Algal Thrombolites, Dolomitization and Porosity Development in the Knox Dolomite: Implications for Carbon Dioxide Sequestration in Kentucky
The Cambro-Ordovician Knox Group in Kentucky contains porosity zones that form saline reservoirs which could be suitable for sequestration of large volumes of supercritical carbon dioxide. Both the Copper Ridge and Beekmantown Dolomites are a repetitive sequence of shallow-water, hypersaline, carbonate rocks consisting of subtidal algal-dominated bioclastic calcarenites containing abundant oolites and quartz sand. Thrombolite colonies formed in deeper subtidal environments, which were extensive over areas of several square miles.
Diagenesis of Knox sediments consisted of multiple episodes of dolomitization, silicification, fracturing, hydrothermal dolomitization (HDT), and base metal mineralization. Hydrothermal dolomites and mineralization occurred as a late-stage event, creating coarsely crystalline HTD along previously fractured chert and silicious beds near thrombolite colonies, which created permeable pathways and porosity zones that may be correlatable on a countywide basis.
Data from approximately 20 wells were examined, and detailed Knox core information from the Southern Kentucky Mineral District was used to project the porous zones to eastern and western Kentucky. Algal thrombolites were discovered in a mineral exploration shaft in the Knox Dolomite in Cumberland County in 1978 and algal features have been noted in underground mines in Tennessee. Samples from the Shell Oil No. 1 Bales and the Dupont No. 1 WAD wells also suggest algal type porosity. This algal type porosity is distinctive because of its fenestral fabric formed along parallel laminations. Cavern zones were preferentially developed in these algal colonies and along fractures and other migration routes formed by HTD fluids. Some of these HTD zones exist in the lower, middle, and upper Copper Ridge Dolomite, in the lower Beekmantown/Rose Run interval, and in the middle Beekmantown Dolomite.
The porosity zones at the top of the Knox are related to the development of paleokarst and paleoaquifer features and may be restricted to erosional remnants, whereas the lower HTD cavern zones would be of larger extent. There is a northeast-trending fracture system in the Knox and tentative correlations of maximum porosity in these cavern zones would be along a northeastern trend. Many of these caverns were zones of high water flow, drill stem testing, injection testing, oil and gas shows, or zones of lost circulation, suggestive of high porosity and permeability.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009