A gravity interpretation of the Central North Sea

Kimbell, G.S.; Williamson, J.P.. 2015 A gravity interpretation of the Central North Sea. British Geological Survey, 66pp. (CR/15/119N) (Unpublished)

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Abstract/Summary

A gravity investigation of the Central North Sea has been undertaken with the aim of supplementing a parallel seismic investigation (Arsenikos et al., 2015) by targeting those areas where the seismic information was sparse or of poor quality. By stripping the gravity effect of the Zechstein and younger sequence it was hoped that concealed Upper Palaeozoic basins could be identified in the residual gravity signatures and distinguished from anomalies associated with Late Caledonian granitic plutons. Density logs from a set of wells across the region were compiled and used to calibrate a density model for the cover sequence. This model employed a combination of compaction trends and burial anomalies in the post-Zechstein units and a relationship between overall thickness and average density in the Zechstein unit. It was used, together with a depth-converted structural model from the seismic interpretation, to calculate the gravity effect down to base Zechstein. This, along with a long-wavelength background field, was subtracted from the observations to leave a residual gravity anomaly that was inverted to produce a 3D model of variations in the thickness of a pre-Zechstein layer that incorporated the effects of both basins and granites. The modelling results were analysed in combination with magnetic imaging and available mapping of intra-Upper Palaeozoic seismic reflectors. Granites were often easy to identify on the basis of a low in the gravity inversion surface that coincided with a structural high defined seismically and, in some cases, a magnetic signature. There are, however, some more ambiguous features that cannot be confidently classified without further information. Relatively low density rocks within the Lower Palaeozoic basement and zones of high density basement or pervasive high density intrusive rocks introduce distortion into the model, and the identification and separation of these influences requires more detailed combined seismic, gravity and magnetic modelling. Potential targets (areas of pre-Zechstein sedimentary thickening) were identified in Quads 19-20, Quads 26-28, and just to the north of an 150 km offshore extrapolation of the line which forms the southern margin of the Tweed Basin in the onshore area (the Pressen-Flodden-Ford faults). Geophysical anomalies in the Q36-37 area suggest a complex interplay between sedimentary and igneous features and would also benefit from further investigation. A ‘ramp’ in the gravity inversion surface appears to be linked, at least in part, to lateral density variations associated with overcompaction along the Sole Pit axis. The geophysical feature extends beyond previous mapping of that axis and is overlain by the Breagh gas field, so is an appropriate target for more detailed study (which could address the possibility of a basement influence on the observed anomalies). The results obtained indicate that gravity/magnetic interpretation provides a useful supplement to seismic reflection surveys, even where the latter form the primary exploration method. There are, for example, features at the southern margin of the Forth Approaches Basin and possible intra-basinal structures within the North Dogger Basin that could add to our understanding of those areas. The new government-funded seismic/gravity/magnetic surveys over the Central North Sea, which were conducted in 2015 and will be released in 2016, will provide the ideal resource with which to follow up the results of this investigation.

Item Type:	Publication - Report
Funders/Sponsors:	British Geological Survey
Additional Information. Not used in RCUK Gateway to Research.:	This report is a published product of the 21st Century Exploration Road Map (21CXRM) Palaeozoic project. This joint industry-Government-BGS project comprised a regional petroleum systems analysis of the offshore Devonian and Carboniferous in the North Sea and Irish Sea. This report made open April 2017
Date made live:	03 Apr 2017 15:26 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/516759

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