Juergen Schieber¹

(1) University of Texas at Arlington, Arlington, TX

ABSTRACT: Sequences In The Late Devonian Chattanooga Shale Of Tennessee And The Impact Of Depositional History On Their Sealing Capacity

Long considered an essentially continuous deep water black shale succession, the Late Devonian Chattanooga Shale of Tennessee reveals a complex depositional history. It can be subdivided into three packages that are separated by significant subaerial erosion surfaces. Tracing of shallow marine erosion surfaces allows further subdivision of these packages into a total of 14 sequences. The entire succession does not exceed 15 meters thickness and individual sequences rarely exceed 3 meters. Due to intermittent erosion, most sequences do not occur over the entire area. All shale intervals exhibit distinct compositional, sedimentary, and textural features that influence their capacity to impede fluid movement. Abundant organic matter is associated with low fracture density, whereas abundant silt and sand beds are associated with increased fracturing. Interbedded gray mudstones show abundant fractures, but continuity is limited due to the more plastic behaving black shale interbeds. Early diagenetic cements of dolomite, quartz, phosphate, and pyrite reduce porosity, whereas layers of wave winnowed silt and sand improve lateral permeability. Vertical interconnection of permeable layers by bioturbation causes improved permeability perpendicular to bedding. Intermittent subaerial exposure of some shale units during the Devonian has also resulted in fracturing due to unloading and to leaching of pyrite and dolomite along fractures and laminae, both resulting in lowered sealing capacity. The sealing capacity of the various black shale intervals in the Chattanooga succession is determined by a combination of oceanographic (organic matter, cement minerals), sedimentologic (winnowing), and biological (bioturbation) factors, and by their history along the continuum of deposition, exposure, erosion, weathering, and final burial. Detailed examination of the interplay of these parameters should reveal valuable principles for the assessment of the sealing capacity of shale units elsewhere.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado