CARBONATE LITHOLOGIES OF THE BARNETT SHALE

Carbonate-rich lithologies are an important aspect of the considerable lithologic heterogeneity of the gas-producing Barnett Shale. Carbonate-rich rocks within the Barnett Shale are themselves diverse, ranging from materials composed primarily of authigenic carbonate (both calcite and dolomite) to ones dominated by skeletal debris and carbonate allochems such as pellets and intraclasts. Carbonate lithologies dominate only in local zones, at the scale of a hand specimen or thin section, and probably do not constitute a volumetrically significant portion of the gas-producing reservoir rock. Carbonate components in the Barnett Shale are significant, however, in terms of the information they record about depositional and early diagenetic conditions. Evidence for dramatically displacive growth of authigenic carbonates suggests the former presence of hydrous carbonate phases, which, in turn, imply very early precipitation under conditions of low temperature, low sedimentation rates, and high organic alkalinity. Reworking of diagenetic carbonate as silt- to sand-size sediment, sediment in-filling of fractures in cemented beds and concretions, and highly random orientations of phyllosilicate grains within carbonate units all support the early timing of carbonate authigenesis in the Barnett Shale. Carbonate lithologies dominated by skeletal components contain impoverished faunas consistent with low oxygenation levels.