Studying the basement structure of the Ramså field on Andøya, northern Norway

Whereas offshore on the Norwegian shelf and between the islands and the mainland deep Mesozoic basins still exist, Mesozoic sediments are generally absent on mainland Norway due to erosion processes. The Ramså field on Andøya, Vesterålen, Northern Norway is the only place in Norway with exposed Mesozoic rocks on land and thus provides an excellent place to study extensively the link between basement structures and sedimentary successions. A combined observation and interpretation of faulting and basement surfaces as well as a better knowledge of the different basement types allow a more accurate and differentiated interpretation of the tectonic setting and evolution of the region. With the aim to gain a better knowledge of the basement and the sedimentary structures, new high resolution aeromagnetic data and densely measured gravity data were acquired on Andøya, processed and interpreted to update the present geotectonic picture for the Ramså field. Magnetic data were used to map the structural elements of the area and to link them with geological observations. Furthermore Euler deconvolution techniques were applied to estimate the depth to magnetic sources and to identify fault systems. Gravity data were most sensitive to the varying sediment thickness and were consequently helpful to map the depth-to-basement. Stratigraphic information from four wells, drilled down to and into different basement types were used to establish a structural layer model of the Ramså field, which were assigned with petrophysical parameters based on NGU's comprehensive petrophysical database. The Ramså field is generally considered to represent a down-faulted block, bounded by sharp and steep faults to the surrounding exposed basement blocks. But the structural models, so far based on surface observations and well correlations, were never controlled and refined by using gravity and magnetic data. State of the art techniques and densely measured potential field data have the capabilities to update and improve the geometry of the Ramså field. A combined interpretation and modelling of gravity and magnetic data was utilized to produce a depth-to-basement map for the Ramså field. Structural elements from the magnetic map were incorporated and provided additional information of the tectonic development of the area. Furthermore at least three different types of basements are present, containing a key to the evolution of the area and might have an important impact for the interpretation of the Mesozoic basins offshore the Lofoten-Vesterålen region. The new insights can be linked with observations in offshore regions and contribute to a better understanding.