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|Coal-bed tagging for the KGS borehole database|
The Kentucky Geological Survey’s coal information database includes coal thickness measurements from both outcrop and drill hole sources. Correlations determined by geologists have been designated as abbreviated codes and corresponding regional bed names in the databases. Users of the data need to be aware of the complex nature of the correlations, particularly those associated with coal zones.
While a few coal beds, like the Springfield in western Kentucky, have regional continuity and are independent of other coals, many others are associated with closely related beds within zones. Coal zones contain two or more individual benches of coal separated by rock parting material of variable thickness and composition. Each zone is overlain and underlain by rock strata, commonly of marine origin, that have considerable extent. Two or more benches of a zone may merge to form a thick, mineable bed of some geographic extent, though single benches can also be mineable (see illustration).
As a result of this pattern, mineable coals within a zone are typically composed of different combinations of benches on a regional basis. Conversely, distinct benches may be associated with more than one mineable bed. These characteristics of coal zones lead to ambiguities in assigning historical coal-bed names that were originally developed using a simple, tabular model of stratification.
A system of tagging is utilized for the KGS borehole database to distinguish bench stratigraphy from mineable coal bodies. With this schema, bed names are abbreviated as three-character codes (e.g. Upper Elkhorn No.3 = UE3) followed by a modifying code of up to four characters. Numeric modifiers are reserved for bench identifiers and alphabetical modifiers represent mineable beds. In the example below, the mineable bed ‘ FCL B’ is composed of benches 1 and 2 or 1, 2 and 4. Mineable bed ‘FCR A’ is composed of benches 4 and 6 or 6 and 8. Assignment of ‘mining’ tags does not necessarily indicate that the bed is economically mineable, rather it is a mechanism of recording equivalent coal horizons for mapping purposes.
Use of this tagging system simplifies data extraction for principal coal beds while preserving the details of bench architecture. Users should be aware that regional bed maps prepared for coal assessments at KGS can be simplifications of the variability found within coal zones and that continuity throughout the map cannot be assured.