TWIN - Tropical Weathering In Norway

The Norwegian Petroleum Directorate and the Geological Survey of Norway have launched a joint research and mapping project on deep weathering of basement rocks in Norway. The two institutions have realised that this phenomenon is widespread in parts of mainland and offshore Norway. The project involves mineralogical, chemical and petrophysical characterization of known occurrences of deep weathering, as well as development and testing of age dating techniques and geomorphological and geophysical mapping tools.

The remnants of deeply weathered basement on the mainland occur mainly as clay minerals, including smectite and kaolinite, along structurally defined weakness zones (faults and fracture zones). Locally, clay-rich zones with thicknesses up to 200-300 metres are observed. The partial or total alteration of bedrock to clay minerals is to a large extent the result of chemical weathering caused by percolating acidic groundwater in humid climate, most probably during tropical to sub-tropical conditions in the Mesozoic. Subsequent exhumation has partly removed the evidence of deep weathering on the mainland. Moreover. several exploration wells drilled offshore have proven weathered basement below younger sedimentary rocks.

We have selected five representative locations for more detailed studies: Vesterålen in northern Norway, the Beitstad-Inderøya area in mid-Norway, Stadlandet-Vågsøy in western Norway, Lista in southernmost Norway and the Kjose-Larvik area in eastern Norway. Four of the areas are located adjacent to offshore areas with seismic indications of Mesozoic deep weathering. Offshore oil exploration wells, shallow boreholes or sea bottom sampling may provide crucial information with regard to timing, extent and character of the weathering. Andøya in Vesterålen is of special interest since weathered basement occurs immediately below outcropping Mesozoic sedimentary rocks.

Improved understanding of age and nature of weathering and clay alteration of basement bedrock has a wide range of applications onshore (e.g. rock stability of natural mountain slopes and manmade construction sites and tunnels in addition to groundwater utilization) as well as offshore (e.g. bottom seal of hydrocarbon traps over basement highs, migration paths of hydrocarbons and potential petroleum reservoirs). The numerous sounds, fjords and islands along the Norwegian coast could to a large extent be conditioned by exhumation and erosion of weathered basement. The study will provide a connection between geomorphological elements along the Norwegian coast on- and off-shore.