The Role of Shales as Cement Sources and Sinks

The debate over the sources of cement can not be considered complete without a discussion of the related controversy over the isochemical nature of shales. Is shale diagenesis an isochemical process or do reactants and products move readily between shales and interbedded sandstones? Given the fact that in a large number of sedimentary basins, the volume of shale greatly exceeds the volume of reservoir sandstone, this is a critical question, and one that remains a notable gap in our understanding of basinal diagenesis.

Since the 1970's, various researchers have addressed this question and come to differing conclusions, although recent publications seem to sugest that shales are not isochemical. For example, Milliken and Land (1993) present petrographic and geochemical evidence from Oligo-Miocene mudstones in South Texas for wide spread dissolution of carbonate grains and early diagenetic carbonate cement, unaccompanied by local re-precipitation. The authors argue that such removal of carbonate must have required a massive acid source extraneous to the shales or adjacent sandstones where no acid sources could be identified. Other authors (see, e.g. Whiteman and Harrison, in press, and Winsch and Kvale, 1994; Bloch and Hutcheon, 1992) also observed mass transfer within shales, and failed to detect significant cementation gradients away from the shales. Finally, recent work by Awwiller (1993) indicates that the KO content of Gulf Coast shales actually increases 2-4% with burial to 4,000 m. His calculations suggest that some of the increase can be explained by a greater abundance of silt-sized K-feldspar grains in the deeper portions of the basin. However, at least some of the K must be imported, presumbly from interbedded sandstones. The implication is that Gulf Coast mudrocks acted as open systems during burial diagenesis. Mass balance calculations based on the K content of present-day porewaters suggest that as much as 10 pore volumes of fluids may have passed through the most K-enriched shales (Awwiller, 1993), a number that has generated lively debate among shale researchers. This debate is likely to continue, because it has significant implications for whether shales serve as sources of diagenetic cements in sandstones.