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3-D Characterization of Fractures in Mesaverde Reservoirs, or Why the Sugar Cube Reservoir Model Doesn't Always Work

John C. Lorenz, Kris L. Smock

Data suggest that fractures in reservoirs of the nonmarine Mesaverde rocks in the subsurface of east-central Piceance Creek basin (northwestern Colorado) are vertical, unidirectional, and subparallel. There is a continuum of fracture sizes and spacings from microscopic fractures to those extending a few tens of feet vertically and horizontally. Fractures often do not extend vertically across small-scale lithologic discontinuities within the fluvial reservoirs. The presence of orthogonal fractures in the subsurface may be less common than generally believed. Primary orthogonal fractures require special structural conditions for development in the subsurface; unidirectional fractures can develop in flat-lying rocks in areas of high pore pressures, and under relatively low d fferential horizontal stress conditions. Outcrop fracture characteristics must be extrapolated into the subsurface with caution because secondary orthogonal fractures can be created at the surface during erosion and stress relief simultaneously with the vertical and horizontal extension of an existing unidirectional fracture set.

Mesaverde reservoir fractures are interpreted to be poorly interconnected, with connections primarily due to the intersection of subparallel adjacent fractures. Different fracture networks exist at different lithologic levels in the reservoir, and few fractures extend vertically from one level to the next. Reservoir fractures are of irregular permeability, spacing, and size along their length, and no horizontal or orthogonal fractures exist. This lack of orthogonal fractures explains why "sugar cube" fractured reservoir models often have to be modified to match reservoir behavior. The connecting orthogonal fracture system, whereas present in outcrop, may not exist at reservoir depths.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.