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Description:
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Syndepositional tectonic movements and geometry of the Illinois Basin had a profound influence on thickness , lithofacies distribution and dolomitization in the Silurian rocks in western Kentucky . The northern portion of the Illinois Basin {in Indiana and Illinois ) was the site of extensive development of reef trends and numerous isolated reefs with associated petroleum entrapment . However , no reefal trends have been reported from the southern portion of the Illinois Basin in western Kentucky .
The Kentucky portion of the Illinois Basin consists predominantly of the Moorman Syncline and its shelves , which are separated from the northern part of the minois Basin by the Rough Creek Fault Zone . Syndepositional Tectonic movements not only controlled the basinal geometry , but also influenced the depositional environments in the Moorman Syncline . The basinal geometry of the Moorman Syncline accounted for a unique geologic setting and a possible reefal trend , altogether different from that of the northern portion of the Illinois Basin .
Silurian rocks in the study area consist of more than 600 feet of predominantly clastic -free carbonates with a few thin shaly intervals . Percentages of grainstone /packstone facies and dolomite generally increase toward margins of the basin , whereas percentages of wackestone /packstone facies and calcite increase toward the center of the basin .
Syndepositional subsidence of the basinal block , along a segment of the Rough Creek Fault Zone in Grayson County , created a local steep shelf -margin within an otherwise gently sloping carbonate platform . This , in turn , influenced the thickness of the sedimentary -fill , facies distribution and dolomitization pattern . Thickness of the reef -bearing interval (the St . Clair and Moccasin Springs formations ) increases locally on the basinal side of the syndepositional fault in Grayson County . Percentages of grainstone /packstone facies and dolomite also increase anomalously in this area .
Four different but interdependent criteria : basinal geometry , geometry of sedimentary -fill , lithology and distribution of dolomite support the existence of a probable reef trend in Grayson County . This inferred reef trend in Grayson County is the most promising site for exploration of hydrocarbons entraped within the Silurian reefs or in draped structures in the overlying rocks . |