KONTINENTALSOKKEL

An interpretation of aeromagnetic, gravimetric and petrophysical data from the Norwegian Sea between 67°N and 71°N is reported. Most of the magnetic and gravity anomalies within the survey area may be interpreted in terms of a fault-related basement relief. Depth to basement estimates based on gravity and aeromagnetic data are 5 km and 10 km, respectively, in the Ribban Basin. The discrepancy between the two estimates is most likely caused by downfaulted Caledonian Nappes, Devonian sediments or amphibolite-facies gneisses. The gravity modelling indicates that the Lofoten - Vesterålen area constitutes a basement high within the rifted and thinned crust of the continental shelf, similar to the basement highs within the Vøring Basin to the South. The depth to the basement is calculated to c. 10 km in northern part of the Træna Basin and c. 9 km in the Røst Basin. A correlation between Jurassic-Cretaceous faulting and underlying basement topography indicates that the deep structural setting may have guided the location of the later faulting. Within the Utrøst Ridge - Ribban Basin the basement map shows a complex pattern with several up- lifted and rotated basement blocks. The Vestfjorden Basin seems to consist of two sub-basins or half grabens; one to the north with a boundary fault to the east and one to the south with a boundary fault to the west. To the west of the Bodø High, and northwards along the shelf edge, there is a continuous zone of high frequency anomalies interpreted as the eastward termination of intrasedi- mentary volcanic rocks. The aeromagnetic and gravity data indicate that the Precambrian rocks in the Lofoten-Vesterålen area, despite Mesozoic reactivation along possible Caledonian zones of weakness, are situated within the margin of the Baltic Shield. They probably constitute a deep section of the Transscandi- navian Granite-Porphyry Belt.

An integrated study of geophysical data from the offshore and onshore areas of northern Norway between 67° and 71°N was carried out as a collaboration project between Elf Petroleum, Norsk Hydro, The Norwegian Petroleum Direc- torate, Statoil and the Geological survey of Norway. The interpretation of aeromagnetic, gravimetric and petrophysical data was integrated with mapping and palaeomagnetic dating of post-Caledonian faults on land.

This report presents results from 1659 magnetic susceptibility measurements on cores from the IKU shallow drilling programme in Nordland IV, V, VI and VII areas in addition to susceptibility, remanence and density measurements on approximately 4100 bedrock samples from the coastal area of Nordland and North Trøndelag (crystalline basement). Together the measured cores represent a > 1900 m rock column through the Upper Permian, Triassic, Jurassic, Creta- ceous, and Lower Tertiary. The Pleistocene cover has also been measured. The magnetic susceptibility data have compared for different lithologies and sediment composition and mineralogical analysis by SEM and XRD methods have been performed in order to identify the mineralogical source of the magnetic properties. These results have been used to aid in interpretation of aeromag- netic measurements from the NAS-94 Project.

An interprtation of the Nordland Aeromagntic Survey (NAS-94) in the Norwegian Sea between from 64° 30' and 68°N is reported. The NAS-94 survey has revealed several sets of high frequency anomalies not discernible in the old NGU data sets. This is due to the use of more sensitive magnetometers, closer line spacing, more accurate navigation and improved processing. When compared with petrophysical measurements on cores from IKU's shallow drilling programme, the anomaly pattern is interpreted to represent three types of magnetic sour- ces; 1) Quaternary overburden of varying thickness and bathymetric features, 2) buried magnetic sedimentary rocks, subcropping sedimentary units and pos- sible magnetic fault zones, 3) magnetic basement in the Lofoten Horst, Nord- land Ridge, southern part of the Trøndelag Platform and mainland Norway.

A combined interpretation of new aeromagnetic data (NAS- 4), existing offshore and onland gravity and new petrophysical data between 64°30' and 68°N is repor- ted. Data from the Nordland Aeromagnetic Survey (NAS-94) has revealed several anomalies caused by magnetic sources within the sedimentary sequences. When compared with petrophysical measurements on cores from the shallow drilling programme of IKU Petroleum Research, the anomaly pattern is interpreted to represent three types of sources; 1) Quaternary overburden of varying thickness and bathymetric features, 2) buried magnetic sedimentary rocks sub-cropping sedimentary units and possible magnetic fault zones, 3) magnetic basement in the Lofoten Horst, Nordland Ridge, south central part of the Trøndelag Platform and mainland Norway.

