Quantitative Assessment of Exhumation Magnitude and Paleobathymetry Using Mckenzie’s Lithospheric Stretching Technique

This paper presents preliminary work and an exhumation and paleobathymetry solution as applied to basin modelling within the northern portion of the Williston basin.

Exhumation and paleobathymetry corrections are paramount to the authenticity of the total petroleum systems models of partially exhumed basins as applicable constraints, risk and the tendency to overestimate undiscovered resource potentials pose significant challenges.

An attempt is made to quantify the magnitude of exhumation and estimate paleobathymetry within Williston basin Phanerozoic successions as an application of the McKenzie lithospheric stretching technique. McKenzie lithospheric stretching technique has been said to be a valid framework for the evaluation of exhumation in intracratonic rift basins of which Williston Basin is a suitable example.

Theoretical subsidence curves are modelled for each selected well and fitted to their respective backstripped and decompacted subsidence curve by adjusting the stretch factors (β) for the crust and the mantle. Paleobathymetry is further estimated by adjusting the initial zero input to other numerical values where erosion is not reported. The resulting departure from the theoretical curve is a measure of denudation as deterministic bathymetric values are extracted as paleobathymetry. Sub-Cretaceous, sub-Triassic, sub-Devonian and Mid Ordovician erosions are estimated.

While the Willison basin Phanerozoic strata has undergone substantial erosions at the aforementioned periods, they are generally assumed to be deposited close to sea level. Most units with the exception of Bakken and possibly Lower Lodgepole formation are deposited in water depth less than 100m hence, a notion that bathymetry effects could be considered negligible in Williston basin burial history models.

The results show that paleobathymetry values vary with locations within the basin. An example is the estimated values from three selected wells (two located within Saskatchewan and the third close to the centre of the basin in North Dakota) that yield Bakken paleobathymetry of 150m, 270m and 385m respectively. This is in agreement with suggested water depth as deep as 250m for Three Forks Bakken and Lower Lodgepole in Saskatchewan portion of Williston basin. The assessment is done under three scenarios: assumed negligible (zero) bathymetry, modelled Bakken bathymetry and modelled bathymetry for all non erosion intervals.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California