Devils Tower Area: Rationalization of Geophysical Subtleties with Geologic Concepts Provides Basis for Successful Drilling Program
Paul K. Wieg
Dominion, Houston TX
The Mississippi Canyon well 773 #2 discovered Devils Tower field in early 2000. The field has grown to an estimated reserve of 75-150 MMBOE. Eight oil reservoirs are geophysical mapping horizons of Lower Pliocene to Upper Miocene age. Seven are regarded as commercial. The field is comprised of multiple pay sands that were deposited at the south end of a small minibasin in a deepwater field configuration. Oil gravity is 18 to 38 API. Most prolific oils are 28 to 32 API. Accumulations are undersaturated by 2500-5000 pounds per square inch, of low asphaltene content, and relatively low in sulfur, < 0.5%. Marine depth is approximately 5600 feet, with production 6000 to 9000 feet below mudline, facilitating inexpensive drilling.
Devils Tower was discovered using amplitude mapping of full-stack, 3 dimensional seismic data. Petrophysical analysis subsequently defined sands as "class III”. That is, hydrocarbon-bearing intervals display well-defined amplitude anomalies. Major pay zones are routinely recognized. However other pay zones are common, even prominent, though seismic amplitudes alone poorly define them. Stratigraphic models and trap concepts have been critically important in delineating these zones. Reservoir geometries and connectivity risks have been assessed by advanced 3 dimensional visualization techniques, e.g. studies of seismic volumes with an opacity viewer.
Immediately adjacent to Devils Tower is the Triton development area, Mississippi Canyon 728 and 772. It consists of several stacked pay intervals, generally coeval with Devils Tower pays. Triton geophysics differs because of hydrocarbon-charged marls. Although many sands are thin, they appear to have good lateral connectivity as sheets of large areal extent.
AAPG Search and Discovery Article #90032©2004 GCAGS 54th Annual Convention, San Antonio, Texas, October 10-12, 2004