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The AAPG/Datapages Combined Publications Database

Williston Basin Symposium

Abstract

NDGS-AAPG

Fifth International Williston Basin Symposium, Core Workshop, June 14, 1987 (SP9A)

Pages iv - 31

WINNIPEGOSIS PLATFORM MARGIN AND PINNACLE REEF RESERVOIRS, NORTHWESTERN NORTH DAKOTA

James R. Ehrets, Jackalope Geological Ltd. 1327 Spruce Street, Suite 3, Boulder, CO 80302
Don L. Kissling, Jackalope Geological Ltd. 1327 Spruce Street, Suite 3, Boulder, CO 80302

ABSTRACT

Differentiation of the Elk Point shelf into distinct platform and basin regimes during deposition of the Middle Devonian Winnipegosis Formation stimulated the development of a reef and slope complex along the platform margin and allowed for sustained growth of pinnacle reefs in a subsiding basin. The platform margin is subdivided into slope, reef, and peritidal facies which prograded as sedimentary wedges into the basin. Platform margin reef fabrics indicate early lithification, providing structural integrity and differentiation into relatively distinct fore reef, reef crest, and reef flat subfacies. Reef development was a function of vertical accretion and basinward progradation in conjunction with general platform physiography around the basin. Conspicuous argillaceous slope facies which bound reef sections represent extrinsic events during normal platform margin deposition, resulting in the redeposition of eroded coastal mud on the slope. Flora and fauna within these marker beds reflect the evolution of environmental conditions culminating in the widespread exposure of platform areas.

Pinnacle reefs, initiated as shallow-water mounds rich in codiacean algae and peloids, apparently evolved a variety of reef subfacies in response to basin subsidence. The upward transition from mound facies to coral-stromatoporoid rim and stromatolite crest subfacies reflects accelerated subsidence of the basin which accompanied platform/basin decoupling. Abundant drusy cements indicate that relatively steep reef peripheries were maintained by organic binding and submarine cementation. Reef flat subfacies developed upon establishment of generally shoaling conditions. During all stages of pinnacle reef development, proximal to distal flank subfacies formed by commingling debris shed off reefs with sediment endemic to the basin floor environments adjacent to reefs.

Evaporative drawdown of the Elk Point basin at the close of Winnipegosis deposition resulted in exposure of the broad platform region as well as the upper portions of pinnacle reefs. The subsequent establishment of vadose and related diagenetic environments resulted in the creation of platform margin and pinnacle reef reservoirs, but styles of reservoir generation and reservoir host facies differ for the two regimes. The reservoir at Temple Field is developed in non-argillaceous slope mudstone facies which underlies platform margin reefs. Early lithified reefs not only served to direct dolomitizing fluids into slope facies. but also played a key role in limiting halite invasion into the slope reservoir. Vertical zonation of diagenetic environments in pinnacle reefs resulted in a variety of vadose and meteoric phreatic processes in the upper parts of reefs, including leaching, lithification, and dolomitization. Consequently, coral-stromatoporoid subfacies display the greatest diversity of pore types and magnitude of porosity. Except for occurrences of overdolomitized reefs, preserved intergranular porosity in underlying algae-peloid mound facies was not significantly altered by marine phreatic processes in either undolomitized North Dakota reefs or dolomitized Saskatchewan reefs.

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