Distribution of Regional Fractures and Fracture Permeability Controlled by Sedimentological Heterogeneities, Mesaverde Reservoirs, Colorado

John C. Lorenz, Sharon J. Finley

Nonmarine sedimentation processes in lenticular Mesaverde sandstones of the Piceance basin, Colorado, created an irregular distribution of subunits within the present low-permeability reservoirs. The different lithologies have not only different porosities and permeabilities, but different mechanical and stress properties as well. These strata were fractured during horizontal tectonic compression, creating a regional fracture system. Because of low stress conditions during fracturing, the extent and distribution of fractures were governed by the differences in rock properties, pore pressures, and in-situ stresses that were associated with the different reservoir heterogeneities. Well tests and laboratory measurements show that the fractures create economic (millidarcy) re ervoirs out of submicrodarcy matrix rock. However, average fracture spacing and height--as measured in 4,000 ft of core and observed in outcrop--are commonly less than gross bed thickness by a factor of at least 10, and fractures are irregularly distributed within the reservoirs. No fractures extend the full thickness of any sandstone greater than 6 ft thick, whereas most reservoirs of interest are 20-50 ft thick. Between 20 and 25% of all vertical fracture terminations in core occur within a reservoir sandstone at mudstone partings, rip-up clast or carbonaceous zones, or at sandstone grain-size changes. Another 10-35% occur at no apparent lithologic discontinuity. Both of these categories may represent en echelon offsets of the fractures beyond the 4-in. diameter core, but hydraulic continuity across an offset of the first type is doubtful. Forty to 70% of fracture terminations are at contacts between a sandstone and an adjacent thick mudstone, in large part because 80% of the observed fractures occur in sandstones that are less than 10 ft thick. This type of fracture and permeability system may be present but unrecognized in other strata that are otherwise structurally undeformed.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988