GEOLOGICAL MODELL ING OF CLAY D IAGENESIS IN SANDSTONES

ABSTRACT: Porewater composition is the main control on diagenetic reactions in sand-stones. Porewater has two possible contrasting primary sources: (i) fresh meteoric water, which is dilute and acidic, (ii) sea-water, which is alkaline and more concentrated than meteoric water. During burial, unstable minerals equilibrate with these porewaters, thus increasing the concentrations of dissolved species. A simple manometer model is used to describe the diagenesis of interconnected (fluvial or deltaic) sandstones. This model illustrates the following eological relationships: (a) a hydraulic head causes meteoric waters to penetrate deep into sedimentary basins, typically generating authigenic kaolinite; (b) decrease ofthe hydraulic head (by lowering the land level or by raising sea level) causes concentrated brines to rise within the basin, typically forming illitic cements; (c) enclosed sandstones (marine facies) are isolated from meteoric water flux and only receive fluxes when fault-induced or when uplifted. Kaolinite morphology and distribution are identified as being flux- or diffusion-controlled. The aim of this paper is to review some of the published data on sandstone diagenesis along with additional data from Hurst (1980a). Models for sandstone diagenesis are constructed using these data. It is beyond the scope of this paper to discuss all diageneti