Using field-based analogues to de-risk exploration in volcanically-affected basins

Continental flood basalts that commonly blanket vast areas within prospective basins pose many challenges to hydrocarbon exploration, development, and ultimately production. Differences in drilling strategies (e.g. using fixed cutter, roller cone, or hybrid drill bits) through the buried volcanic sequences and the often widely spaced nature and paucity of well data means interpretations can be limited. This may also be compounded by the scale at which (e.g. decametre) seismic imaging can resolve volcanic facies. Intra-volcanic discoveries, most notably in the Faroe-Shetland Basin, has driven research to understand, for example, sedimentation and drainage pathways in flood basalt provinces, reservoir architectures, and the effects of volcanic debris on reservoir properties and/or sealing potential. Fieldwork undertaken in the Ethiopian Flood Basalt Province (EFBP), the Faroe Islands Basalt Group (FIBG) and the East Greenland Plateau Lavas (EGPL) has begun to address these questions and is bringing together observations from beneath, within, and above these provinces. Developing detailed volcanic stratigraphies in the FIBG and EFBP has shown they share many common features. It has enabled us to understand the spatial and temporal distributions of the volcanic and sedimentary lithofacies and the influence of pre-existing structure on these variations. This is helping, not only, to potentially predict the location of clean intra-volcanic, but also sub-volcanic reservoirs. Although the vast majority of interlava units are volcaniclastic in composition, detailed petrographic and heavy mineral studies are recording non-volcanic material preserved in these rocks in addition to clean interbedded sandstones. The provenance of this non-volcanic material is, for example, being deduced for the Faroese and Ethiopian samples and field/photogrammetric studies are highlighting the prevalence of channels and drainage pathways within the associated volcanic sequences. In East Greenland and Ethiopia, petrographic and geochemical analyses of crystalline volcanic clasts are also differentiating extraformational sources. In Ethiopia, for example, mafic, alkaline, and felsic debris, or mixtures thereof have been recorded within mass flow and fluvial deposits preserved in interlava sedimentary sequences that are locally >200 m thick. The variability in composition has implications for drainage development within lava fields as well as reservoir/sealing properties due to their differing susceptibilities to alteration. An extreme example is observed in East Greenland, where subarkoses with porosities of >20% are interbedded with cemented volcaniclastic sandstones suggesting they not only underwent different diagenetic histories, but very little mixing occurred between the two systems. These multi-disciplinary studies from a variety of analogous settings are beginning to elucidate the types, scale, and controls on sedimentation patterns in volcanic provinces and are providing parameters for de-risking exploration in these unconventional basins, including those of the UK continental shelf.