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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Climate Control on Reservoir Distribution in the Upper Devonian Three Forks Formation, North Dakota and Montana

Previous HitSvenTop Egenhoff1; Ali Jaffri2; Patrick Medlock3

(1) Geosciences, Colorado State University, Fort Collins, CO.

(2) VNG Norge, Stavanger, Norway.

(3) Brigham Exploration Company, Austin, TX.

The Three Forks Formation is currently one of the main targeted reservoirs in the Williston basin. Estimates go as high as 2 bbl of recoverable oil in this unit, however, a detailed sedimentology of the Three Forks is still lacking. Here we introduce a process-oriented model for this Upper Devonian formation and show how its sediment architecture reflects climate fluctuations allowing to predict the vertical and lateral distribution of reservoir facies.

The Three Forks consists of six facies groups: terrestrial paleosols, sabkha, subaerial gravity flow, intertidal, peritidal, and subtidal. Of these only the sabkha, gravity flow and peritidal deposits are common and present everywhere in the basin. Controlled by climate and sea-level changes, two different proximal to distal facies transects characterize the Three Forks sedimentary system. During arid times little sediment is derived from the cratonal source areas, and a wide sabkha developed, locally with intertidal algal mats, that grades laterally into peritidal mixed carbonate-siliciclastic sediments and further seawards into subtidal storm deposits. Its humid counterpart shows the same peri- and subtidal facies, but its landward portion consists of a subaerial plain receiving abundant debris flows from the hinterland, substituting the sabkha, and paleosols characterizing local patches of non-deposition.

Sequence stratigraphic correlation of the succession shows an overall deepening of the environment reflected in backstepping parasequences. The non-reservoir sabkha facies is overlain by intercalated peritidal reservoir and continental non-reservoir deposits, arranged in a cyclic fashion. The top of the Three Forks succession is formed by peritidal reservoir facies in the outer parts of the basin, and subtidal non-reservoir facies in its center.

While sabkha and some peritidal deposits with associated evaporites show arid conditions the increase in sediment supply by debris flows reflects a change to a humid environment with paleosol development. The stratigraphic distribution of sabkha deposits overlain successively by debris flow units, evaporite-bearing peritidal deposits and subtidal sediments records two shifts from arid to humid conditions, and back. These climate shifts are in phase with sea-level where million-year highstands mark humid environments, and lowstands arid conditions.