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ABSTRACT: Exploring for Transgressive Barrier Strat Traps in the Williston Basin

John H. Barwis

Transgressive barrier islands on relatively stable margins produce thin, discontinuous sand bodies. Because preserved sand bodies may be relatively small and may display any number of orientations, stratigraphic components of traps are of primary importance. Exploration and production in these situations carry relatively high reservoir risk. The keys to minimizing this risk are to combine detailed facies analyses of individual reservoirs and seals with broad-scale sequence analysis of the entire basin, and to recognize the role played by sea level changes in determining facies relationships and sand-body architecture at both scales.

The Medora-Dickinson trend is a string of fields that produce oil from the Tyler Formation, which locally comprises lenticular sandstones deposited along a microtidal coastline. The trend represents remnants of a transgressively modified, sand-poor barrier-island system deposited on a relatively stable cratonic margin. Reservoirs originated as tidal-channel point bars and flood-tidal deltas, and were preserved as thin, linear to podlike sandstone bodies configured in a wide variety of structurally influenced stratigraphic traps. Production in from quartzarenites encased in lagoonal-shale source rocks.

Tyler stratigraphy records the depositional response to a Carboniferous sea level fall and subsequent rise, a single chronostratigraphic sequence on which higher frequency cyclicity was superimposed. At least ten regionally correlative base-level rises are evident within the Tyler. Separate trends of stratigraphic traps should exist for each of these relatively highstands, each associated with mappable paleoshorelines. Once a shoreline trend is identified, random drilling would result in a 20% success rate.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990