Seismic Characterization of Gant Sand Injectites in the North Sea and West of Shetland: Reservoirs, Conduits and Drilling Hazards

Huuse, Mads¹, Joe A. Cartwright², Simon J. Shoulders³, Rutger Gras⁴ (1) University of Aberdeen, Aberdeen, United Kingdom (2) Cardiff University, United Kingdom (3) BP, (4) Schlumberger Information Solutions, Netherlands

Over the last five years, following more than 3 decades of hydrocarbon exploration in the North Sea, giant sand injectites have become widely recognized in the northern North Sea and in the Faroe-Shetland Basin. The injectites are seen on seismic data as crosscutting reflections, often of anomalous amplitude due to HC charge or cementation of the sandstones, but recently it has been shown that km-scale concordant sill bodies also occur a kin to igneous laccoliths. Typical geometries include conical and saucer-shaped injectites, which despite their size were previously dismissed as artefacts or simply overlooked. Some of these form oil fields in their own right. Typcial dimensions range from 500-2000 m diameter and 50-300 m height. The sheets are typically inclined at 15-40 degrees to the bedding, probably reduced by compaction from an original intrusion angle of the order of 45-60 degrees. On typical seismic display scales (5-10:1) the conical injectites appear V-shaped and may resemble channel features. However, they are clearly polygonal or circular in plan view and thus cannot be of erosional origin. Borehole calibrations of several anomalies in the northern North Sea show that crosscutting seismic anomalies with circular to polygonal plan geometry typically correspond to 10-60 m thick massive sandstones. The sandstones may be cemented or completely uncemented with porosities up to 30-40% and Darcy-range permeability and thus may provide highly permeable reservoirs and pathways for fluid migration through otherwise poorly permeable shale intervals. We show examples of giant, saucer-shaped sand injectites that constitute reservoirs, and of conical sand injectites that serve as efficient migration paths over hundreds of metres. Finally we argue that sand injectites may constitute drilling hazards when using high-ressure/heavy muds to drill through polygonally-faulted mudstones.