Deep-Water Clastic Sand Injectites—Traps and Relevance to Basin Analysis

Hurst, Andrew¹, Joseph Cartwright², Mads Huuse² (1) Aberdeen University, Aberdeen, United Kingdom (2) University of Wales, Cardiff, United Kingdom

Sand injectites are recognised as important modifiers of reservoir geometry in many deep-water clastic systems, and in particular in the Paleogene, Lower Cretaceous and Upper Jurassic of NW Europe. Improvement in the resolution and coverage of 3-D seismic has led to more common recognition of abundant high-angle to approximately vertical bedding-dis-cordant features, many of which are sand-filled and some of which are prospective. Sand injectites have unconventional reservoir geometries and enigmatic internal structures, which have caused explorationists to treat them circumspectly. Failure to differentiate decametre­scale injectites from sand-rich turbidites has led to development of inappropriate reservoir models and delayed the appreciation their general geological significance. Once formed, typ­ically during the first 500 m of burial, sand injectites are highly permeable conduits within otherwise low permeability strata; hence, they have a first-order affect on fluid migration. In some situations injectites may facilitate orders of magnitude faster rates of hydrocarbon migration than may otherwise be predicted. Injectites form intrusive traps, which may com­bine with more conventional traps or occur in isolation. Up to 2-3 x 107 m3 of sand may be injected in large injectite complexes, which constitute exploration targets in many petrole­um basins. Evaluation of reservoir quality and the presence of hydrocarbons is complicated by the unusual geometry and the lack of previous exploration of intrusive traps. Examples and analogues will be used to demonstrate characteristic features of sand injectite plays, their play potential, and their significance in basin analysis.