Depending on the context and discipline, the terms ‘clays’ or ‘clay’ may be used in three different ways: as a particle-size term, as a mineral term, and as a rock term ( Moore, 1996 ). The field of oil-sands research makes no exception and various uses can be encountered in the literature, including terms such as ‘clay-sized minerals,’ ‘fine clays,’ ‘ultrafine clays’ (or ‘ultrafines’), ‘estuarine clay,’ ‘marine clay,’ etc. In this chapter, an overview is given of: (1) the nomenclature associated with clays and clay minerals; (2) the structure and crystal chemistry of phyllosilicates; and (3) the geology and clay mineralogy of oil sands.

Abstract: The lacustrine carbonate reservoirs of the South Atlantic host significant accumulations of chemically reactive and Al-free Mg-silicate minerals (e.g. stevensite, kerolite and talc). Petrographic data from units such as the Cretaceous Barra Velha Formation in the Santos Basin suggest that Mg-silicate minerals strongly influenced, and perhaps created, much of the observed secondary porosity. The diagenetic interactions between reactive Mg-silicate minerals and carbonate sediments are, however, poorly known. Here we develop a conceptual model for the origin of secondary porosity in the Barra Velha Formation guided by considerations of the chemistry that triggers Mg-silicate crystallization, as well as the geochemical and mineralogical factors that act as prerequisites for rapid Mg-silicate dissolution during early and late diagenesis. We conclude that sub-littoral zones of volcanically influenced rift lakes would have acted as the locus for widespread Mg-silicate accumulation and preservation. Organic-rich profundal sediments, however, would be especially prone to Mg-silicate dissolution and secondary porosity development. Here, organic matter diagenesis (especially methanogenesis) plays a major role in modifying the dissolved inorganic carbon budget and the pH of sediment porewaters, which preferentially destabilizes and then dissolves Mg-silicates. Together, the sedimentological, stratigraphic and geochemical predictions of the model explain many enigmatic features of the Barra Velha Formation, providing a novel framework for understanding how Mg-silicate–carbonate interactions might generate secondary porosity more broadly in other lacustrine carbonate reservoirs across the South Atlantic.