Bakken Tight Oil Resource Play –
Exploration Model
Colorado School of Mines Bakken Team
J. Frederick Sarg
S. A. Sonnenberg
M. Batzle
M. Prasad
DOE Award No.: DE-NT0005672
November 11, 2011
Bakken Exploration Model - Introduction
The following slides summarize the key learnings of this
NETL/CSM joint project on the Bakken Petroleum System.
Parameters and criteria have been developed to identify
areas where new traps may be found, and where enhanced
productivity of Bakken oil is possible. The criteria are
grouped into four categories – (1) source and maturation,
(2) reservoir quality of matrix, (3) natural fractures, and (4)
trap and seal. This summary focuses on the Bakken
Formation, but the following conclusions may be applicable
to the underlying Three Forks Formation. A paper is in
preparation that will describe in more detail, project
conclusions and this exploration model.
Source Rocks - Organic-Rich Bakken Shales
• Kerogen Type I and II.
• TOC averages ≈11%, ranges from 8-16% over much of the
Williston basin in North Dakota, and can attain values into the
mid-30’s.
• Mature over much of the basin, and still generating oil. Large
fields (Elm Coulee, Sanish, Parshall) are situated at transition
from mature to immature level of maturation. Volume
increases during maturation by as much as 30%. Volume
decrease at the transition to immature source rock is important
to providing updip top and lateral seal.
• Oil migration from the Upper Bakken shale probably has been
over short distances (km to 10’s of km), and has been mainly
into the underlying Middle Bakken. Much of the oil remains
within the Bakken Petroleum System.
Reservoir Quality – Middle Bakken
& Upper Bakken Shale
• Middle Bakken facies B, C, D, and E can all be reservoirs, if quartzrich and dolomite-rich (D) or dolomitized (B, C, and E).
• Porosity averages 4-8%, permeability averages 0.01-0.001 md or
less.
• Matrix reservoir quality is enhanced by dolomitization. Porosity is
intercrystalline and tends to be > 6%. Permeabilities may reach to
0.15 md or greater. This is a “sweet spot” determinant (e.g., Elm
Coulee, Parshall, and Sanish).
• Horizontal micro-fractures form along bedding lamination of the
Middel Bakken C and E facies
• The Upper Bakken shale is siliceous, increasing brittleness and
enhancing fracability. During oil generation, the volume increase
causes overpressure (0.6 - 0.73 psi/ft pressure gradient) and
horizontal micro-fractures in the shale enhancing permeability.
Bakken Natural Fractures – Permeability
Enhancement
• Regional fractures (faults) form an orthogonal set with NE-SW
(dominant and parallel to σ1) and NW-SE orientations. Many
horizontal wells are drilled perpendicular to σ1 direction to
intersect these fractures.
• Local structures formed by basement tectonics or salt dissolution
form both hinge parallel and hinge oblique fractures that may
overprint and dominate the local fracture signature.
• Horizontal fractures formed by oil expulsion in shale and along
bedding plane lamination of Middle Bakken C and E facies provide
permeability pathways.
Trap and Seal – Bakken
• Top Seal – Lodgepole Formation, and where immature,
Upper Bakken Shale
• Base and Lateral Seals
1. Middle Bakken B & C, where undolomitized.
2. Lateral facies change to impermeable rock.
• Trap
1. Hydrodynamic at mature to immature boundary
(e.g., Parshall).
2. Stratigraphic pinch out (onlap) of Middle Bakken
(e.g., Elm Coulee).
3. Porosity pinch out of Middle Bakken dolomitic facies
(e.g., Elm Coulee).
4. Lateral facies changes in Middle Bakken (e.g.,
Viewfield).