Abstract: Origin, Distribution and Character of Fractures within the Lisburne Group, Northeastern Sadlerochit Mountains, Northeastern Alaska

C. L. Hanks, J. C. Lorenz, A.P. Krumhardt

The Carboniferous Lisburne Group is the naturally fractured carbonate reservoir of the Lisburne oil field of the North Slope of Alaska. Understanding the origin, distribution, and character of these fractures could play a major role in increasing the productivity of this giant field. The nearest exposed section of the Lisburne Group is in the eastern Sadlerochit Mountains 75 miles to the southeast. Although the structural setting of the Lisburne Group exposed in the Sadlerochit Mountains and that of the Lisburne carbonates in the subsurface to the northwest are very different, the regional variations in gross fracture patterns as well as the control of stratigraphy on fracture character and distribution can be evaluated more easily in surface exposures than in the subsurface.

Preliminary analysis suggests two dominant sets of calcite-filled fractures occur in the eastern Sadlerochit Mountains. An early, north-northwest striking set of fractures is probably related to north-northwest directed regional compression and thrusting during Late Cretaceous and early Tertiary time. This fracture set is well developed in the eastern Sadlerochit Mountains and elsewhere in the northeastern Brooks Range and is probably widespread in the subsurface north of the range front. The second set strikes east-northeast, postdates the north-northwest-trending fractures and appears to be related to local faulting and related folding and thus is more areally restricted.

In both sets of fractures, lithology plays a major role in the distribution of the fractures, with fracture density higher in calcareous mudstones and/or dolomitic horizons. There is no obvious correlation between fracture density and bed thickness. A large percentage of fractures in both sets terminate against bedding planes or stylolites. From these observations we conclude that vertical variations in lithology at one location as well as lateral variations in stratigraphy from site to site potentially can result in extreme vertical and lateral porosity and permeability heterogeneity in this naturally fractured carbonate reservoir.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994