Reservoir Characterization of the Mississippian Sun River Dolomite: Reagan Field,

Glacier County, Montana and southern Alberta

By

Mark D. Sonnenfeld, K. Lyn Canter, and Laura C. Zahm

iReservoir.com, Inc, Greenwood Village, CO

Lara Conrad, John Parkin, and Craig Siemens

PanCanadian Energy, Calgary

Most studies of the Sun River Dolomite (SRD) emphasize the following reservoir controls: 1) a strong dolomitization overprint that is only weakly facies-selective, and 2) the importance of karst at the top-Madison unconformity. Although these processes applied at Reagan Field, primary facies distribution remains the principal control on reservoir heterogeneity, especially where enhanced thickness of the main pay (peloid-dasyclad dolograinstone) is controlled by axes of erosion at an intra-SRD unconformity. The sedimentologic-stratigraphic model was the first step in an integrated reservoir characterization workflow leading to a full-field 3-d reservoir simulation based on 38 cores from Reagan and surrounding fields, coupled with observations made during a 2-day analog outcrop study in the Little Belts and Sun River Canyon. Nine depositional facies were grouped into three gross intervals: 1) an open-marine-dominated lower interval, culminated by bioclastic shoal development; 2) a middle interval characterized by aggrading cycles of restricted lagoon deposition, terminated by a significant paleokarst; and 3) an upper unit characterized by stacked sequences of high-energy transgressive shoal/channel capped by restricted lagoon deposits.

Key geologic insights that significantly impacted the reservoir model were: 1) most gamma-ray markers are hypersaline, microporous lagoonal dolomudstones that are vertical permeability baffles effective at controlling both gas- and water-coning; 2) recognition of an erosional base to the most prominent grainstone pay interval; 3) recognition of the role of paleostructure on axes of erosion; and 4) control of the field’s sweet spot by intra-SRD erosion in a paleo-low--later inverted to become the Laramide structural high.

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