--> Abstract: Stratigraphy, Diagenesis and Fracture Distribution of the Three Forks Formation, Williston Basin, by Vasilisa V. Nekhorosheva, Allaudin A. Gantyno, Stephen Sonnenberg, and J Frederick Sarg; #90124 (2011)

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

Stratigraphy, Diagenesis and Fracture Distribution of the Three Forks Formation, Williston Basin

Vasilisa V. Nekhorosheva1; Allaudin A. Gantyno1; Stephen Sonnenberg1; J Frederick Sarg1

(1) Colorado School of Mines, Golden, CO.

The Three Forks Formation could yield as much as 2 billion barrels of petroleum (NDGS, 2010). Previous outcrop studies date from the 1960s leave this formation almost uninvestigated in modern sedimentologic and stratigraphic terms.

The aim of the current work is to provide regional correlation of lithofacies, and to document fracture distribution and trends in the formation based on ten outcrops in Wyoming, Montana and South Dakota, and cores from 21 wells in the Williston basin and a core from the Big Horn basin. The proposed stratigraphic correlation of the Three Forks Formation shows lateral and vertical facies changes based on both outcrops and cores.

Key stratigraphic surfaces are recognized from the core data and traceable throughout the study area in well and outcrops. These major stratigraphic surfaces are a sequence boundary at the base of the Three Forks, a transgressive surface in the middle part of the Three Forks, and a sequence boundary at the top of the formation. Five interpreted facies associations are from the shallowest to the deepest: (1) upper supratidal sabkha; (2) lower supratidal sabkha; (3) upper intertidal mud flat; (4) lower intertidal mud flat; and (5) open marine.

Upper supratidal sabkha, has shallowing upward stacking pattern. Mosaic, nodular and bedded anhydrite together with reddish and greenish silty claystone are dominant. Lower supratidal sabkha, is greenish and reddish silty dolomitic claystones.

Upper intertidal mud-flat is composed of pinkish-grey siltsone with shrinkage cracks. Lower intertidal mud-flat to shallow subtidal is composed of grey well-cemented bioturbated sandy siltstone. Open marine, consists of brown to grey bioturbated silty very fine-grained sandstone.

The lower and the middle parts of the Three Forks are dominated by supratidal sabkha deposits, whereas, the upper part is dominated by the intertidal and occasional open marine deposits. Diagenetic features include dolomitization, anhydrite precipitation and cementation, compaction, clay cementation, dissolution and pyritization.

Fracture patterns were described and compared with fracture distribution in overlying Bakken Formation. In most of the outcrop locations there is a systematic fracture distribution associated with major faults. Fractures have predominant strikes of 50, 120, 250, and 320 degrees. General spacing between major fractures of similar direction is 1m and less. Laminated layers tend to show more fractures than more massive beds.