Log responses in basalt successions in 8 wells from the Faroe-Shetland Channel - a classification scheme for interpretation of geophysical logs and case studies

Sub-basalt petroleum plays are becoming significant in the Faroe Region and even effort are put on recognizing basaltic successions with intervals of reservoir quality. However, only few studies have been made of wireline data from exploration wells penetrating basalt successions although detailed understanding of the physical properties of basalt successions are important for seismic processing and imaging, interpretation of the geological evolution and well planning. Basaltic successions of various thicknesses and characters have been penetrated in exploration wells in the British sector of the Faroe-Shetland Channel and around the Northeast Rockall Basin. The wireline logs through basaltic successions in eight released wells were selected for a systematic study of the log-response of basaltic rocks in this region. Physical properties recorded by wireline logs show pronounced variation of similar or broader ranges than in most sediment systems. Based on overall log character five distinct classes of basaltic units are distinguished: simple lava beds, compound lava units, volcaniclastic units, hyaloclastic units and hypabyssal intrusives.

Further characterisation of the individual basaltic unit is carried out using a characteristic trend/pattern or value found on one or more log traces throughout a unit, within a part of a unit or across the boundary between two units. In general terms log characteristics in basaltic successions may be thought of as equivalent to intrafacies components.

The log characteristics used in this study are selected in such a way that they are as far as possible considered diagnostic of typical lava morphological features of basaltic eruptives.

In the studied wells, the classification and characterisation of log responses allow a description of the eruptive style and the environment of lava emplacement, which is in good agreement with descriptions based on palynological and petrological investigations of the successions.