Interpretation of Upper Muddy Transgressive Shoreline Reservoirs in the Southern Powder River Basin: Development and Application of New Exploration Concepts to a

Mature Petroleum Play

By

Paul E. Devine

Key Production Co., Inc, Denver, CO

The Lower Cretaceous Muddy was formed following a sea-level lowering across the present basin. The lower Muddy includes incised valleys with continental fill from the subsequent eastward transgression. Transgression was interrupted by a series of shoreline progradations in the upper Muddy that are progressively younger from west to east. Subsurface exploration for shoreline reservoirs has typically involved mapping gross sandstone values and interpreting sedimentary environments from the resulting patterns. Present-day sandstone distribution in each parasequence, however, is not solely a depositional effect, but is also a function of preservation beneath an erosional disconformity. Without a method for handling this complexity, the preservation component is usually ignored.

Through analysis of major fields producing from two upper Muddy shoreline trends I have developed methods for separately evaluating both deposition and preservation. The depositional component is delineated by an isopach of the shale interval beneath the shoreline sandstone. For each parasequence, the thinnest shales demonstrate a strike-elongate trend representing the final stabilized position of the retrograded shoreline (barrier island). Previous studies have suggested that the locations of prospective Muddyage deposits correlate with accommodation from Paleozoic salt dissolution. My work shows that upper Muddy preservation only corresponds to paleostructural lows if the valley-fill is either thin or was thickened by erosion. For preservation, I use paleostructure, valley-fill and incisement data in an algorithm that predicts interval thickness without regard to facies content. Finally, the two components are combined into a map of sand potential that explains the occurrence of existing fields and exposes exploration targets.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming