Facies Control on Diagenesis and Reservoir Quality of Fluvial Sandstones; an Example from the Middle Jurassic of Yorkshire

Peltonen, Christer B.¹, Knut Bjørlykke¹ (1) University of Oslo, Oslo, Norway

Two sandstone facies from the Middle Jurassic Ravenscar Group (onshore Yorkshire, England) were compared with respect to mineralogy and diagenesis. Samples from fluvial channel sandstones contained no K-feldspar, and had relatively high amounts of kaolin (14.3% avg.), whereas the crevasse-splay sandstones had significant amounts of K-feldspar (4% avg.) and a low relative amount of authigenic kaolin (7% avg.). Petrographic evidence suggests that kaolinite was precipitated prior to quartz cementation. The more permeable channel sandstones would have served as conduits for meteoric water flow during shallow burial, whereas the crevasse-splay sandstones had limited hydraulic conductivity and therefore lower fluxes. This explains the differences, with respect to dissolution of feldspar and precipitation of kaolin, within these two fluvial sandstone facies. The degree of leaching of feldspar and other unstable silicate minerals such as mica may therefore be taken as a measure of the degree of meteoric water flushing and thereby the degree of sandstone connectivity. Sandstones showing evidence of only small or moderate amounts of leaching may represent bodies which are poorly connected to the main aquifers. The absence of K-feldspar in the channel sandstones provides a good basis for prediction of reservoir quality at greater depth (>130 degC). Pore-filling illite would not have developed if these channel sandstones were buried more deeply due to lack of a local potassium source. The crevasse-splay sandstones on the other hand would have greater potential for authigenic illite growth, and the difference in reservoir quality of these two facies would therefore be even greater. This information may be used during production and evaluation of fluvio-deltaic and shallow marine reservoir sandstones.