OCCURRENCE AND SIGNIFICANCE OF CARBONATE IN THE MARCELLUS SHALE

The occurrence of carbonate minerals in outcrop samples of Marcellus Shale are observed under microscopes, and its significance to the nature and the gas potential of shale is evaluated. These shales are taken from the eastern margin of the Appalachian Basin in the States of West Virginia and Virginia. They are mildly deformed during the Allegheny Orogeny. The tectonic disturbance created small folds, faults and cleavages in the shale. However, recrystallization of carbonates in the slightly deformed shale is insignificant. There are three major occurrences of carbonate minerals: detrital grains, micritic matrix, and veinlets. The detrital carbonate grains are mostly sand-size microfossils with authigenic calcite filling. The carbonate veinlet runs parallel or across bedding and is composed of silt size calcite crystals. It could be related to the fracturing of shale occurred during the stage of overpressuring. The micritic crystals of carbonates are dispersed in the shale matrix, and have two different origins: nearly syndepositional crystallization at the very early stage of shallow burial; or neocrystallization which took place in deep burial and could use the carbon derived from the thermal decomposition of kerogen. In either case, the matrix type of carbonate has the closest spatial relationship to kerogen and can provide micropores for the gas storage. These three occurrences of carbonate in the Marcellus Shale may have distinctly different composition in carbon and oxygen isotopes, and should be carefully separated during an isotope analysis.