Print this pageCoal-Seams as Sensitive Recorders of Base-Level Change, and Implications for Predicting Vertical and Lateral Variations in Coal Composition: Examples from the Pennsylvanian, Kentucky, Usa
Coal seams formed in coastal plain and delta-top settings preserve a high-resolution record of water table (and hence base-level) fluctuation throughout their deposition. Detailed (cm-scale) study of their petrography provides a means of identifying intra-seam surfaces that separate spatially and temporally distinct phases of mire development, and mark abrupt changes in paleoenvironmental conditions.
The regionally-extensive, up to 1.5 m-thick, Westphalian B Fire Clay coal was deposited in a delta-top environment, in an ever-wet, palaeoequatorial setting in the Central Appalachian Basin. The petrographic composition of three hundred samples collected from eleven localities along a 150 km depositional dip transect was analysed in order to determine the internal stratigraphy of the coal seam. Correlation between localities was aided by the presence of a volcanic ash-fall horizon (“tonstein”) within the coal, which provides an independent time-line (~310 Ma).
The results of these analyses reveal cyclic changes in the composition of the Fire Clay coal, which are correlatable over distances of more than 100 km. Each of the correlatable sub-units of coal shows a trend of peat deposition under progressively drier conditions. The top of each unit is represented by a “dry maceral assemblage”, high in inertinite and resistant peat components, indicating high rates of oxidation and biomass-loss. The base of each subsequent unit is marked by an abrupt change to a “wet maceral assemblage” interpreted as the response to an abrupt rise in the water-table. Each of these genetic sub-units has a different spatial distribution, indicating that the coal represents several spatially and temporally distinct phases of peat accumulation.
Understanding the internal stratigraphy, and the spatial and temporal development of regionally-extensive coal seams in this way may have significant implications for improving the predictability of vertical and lateral changes in coal composition, which in turn may have commercial applications for mining and coalbed methane exploitation of coal bed resources. In order to visualise the results of the Fire Clay coal study, the data collected for this project have been used to construct a detailed 3D model using Petrel reservoir modelling software. This model clearly demonstrates the composite nature of the Fire Clay coal, and could be used as tool to predict coal composition in as yet un-exploited parts of the seam.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009