Victor, Ash Draw, and West Prong Field: A Minnelusa Seismic Stratigraphic Comparison

GALLIVAN, LYLE B., and GEORGE E. HANDLEY, Eclipse Exploration Corporation, Denver, CO

Seismic stratigraphy has become an acceptable but not infallible tool in Minnelusa exploration and development. Victor, Ash Draw, and West Prong fields were discovered and/or developed using seismic stratigraphy. Each field produces from a different sand member of the Minnelusa Formation, as well as a different area of the Powder River basin. Thus, each field illustrates a different seismic signature.

Victor field produces from only 15 ft of the lower B sand since it is close to the limit of B sand deposition. Facies change of this eolian sand into interdunal anhydritic dolomites provides the trap. Synthetic models indicate that 70-80 Hz data is necessary to resolve a subtle trough development that never achieves strong amplitude due to the thinness of the sand. Seismic data confirm this and show that thickness and velocity variations in the underlying B dolomite and overlying Opeche shale can mask or generate false sand character.

Ash Draw field, near the Montana/Wyoming border, produces where a low-velocity C sand truncates into a higher velocity Opeche shale. One east-west seismic line shows this change as a top-loaded upper C sand generated trough changes into a doublet, with a bottom-loaded trough below, due to the middle C sand porosity.

West Prong field documents the most dramatic seismic anomaly. It produces from an A sand member 40 ft thick with 25% porosity. This A sand thins as the lower B sand thickens and is truncated by a thick Opeche shale section. Due to the greater thickness of each unit a distinct seismic shift and strong amplitude is seen over each event.

Seismic stratigraphy can enhance a Minnelusa exploratory program and reduce development risk, especially where sand thickness exceeds 25 ft. Each sand member and area of the basin, however, must be treated separately.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)