The application of a solution-mineral equilibrium model to the diagenesis of Carboniferous sandstones, Bothamsall oilfield, East Midlands, England

Abstract

Summary Within the fluviatile reservoir sandstones of the Bothamsall oilfield, authigenic cements of early quartz, kaolinite and illite predominate. Detrital feldspars show varying degrees of alteration and muscovite micas are commonly altered to intergrowths of kaolinite and illite. Solution-mineral equilibria in the idealized system of K 2 O-Al 2 O 3 -SiO 2 -H 2 O are used to relate the simple silicate mineral assemblage observed to possible pore-fluid compositions. Previous studies considered aluminium to be immobile in this system. Petrographic evidence of the paragenetic sequence at Bothamsall indicates that aluminium is mobile, albeit over short distances. This study therefore considers aluminium mobility in the solution-mineral equilibria. Graphical plots of total aluminium versus pH for given potassium ion activity, at a temperature of 298 K, are constructed to illustrate the stability fields of the phases identified. These show that aluminium can be an important component, together with pH and potassium-ion activity, in affecting mineral stabilities in this system. The existence of several co-stable phases at Bothamsall considerably reduces the possible permutations for pore-fluid chemistry; kaolinite-illite alteration of mica proves particularly sensitive in this respect. The relative merits of two different pore-fluid models are compared: one, where the pore-fluid composition is considered to be constant in an open system, and another where the pore-fluid composition is assumed to vary in response to in situ mineralogy in a closed system. The former would require a very restricted range of pore-fluid compositions to result in the diagenetic modifications observed, whereas a wide range of pore fluids could evolve in a closed system driven by feldspar dissolution. This pore-fluid evolutionary model is preferred.