^PSCathodoluminescent Textures and the Origin of Quartz in the Mississippian Barnett Shale, Fort Worth Basin, Texas*

By

Petro K. Papazis¹ and Kitty Milliken¹

Search and Discovery Article #50009 (2005)

Posted August 14, 2005

*Poster presentation at AAPG Annual Convention, Calgary, Alberta, June 19-22, 2005.

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        Poster 1             Poster 2

¹John A. and Katherine G. Jackson School of Geosciences, Department of Geological Sciences, The University of Texas at Austin, Austin, Texas ([email protected])

Abstract 

Mudrocks from the Mississippian Barnett Shale in north-central Texas contain a substantial proportion of silt-size (10-62 mm) quartz crystals with diverse cathodoluminescent (CL) textures. Samples used in this study include 32 polished thin sections from two cores from Erath and Wise counties and from outcrop samples in Lampasas and San Saba counties in central Texas. CL textures show that the quartz component in these mudrocks is present in several distinct forms. Many brightly luminescent detrital grains have weakly luminescent rims. Irregularly-shaped regions several tens of microns across consist of siliceous microbreccias in which relatively bright luminescent angular clay-size quartz fragments are surrounded by a matrix of dark luminescent quartz. Authigenic quartz is also present as darkly luminescent material in healed intragranular fractures within brightly luminescent detrital grains. Finally, quartz is localized as replacements of marine skeletal debris. Overall, authigenic quartz dominates the quartz fraction in mudrocks of the Barnett. Thus, silica geochemistry and diagenesis is an area of compelling interest for understanding the physical properties of the Barnett.

Geologic framework.

Geologic Setting 

The Barnett Shale lies in the Fort Worth basin of north-central Texas. It is a middle-late Mississippian age petroliferous black shale. This foreland basin experienced tectonism during the early Pennsylvanian. To the east the basin is bounded by the Ouachita fold-thrust belt, to the south by the Precambrian crystalline Llano Uplift, and to the west and north by a series of arches activated and reactivated at different times throughout the Paleozoic.

The Barnett lies stratigraphically above the Ordovician Ellenburger Dolomite, Simpson Group, and Viola Limestone. The contact between the Barnett and Ellenburger Group is an erosional unconformity. Within the Mississippan section, the Barnett is stratigraphically equivalent to the Chappel Limestone in many portions of the basin. Overlying the Barnett is the Pennsylvanian Marble Falls Limestone.

Approach 

Petrographic characterization of components within mudrocks is challenging with conventional transmitted light microscopy because it is difficult to image individual particles that are smaller than the thickness of a conventional thin section. Scanning electron methods including secondary electron, back-scattered electron, and cathodoluminescence imaging have provided important insights into the character of shale components (e.g., Milliken, 1994; Schieber et al., 2000). This preliminary study has integrated all of these petrographic methods in an attempt to understand the quartz that makes up a significant fraction of the Barnett Shale.

Conclusions 

1) Detrital quartz in the Barnett Shale occurs as silt-size quartz grains and as particles aggregated within the tests of agglutinated foraminifera.

2) Authigenic quartz is represented as veins, replacements of skeletal debris, healed fractures within individual detrital grains, localized overgrowths on detrital quartz, and as cement within agglutinated foram tests.

3) Identification of abundant clay-size quartz crystals within the Barnett awaits the application of stable isotopic techniques because these crystals are too small for application of readily available imaging techniques.

References 

Milliken, K. L., 1994, Cathodoluminescent textures and the origin of quartz silt in Oligocene mudrocks, South Texas: Journal of Sedimentary Research, v. A64, p. 567-571.

Montgomery, S. L., D. M. Jarvie, K. A. Bowker, and R. M. Pollastro, 2005, Mississippian Barnett Shale, Fort Worth basin, north-central Texas: Gas-shale play with multi-trillion cubic foot potential: American Association of Petroleum Geologists Bulletin, v. 89, p. 155-1 75.

Schieber, J., D. Krinsley, and L. Riciputi, 2000, Diagenetic origin of quartz silt in mudstones and implications for silica cycling: Nature, v. 406, p. 981-985.