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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 59 - 88

SILURIAN INTERLAKE GROUP: A SEQUENCE OF CYCLIC MARINE AND FRESHWATER CARBONATE DEPOSITS IN THE CENTRAL WILLISTON BASIN1

Esther R. Megathan, Esther R. Magathan & Associates , Consulting Geologists, 535 Garfield Street, Denver, Colorado 80206

ABSTRACT

Dramatic paleoenvironmental changes are recorded by Interlake facies, fossil assemblages, diagenetic textures, and cements in the central Williston Basin. They demonstrate that depositional conditions during the Silurian initially oscillated between open marine and hypersaline, but were ultimately replaced by freshwater environments. These changes significantly affected the hydrocarbon potential of Interlake rocks because they resulted in geographically and stratigraphically confined source beds and seals, and in a variety of reservoirs whose properties were influenced both by facies and diagenesis.

Seven Interlake formations, represented in three cores from the Hesson anticline area, illustrate the carbonate facies sequence typical of the central part of the basin. The Lower Interlake Strathclair, Fife Lake, and Guernsey Formations incorporate turbidites, organic-rich basin laminites, anhydrites, and quartz-bearing 'event' markers, recording depositional cycles initiated in deep waters of normal marine salinity, and terminated in shallower, hypersaline water. The Middle Interlake Cedar Lake Formation typically is of dolomite sequences alternately rich in, and barren of, marine fossils, suggesting that salinity variations continued but were less extreme than in Lower Interlake time. The Upper Interlake Mendenhall, Missouri Breaks, and Sherven Formations contain only freshwater fossils, and document the filling of a terrestrial inland basin by lake, marsh, and stream sediments cannibalized from older carbonates exposed around the basin margin.

These highly variable depositional regimes have resulted in excellent source, reservoir, and seal facies within Interlake formations. Rocks with source rock potential accumulated during several phases of marine and freshwater deposition. Reefs and associated skeletal deposits, when dolomitized, form good reservoirs in Lower and Middle Interlake formations, and porous marsh, stream, and karst deposits, where flushed by undersaturated groundwaters, are excellent reservoirs in the Upper Interlake. Interlake seals are products both of facies and diagenesis: several depositional evaporites confine Lower Interlake reservoirs, whereas Upper Interlake reservoirs are confined both by tightly cemented quartzites and by halite remobilized during late burial diagenesis. Regionally, the entire Interlake sequence is confined by impermeable facies of the overlying Ashern Formation.

Major discoveries in recent years have established several Interlake formations as attractive exploration targets, yet wildcatting has been limited due to the lack of coherent regional stratigraphic, facies, and diagenetic models. Core studies offer the only sound basis for developing these models, and have been successfully integrated with sample studies and log suites to regionally correlate and map prospective reservoir, source, and seal facies in the most prospective central part of the basin.

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