Mechanical Stratigraphy in the Monterey Formation and its Control on Fracture Geometry, Distribution and Scaling

Michael R. Gross and Jon R. Schwalbach
Florida International University, Department of Earth Sciences, Miami, FL 33199

Fracture architecture in sedimentary rocks is controlled by a number of factors including lithology, mechanical properties, structural position, tectonic stresses and strain history. In the lithologically diverse and thin to medium bedded Monterey Formation, mechanical stratigraphy plays a critical role in the development of fracture and fault populations at the reservoir scale. Fractures often terminate at discrete boundaries such as lithologic contacts or bedding slip surfaces, resulting in fracture geometries with high length to width ratios. Further, extensional strain in the Monterey Formation, often the product of fault-related folding, is accommodated by different types of brittle deformation. Thus beds of siliceous chert and dolostone may contain joints and veins, whereas adjacent mudstone units are characterized by conjugate normal faults. The result is a mechanical stratigraphy consisting of alternating jointed, faulted and unfractured units. In addition, multilayer fracture and fault zones develop at elevated strains, and correspond to larger mechanical units that incorporate numerous smaller units in a multi-scale hierarchy. These highly permeable zones consist of closely-spaced fractures that serve as primary pathways for fluid flow across mechanical layer boundaries.

AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California