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Abstract

AAPG Bulletin, V. 91, No. 7 (July 2007), P. 1005-1023.

Copyright copy2007. The American Association of Petroleum Geologists. All rights reserved.

DOI:10.1306/03130706066

Late opening-mode fractures in karst-brecciated dolostones of the Lower Ordovician Ellenburger Group, west Texas: Recognition, characterization, and implications for fluid flow

Julia F. W. Gale,1 Leonel A. Gomez2

1Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, J. J. Pickle Research Campus, 10100 Burnet Road, Austin, Texas 78758; [email protected]
2Department of Geological Sciences, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78713; present address: ExxonMobil Upstream Research Company, 3120 Buffalo Speedway, Houston, Texas 77098; [email protected]

ABSTRACT

Two distinct groups of fractures in an Ellenburger Group reservoir in Barnhart field, Reagan County, west Texas, were identified. The oldest fractures (FBR) are the most numerous; have irregular shapes, sediment, and baroque dolomite fill, and no preferred orientation; and have been attributed by previous workers to brecciation associated with the collapse of Lower Ordovician paleocave systems. Younger, subvertical, opening-mode fractures (FY) that have consistent east-southeast and south-southwest strikes postdate the baroque dolomite cement. FY fractures therefore formed during the late stages or after the Pennsylvanian Ouachita orogeny. We analyzed FY fracture orientation, intensity, and openness using well image logs, oriented rotary-drilled sidewall cores, and a full-diameter core. FY fracture aperture sizes range from several micrometers to a few millimeters, and the fracture intensity is consistent within and between the wells studied at 1.8–4.0 times 10minus2 fractures/mm2 for fractures ge1 mm (ge0.04 in.) wide.

Dolomite cement that is synchronous with FY fracture opening seals fractures in some locations, but is limited to fracture linings and mineral bridges in other places. Calcite, which grew after FY fractures stopped opening, is variably present and postdates dolomite cements. Where present, calcite occludes most remaining FY fracture porosity. Diagnosing the presence of postkinematic calcite is therefore an important step in being able to predict open fractures and was done for part of Barnhart field using rotary-drilled sidewall cores.

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