Diagenesis: Clay Cements
2015. "Diagenesis: Clay Cements", A Color Guide to the Petrography of Sandstones, Siltstones, Shales and Associated Rocks, Dana S. Ulmer-Scholle, Peter A. Scholle, Juergen Schieber, Robert J. Raine
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Clay minerals are a complex family of aluminosilicates, and generalized chemical formulas for them can be found in Table 11.2 (page 247). They can be either platy or fibrous with a high degree of chemical substitution. The word “clay” also has grain size connotations, so when referring to clay cements, it is best to call them “clay minerals”. Clay minerals occur in siliciclastic rocks as detrital and/or diagenetic components, which are commonly difficult to differentiate. Diagenetic clay minerals, the focus of this chapter, form in several ways: 1. Alteration of unstable silicate minerals, such as feldspars; 2. Pseudomorphic or neomorphic transformation of detrital or precursor diagenetic clays; or 3. Direct precipitates. They are of great interest to the oil and gas industry, because they can have a significant impact on sandstone reservoirs, commonly lowering porosity and permeability and increasing the possibility of formation damage. However, in many cases, clays (especially chlorite) form eogenetic to early mesogenetic grain-coating cements that can impede later cementation and preserve exceptional porosity at depth (e.g., Pittman et al., 1992; Ehrenberg, 1993; Anjos et al., 2003; Berger et al., 2009; Gould et al., 2010; Ajdukiewicz and Larese, 2012).