one another. However, continuity may be a problem in other, parts of the reservoir in this systems tract. Sand complexes exhibit serrated well-log patterns, which suggests that individual reservoirs are separated by mudstone intervals. These portions of the systems tract are characterized by dip-directed amplitude patterns which exhibit lower amplitude values than those in the central channel complex.

The upper lowstand systems tract (prograding wedge) in this field (Figure 2) exhibits amplitude patterns which trend along strike. The isochron map of this systems tract shows that depositional thicks are developed along strike as well. High amplitudes in this systems tract image hydrocarbon-charged sands which were probably deposited in wave-dominated, shelf edge deltas. Sandstone continuity is very good throughout this reservoir interval.

Accurate data concerning reservoir morphology and continuity are key to any exploration or development program. Historically the way to get these data was to analyze nearby wells and to make use of them to interpret reservoir quality in undrilled areas of the field. These analyses are still conducted. However, SCA and Seis Strat use the well-log data to properly calibrate the seismic signature in terms of both reflection configuration patterns and seismic attributes to specific lithologic properties. Once this calibration, process is complete 3-D seismic data, can be used to evaluate undrilled areas of a field in terms of their, reservoir morphology, with greatly increased accuracy.