The case of the missing clay, aluminium loss and secondary porosity, South Brae Oilfield, North Sea

Abstract

AbstractUpper Jurassic sandstones of the South Brae Field were deposited as a submarine fan complex. The earliest formed concretionary ferroan calcite cement passively encloses detrital feldspars which originally formed more than 10% of the rock. Later non-ferroan calcite at the concretion margins was precipitated from a more aggressive fluid. This fluid dissolved up to half of the feldspars and micas originally present, but little or no clay was precipitated. Aluminium must have been lost from the system.A late dissolution event has enhanced porosity by up to 8%. Feldspar was again reduced in volume by about half, leaving only ∼2% in the rock today. Very minor amounts of fibrous illite and kaolinite (<1%) form the last diagenetic cement. Aluminium must have again been lost from the system.As the Kimmeridge Clay Formation (KCF) encloses and interdigitates with the South Brae sandstones, a local source of organic acids is quite possible. These acid solutions may have increased the mobility of Al. The Al from the feldspars must have therefore been transported vertically into the KCF, or more probably transported laterally by compactional flows out of the basin (up to 10 km) during release of overpressured basinal water.