Carbonate cements eogenetic

Factors that control the geochemistry, abundance and distribution of carbonate cements are of prime importance in the understanding and prediction of porosity-permeability variations and in tracing the geochemical evolution of pore waters during the burial of sandstones and associated sediments. Moreover, the stable isotopic composition of nearsurface, eogenetic carbonates (e.g. in soil profiles) provides important clues to the palaeoclimatic conditions (e.g. Ceding, 1984). 

Like other diagenetic minerals in siliciclastic sequences, eogenetic carbonate cements may display a strong relationship with depositional facies in continental and marine settings. 

When carbonate cements are subjected to physicochemical conditions that vary considerably from those under which they formed, they may dissolve and re-precipitate at various scales. Carbonate dissolution and the creation of secondary porosity may occur during eodiagenesis or telodiagenesis or in response to progressive burial. Eogenetic secondary pores may survive subsequent burial and compaction in sandstones that have been subjected to early overpressuring or hydrocarbon emplacement, or if dissolution is incomplete, and leave evenly distributed remnants of carbonate cement. 

Sandstones with potentially better porosity preservation are characterized by (i) coarser grain size and better sorting (ii) lower tendency to host extensive eogenetic carbonate cement than the finer sediments, which are more represented by well 34/4-1 samples and (iii) chlorite rims evolved from the infiltrated clay coatings, which are more abundant in coarse-grained sands which inhibited precipitation of pore-occluding quartz and carbonate cements. 

Fig. 15. Palaeogeographical and palaeoclimatic imprints on the eogenetic fluid composition and the distribution of early carbonate cements in the Serraria deposits. Modified from Garcia (1992).

Eogenetic magnesite cement in sandstones is relatively rare because its formation requires pore waters to be enriched in Mg " " and depleted in Ca " ", S04 and Cl". These conditions may occur in arid climates in which marine pore waters evaporate and become successively saturated with respect to calcium carbonates, calcium sulphates and halite, such as in sabkha settings (Kinsman, 1969 Morad et al., 1995). Continental brines enriched in Mg + are also suitable for the formation of eogenetic magnesite due to the low sulphate and chloride ion concentrations. Most recent magnesite cements form in the fine-grained sediments of alkaline/saline lakes (Last, 1992 Warren, 1990) and, less commonly, in freshwater lacustrine sediments (Zachmann, 1989). 

A late structural inversion defined in particular in slightly subsided marginal basins, especially on the northern flank of the Illizi Basin, caused meteoric waters to penetrate into the respective semdstones. Under the conditions of the original distribution of temperature, pressure and pore water chemistry the eogenetic and meso-genetic mineral complexes became unstable and started to react with the actual subterranean waters in an attempt to achieve a new equilibrium. These weakly mineralized waters with a near-neutral pH became able to leach the carbonate, sulfate and halite cements and generated a secondary porosity. 

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