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Great American Carbonate Bank: Knox Group in the Black Warrior Basin

By
Richard D. Fritz
Richard D. Fritz
SM Energy, Tulsa, Oklahoma, U.S.A.
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Patrick Medlock
Patrick Medlock
Consultant, Austin, Texas, U.S.A.
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Michael Kuykendal
Michael Kuykendal
Newfield Exploration, Tulsa, Oklahoma, U.S.A.
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James Lee Wilson
James Lee Wilson
Consultant, New Braunfels, Texas, U.S.A.
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Published:
January 01, 2012

Abstract

The Knox Group in the Black Warrior Basin comprises the southeastern part of the great American carbonate bank (GACB) and consists mostly of carbonates. The Black Warrior Basin is a Carboniferous foreland downwarp developed over a passive margin of early Paleozoic age. Similar to other parts of the GACB, the thick widespread Cambrian–Ordovician Knox Group was deposited as mostly shallow-water, restricted, marine carbonates.

The Knox depositional model is that of an extensive regressive tidal flat, made up of shallow subtidal, intertidal, and rare supratidal facies. These facies shallow upward and comprise numerous cycles in the Knox. There exists a tremendous variation in thickness of the cycles that can be as thin as 3 to more than 100 ft (1 to >30 m) thick. These cycles can be further grouped into packages of sequences that are mostly composed of intertidally dominated or subtidally dominated cycles. Large-scale regional changes in relative sea level may have a large influence on the type of cycles and sequences that formed during the Knox. Knox strata, especially within third-order sequence boundaries, are correlatable across the basin. Detailed local to regional correlation of the facies bundles can be made with gamma-ray and resistivity logs; however, facies are commonly obscured by strong diagenetic overprints that make detailed correlation difficult.

Numerous unconformities occur within the Knox Group at major sequence boundaries. The super-Knox unconformity is recognized as evidence of a globally eustatic sea level drop and has been used to mark the boundary between the Sauk and Tippecanoe depositional megasequences. Paleokarst is observed regularly within the Knox carbonates, especially along major sequence boundaries with related unconformity surfaces. Paleokarstic features in the Knox Group have been identified in outcrop in central Alabama, with the Knox containing a sinkhole filled with Middle Ordovician strata. Numerous cores contain collapse breccias that are interpreted to have formed in response to karst conditions. With some paleokarst collapse breccias occurring 3000ft (914 m) below the top of the Knox, itislikely that some of these breccias formed in response to intra-Knox unconformities.

In the Knox, diagenetic changes are a continuum that begins with early diagenesis, including hypersaline or evaporative, vadose, and phreatic conditions, and followed by deep phreatic to late thermal diagenesis. Evidence exists that porosity formed (some of which may be thought of as karst) during each of these diagenetic phases. Conversely, precipitation events and dolomitization also occurred throughout various levels of the profile. Volumetrically, dolomite is the most abundant mineral. Knox dolomite can be subdivided into early (syngenetic to penecontemporaneous) hypersaline dolomite, shallow burial mixed-water (phreatic) dolomite, and deeper burial to thermal (baroque and xenotopic) dolomite.

Reservoir development is typically along sequence boundaries, especially where facies have strong diagenetic overprints from dolomitization and dissolution associated with paleokarstic events. The best reservoirs are structurally related, with strong fracture overprints.

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Contents

Memoir

Great American Carbonate Bank: The Geology and Economic Resources of the Cambrian—Ordovician Sauk Megasequence of Laurentia

James Derby
James Derby
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Richard Fritz
Richard Fritz
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Susan Longacre
Susan Longacre
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William Morgan
William Morgan
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Charles Sternbach
Charles Sternbach
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American Association of Petroleum Geologists
Volume
98
ISBN electronic:
9781629810201
Publication date:
January 01, 2012

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