LAUSR

Chlorite, an Fe- and Mg-rich aluminosilicate clay, may be either detrital or authigenic in sandstones. Detrital chlorite includes mineral grains, components of lithic grain, matrix and detrital grain coats. Authigenic chlorite may be grain-coating, pore-filling or grain-replacing. Chlorite can be observed and quantified by a range of laboratory techniques including light optical and scanning electron microscopy and X-ray diffraction; the presence of chlorite in sandstone can be identified by the careful integration of signals from downhole logs. Grain-coating chlorite is the only type of chlorite that can help sandstone reservoir quality since it inhibits quartz cementation in deeply buried sandstones. Grain coats are up to about 10 m thick and typically isopachous on all grain surfaces; they result from rapid indiscriminate nucleation at high levels of chlorite supersaturation in the pore waters and then growth of appropriately oriented nuclei as ultra-thin, roughly equant crystals. Chlorite can have many possible origins, but it is likely that grain-coating chlorite results from closed system diagenesis at the bed scale. Chlorite sources include transformation of detrital Fe-rich berthierine, transformation of Mg-rich smectite, reaction of kaolinite with sources of Fe and breakdown of volcanic grains. The specific origin of chlorite controls its composition, with marine sandstones having a berthierine source and continental sandstones having a smectite source. Incorporation of precursor clays required for chlorite growth can be achieved by a variety of processes; these most commonly occur in marginal marine environments possibly explaining why Fe-rich chlorite coats are most commonly found in marginal marine sandstones.