Abstract Volcaniclastic rocks are commonly overlooked as reservoirs or seals in hydrocarbon plays because their compositions are variably unstable and reactive during burial diagenesis. This study investigated the petrography and petrophysical characteristics of 60 volcaniclastic and four siliciclastic samples from three Paleogene volcanic provinces – East Greenland, Faroe Islands and Ethiopia. The volcaniclastic samples have highly variable helium porosities (average 25.2%), but negligible total optical porosities (average 1.9%), implying reduced reservoir potential. The samples have, however, highly variable air permeabilities (average 11 mD), suggesting that they could make tight reservoirs. The permeabilities are related to either early calcite cements or the devitrification of volcanic glass. Mercury injection capillary pressure data were collected for a subset of 33 samples that at leakage/breakthrough saturations could, under near-surface conditions, hold oil column heights of between 4 and 1181 m (average 240 m). The best seals consistently have zeolite contents of >20 vol% owing to their small pore throat radii. Conversely, the worst seals are dominated by smectite and a conspicuous absence of zeolite minerals. The zeolite-rich volcaniclastic rocks could, therefore, make good shallow seals. These features also apply to CO 2 storage, but questions remain about the reactivity of the volcanic material and secondary minerals with injected CO 2 , but also the adsorbent properties of zeolites, particularly clinoptilolite, in the presence of CO 2 .

Abstract The margins of the North Atlantic rift are covered by an extensive succession of volcanic rocks, with up to 5 km of continental flood basalts, hyaloclastites and interbedded sedimentary rocks. The volcanic succession deteriorates seismic imaging and has hampered petroleum exploration in these areas. Focused research and pioneering exploration activity, however, has improved the understanding and development of new play models in volcanic-influenced basins. In 2004, the Rosebank discovery finally proved that intra-volcanic siliciclastic sandstones of the Flett Formation may form attractive hydrocarbon reservoirs in the Faroe–Shetland Basin. The Kangerlussuaq Basin in southern East Greenland offers a unique opportunity to study the interaction of siliciclastic sediments with lavas and various volcaniclastic units. It is demonstrated that: (1) laterally extensive siliciclastic sedimentary units are present in the lower part of the volcanic succession; (2) the morphology of the lavas controlled variations in sandstone geometry and thickness; and (3) deposition of the interbedded sediments and lavas occurred in a low-relief environment close to sea level. The mineralogical composition of the intra-volcanic sediments is highly variable, ranging from siliciclastic to purely volcaniclastic. Diagenetic studies suggest that the nature of the volcanic component in volcaniclastic sandstones is more important to reservoir properties than the relative concentration.