CLOKE, I., DAVIES R. J., LINE C., HORNAFIUS S., and MCLACHLAN K., Mobil North Sea Limited, London, UK

Abstract: Petroleum System of the Faroe-Shetland Basin

The Role of Inversion in the Development of the Petroleum System of the Faroe-Shetland Basin. Although historically within the Faroe-Shetland basin, major basement highs and intra-Paleocene stratigraphic traps have been the most prolific style of hydrocarbon entrapment, within the north-west section of the basin, inversion has played a key part in the development of the hydrocarbon system.These features are located in deep water (>1000m) and are important for hydrocarbon exploration since they are potentially four-way dip closed traps with multiple, stacked exploration targets.These structures were studied using a combination of seismic sequence mapping, potential field modelling and structural restoration techniques.

The Mesozoic rift-basin architecture was interpreted from various 3D seismic datasets and this detailed interpretation then extrapolated regionally throughout the Faroe-Shetland basin using a combination of 2D seismic, filtered gravity and magnetic data (Figure 1). Analysis and dating of growth packages into major basin bounding faults indicates Permo-Triassic, prolonged Cretaceous extension and a later lower Paleocene rift event. This series of rift events resulted in considerable rift-related topography which was infilled by younger sequences. Submarine fan systems of Paleocene and Eocene age were mapped throughout the Faroe-Shetland basin using 3D and 2D seismic data and tied to wells.

The presence of long wavelength asymmetrical anticlines onlapped by reflectors of Mio-Oligocene age and spatial thickness variations in the post-Eocene successions are used to infer contraction from at least the Mio-Oligocene, but possibly earlier, to the present day. Correlation between potential field data, seismic data and structural modelling demonstrates that the anticlines are the result of inversion of pre-existing Mesozoic extensional halfgraben. Spatial thickness variations of syn-inversion growth sequences and asymmetry of the inversion anticlines have been used to further constrain Mesozoic fault position and basin structure at depth. Cross sections constructed from seismic sections were structurally restored using vertical and oblique slip methodologies and show that only small amounts of shortening (<10%) consistent with basin inversion were required to generate substantial positive relief on the Tertiary fan systems (Figure 2). Restoration of several anticlines using different onlap marker horizons shows that inversion is diachronous throughout the Faroe-Shetland Basin.

Basin inversion had a profound impact on at least three elements of the hydrocarbon system, these being trap formation, reservoir presence and distribution and importantly the timing of migration of hydrocarbons.

Impact on Trap Formation. Inversion resulted in the formation of a series of NE-SW doubly-plunging assymetrical folds with complex and poorly understood structural configurations. Clear fluid contacts identified within the Middle Eocene succession are confined to four-way-dip closed anticlines. Most shortening took place during the OligoMiocene and we see this being contemporaneous or closely followed by migration.

Impact on Reservoir Distribution. At a regional scale the location of these Tertiary age fan systems is directly controlled by compaction over the underlying rift-basin topography (Figure 3).This resulted in narrow aggradational submarine fan systems confined to structural, compaction generated depressions developed in the hanging-wall of extensional fault systems. At a local (prospect scale), the distribution of sand prone submarine channels was controlled by the initial development of inversion driven hangingwall anticlines. Amplitude extraction timeslices covering a Middle Eocene fan system within the Faroe-Shetland basin clearly show that major (3km wide) SW-NE advancing submarine channels were deflected around local inversion related basin floor highs that had developed within the immediate hangingwall of underlying NE-SW trending Mesozoic extensional faults.

Impact on Timing of Migration Inversion is suggested to have contributed to the breaching of seals horizons within the Paleocene and Cretaceous successions and allowed remigration from deeper holding tanks into shallower,Tertiary closures. Gas chimneys identified on seismic sections and "fluid contacts", supported by AVO, identified within the Eocene succession demonstrate that vertical migration is taking place at the present day.

To the northeast in the Møre Basin this style of hydrocarbon trap has been proven by the Ormen Lange discovery well. Significant potential exists in similar features within the North-West Faroe Shetland Basin and these may be proven in the near future.

Figure 1. Faroe-Shetland Basin.

Figure 2.

Figure 3.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England