Integrated sedimentological and whole-rock trace element geochemical correlation of alluvial red-bed sequences at outcrop and in the subsurface
Red-bed alluvial systems are becoming increasingly important as hydrocarbon plays in the UK Northern North Sea. Commonly such ephemeral systems are hard to define in terms of reservoir architecture, due to the difficulty in correlating such diverse and palaeontologically-barren sequences. This project aims to improve understanding of one such system, the Late Jurassic Cormorant Formation, of the Northern North Sea, through detailed sedimentological study of two outcrop analogues. The whole-rock trace element geochemical correlation technique in a variety of settings. The Lower Old Red Sandstone Moor Cliffs Formation of the Anglo-Welsh Basin provides ideal conditions for testing the whole-rock trace element geochemical correlation technique. The Moor Cliffs Formation is a low net:gross alluvial red bed suite, which by virtue of Variscan deformation, outcrops in a well-exposed, easily accessible cliff section at Priests Nose, near Manorbier, Pembrokeshire. A 100m section was sampled to determine variables that may affect whole-rock trace element geochemical correlation. The results prove that whole-rock trace element geochemical correlation can be applied to alluvial successions, despite pedogenic modification and deep burial. The Late Triassic Blomidon Formation of the Fundy Basin, Nova Scotia, provided a second outcrop example of an alluvial red-bed sequence, which was deposited in similar palaeo-climatic and tectonic conditions to the Northem North Sea Triassic. The Blomidon Formation contained a range of fluvial styles from confined channels to unconfined sheetfloods. Three sections were studied along the hanging-wall margin of the Fundy Basin, separated by up to 150km along strike. Each section provided a contrasting basinal setting, allowing comparison of facies along strike and down depositional dip. Sections were measured in detail to quantify bed geometries and facies variants, facilitating architectural analysis. Specific attention was paid to features that may be diagnostic in sub-surface cored sections of the Northern North Sea Triassic. Correlation was possible on a number of scales, using laterally continuous ephemeral marker beds within the Blomidon Formation. Detailed facies evaluation has allowed the division of the Blomidon Formation into four distinctive facies packages that vary considerably in sandstone net:gross. Each facies assemblage is defined by variations in fluvial style and occurrence of evaporite rich, ephemeral lacustrine and rare aeolian sediments. It was possible to produce a broad, basin-wide correlation scheme for the Blomidon Formation, based on these four facies packages. Detailed facies analysis of three cores from the Cormorant Formation, Tem Field, Northern North Sea allowed definition of reservoir architecture, based on models derived from outcrop analogues. The results suggest that correlation based on individual horizons and facies packages is possible in alluvial red-bed sequences through detailed sedimentological study. The whole-rock trace element geochemical correlation technique can also provide additional datasets to enhance correlation in the subsurface.