The nature, origin and distribution of authigenic clay minerals from Middle Jurassic Ravenscar and Brent Group sandstones

Abstract

Middle Jurassic sandstones contain a variety of authigenic clay materials. The origin and distribution of these are related to the influence of several factors, principally depositional pore-fluid chemistry, sandbody geometry and the migration of aggressive fluids. Pore-lining illite, pore-lining chlorite and pore-filling vermiform kaolinite in the Ravenscar Group mutually exclude each other because of depositional pore-water chemistry; seawater in the case of the illite, anoxic freshwater in the case of chlorite and oxygenated freshwater in the case of the kaolinite. Blocky pore-filling dickite occurs ubiquitously within the large connected sandbodies. Its origin may be related to the migration of aggressive fluids and its distribution to depositional sandbody geometry. Mixed-layer chlorite-vermiculite also occurs, and is believed to have formed from chlorite during Recent weathering. Similar patterns occur in Ninian Field Brent Group sandstones, although the situation is more complicated (in these sandstones the kandite subgroup minerals are undifferentiated in this paper). Here too, pore-lining illite occurs in marine sandstones and pore-filling vermiform kandites in non-marine sandstones. However, vermiform kandites also occur in the marine sandstones, perhaps due to freshwater-table development following progradation. The more blocky kandites occur in large connected sandbodies. Finally, a second phase of illitization occurs, postdating blocky kandites, perhaps caused by alkaline formation waters. The occurrence of chlorite in the Broom Formation is anomalous and its possible origin is discussed.

Clay Diagenesis and Oil Migration in Brent Group Sandstones of NW Hutton Field, UK North Sea

“…The fine-grained, micaceous, crevasse-splay sandstones of the Ness Formation and the Rannoch Formation contain the highest amounts of illite, suggesting an overriding lithological and depositional mineralogical control on iUitization. Sommer (1978) and Kantorowicz (1984) report similar patterns of illitization within the different Brent Group Formations and its relationship to kaolinite and hydrocarbons.…”

“…Chlorite is more problematic. Sommer (1978), Budding & Inglin (1981) and Kantorowicz (1984) report the highest contents in the Broom Formation which Kantorowicz (1984) suggests is due to early, pre-burial suboxic zone diagenesis.…”

“…A large-scale post-burial influx of meteoric waters into the Brent Group sandstones of NW Hutton seems unlikely as the overlying Upper Jurassic shales are not breached locally by the base-Cretaceous unconformity which has been regionally invoked as the main period of meteoric water influx during Brent diagenesis (Blanche & Whittaker, 1978;Bj~rlykke, 1984;Kantorowicz, 1984;L~noy et al, 1986;Thomas, 1986;Jourdan et al, 1987;Glasmann et al, 1989). For the nearby Heather Field, which has also remained unbreached by post-Brent erosion, Glasmann et al (1989) resort to a flow of meteoric water down faults from the East Shetland Platform, which appears difficult to envisage and, even if such a mechanism were operable, Curtis (1983) and Giles & Marshall (1986) have demonstrated that such longtravelled meteoric waters would have been effectively neutralized early in their flow paths.…”

Diagenetic Kaolinite/Dickite (Betic Cordilleras, Spain)

“…Kaolinite cement developed extensively in sandstones as a consequence of feldspar and mica dissolution (Ruiz Cruz and Puga, 1992). Dickite cement is usually related to burial (Dunoyer de Segonzac, 1969;Kantorowicz, 1984;Chamley, 1990). In the zone studied, the Permo-Triassic sediments have been slightly buried, but pressures approximating 2-3 Kb were experienced.…”

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“…La formación de dickita puede relacionarse con la diagénesis de enterramiento (Curtis, 1983;Kantorowicz, 1984). Precisamente este mineral aparece en perfiles del Aljibe y de Bolonia e~que las~aracte rísticas de las ilitas y las transformacIOnes sufndas por los interestratificados ilita-esmectita indican un grado de diagénesis más elevado (Rodríguez Jiménez y Ruiz Cruz, 1988).…”