Abstract This paper reviews petrographic evidence concerning the mechanisms by which mudrocks lithify. Mudrocks clearly undergo processes analogous to compaction and cementation in sandstone and limestone, although the relative importance of these two processes in the diagenesis of mudrocks remains uncertain. Cement in mudrocks can be demonstrated to fill both primary and secondary pores. Inter- and intragranular cements are observed in mudrocks as well as cement within fracture fills. Thus, cements in mudrocks take the full range of form and distribution as observed for cements in sandstone and limestone. Displacive precipitation is a chemical-mechanical process observed with particular frequency in mudrocks, which contrasts with the common cementation processes in sandstones and limestones. Overall, the same authigenic minerals common in sandstone and limestone dominate the authigenic assemblages in mudrocks. Sediment accumulation rate is a significant factor in mudrock cementation. In situations of slow sediment accumulation, cement emplacement, typically in the form of highly localized carbonate and phosphate minerals, takes place near the sediment-water interface. In contrast, rapidly deposited mudrocks tend to lithify at greater depths in response to thermally controlled diagenetic reaction of the detrital assemblage. Authigenic quartz in mudrocks, a topic of particular interest for an understanding of quartz are readily documented in mudrocks, but convincing demonstration of intergranular quartz cement remains elusive. High-resolution imaging by transmission electron microscopy may be required to fully resolve issues surrounding the emplacement of authigenic quartz into the minute pores of mudrocks.

Abstract Carbonate-rich lithologies of the gas-producing Upper Mississippian Barnett Shale, Fort Worth Basin, Texas, are diverse and include lithologies with carbonate components that are primarily authigenic, as well as those that have carbonate components dominated by skeletal debris and other allochems such as peloids and intraclasts. Compositionally, carbonate-bearing lithologies of the Barnett Shale (including the informal unit known as the Forestburg Limestone) can be viewed as mixtures of authigenic or allochemical carbonate and siliciclastic sediment derived mostly from outside the basin. With the exception of the Forestburg Limestone, these varied carbonate lithologies dominate only in local zones, at the scale of a hand specimen or thin section, and do not constitute a volumetrically significant part of the gas-producing reservoir rock. Carbonate lithologies are significant, however, for clues they provide on environmental and early diagenetic conditions during accumulation of the Barnett Shale. Carbonate lithologies dominated by skeletal components contain distinct and impoverished marine faunas that are consistent with low oxygenation levels. The generally early timing of carbonate cement precipitation is supported by the reworking of diagenetic carbonate as silt- to sand-size intraclasts, sediment infilling of fractures in cemented beds and concretions, displacive fabrics, and highly random orientations of phyllosilicate grains within carbonate units. In some cases, detrital allochemical carbonates provided nucleation substrates for precipitation of highly displacive authigenic carbonate that was extensively reworked into microspar-size sediment particles. The elemental and isotopic chemistries of authigenic carbonates are consistent with near-sea-floor authigenesis driven by microbial cycling of organic carbon into carbonate minerals under generally reducing and low-temperature conditions.