Scott W. Tinker1
(1) Marathon Oil Company, Littleton, CO
Abstract: Reservoir-scale seismic stratigraphy in prograding systems: Time or rock?
Seismic-stratigraphic concepts have allowed for significant advances in sequence-stratigraphic understanding over the past two decades. Two significant lessons of exploration scale seismic stratigraphy are that seismic discordance is a main criterion for determining a sequence boundary, and that seismic reflections tend to parallel stratal surfaces and therefore parallel time lines. As seismic-stratigraphic interpretations are increasingly integrated into detailed 3-D reservoir characterization studies, these two lessons warrant further examination.
The highly progradational Permian Capitan system is exposed in the Guadeloupe Mountains of New Mexico. Detailed, quantified, sequence-stratigraphic work in McKittrick Canyon indicates two composite sequences composed of 10 high-frequency sequences. No major erosion has occurred at the sequence boundaries, and stratal geometries show no discordance such as toplap, onlap, or downlap. The outcrop data are ideal for 2-D synthetic-seismic modeling. At the exploration scale, the synthetic seismic represents the overall prograding clinoform geometries. However, at the reservoir scale, seismic reflections are misleading and indicate an apparent sequence boundary with associated toplap at all synthetic-seismic frequencies below 100 Hertz.
The lessons learned from outcrop can be applied in the subsurface in the Pennsylvanian Canyon and Cisco system in South Dagger Draw field. An integrated sequence-stratigraphic interpretation using logs, cores, predicted facies, and 3-D acoustic impedance (AI) data, guided by a depositional model derived from description of cores and nearby outcrops, indicates complex sigmoid-oblique, prograding clinoforms. Interpretation of either the amplitude or AI data would be erroneous at the reservoir scale, because seismic reflections follow the asynchronous prograding lithofacies, not the depositional stratal geometries. Therefore, the seismic incorrectly indicates relatively flat horizons with associated toplap, similar to the misleading synthetic seismic from outcrop. A 3-D reservoir model based solely on seismic interpretations would result in improper flow-unit connections and incorrect matrix distribution.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana