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Diagenesis and Late-Stage Porosity Development in the Pennsylvanian Strawn Formation, Val Verde Basin, Texas, U.S.A.

By
K. David Newell
K. David Newell
Kansas Geological Survey, University of Kansas, 1930 Constant Ave., Lawrence, Kansas 66047, U.S.A., e-mail: dnewell@kgs.ku.edu
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Robert H. Goldstein
Robert H. Goldstein
Department of Geology, University of Kansas, 1476 Jayhawk Blvd., Lawrence, Kansas 66045, U.S.A., e-mail: gold@ku.edu
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Carl J. Burdick
Carl J. Burdick
Conoco (U.K.) Limited, Rubislaw House, Anderson Dr., Aberdeen AB15 6FZ, Scotland Present address: Huntington Energy, L.L.C., 6301 Waterford Blvd., Suite 400, Oklahoma City, Oklahoma 73118, U.S.A.
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Published:
January 01, 2003

Abstract

The Middle Pennsylvanian (Desmoinesian) Strawn Formation in the Trans-Pecos area of Texas was deposited during relative tectonic quiescence that prevailed before rapid infilling of the Val Verde Basin. It represents one of a series of backstepping carbonate ramps formed on the craton side of this foreland basin.

Strawn Formation carbonate rocks in three cores—Conoco Anna McClung #3-1, Alex Mitchell #2-1R, and Creek Ranch #10-1—show several shallowing-upward sequences, each a few meters thick. The Creek Ranch core displays the deepest-water characteristics of the three cores; the lower part of this core is dominated by graded bedding. The Mitchell and McClung cores contain skeletal-rich carbonates. Both of these cores display characteristics of shallow-water bank or lagoonal environments.

All three cores have approximately the same diagenetic history. Primary fluid inclusions indicate early porosity-occluding interparticle and mold-filling calcite precipitated from water with a narrow range of salinities. Modal salinities are that of seawater, but slightly lesser salinities (indicating mixing of seawater and meteoric water) and slightly greater salinities (indicating evaporative concentration of seawater) are also indicated. The influence of meteoric groundwater can be detected by stable-isotope analyses of the early cements at stratigraphic levels that correlate to the tops of the major shallowing-upward depositional sequences. However, subaerial exposure surfaces are not demonstrated in these cores but were likely to be present updip.

Most porosity is cement-reduced vugs, dissolution-enlarged (and cement-reduced) molds (> 1/16 mm, < 4 mm), and fractures. Minor intraparticle, intercrystalline, and shelter porosity is also present. Reservoir porosity is caused by fracturing and a late-stage dissolution event. Dissolution in the Creek Ranch core is not as pronounced as in the other cores because of a dearth of skeletal material. Porous zones in the McClung and Mitchell cores are associated with open fractures spatially, which commonly interconnect with nearby molds and vugs. This complex porosity system occurred after stylolitization, as evidenced by “cuticles” of insoluble stylolitic residue that bridge across small dissolution-enlarged fractures. Porosity detected by wireline logs therefore is mostly effective porosity. The open-fracture network may have been caused by thrusting of the Strawn Formation, most likely in Permian time.

Late-stage cement reduction of porosity occurs in two stages—first by calcite spar, then saddle dolomite. These cements are unevenly distributed. Both of these cements contain primary oil-filled fluid inclusions. Homogenization temperatures of primary aqueous fluid inclusions in saddle dolomites indicate that the Strawn Formation has been subjected to a temperature of at least 136°C (roughly 45°C over present formation temperature), which correlates to a vitrinite reflectance equivalent of 1.22%. Homogenization temperatures, in conjunction with oxygen isotope compositions, indicate that fracture-filling calcite spars and the later saddle dolomites precipitated from isotopically positive fluids, which were probably connate waters that had undergone extensive rock-water interaction.

These observations suggest that thrusting of carbonate shelf strata, in a proximal foreland setting, was responsible for creation of latestage fracture porosity. In turn, tectonic expulsion of undersaturated, heated, connate water into the Strawn Formation enhanced the porosity. As this expulsed water cooled, it reached saturation with respect to calcite and dolomite, and these cements partly filled the available porosity. These processes of reservoir creation might be expected in other proximal foreland settings.

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Contents

SEPM Special Publication

Permo-Carboniferous Carbonate Platforms and Reefs

Wayne M. Ahr
Wayne M. Ahr
Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77843-3115, U.S.A.
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Paul M. (Mitch) Harris
Paul M. (Mitch) Harris
ChevronTexaco E&P Technology Company, 6001 Bollinger Canyon Road, San Ramon, California 94583-0746, U.S.A.
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William A. Morgan
William A. Morgan
ConocoPhillips, Inc., P.O. Box 2197, Houston, Texas 77252-2197, U.S.A.
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Ian D. Somerville
Ian D. Somerville
Department of Geology, University College - Dublin, Belfield, Dublin 4, Ireland
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SEPM Society for Sedimentary Geology
Volume
78
ISBN electronic:
9781565763340
Publication date:
January 01, 2003

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