Onshore-offshore Neotectonics in Nordland, Northern NO – Implications for Petroleum Exploration

Odleiv Olesen

The coastal area of Nordland, northern NO, is a region with increased seismic activity relative to other parts of NO. There is a parallel and shallow zone of seismicity along the coast largely reflecting extensional stress conditions. Below the Pleistocene wedge along the continental edge to the west there is another seismic zone with deeper compressional events. The coinciding patterns of sediment loading/unloading and compressional/extensional earthquakes indicate that there is a causal relationship between the two phenomena. One of the most significant exploration problems in the Helgeland and Ribban basins relates to the Neogene uplift and erosion. An improved understanding of present-day deformation in terms of stress, fluid pressure, gas expansion and transient thermal states will therefore be crucial for the evaluation of the petroleum potential. Several independent datasets in the outer Ranafjorden region indicate that the area is under WNW-ESE present-day extension. A six-station seismic network established by NORSAR in this region detected during a two-year period (1997-1999) c. 300 earthquakes, often occurring as swarms. Fault plane solutions indicate E-W extensional faulting. The outer Ranafjorden district is also the location for the largest earthquake recorded in Fennoscandia in historical times, i.e. the c. 5.8 magnitude in 1819. Three measurements of uplift of acorn barnacle and bladder wrack marks on the islands of Hugla and Tomma in the outer Ranafjorden area show anomalous low land uplift from 1894 to 1990 (0.0-0.07m) compared with the uplift recorded to the north and south (0.23-0.30 m). An irregular subsidence pattern in the order of c. 1 mm/year is also observed on DInSAR permanent scatterer data in the same area. The seismicity occurring at a depth of 2-12 km could therefore create the observed irregular subsidence pattern at the surface. Three 15-20 km-long GPS profiles were established across outer, central and inner Ranafjorden to measure the active strain . Campaign measurements in 1999 and 2008 indicate that the bench marks along the western profile have moved c. 1 mm/year to the W relative to the stations along the two eastern profiles. Weathered bedrock can be found up to an altitude of c. 500 m above sea level along the Nordland coast and is most likely exhumed during Pleistocene erosion. This indicates that the present-day large-scale topography may to a large extent be inherited from the Mesozoic. The unloading of the crust along the coastal areas of Nordland resulted most likely in flexuring and accompanying fracture extension and seismicity. The pressure decrease associated with removal of sedimentary overburden may have caused expansion of gas and resulted in expulsion of oil from the offshore traps. Where uplift and tilting result in local extension, seal breaching and spillage may also occur.

AAPG Search and Discovery Article #90177©3P Arctic, Polar Petroleum Potential Conference & Exhibition, Stavanger, Norway, October 15-18, 2013