Reservoir Quality Evolution in Upper Devonian Strata of the North Sea

Abstract

In the UK North Sea, the Devonian has been perceived little potential to be hydrocarbon res-ervoir for a long time. Since 1970s, a number of discoveries, though their initial targets were not Devonian, have proved that the Upper Devonian Buchan Formation is porous, permeable and productive with considerable reservoir quality heterogeneity. The Alma Field (Block 30/24, UK Central North Sea) possessed relatively complete oil-bearing Buchan Formation sandstones with anomalously high reservoir quality, which provides a good opportunity to study this poorly understood Palaeozoic reservoir. The study has employed various methods from macro to micro scale by integrating analogue outcrops, drilled cores, thin section obser-vations, SEM analysis and several geochemistry methods (EDX, XRD, cathodoluminescence, stable isotopic analysis and fluid inclusion thermometry). It has been recognized that the braided-fluvial channel and aeolian dune sandstones form the main types of reservoir with good horizontal connectivity in the Buchan Formation of the Alma Field. The Buchan For-mation sandstones have experienced, due to consistently shallow burial depth until Paleogene and then rapidly buried into today’s maximum depth (2.7 km – 3.2 km), less significant com-paction and highly variable cementation in fluvial and aeolian sandstones. Dolomite cement occurred prevalently in both sandstones and the overlying Permian Zechstein dolomite is the most possible carbon source. The quartz overgrowth is extensive in fluvial sandstones but absent in aeolian sandstones; this difference is due to the presence of early-formed grain coat-ing illite/smectite (I/S), which is only occurred in aeolian sandstones, originated from fluvial distal sector and formed by the mechanical infiltration. This grain coating I/S have effectively inhibited quartz overgrowth which maintained anomalously high reservoir quality in aeolian sandstones. Including but not limited to Alma Field, the outcomes of this study should have broad applications to the future hydrocarbon explorations targeting Devonian in the Central and Northern North Sea.