Abstract

Mesozoic sandstones cored in a well in the northern North Sea Basin show at least 78 superimposed fining-upward units varying from about 1 to 5 m thick, interpreted as fluvial deposits. Repetitive fluctuations in cementation, porosity, and permeability were controlled directly or indirectly by conditions during and soon after deposition. At the base of each unit poorly sorted sandstones bearing shale clasts tend to be cemented by early diagenetic dolomite and have relatively low porosity and permeability. Above them, well-sorted fine- to medium-grained cross-bedded sands forming the major part of each unit contain little dolomite and are moderately porous and permeable. At the top of each unit, ripple-bedded micaceous sands have low porosity and permeability because of their silt and clay content. Authigenic clay may be present in all parts of the units, but is particularly prominent as a thin layer around detrital grains in the clean non-dolomitic sands forming the centre of each unit where it inhibited later quartz overgrowths; lacking the support of cements or matrix, these sands were the most susceptible to compaction on deeper burial.

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