Abstract: Carbonate Sequence Stratigraphy on the Development Geology Scale: Outcrop and Subsurface Examples from the Permian Grayburg Formation, Permian Basin

Robert F. Lindsay

Grayburg outcrop studies in the Guadalupe Mountains combined with subsurface reservoir studies in the Permian basin have helped define updip, downdip, and strike-view variations in reservoir architecture in a carbonate ramp setting. The hierarchy of sequence stratigraphy was identified within the Grayburg sequence, identifying simple sequences, parasequence sets, parasequences, beds, and laminae.

Sequence stratigraphy slices the reservoir horizontally, becoming more complex updip, whereas depositional facies slice the reservoir vertically into dip-view compartments and strike-view strips. The resulting thin compartments and strips are nature's versions of the grid cell blocks which the petroleum industry uses in reservoir simulation.

Once the Grayburg sequence stratigraphic model was defined, additional geologic and engineering data were superimposed upon it, such as carbonate facies and rock types, siliciclastic beds, log-derived grain-rich vs. mud-rich intervals, the transition from reservoir quality to non-reservoir rock, pressure distribution, edge water, bottom water and connate water distribution, and the structural position within the reservoir. These various reservoir data were combined to outline the overall geometry of remaining floodable portions of reservoirs.

Ultimately, high-porosity and high-permeability flow units were identified that were responsible for cycling water between injection and production wells during waterflood operations and edge water encroachment into reservoirs during primary recovery. Profile modification was used to eliminate or reduce the influence of these flow units. This procedure redirected injection water to recover bypassed and unswept mobile oil.

The use of sequence stratigraphy in the management of carbonate reservoirs is a powerful tool if properly applied. A better understanding of these relationships can help improve ultimate recovery from carbonate reservoirs and explain why and how reservoirs perform under primary, secondary, and tertiary recovery operations.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994