MIDDLE PALEOZOIC, HYDROCARBON-BEARING MUDROCK SYSTEMS IN THE SOUTHERN MIDCONTINENT: RECORD OF CONTINENTAL COLLISION, PLATFORM DROWNING, AND UPWELLING

The Barnett Shale Fm of the Ft. Worth Basin of central Texas has drawn worldwide attention because of impressive successes in obtaining commercial gas production from this Mississippian hydrocarbon source rock. To many, the Barnett has become the universal standard reference for understanding mudrock (“shale”) systems. However, the Barnett and the underlying Devonian Woodford Fm comprise part of a unique continental margin mudrock system that differs substantially in mineralogy, organic chemistry, and depositional setting from many other “shale” successions in North America, including those of comparable age.

The Woodford-Barnett system is part of a geographically extensive, distal platform, depositional succession that formed along the southern margin of the Laurussian paleocontinent as a result of collisional tectonics associated with approach of the Gondwanan plate. Platform drowning began at least as early as the middle Silurian but reached a peak in the late Devonian and Mississippian. Sedimentation in the continental margin foreland basin formed by this collision and drowning event was dominated by slow accumulation of (1) hemipelagic, fine-grained, detrital siliciclastics and (2) biogenic silica associated with ocean upwelling.

Both the Barnett and the Woodford display proximal/distal trends in facies, mineralogy, and depositional style that reflect original topography, sediment sources, and nutrient input. Proximal facies typically contain higher volumes of clay minerals, detrital silica, and skeletal debris, as well as more common indications of infaunal burrowing and traction sedimentation. Distal facies are dominated by biogenic silica, and contain little, if any, evidence of in situ biotic activity or high energy sedimentation.

Distal Barnett and Woodford rocks share two important keys to potential hydrocarbon production success: an abundance of biogenic silica which creates rock mechanical strength, and relatively high levels of thermal maturity, which increases the likelihood of significant thermogenic gas generation. Many other “shale” successions contain neither of these attributes.