Sedimentology and reservoir architecture of a widespread siliciclastic intralava unit, Kangerlussuaq, East Greenland

The Rosebank hydrocarbon discovery in 2004 proved that intra-lava sandstones form attractive reservoirs in the Faroe–Shetland Basin and the new volcanic play triggered the need for suitable analogues to describe and assess sedimentology, reservoir architecture, compartmentalization, and connectivity of intra-lava siliciclastic units. The onshore Kangerlussuaq Basin in East Greenland offers the opportunity to study Paleogene intra-lava siliciclastic sandstones and their interaction with lavas, on the scale of the Rosebank Field. The focus of this study is a siliciclastic-dominated intra-lava unit, 4–10 m thick, exposed in almost vertical cliff sections over distances of several kilometers. The unit reflects a short return to siliciclastic deposition following initiation of volcanic activity and extrusion of the first lava flows in the area. Deposition took place as shoreface and delta progradation in a marine-influenced, shallow embayment. Lateral variations in sedimentary facies distribution and geometry are prominent and were largely governed by an interplay of base-level variations and autocyclic processes, the surface roughness and type of substratum on which deposition took place, and differential block movements before and during deposition. Presence of local topographic barriers are of key importance and influenced the lava–sediment interaction and the resulting 3D-geometry of lava flows and sediment bodies. In addition, compartmentalization of the intra-lava sandstone unit is observed and is controlled by the offset across normal faults, intersecting dikes, and to a lesser extent by invasive and eroding lavas. A depositional model is suggested that incorporates the detailed sedimentological and 3D photogrammetric observations and presents a possible explanation for the contrasting architecture of the intra-lava unit observed in three areas located a few kilometers apart. The model embraces the complex interplay between siliciclastic and volcanic settings and reveals important aspects to consider when recoverable volumes of hydrocarbons are estimated in intra-volcanic subsurface reservoirs in volcanic rifted margins with poor seismic imaging of the relatively thin intra-lava reservoirs.