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The Formation and Diagenesis of Mg-Clay Minerals in Lacustrine Carbonate Reservoirs


Mg-rich clay minerals are relatively common components in modern and ancient lake systems, but are unusual in carbonate rocks. Although clays are traditionally viewed as having a negative impact on reservoir characteristics (e.g. promotion of pressure solution, localised cementation and clay seams), Mg-rich clays are prone to a series of diagenetic reactions that instead lead to the production of secondary porosity. Because of their unusual chemical composition and structure, Mg-clays are chemically more labile and wholly distinct from the more common Al-rich varieties (e.g. illite). Modern analogs and experimental syntheses show that Mg-clays act as valuable tools in constructing facies models, yet they exhibit complex and rapid responses to depositional chemistry (e.g. salinity, pH, Mg/Si ratios, temperature, detrital input and the presence of biological substrates). Once initiated, the precipitation of Mg-clays from water takes place rapidly via the formation of a poorly crystalline, viscous and extensively hydrated “gel”; an ideal substrate for the generation of spherulitic calcite. Dehydration of the gel phase is capable of releasing up to 25-30 wt. % H2O from crystallite surfaces and interlayers, acting as an important source of diagenetic water to buried sediments. In addition to burial dehydration, Mg-clays in general, and stevensite in particular, may be prone to a number “self-initiated” acid producing reactions that lead to corrosion upon further diagenesis. Ongoing experimental work is aimed at unraveling these pathways, some of which may be uniquely attributed to the burial of Mg-rich clay structures (e.g. gel annealing and interlayer cation migration). Once destabilised, however, the distinctly Al-free chemical composition of most Mg-clays means dissolution is nearly always congruent with few secondary products beyond silica and dolomite. Our view of Mg clay diagenesis in lacustrine carbonate reservoirs is consistent with a number of sedimentary textures observed in Pre-Salt carbonates of the Santos and Campos basins. Nevertheless, understanding the interactions between Mg-clays and lacustrine carbonates is a challenging frontier for research and warrants new approaches to understanding geochemical pathways during diagenesis.