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 role of the subsurface team is to reduce technical risk and uncertainty associated with profile delivery, for both hydrocarbon production and the injection of water or gases, enabling confident reservoir management and investment decisions. Core is a key resource for delivering this objective, providing the only opportunity to directly observe and measure the reservoir rock. However, it is expensive to acquire and the decision to take new core should be clearly linked to a business objective. A combination of depositional and diagenetic factors can result in carbonate reservoirs being highly heterogeneous, making them a challenge to find and develop. Core studies can significantly improve understanding of the controls on heterogeneity and its distribution within the subsurface, but to maximize value, they should be fully integrated with wireline logs, seismic and production or test data to predict likely reservoir behaviour. For example, thin (<1 m) high or low permeability zones can dominate production yet remain very subtle or undetectable in wireline log or seismic data alone. During exploration and appraisal, sufficient data and knowledge are required to enable appraise/develop or walk-away decisions. Appropriate geological description of core material helps to establish original volumes in place, net-to-gross, depositional setting, age, reservoir architecture and large-scale flow zones. Core material can also help establish other key reservoir parameters such as saturation, reservoir fluids, seismic rock properties and geomechanical properties as well as influence decisions such as well design, development strategy and facility requirements. As a field is developed and production matures, reservoir behaviour may change and detail of the sedimentology, structure, diagenesis, baffles and thin heterolithic permeability zones may become more important. In carbonate reservoirs, this often requires the integration of existing legacy core or new core material and targeted surveillance data, such as injection and production logging tool (ILT/PLT) or saturation logs, to understand the stratigraphic, depositional and diagenetic controls on flow units and improve predictive capability. Moving a reservoir from natural depletion onto waterflood or enhanced oil recovery (EOR) may need more advanced core-based data from core flood experiments to deliver maximum value from the reservoir. New core material, or detailed review and new sampling of existing or analogue legacy core, may be required to understand complex rock–fluid interactions and the value that waterflood or EOR could bring to a development. To maximize the value and insights gained from core, it is essential to use it throughout a field's life. High quality description and a well-considered sampling regime should be carried out at the earliest opportunity to provide baseline data. However, periodically revisiting the core enables the entire subsurface team to keep testing hypotheses and refreshing models. Where there is no core material available in the field, analogue field core can be especially useful both to narrow uncertainty and explore possible alternative models. A multi-disciplinary approach integrating new data with core observations helps refine and improve predictions and can lead to the identification of significant additional opportunities as reservoir behaviour matures and field development progresses.