--> ABSTRACT: Strain Patterns in Folds: Applications to Fractured Reservoir Analysis, by Roy Kligfield, R. Ratliff, and P. A. Geiser; #91030 (2010)

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Strain Patterns in Folds: Applications to Fractured Reservoir Analysis

Roy Kligfield, R. Ratliff, P. A. Geiser

Fold formation associated with extensional and compressional orogeny results not only in bed rotation but also in internal distortion of beds. Deformation mechanisms permitting such shape changes include brittle fracturing, pressure solution, grain boundary sliding, and plastic deformation. The selection of deformation mechanism depends upon a number of factors, including rock type, cement type, pore pressure, strain rate, and ambient temperature and pressure at the time of folding.

Flexural slip and flexural flow are among the most common folding mechanisms affecting layered sedimentary rocks in hydrocarbon producing regions. The strain produced by these processes is that of simple shear, with slip dominantly occurring along layer boundaries (flexural slip) or at the grain scale (flexural flow). In order to specify the strain patterns of these fold types, both the geometry of the structure and a suitable pin line must be known. Pin lines may be chosen either in the hinges of folds, as in the case of chevron folds that amplify until they reach their locking positions, or within fold limbs. Field studies in the Wyoming-Idaho and Helvetic thrust belts indicate that both backlimb pins and pins within hinge zones are common. The choice of pin line produces very different strain patterns within the same fold geometry. The display of strain patterns has been automated through the use of an interactive microcomputer system (GEOSEC20), enabling the operator to view the effects of differently chosen pin lines on the fold geometry.

These concepts are illustrated by applications to oil and gas producing fields whose geometries are known through published studies: the Yellow Creek, Anschutz Ranch, and Whitney Canyon fields. In most cases, the choice of a backlimb pin produces a strain pattern characterized by intense deformation of the inverted limbs. Such deformation patterns produce intense fracturing and brecciation on analogous surface structures. These strain patterns also produce significant changes in fracture density, permeability, and porosity within the producing fields, depending upon the operative deformation patterns within folds can be used as a predictive model for subsurface exploration and secondary recovery, and tested with industry reservoir production data.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.