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A 33-m (108-ft) core of the Upper Silurian Bass Islands formation from a well in Johnson County, Kentucky, consists mainly of finely crystalline dolomite and intraclastic dolomite. These rocks reflect low-energy, hypersaline coastal environments which bordered the Cincinnati arch. Environments included the supratidal mud flat (evaporite minerals and desiccation features), intertidal flat (algal stromatolites), tidal channels (intraclasts), and beach ridges (peloids and intraclasts). The original lime sediments are believed to have been totally dolomitized penecontemporaneously with deposition.
Several shows of natural gas were reported from the formation in this well. The entire "Corniferous" group, including the Bass Islands carbonates, was treated and had an initial production of 464 MCFGD. Porosity is generally poor throughout the formation, but it is as high as 9% in some zones. Porosity occurs as micropores (50µ) associated with dedolomitization, and mesopores (up to 5mm) interpreted to be solution-enlarged molds of carbonate grains and evaporite minerals, horizontal fractures along bedding planes, and incompletely filled vertical fractures.
Dedolomitization probably occurred with the development of an Early Devonian paraconformity, when the Bass Islands was buried to a depth less than 40 m (130 ft). Evaporitic sulfate minerals were attacked by anaerobic bacteria and replaced by silica in this near-surface diagenetic setting. Both reactions released calcium to the pore water. As the calcium/magnesium ratio increased, calcite began to replace dolomite. The dedolomitized calcite generally has a poikilotopic texture but also has a porphyrotopic texture. At some later time, outside pore water entering through fractures in the carbonate formation partially leached some of the dedolomitized calcite, thus creating most of the porosity preserved in the Bass Islands.
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