--> Abstract: Geologic Model for the Bakken Horizontal Play, Williston Basin, North Dakota, USA, by L. N. Smith and L. P. Slagle; #91012 (1992).

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ABSTRACT: Geologic Model for the Bakken Horizontal Play, Williston Basin, North Dakota, USA

SMITH, LARRY N., and LETHA P. SLAGLE, Shell Western E & P Inc., Houston, TX

Horizontal drilling of the Mississippian/Devonian Bakken Formation, which consists of two black shales separated by a silty dolomite, defines a productive fairway that has been difficult to explain geologically. Petrophysical, geochemical, pressure, and maturity data define an 8,000,000 ac region that is mature, charged with oil, and geopressured. Drilling has proven only a 550,000 ac fairway to be productive. Outside the fairway, discoveries are limited to historically productive, robustly flexed areas developed with vertical wells, such as Antelope field. Horizontal wells on less robust flexures have made nonsustainable production or dry holes. These failures are thought to result from inability to locate areas of high fracture density outside the fairway. We present here an empiric l geologic model that explains increased fracture density in the fairway.

The unique geologic feature of the fairway is a relatively thin and low porosity Bakken section. Although all members thin southwest toward an onlap edge, the Billings nose region contains an exceptionally thin middle member. The upper shale and middle member display decreased density porosity in this area, probably as a result of early cementation on a platform located near the present Billings nose. Rock mechanic and burial modeling data predict that calcareous beds above and below the upper shale have greatest fracture density where thinnest and lowest in porosity. Boundaries of the fairway correspond to regions where the middle Bakken thickened during synsedimentary dissolution of subjacent salt. This model predicts that above some threshold of middle Bakken bed thickness, horizon al wells will not be productive owing to excessive fracture spacing.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)