Frontier exploration in Western and Northwest Europe

Abstract

Extended Summary Much of the offshore of Western and Northwest Europe can be considered as unexplored and underexplored and is thus a major frontier area in terms of oil exploration. If the interior shelf basins of the Celtic Sea and English Channel are excluded, the total number of commercial wells is less than 100 with the majority of these being concentrated in the inner shelf of the Aquitaine, Lusitanian and Faeroe-Shetland Channel Basins. The outer shelf and the upper slope to a realistic depth unit of 1000 m has hardly been tested at all. In these water depths, a significant and relevant contribution to our understanding of the regional petroleum geology has come from the results of DSDP/IPOD drilling on Legs 478, 48, 80 as well as the allied regional seismic surveys made by various European academic and government institutions. In considering the geology of this frontier area, it is appropriate to begin by recognising that the present day distribution and architecture of the principal basins as well as the development of the principal reservoir and source intervals is dependent on the complex history of rifting and spreading that has structured the Atlantic margins north of the Azores-Gibraltar line. Basin development in the area between the Faeroe-Shetland channel and Southern Portugal was initiated in early Permian time along lines influenced by Late Hercynian shears. The first but relatively minor phase of rifting in Triassic-Liassic time reflects the more intense phase that ultimately opened the Atlantic between America and Africa. The next and most major phase of rifting that led to the opening of the north Atlantic was probably initiated in Late Oxfordian time culminating in Late Jurassic-Early Cretaceous time. Spreading had probably begun in the Bay of Biscay, west of Iberia in the Rockall-Trough-Faeroe-Shetland Channel by Aptian time but the kinematics of the opening are not fully understood (Roberts et al. 1981). By Late Cretaceous time, spreading had ceased in the Bay of Biscay and Rockall Trough and a new axis of spreading opened the Labrador Sea. Between Late Cretaceous and Oligocene time, Iberia moved westwards overthrusting the southern Bay of Biscay during the Pyrenean orogeny. In the Lusitanian Basin, Triassic through Liassic sedimentation may have been controlled by the first rifting phase. Late Jurassic and Early Cretaceous sandstones were deposited as a consequence of the main rifting phase. Extension onshore and offshore was controlled in part by the pattern of Late Hercynian shears (Groupe Galice 1979). In contrast to the Lusitanian Basin and the north margin of Biscay, the Iberian margin of Biscay is dominated by overthrust tectonics for the whole of its length. The margin can be simply described as a rifted margin of Late Jurassic-Early Cretaceous age overthrust to the northwest during the Pyrenean orogeny. Principal oil exploration targets in this area include Late Cretaceous turbidites associated with overthrusting and Urgonian limestone deposited the rift phase (Soler et al. 1981). The geological history of the north margin of Biscay and the Gobun Spur has been documented by Roberts et al. (1981). A series of large tilted and rotated fault blocks trending generally NW-SE are thinly covered by post Aptian sediments. These tilted blocks represent the northwestward prolongation of similar features known in the offshore Aquitaine Basin (Montadert et al. 1974). The tilted blocks evidently contain pre and synrift sediments but source and reservoir potential are unknown. Further north the Faeroe-Shetland channel is an area of active exploration. The Channel represents the northward continuation of the Rockall Trough in which there has been no significant exploration activity. Studies of the subsidence history of the Faeroe-Shetland Channel suggest formation by rifting in Late Jurassic-Early Cretaceous time with the onset of thermal subsidence (spreading?) in about Aptian time. During Palaeocene-Eocene time, eruption of lavas from centres in the Faeroes area masked the older section in the channel (Ridd 1981). Established success in the better explored areas of offshore Europe may be a signpost to future success in the underexplored areas. However a full assessment of these areas lies beyond the scope of this review of basin development.