Abstract

Until 2005, there were approximately 75 wells producing Bakken oil in southeastern Saskatchewan’s portion of the Williston Basin producing approximately 200m3/day (1258 bbls/day) of oil. These were mostly in the Rocanville Field, discovered during the mid-1950s. Since 2005, the Bakken in southeast Saskatchewan has been extensively drilled with over 1,900 wells completed, and a current production of over 62,000 bbls/day of 38-40 API oil with a 50-60 percent water cut. The Bakken has become one of Canada’s premier light oil plays.

In southeastern Saskatchewan, the Bakken Formation is comprised of upper and lower shale members and a middle siltstone to sandstone member. The upper and lower shales are likely not thermally mature in most of the study area; however, they are a significant oil source rock down-dip from the main Saskatchewan plays.

The most recent oil production (since 2005) is from the Middle Member’s siltstone beds, while previous production at Rocanville was from sandier intervals within the unit. The Middle Bakken is a clastic unit deposited through late Devonian and into the earliest Mississippian. It is generally comprised of calcareous, very fine-sandstones and siltstones with shale-partings. Production from the Middle Bakken appears to be controlled by the stratigraphic architecture and relative degree of cementation. Most importantly, in the areas of recent development, discontinuities within the Middle Bakken appear to play an important role in determining the best reservoir rocks.

The Middle Member of the Bakken is subdivided into three units (A, B, and C) each displaying different sedimentological, ichnological and diagenetic characteristics. These units are distinguishable on geophysical well logs and have therefore been mapped in detail. This mapping, along with core study and the application of sequence stratigraphic principles has led to the development of a depositional model. This model suggests Unit ‘A′ represents a regressive prograding outer shoreface system while Unit ‘B’ deposition is characterized by continued regression followed by the development of a regressive surface of marine erosion (RSME). This is followed by a transgression as a result of a relative sea level rise, allowing for the deposition of unit ‘C’ and subsequently the Upper Bakken shale.