Talk Title: Integrated Analysis of the Bakken Petroleum System: U.S. Williston Basin
Speakers: Jack Flannery and Jeff Kraus, Tethys Geoscience
Publication: The Outcrop, November 2005, p. 5
As much as 300 billion barrels of oil have been generated from Upper Devonian-Lower Mississippian Bakken shales in the U.S. Williston Basin. Only 50 million barrels (< 1 %) have been produced from the Bakken in Montana and North Dakota. Geochemical investigations demonstrate that very little Bakken-sourced oil has leaked in to surrounding reservoirs. This basic understanding of Bakken potential has intrigued explorers for decades.
Recent industry activity has been focused on the middle Bakken siltstone trend in Richland County, Montana. Operators there are enjoying impressive success rates from wells that test 500 barrels of oil per day, on average. Horizontal drilling, completion, and fracturing technology are generally credited with opening up the historically disappointing play. Companies are now extending the play in to other parts of the Basin. Future success will rely largely upon developing a thorough understanding of the play as it is currently being exploited and, especially, upon using that understanding to identify key geologic controls of Bakken prospectivity that can be capitalized on elsewhere.
We discuss the Bakken play within the context of the geology and exploration history of the Williston Basin. Regional structure and isopach maps, along with geochemical, thermal, and rock properties data, are used to construct a three-dimensional thermal and fluid flow model of the basin. The model provides unique insight into the evolution of the Bakken petroleum system and allows us to predict reservoir quality, source maturation, and volumes of oil expelled and currently trapped within the middle Bakken.
Integration and spatial analysis of modeled results, regional maps, and measured data in the geographic information system shed light upon the fundamental geologic variables and relationships that control Bakken prospectivity. Key factors include:
- Maximum Temperature – Maximum temperatures were reached in the Basin during the early Tertiary. Temperatures in the Bakken had to exceed 100°C to cause oil expulsion from organic-rich shales. Expulsion of oil hydraulically fractured characteristically tight middle Bakken reservoirs, enhancing porosity and significantly improving reservoir quality.
- Stratigraphic Architecture – The play is most effective where the middle Bakken siltstone pinches out along the southern and southwestern edge of the Basin. Significant potential also exists toward the Basin depocenter, where the middle Bakken becomes sandier and may be as thick as 120 feet.
- Small-Scale Porosity Development – In addition to fractures and fracture networks, the development or preservation of intergranular porosity appears to effect production, as inferred from initial production test oil flow rates.
Based upon these observations, we interpret potential for additional middle Bakken exploration down dip from the current siltstone play where the middle Bakken thickens and becomes sandier. Lease and land data, in conjunction with play maps and drilling results, can also be used to understand industry and company activity trends, and to help build an integrated technical-commercial strategy for the Bakken play.