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.

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.