Stratigraphy of the Three Forks Formation and Implications for Exploration

Alyssa Franklin
Colorado School of Mines

The Three Forks Formation is a tight oil reservoir sourced by the Bakken Shale in the Williston Basin. This study seeks to analyze depositional controls of reservoir facies distribution across the basin and apply that understanding to evaluate the potential of extending the Three Forks play from North Dakota into Montana. This study was conducted by describing 23 cores: 14 from Montana and 9 from North Dakota along with interpreting digital logs from across the U.S. portion of the basin. Lithofacies indicate deposition in a wind-driven wave-dominated, storm-influenced, micro-tidal epeiric shelf that was partially restricted due to movement along the Punnichy Arch - Swift Current Platform complex. Landward facies equivalents include coastal plain to evaporitic mudflat deposits. Facies are observed in laterally expansive, continuous belts across the basin, often bound by correlative surfaces. They are interpreted to have been deposited on a broad, shallow epeiric shelf with minimal accommodation. This depositional geometry in fact prohibits progradation and instead results in highly punctuated and aggrading deposits. Subtle changes in eustatic sealevel results in dramatic facies shifts across the broad basin with minimal slope. This depositional geometry also resulted in the tripartite energy zonation with lower energy zones separated by a shoreline detached high energy zone (see Irwin, 1964; Poppelreiter and Aigner, 2003). The dominant energy in this system varies between wind-driven waves and storms with microtidality. Reservoir quality, source maturity, pressures, and entrapment mechanisms have been considered to identify prospective areas for the Three Forks play in Montana.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013