--> ABSTRACT: Evidence for Barrier Island (Vs. Strand Plain) and Valley-Fill Origins of and Production Heterogeneities in Bell Creek Field, Montana, by Roderick W. Tillman and D. H. Dailey; #91022 (1989)

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Evidence for Barrier Island (Vs. Strand Plain) and Valley-Fill Origins of and Production Heterogeneities in Bell Creek Field, Montana

Roderick W. Tillman, D. H. Dailey

Bell Creek field, Montana, produces from Albian (Lower Cretaceous) Muddy barrier island and valley-fill sandstones. The origin(s) for the producing sandstones are somewhat controversial. Evidence presented herein supports a barrier island origin for the major producing reservoir at Bell Creek. Varying scales of heterogeneities, which are deterrents or barriers to fluid flow, are associated with the barrier. Recent work in 1988 suggests that to the south, in Wyoming, time-equivalent deposits to the marine sandstones at Bell Creek are strand-plain deposits and by inference suggests that Bell Creek is not a barrier but a strand-plain deposit. Characteristics common to Bell Creek and supportive of a barrier island are (1) washover facies, interbedded with lagoonal mudstone, occur in cores on the landward northeast side of unit A, (2) foraminiferal species indicate less-than-normal marine salinities in lagoonal mudstones, (3) shoreface sandstones thin or are absent in wells on the landward side of the field, (4) in analogous outcrops and in cores, foresh re, and/or upper shoreface deposits locally occur above middle shoreface deposits, and (5) backshore sandstones with landward southeast flow directions occur above washover deposits in analogous outcrops.

Subordinate production at Bell Creek comes from suprajacent valley-fill deposits, which also form barriers to flow and subdivide the field into separate producing units. Two periods of valley cutting are suggested.

Field-wide heterogeneities, interwell heterogeneities, and very local (single-well) heterogeneities contribute to highly variable, total cumulative production ranging from 80,000 to 200,000 bbl/well. Densely spaced northwest-southeast and northeast-southwest-oriented smallthrow faults form local barriers to flow. Diagenetic clay and depositional clay stringers are widely distributed in a somewhat predictable manner.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.