The Playa Lake Depositional Model for the Three Forks Fm

Abstract

The Late Devonian Three Forks formation of the Williston Basin is an important unit in the Bakken petroleum system, with estimates suggesting more than 3.7 billion barrels of recoverable oil. We propose that this formation was deposited in a continental environment with little to none marine influence, in contrast with widespread models that invoke restricted shallow marine, coastal sabkha or tidal flat environments. Our model allows us to explore stratigraphic controls on the distribution of hypersalinity and sour gas in the basin, as well as the vertical and lateral variability of reservoir quality in terms of petrophysical and geomechanical properties. The Devonian was a predominantly warm and arid period, with the Williston Basin located in ever so more arid equatorial position and affected by periods of marine restriction and complete isolation from the ocean. This resulted in the accumulation of a complex mosaic of silt-sized dolomite and quartz, clay and evaporite lithologies and a series of bull's-eye isopach configurations. Based on stratigraphic, sedimentological and petrographical analyses of 28 core, we conclude the Three Forks Fm was deposited in subaerial and subaqueous environments, without tidal or marine influence, that are comparable to those found in playa lake or continental sabkha environments. This isolated and dry environment was sporadically affected by large floods that covered the basin terrigenous sediment, debrite-like deposits with intraclasts, scour and erosion surfaces. Floods were followed by a drying period when the deposition of the mineral sequence dolomite-anhydrite-halite in ponds distributed across the basin surface. The drying period is recorded by subaqueous dolomites to subaerially-exposed and transported sediments with ripples, dewatering structures, scour surfaces, evaporite cemented surfaces, evaporite-removal breccias, halite pseudomorphs, cracks, etc. The vertical succession records an upward trend of increasing proportions of subaqueous facies, indicating a transition from arid to relatively less arid climatic conditions during Three Forks deposition.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016