Inter‐tuff correlation and architecture within the Moor Cliffs Formation, Lower Old Red Sandstone, southwest Wales: a multi‐disciplinary approach for reservoir characterization

Abstract

AbstractThe laterally continuous coastal exposures of the Moor Cliffs Formation of the Anglo‐Welsh Basin have allowed a highly detailed 2D reconstruction of the floodplain. Whole‐rock trace element geochemical and spectral gamma‐ray (SGR) correlation are combined with traditional sedimentological analysis allowing outcrop and subsurface techniques to be compared. Micaceous, quartz‐rich and well‐sorted fluvial channel sandstones form up to 10% of the thickness in this low net‐to‐gross succession. High Th/K ratios derived from SGR analysis distinguish sandstones from the surrounding fine‐grained sediments, but lateral facies variations prevent the correlation of fluvial channel intervals using SGR without outcrop control. Red‐brown micaceous siltstone and claystone constitute 65 to 95% of the outcrop thickness and contain abundant pedogenic features characteristic of modern Vertisols. The most mature Vertisol horizons and the non‐pedogenically altered siltstones/claystones do not show significant differences in their SGR response. Intraformational conglomerates comprising carbonate and siltstone clasts within a siltstone matrix are common above laterally extensive erosion surfaces, and can be clearly recognized from a sharp spike in Nb/Y and Zr/Y ratios. Major tuff horizons, which form chronostratigraphic marker horizons throughout the Moor Cliffs Formation, are also characterized by a distinctive geochemical signature, enabling calibration of the geochemical and sedimentological datasets. Geochemical and SGR analytical techniques are complementary and assist in enhancing the confidence of correlations created by traditional techniques in the Moor Cliffs Formation. Where multiple correlation scenarios have been interpreted due to a lack of diagnostic marker beds or biostratigraphic control, the SGR and geochemical data can increase confidence in a particular interpretation and hence decrease uncertainty in geological models of low net‐to‐gross systems in the subsurface. Copyright © 2004 John Wiley & Sons, Ltd.