Abstract: Origin of Fractured Cretaceous Conventional and Unconventional Reservoirs, Southern Powder Basin, Wyoming

Gary C. Mitchell, Mark H. Rogers

Cretaceous conventional and unconventional fractured reservoirs in the southern Powder River basin, Wyoming, are associated with small throw (10 to 30 feet; 3 to 10 m) normal faults. The faults are nearly vertical, trend northwest-southeast and northeast-southwest and are probably basement derived. The faults are most easily identified in Cretaceous marine shales and are exposed at the surface in Tertiary units.

Erosion and subsequent deposition of Cretaceous sandstones, limestones and shales affected by the extensional normal faults form stratigraphic traps. The reservoirs are interbedded with, or composed of, mature source rocks that have generated and expelled significant hydrocarbons. Overpressuring from the maturation and expulsion processes is still present and has preserved open fractures and porosity in reservoirs from the Lower Cretaceous Fall River through the Upper Cretaceous Niobrara formations.

The faults have offset thin sandstone reservoirs forming permeability barriers. The faulting and associated fractures have provided vertical pathways for organic acids that assisted formation of secondary porosity in Upper Cretaceous sandstones. The fracturing of mature source rocks provides areally extensive unconventional reservoirs. Fracturing associated with the extensional normal faults provides significant exploration and exploitation potential for the use of horizontal drilling techniques to evaluate multiple, fractured, overpressured conventional and unconventional reservoirs that may contain large reserves.

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