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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Stratigraphic Details Illuminated Using Modern
3D Seismic Techniques in Upper Cretaceous Lenticular
Reservoirs, Optimizing EOR Production in the
Gulf Coast with CO2
Denbury Resources, Inc.
Plano, Texas
The five major Lower Tuscaloosa (Upper Cretaceous age) oil fields of southwest Mississippi were discovered in the 1940s and 1950s and were developed on 40-acre spacing to provide apparently abundant well control to delineate reservoir limits. However, upon closer inspection, via detailed subsurface work utilizing existing well control and available whole core data, abrupt reservoir variations in these stacked fluvial and transitional marine sand sequences became evident. A review of these depositional processes revealed the limits of relying on apparently high-density well control alone when implementing an EOR (tertiary) CO2 project. Early recognition of flow units is critical to the success of a CO2 flood. The CO2 process is extremely sensitive to pressures and pressure variations, as viscous fingering and gravity segregation can have an adverse effect on flood performance. This realization drove Denbury Resources, Inc. to formulate a proprietary 3D seismic plan in an effort to optimize well locations by accurately defining reservoir limits (edges) while imaging multiple channels.
In 2001, Denbury Resources began to acquire proprietary 3D shoots for each of their five Lower Tuscaloosa CO2 tertiary floods in southwest Mississippi. The sequence of 3D shoots beginning with the Little Creek complex in 2001, and followed by Mallalieu in 2005, the McComb area in 2006, and Brookhaven and Cranfield in 2007 depict the acquisition and processing advances realized in less than a decade. Full utilization of the geological well data set combined with the latest 3D interpretation software provides a visualization method to spatially depict these complex reservoirs and the future opportunity to monitor the CO2 sweep efficiency via time-lapse 4D seismic*.
Although each of Denburys 3D surveys has resolvability issues due to limited bandwidth and frequency content, this does not impede the detection of channels and determining relative reservoir thicknesses. Channel detectability is readily accomplished by seismically picking a regional conformable stratigraphic event below the Lower Tuscaloosa reservoirs, flattening on that picked event, and then generating stratigraphically conformable (stratal) slices. These reservoir sands tune at certain frequenceies which correspond to a temporal thickness related to actual sand thickness and fluid content. We have combined four seismic attributes including up to three frequency volumes to detect these changes in reservoir thickness and/or fluid content. This combined image simultaneously displays spec decomp attributes in 3D space and the interpreter can animate through the slices in subsampled time increments to reveal increased stratigraphic detail.
Denbury Resources latest generation of reservoir maps incorporate the results of the 3D spectral decomposition interpretation and has significantly enhanced the understanding and management of the five Lower Tuscaloosa EOR (tertiary) CO2 floods.
*A companion two-year joint study now under way is utilizing Denburys newest 3D control, which was shot in 2008 over a portion of Delhi Field (Tuscaloosa and Paluxy) in northeast Louisiana. The Reservoir Characterization Project Phase XIII, via the Colorado School of Mines, includes a relevant time-lapse multi-component 9C4D seismic monitoring program to better image, depict, and manage a CO2 flood in a highly heterogeneous series of multi-stacked lenticular bar and channel reservoirs.
C Channel Stratal Slice via Spectral Decomposition in GeoModeling