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Facies, Fractures, Pressure, and Production in the Eagle Ford Shale (Cretaceous) between the San Marcos Arch and the Maverick Basin, Texas, U.S.A.

By
John A. Breyer
John A. Breyer
Marathon Oil, 5555 San Felipe Rd., Houston, Texas 77056, U.S.A. (e-mail: jabreyer@marathonoil.com)
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Richard A. Denne
Richard A. Denne
ALS Oil and Gas, 6510 Guhn Rd., Houston, Texas 77040, U.S.A. (e-mails: richdenne@att.net and tobi.kosanke@alsglobal.com)
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Tobi Kosanke
Tobi Kosanke
ALS Oil and Gas, 6510 Guhn Rd., Houston, Texas 77040, U.S.A. (e-mails: richdenne@att.net and tobi.kosanke@alsglobal.com)
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Joan M. Spaw
Joan M. Spaw
Marathon Oil, 5555 San Felipe Rd., Houston, Texas 77056, U.S.A. (e-mail: jspaw@marathonoil.com)
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Jonathan Funk
Jonathan Funk
EOG Resources, 19100 Ridgewood Pkwy., San Antonio, Texas 78259, U.S.A. (e-mail: jonfunk@eogresources.com)
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Pete Christianson
Pete Christianson
Marathon Oil, 7301 NW Expressway, Oklahoma City, Oklahoma 73132, U.S.A. (e-mail: pchristianson@marathonoil.com)
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Debbie A. Bush
Debbie A. Bush
Murphy Exploration and Production, 9805 Katy Fwy., Houston, Texas 77024, U.S.A. (e-mail: deborah_bush@murphyoilcorp.com)
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Ronald A. Nelson
Ronald A. Nelson
Broken N Consulting, Inc., 1444 New Ulm Rd., Cat Spring, Texas 78933, U.S.A. (e-mail: nelson_consulting@hotmail.com)
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Published:
January 01, 2016

Abstract

The Eagle Ford play in south Texas extends along strike from the San Marcos arch in the northeast into the Maverick Basin along the international border with Mexico. The highest initial oil production is in a strike-parallel belt between the Karnes trough and the Cretaceous shelf margin. Three lithologies comprise the bulk of the Eagle Ford Shale in this area: argillaceous mudrock (shale), calcareous mudrock (marl), and limestone. The marls consist mainly of coccoliths and contain more total organic carbon (TOC) and have higher porosities than the other lithologies. The sand- and silt-sized grains in the marls and limestones consist predominantly of planktonic foraminifera, radiolarians, and calcispheres, with lesser amounts of inoceramid fragments and other carbonate grains. The limestones may be partly to entirely recrystallized. The strength and rigidity of the rocks increase with calcite content—the limestones are stronger and more rigid than the marls. Argillaceous mudrock (shale) comprises only a small portion of the Eagle Ford between the San Marcos arch and the Maverick Basin, but is more common in the lower part of the formation along strike to the northeast.

Six unconformity-bounded stratigraphic intervals (depositional sequences) can be recognized and mapped within the Eagle Ford Shale between the San Marcos arch and the Maverick Basin. Significant changes in biostratigraphy and chemostratigraphy within the Eagle Ford take place at these sequence boundaries. The Cenomanian–Turonian boundary occurs within the lower part of the Upper Eagle Ford. Typically, the Upper Eagle Ford contains less vanadium, molybdenum, uranium, and TOC than the Lower Eagle Ford, indicating bottom-water oxygen levels were oxic rather than dysoxic or anoxic during deposition. The Eagle Ford as a whole and each of its major subdivisions thin across an area in southwestern Karnes County coinciding with a structural high on the underlying Buda Limestone. The percentage of limestone within the Eagle Ford and each of its major subdivisions increases over this area. Changes in thickness and facies within the Eagle Ford suggest the area above the high on the time-structure map was a topographic high on the seafloor. Furthermore, changes in bathymetry influenced facies distribution and ultimately production from the Eagle Ford Shale. However, changes in pore pressure and fracture intensity also occur across the high, confounding the effect of facies on production.

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Contents

AAPG Memoir

The Eagle Ford Shale: A Renaissance in U.S. Oil Production

John Breyer
John Breyer
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American Association of Petroleum Geologists
Volume
110
ISBN electronic:
9781629812748
Publication date:
January 01, 2016

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