Salt Imaging in the Nordkapp Basin with Electromagnetic Data

Ketil Hokstad, Eva Andrea Myrlund, and Bente Fotland
Exploration Northern Areas, StatoilHydro, Harstad, Norway.

The Nordkapp Basin is an intra-continental salt basin with approximately 30 complex salt structures. The salt is of Early Permian age, and was mobilized by Early Triassic sedimentation. Tertiary uplift and erosion removed 1200-1400 m of Cretaceous and younger sediments. The hydrocarbon play is mainly salt-related traps in the Carnian. Two wells have been drilled in the basin; the Pandora well which was oil and gas discovery, and the Uranus well which was terminated inside the salt.

The high-velocity salt is surrounded by old and compacted high-velocity sediments. The acoustic impedance contrast at base salt is expected to be small. The hard sea-floor and shallow top salt generate strong free-surface and interbed multiples. Moreover, rugous top salt and internal structure in the salt generate strong diffractions in the seismic data. Consequently, seismic imaging becomes extremely difficult. The salt structures are surrounded by a seismic ``shadow zone'' where interpretation of continuous seismic reflectors is not possible.

The electric resistivity of the salt is very high. In the Uranus well, which penetrated 2 km of salt, 1000 ohm-m and more were logged. This suggests that electromagnetic data is an interesting alternative for delineation of the salt, and may provide complementary information compared to seismic imaging. In 2006 Statoil and Hydro, in a research co-operation with NGI, acquired wide band controlled-source electromagnetic (CSEM) data over the Uranus well line. This research survey introduced a novel approach to the salt imaging problem, and proved that the CSEM imaging concept worked as expected from modelling studies. In 2007 another 15 CSEM and MT lines, including the Uranus well line, were acquired, covering all the main salt diapirs in the Southern Nordkapp Basin. The CSEM data were imaged using 3D CSEM depth migration, and a salt-flood approach, similar to seismic base-salt imaging. Inversion of the MT data was performed using smooth Occam inversion.

At present (March 2009) the CSEM and MT results are interpreted together with 3D gravity, 2D and 3D seismic and VSP data, to rank the salt structures with respect to prospectivity, and to decide possible location for a new 3D seismic acquisition. In the interpretation of the MT and CSEM images, the results from the Uranus well line are used for reference to obtain consistent and comparable interpretations for all the salt structures.

AAPG Search and Discover Article #90096©2009 AAPG 3-P Arctic Conference and Exhibition, Moscow, Russia