Abstract: Modeling Overpressures in Sedimentary Basins: Consequences for Permeability and Rheology of Shales, and Petroleum Expulsion Efficiency

J. Burrus, F. Schneider, S. Wolf

The prediction of overpressures using Institut Francais du Petrole's 2-D numerical model TEMISPACK is applied to several provinces of the world. In the Paris basin, France, normally pressured Liassic shales are shown to have permeabilities around a microdarcy, independently confirmed by laboratory measurements. In contrast, in the Norway section of the North Sea, Williston Basin, Canada, Gulf Coast, and in the Mahakam delta, observed overpressures of 10-50 MPa are consistently modeled with shale permeabilities around 1-10 nanodarcys. This theoretical value fits well with the lowest permeability measured in compacted shales. For these basins, compaction disequilibrium was found to explain most (>85%) of the overpressures. The only exception was the Williston basin in which overpress res observed in the organic-rich Bakken shales are entirely due to hydrocarbon generation.

In Mahakam delta, the rheology of shales is nonlinear, i.e., the strength of shales increases rapidly with depth. Consequently, shale compaction cannot be described by the linear behavior often assumed in hydrology. In the absence of fault barriers, numerical simulations and geological evidence suggest that overpressured source rocks have low or very low expulsion efficiency, irrespective of their organic content. However, shales with a permeability on the order of a microdarcy do not hinder petroleum migration.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994