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Seismic stratigraphic exploration is particularly applicable to exploration for unconformity traps. Typical upstructure truncation and angularity can often be detected with conventional seismic displays. More subtle traps created by lateral truncation of secondary porosity in either limestone or chert are more difficult to explore for using the seismic tool. Often sophisticated data processing techniques like SeislogR aid in identification of the reservoir-seal relation. Comparison of the Seislog expression of a structural unconformity with a low relief erosion surface provides insight into seismic stratigraphic exploration for subunconformity fields.
South Laredo field, Webb County, Texas, serves as a classic upstructure unconformity. Eocene Wilcox sandstones are either progressively truncated at the flanks of the Salado uplift or preserved in rotated fault blocks. On Seislog, unique high velocity sandstone beds progressively lose thickness updip beneath a low-velocity shale. This truncation provides clear evidence for the trap.
In contrast, erosional plays that search for shale overlying porous carbonate rocks or chert often need to recognize units with similar velocity. At Star Lacey field in Kingfisher County, Oklahoma, porous Hunton is not a distinct unit but, on Seislog, exhibits velocity comparable with the overlying Woodford Shale. The seismic trap expression is created by anomalous Woodford thicks which represent the superposition of shale over Hunton porosity. The chert trap at Nicols field, Kiowa County, Kansas, similarly is based on a subtle slope change on the Seislog at the shale reservoir interface. Seislog colors separate the gentle character formed when shale overlies porous chert to the steep slope generated by shale over nonporous carbonate rock.
The expression of the fields on Seislog provides encouragement that seismic data can successfully locate unconformity-type reservoirs. The subtle nature of the features, however, suggests that careful integration with subsurface data is required to maximize exploration efforts.
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