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Chlorite in sandstones
R.H. Worden, J. Griffiths, L.J. Wooldridge, J.E.P. Utley, A.Y.
Lawan, D.D. Muhammed, N. Simon, P.J. Armitage

To appear in: Earth-Science Reviews
Please cite this article as: R.H. Worden, J. Griffiths, L.J. Wooldridge, et al., Chlorite
in sandstones, Earth-Science Reviews(2020)

Abstract
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