This report has been modified from the 'NAS-94 Interpretation Report, Part II: Petrophysical data'. The revised report presents results from 428 magnetic susceptibility measurements on cores from the IKU shallow drilling programme in the Nordland IV, V, VI and VII areas in addition to susceptibility, remanence and density measurements on approximately 4100 bedrock samples from the coastal area of Nordland and North Trøndelag (crystalline basement). Together the measured cores represent a > 1090 m rock column through the Triassic, Jurassic, Cretaceous and Lower Tertiary. The Pleistocene cover has also been measured. The masgnetic susceptibility data have been compared for different lithologies and sediment composition and mineralogical analysis by SEM and XRD methods have been performed in order to identify the mineralogical source of the magnetic properties. These results have been used to aid in interpretation of aeromagnetic measurements from the NAS-94 Project.

The purpose of the present compilation of the magnetic characteristics of the Norwegian continental shelf is to provide background data for developing a system to reduce the well-bore positional uncertainty in offshore areas. IKU Petroleum Research wants to consider the potential influence from magnetic rocks adjacent to offshore exploration and production wells. An aeromagnetic map at the scale of 1:3 million, depth to magnetic basement maps and compila- tion of available petrophysical data are presented. The magnetic anomaly pattern on the Norwegian continental shelf is interpreted to reflect three different types of magnetic sources; a) Magnetic basement continuing from mainland Norway below the continental shelf b) Tertiary and Permian volcanic rocks c) Subcropping and buried sedimentary units of late Palaeozoic, Mesozoic, Tertiary or Quaternary age causing low amplitude, high frequency anomalies. The latter is only seen in modern, high-sensitivity data. The depth to magnetic basement estimates indicate a general depth of 7-10 km under the basins of the Norwegian continental shelf. The magnetisation of the basement decreases to zero at a depth of approximately 20 km where the rocks reach the Curie temperature for magnetite (580oC) We conclude from available petro- physical data that the volcanic rocks in the sedimentary basins are the most important contributor to inaccuracy in the positioning of offshore wells. The magnetic basement is in most cases several kilometres deeper than the reser- voirs of interest to the petroleum industry and their influence will there- fore be relatively small. The magnetisation of the sediments within the basins are one to two orders of magnitude lower than the magnetisation of basement rocks and volcanics.

Fortrolig til 2002 Potential field data in the Norwegian-Danish Basin have been interpreted. The aeromagnetic anomaly patterns is found to represent three types of magnetic sources: 1) Deep magnetic basement 2) Permian volcanics and intrusives, and 3) Buried sand channels of Quaternary age. The positive gravity effect from the shallow Moho below the Norwegian-Danish Basin is not sufficient to com- pensate for the highly reduced gravity effect from the light sediments within the basin. More dense material has to be introduced at depth in the crust below the deepest part of the westermost Norwegian-Danish Basin and get thin- ner towards the east. The basement depth of the bulk of the Norwegian-Danish Basin varies between 6 and 8 km. Depths between 3 km and 9 km were obtained along the Farsund Basin, and the magnetic anomaly pattern indicates a complex system of fault blocks along the Sorgenfrei-Tornquist Zone.

Fortrolig til 2002. The processing and preliminary interpretations of the Skagerrak Aeromagnetic Survey 1996 (SAS-96) in the southeastern North Sea and Skagerrak is reported. The SAS-96 survey has revealed several sets of high frequency anomalies not discernible in the old NGU data sets from the mid seventies. This is due to the use of more sensitive magnetometers, tigher line spacing, more accurate navigation, and improved processing techniques. Several of the interpreted structures have not been previously reported to the authors knowledge. The anomaly pattern is interpreted to represent three types of magnetic sources: 1) deep buried sand channels of Quaternary and/or Tertiary age, 2) Permian volcanics and intrusive, and 3) magnetic basement.

Aeromagnetic and marine gravity data sets for the northern North Sea and western Norway have been interpreted in terms of on- and offfshore tectonic links. Structure both in basement and overlying sedimentary rocks were identified and compiled into three 1:250 000 scale interpretation maps. Two regional NW-SE lineaments were recognised and given the names Marflo Lineament and Troll Lineament. The Marflo Lineaments is interpretated as the shelf-continuation of an oceanic fracture of an oceanic fracture zone. It is suggested that this lineament functioned as a transfer zone in the Mesozoic and is responsible for an offset in the Viking Graben and may separate regions with different fault polarities. The Troll Lineament is a fundamental crustal structure interpreted to have affected the Mesozoic development of the Tampen Spur. The Troll Lineamnet is seismically active.