Formation of grain-coating chlorite in sandstones. Laboratory synthesized vs. natural occurrences

Abstract

Grain-coating chlorites in clastic quartz-rich sandstones have long been recognized as an important porosity-preserving constituent in medium- to deep-burial diagenesis. As little is known about the occurrence and origin of chlorite coatings, chlorite synthesis experiments were performed to study how grain-coating chlorites form in certain sandstones during burial. The starting material was naturally-occuring sandstones from the Oseberg and the Veslefrikk fields offshore Norway, where the same sandstone formation is buried to different depths due to faulting. Grain-coating chlorites exist below ~3000 m burial depth only. At shallower burial (2400 m), an X-ray amorphous iron containing thin clay coating is present.The samples were heated to 200 and 250°C (at water vapour pressure) in a hydrothermal bomb for 2–4 weeks. Both starting material and end-products were studied (electron-) optically in both scanning and transmission microscopes. The TEM showed the Fe-rich precursor material to consist of a fine-grained berthierine-dominated mixed-layer. The neoformed grain coatings in the reacted samples were similar in appearance to naturally-occurring chlorite coatings. The TEM analyses of individual grains documented an Fe-rich chloritic phase with an average composition of Mg0.41Fe3.52Mn0.10Al1.51(Al0.58Si3.42)O10(OH)8. The reacted waters were found to be close to saturation with the newly formed chlorites.Grain-coating chlorite thus appears to form in the natural environment from Fe-rich berthierine precursors at a burial depth corresponding to a temperature around 90°C.

Contrasting red bed diagenesis: the southern and northern margin of the Central European Basin

“…Pore-lining diagenetic chlorites in reservoir sandstones have been interpreted as replacements of earlier smectite, corrensite or berthierine in a number of studies (e.g. Humphreys et al 1989;Hillier 1994;Aagaard et al 2000).…”

Biofilm origin of clay-coated sand grains

“…Clay-coated grains in sandstones are of interest because they affect petroleum reservoir quality; they inhibit quartz cement growth during prolonged burial and heating (Aagaard et al, 2000;Bloch et al, 2002;Worden and Morad, 2000). If, as reported by Bloch et al (2002), burial diagenetic clay coats result from recrystallization of clay coats formed during deposition, then developing a robust understanding of the origin of the primary coats will lead to models capable of predicting reservoir quality in sandstones (Wooldridge et al, 2017).…”

Climatic disaster at the Triassic-Jurassic boundary - a clay minerals and major elements record from the Polish Basin

“…2). Regard less of that, berthierine is con sid ered to be eas ily transformed into chlorite dur ing burial diagenesis (Hill ier, 1994;Aagaard et al, 2000). In deed, chlorite does not oc cur in Kamieñ al though it is rather com monly ob served in other places in which berthierine-to-chlorite trans for ma tion is not ex cluded.…”