- Copyright ©2009. The American Association of Petroleum Geologists. All rights reserved.
Using examples from core studies, this article shows that separate identification of mechanical stratigraphy and fracture stratigraphy leads to a clearer understanding of fracture patterns and more accurate prediction of fracture attributes away from the wellbore. Mechanical stratigraphy subdivides stratified rock into discrete mechanical units defined by properties such as tensile strength, elastic stiffness, brittleness, and fracture mechanics properties. Fracture stratigraphy subdivides rock into fracture units according to extent, intensity, or some other observed fracture attribute. Mechanical stratigraphy is the by-product of depositional composition and structure, and chemical and mechanical changes superimposed on rock composition, texture, and interfaces after deposition. Fracture stratigraphy reflects a specific loading history and mechanical stratigraphy during failure. Because mechanical property changes reflect diagenesis and fractures evolve with loading history, mechanical stratigraphy and fracture stratigraphy need not coincide. In subsurface studies, current mechanical stratigraphy is generally measurable, but because of inherent limitations of sampling, fracture stratigraphy is commonly incompletely known. To accurately predict fractures in diagenetically and structurally complex settings, we need to use evidence of loading and mechanical property history as well as current mechanical states.
Steve Laubach is a senior research scientist at the Bureau Economic Geology where he leads the fracture and structural diagenesis research programs. He also supervises graduate student research in structural geology and diagenesis in the Jackson School of Geosciences. He is the chair of the Jackson School's Energy Geoscience Education and Research Group.
Jon Olson is an associate professor in the Department of Petroleum and Geosystems Engineering. He joined the faculty in 1995. He has six years of industrial experience. He specializes in the applications of rock fracture and continuum mechanics to fractured reservoir characterization, hydraulic fracturing, and rock mechanics.
Michael Gross is a professor at Florida International University specializing in brittle deformation and the use of quantitative, field-based structural methods to joints, faults, and veins in an attempt to understand their formation, distribution, and impact on subsurface fluid flow. His current research activity focuses on fractured reservoir characterization, the influence of mechanical stratigraphy on fracture development, and flow through fracture networks in layered rocks.