Variation in kaolinite morphology with growth temperature in isotopically mixed pore-fluids, Brent Group, UK North Sea

Abstract

A B ST R ACT: Diagenetic kaolinite in reservoir sandstones of the Brent Group precipitated following the dissolution of detrial feldspar. Two distinct morphologies of kaolinite occur: (1) early diagenetic vermiform kaolinite which is often associated with expanded detrital micas: (2) later diagenetic 'blocky" kaolinite. Combined hydrogen and oxygen isotopic studies suggest that vermiform kaolinite precipitated at 25-50~ and blocky kaolinite at 50-80~ from pore-waters of a similar isotopic composition (6t'~O 6.5 to -3.5%0). These pore-waters are interpreted to be either a mixture of meteoric and compactional waters, or alternatively a meteoric water that had evolved isotopically due to water-rock interaction. Kaolinite precipitation occurred predominantly during the late Cretaceous to early Eocene. Influx of meteoric water into the Brent Group, probably occurred during the Palaeocene. Fluid flow across the entire basin was driven by a hydrostatic head on the East Shetland Platform palaeo-landmass to the west. The development of the two kaolinite morphologies is possibly related to the degree of supersaturation at the time of precipitation. At low degrees of supersaturation, vermiform kaolinite precipitated slowly upon detrital mica surfaces. Blocky kaolinite precipitated more rapidly into open pore-space at higher degrees of supersaturation. Precipitation of blocky kaolinite was perhaps triggered by the decay of oxalate.

Indicators of hot fluid migration in sedimentary basins: evidence from the UK Atlantic Margin

“…In addition to the relationship between morphology and temperature of precipitation discussed above, previous isotopic studies of kaolin in sandstones have also demonstrated a direct correlation between morphology and isotope chemistry. In a compilation of North Sea data, Osborne et al (1994) noted that low temperature 'vermiform' kaolinite was characterized by high O values and strongly depleted D. Similar kaolinite compositions were noted in upper Jurassic reservoir sandstones from the Magnus Province, east Shetland Basin (Fallick et al 1993). Higher temperature polytypes of kaolin, including samples described as kaolinite (McAuley et al 1994), 'blocky' or 'intermediate' kaolinite (Glassman et al 1989) and dickite (McAuley et al 1994) (Fig.…”

“…As feldspar dissolves during progressive burial, kaolin morphologies coarsen and become more euhedral due to recrystallization. Kaolinite coarsened by recrystallization (and intergrowths of kaolinite with dickite pseudomorphing kaolinite) are commonly referred to in the literature as 'blocky' or 'intermediate' kaolinite (Osborne et al 1994;Beaufort et al 1998). Intermediate kaolinite may, however, form by hydrothermal processes involving the illitization of early kaolinite by hot fluids (Lanson et al 1996), as well as by burial-related recrystallization.…”

“…Crystal morphology was used to discriminate between low temperature kaolinite, intermediate kaolinitedickite and high temperature dickite (e.g. McAuley et al 1994;Osborne et al 1994). Kaolin polytypes (which for these purposes can be subdivided into kaolinite, intergrowths of kaolinite and dickite and pure dickite) become progressively more 'blocky' in appearance with increasing temperature (McAuley et al 1994;Beaufort et al 1998).…”

Characteristics of diagenesis, isotopic relations and reservoir properties of the Middle Miocene sandstones in the Carpathian Foredeep (Poland and Ukraine)

“…The pre lim i nary al ter ation stages of un sta ble com ponents of the grain fab ric (such as micas and feld spars) led to the kaolinite for ma tion. Tem per a tures of the kaolinite for ma tion may be de ter mined as 25-50°C (Osborne et al, 1994). The pre cip i tation of sil ica may have also started at this tem per a ture.…”

Berthierine in the Middle Jurassic sideritic rocks from southern Poland

“…5G). Kaolinite forms in an acidic en vi ron ment (Osborne et al, 1994) where a sig nif i cant role is played by mete oric wa ters, slightly acidic due to dis so lu tion of CO 2 and organic ac ids (Giles and de Boer, 1990). Dur ing the change from suboxic to anoxic con di tions, berthierine is re placed by py rite in the zone of sul phate re duc tion.…”