MAPPING FOR STRATIGRAPHIC TRAPS ON THE COMPLEX EASTERN SHELF

The Eastern Shelf to the Midland Basin provides a unique opportunity for identifying and exploring for stratigraphic traps. Because of the structural style along the Eastern Shelf, clastic carrier beds such as anastomosing to braided channel belts provide an economic opportunity which remain on the shelf. However, clastic depositional environment behavior can change quickly in the preserved section across the shelf, and the reservoir rock can be absent from one offset well to another. Regional marker beds, such as maximum flooding surfaces within mudrocks, provide such clues to the extent for which clastic cycles can be mapped and correlated.

Stratigraphic frameworks on the Eastern Shelf are complex because of the nature of the varying depositional environments, which are partly controlled by the underlying regional ramp structure. Long and broad regional ramps provide the variability on mapped environments because energy dispersion within these depositional systems is distributed across this gentle dipping shelf affecting the actual placement of sediment. Sequencing of specific facies can be correlated and picked into varied depositional environments and placed into mapped framework models, but this process can only occur if the geologist understands log signatures and facies typing from the well log provided. Facies typing from log signatures are unique identifiers of depositional environments because they represent the preserved rock section and therefore can be mapped. However, in order to drive the placement of the depositional environment, the geologist must derive the environment and map away from the provided clues in the existing wellbore. After all, computer driven contours only contour where the data is and not where the stratigraphic trap lies. Mapping a particular environment away from the wellbore is developed by the interpreter, but geologic risk still remains and is tied back to the preserved rock section. Mapping from these criteria is slow and tedious, but hand mapping processes must be fed into the overall model where maps must be reworked and balanced to include a variation of styles from pessimistic, to realistic, to optimistic contouring. By walking through the evolution of petroleum history on the Eastern Shelf and linking this with current and modern geologic concepts, processes can be developed to assist in unlocking additional economic stratigraphic traps on the Eastern Shelf.