Biogenic Silica in the Devonian Shale Succession of the Appalachian Basin, USA
Blood, Randy; Lash, Gary; Bridges, Lindell C.
Opaline quartz tests of planktonic organisms such as radiolarians are unstable and commonly dissolve in bottom water undersaturated with respect to silicon. Upon dissolution, and under conditions of enhanced productivity export, silica precipitates in a more stable form often intimately associated with organic matter which, upon burial, ultimately becomes the reservoir for gas in many of these organic-rich deposits. Further, the newly precipitated silica permeates the clay fabric of mudstones providing a continuous high-modulus medium that is conducive to the initiation of, and maintenance of, high conductivity hydraulic fractures. Petrographic analysis, including thin section and SEM, and chemostratigraphic analysis of the Middle Devonian Marcellus Shale through Upper Devonian Dunkirk Shale (Lower Huron Shale equivalent) provide evidence of recurrent patterns of biogenic silica enrichment in these organic-rich deposits. Thin section analysis shows radiolarians in various states of dissolution, some being completely replaced by pyrite. Elevated molybdenum to total organic carbon ratios (Mo/TOC) coincide with excursions in silica to aluminum (Si/Al) ratios. We suggest that during times of transgression the Mo reservoir is resupplied via connection with the global ocean, resulting in elevated Mo/TOC. In addition to Mo, transgression provides silica and nutrients needed to stimulate primary productivity in the photic zone. As a result, late transgressive to early regressive systems tract deposits of the Middle and Upper Devonian shale succession become enriched in biogenic silica. These zones, or condensed sections provide high gas-in-place and deliverability and should be considered for horizontal wellbore placement.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013