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Abstract

John C. Lorenz, Scott P. Cooper, 2013, Natural fractures in folded sandstones of the Tensleep Formation, Wyoming, in C Knight. and J. Cuzella, eds., Application of structural methods to Rocky Mountain hydrocarbon exploration and development: AAPG Studies in Geology 65, p. 175213.

DOI:10.1306/13381696St653575

Copyright © 2013 by The American Association of Petroleum Geologists.

Natural Fractures in Folded Sandstones of the Tensleep Formation, Wyoming

John C. Lorenz, Scott P. Cooper,

FractureStudies LLC 99 Rainbow Rd. Edgewood, New Mexico, 87015, U.S.A. (e-mails: [email protected], [email protected])

ABSTRACT

A study of fracture characteristics in sandstones from the Tensleep Formation on a spectrum of thrust-related Laramide Anticlines in Wyoming suggests that an idealized fracture model can be constructed for these reservoirs but that important local variations are common. A conceptual model for fracture genesis and distributions in folded Tensleep reservoirs was derived from a comparison of fracture characteristics in the various anticlines, with key relationships being documented by an important surface and subsurface data set at Teapot Dome. Inherited F0 fracture patterns, predating and unrelated to folding, are present in some structures. A widespread, hinge-normal F1 extension fracture set typically formed in response to horizontal stress prior to and during the early stages of folding, but may strike oblique to the hinge in anticlines where the horizontal compressive stress that caused folding and fracturing was oblique to an inherited basement structure that dictated the axis of the anticline. Later hinge-parallel F2 fractures formed during folding, commonly being concentrated at the fold hinges and in the forelimbs; this set may extend into a curved backlimb but is absent from planar backlimbs. Any of these fracture sets may become reactivated in shear or extension during continued folding. Some fractures also formed originally as shear fractures, primarily in the forelimbs. Early-formed hydraulic injection fractures, Fi, are common at Flat Top Anticline but are rare elsewhere. The limestones and dolomites interbedded between the thicker reservoir sandstones are also fractured, but healing of these fractures suggests that they are not significant conduits for fluid flow in the subsurface. Fracture patterns may change with depth in response to the changing structural configuration and resultant stress conditions. Hinge-normal and hinge-parallel fractures in shallow strata may have formed in response to bilateral extension that is only indirectly related to the compressive stress that initiated basement thrusting. Knowledge of the most likely fracture patterns in different parts of folded Tensleep reservoirs across Wyoming can be used to predict and model fracture-related fluid flow, but this requires a solid understanding of the structural development of a given anticline.

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