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AAPG Bulletin

Abstract

AAPG Bulletin, V. 93, No. 11 (November 2009), P. 14131426.

Copyright copy2009. The American Association of Petroleum Geologists. All rights reserved.

DOI:10.1306/07270909094

Mechanical and fracture stratigraphy

Stephen E. Laubach,1 Jon E. Olson,2 Michael R. Gross3

1Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78713-8924; [email protected]
2Petroleum and Geosystems Engineering, University of Texas at Austin, 1 University Station C0300, Austin, Texas 78712; [email protected]
3Department of Earth Sciences, Florida International University, Miami, Florida 33199; [email protected]

ABSTRACT

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.

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