Integrated Burial and Thermal Reconstruction of the Bakken Shale in the Canadian and Us Williston Basin

Elizabeth Roller, Andy Pepper, Gunardi Sulysto, and David Schmidt
Hess Corporation, Houston, TX

Accurate basin reconstruction and burial/thermal history analysis is an important step in determining the petroleum potential of a sedimentary basin; and in delineating the lowest risk segments of a play fairway such as the Bakken play in the Williston basin. In this presentation we summarize a workflow to derive an accurate, geologically reasonable thermal reconstruction of the Bakken across the basin, stretching from northern USA to southern Canada.

Historically, the maturity of the Bakken source rocks has been estimated and mapped in a number of ways:
(1) Electrical resistivity, tied loosely to present day temperatures (Meissner’s work of the late 70’s)
(2) Geochemical investigations using pyrolysis data (Webster’s and Price’s work of the early-mid 80’s)
(3) Two-dimensional sectional thermal modeling (Burruss et. al., 1984)

Although it has long been recognized that the Alberta foreland has been significantly uplifted and eroded, none of these studies based in the US sector of the Williston basin have addressed the problem.

We approached the thermal history of the Bakken by constructing a map-based model combined with a suite of 1-D burial history temperature models to illustrate the thermal and structural evolution through time. A representative spread of wells were chosen in North Dakota, Montana, and Saskatchewan, selecting those with both present day temperature control, and Bakken core data on which thermal stress indicator (TSI) data can be derived. (We thank North Dakota Geological Survey and the Saskatchewan Mineral Resources Board for access to samples and data.) As pointed out by Price, vitrinite reflectance data is very poor in the Bakken, so we used biomarker-derived thermal stress indicators (hopanes, steranes and aromatic steranes) with known kinetic parameters to estimate paleo-thermal stress - and hence the amount of eroded section.

The Tertiary burial history of the basin was revisited and re-evaluated using coal compaction models from various coal-producing horizons in Cretaceous and early Paleocene stratigraphy.

Two conclusions are immediately evident from the modeling:
(1) There are significant lateral variations in present day (and paleo-) heat flow across the basin, which are best explained by variations in the radiogenic heat contribution, in turn reflecting the composition of the underlying Pre-Cambrian crust.
(2) As in the Alberta foreland, significant post-Paleocene/Eocene deposition was followed by rebound and erosion (most likely taking place during the Oligocene). This observation is supported by the rank of lignites in the surface-mined Paleocene deposits.

For a combination of these two reasons, present-day depth is a very poor indicator of (paleo-) thermal stress in the Williston basin.

The next phase of this project will combine the thermal history with kinetic models of generation/expulsion within the Upper and Lower Bakken Shale units, to predict regions of most favorable charge, fetch and focus in the Bakken fairway.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado