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

¹ Bureau 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; julia.gale{at}beg.utexas.edu
² Department 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; leonel.gomez{at}exxonmobil.com

Julia Gale obtained a Ph.D. in structural geology from Exeter University in 1987. She taught structural geology and tectonics for 12 years at the University of Derby. She moved to the University of Texas at Austin in 1998, working as a research associate first in the Department of Geological Sciences and then at the Bureau of Economic Geology. Her interests include fracture characterization in carbonate and shale hydrocarbon reservoirs.

Leonel Gomez obtained a Ph.D. in structural geology from the University of Texas at Austin in 2007, working on the quantification of spatial arrangement of fractures. Before his graduate studies, he worked for 7 years at the Colombia offices of Shell and Chevron. He has a B.Sc. degree in geology from Universidad Industrial de Santander in his native Bucaramanga, Colombia. Leonel is currently working at the upstream research center of ExxonMobil in Houston.

Two distinct groups of fractures in an Ellenburger Group reservoir in Barnhart field, Reagan County, west Texas, were identified. The oldest fractures (F_BR) 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 (F_Y) that have consistent east-southeast and south-southwest strikes postdate the baroque dolomite cement. F_Y fractures therefore formed during the late stages or after the Pennsylvanian Ouachita orogeny. We analyzed F_Y fracture orientation, intensity, and openness using well image logs, oriented rotary-drilled sidewall cores, and a full-diameter core. F_Y 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 x 10^–2 fractures/mm² for fractures 1 mm (0.04 in.) wide.

Dolomite cement that is synchronous with F_Y fracture opening seals fractures in some locations, but is limited to fracture linings and mineral bridges in other places. Calcite, which grew after F_Y fractures stopped opening, is variably present and postdates dolomite cements. Where present, calcite occludes most remaining F_Y 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.