Characterization of volcanic units using detailed velocity analysis in the Atlantic Margin, West of Shetlands, United Kingdom

Abstract

The offshore Faroe-Shetlands Basin, northwest of Scotland and south of the Faroe Islands (Figure 1), has seen renewed exploration interest following development of Schiehallion, Foinaven, and Loyal fields and opening of the first licensing round of the Faroe Islands. Figure 1. Significant basalt wells, rock property studies, and key velocity inversion profiles in area of study. Interpretation of volcanic data in the area is very challenging, and many local oil and gas prospects involve clastic objectives above or below volcanic zones. High uncertainty about the thickness and morphology of these zones results in high uncertainties in structural geometry, reservoir definition, and hydrocarbon migration and charge risk. Only a few wells have penetrated these volcanic zones to date. Sonic logs from these wells and regional rock property trends for Tertiary-aged clastic sediments suggest that significant rms (root-mean-square) velocity changes occur at or near these volcanic boundaries, boundaries not easily identified with conventional 2-D or 3-D data and interpretation techniques. This case study discusses how interpretation was improved via a focus on velocity coherence analysis. Data were reprocessed using various demultiple and filtering techniques specifically designed to improve velocity coherence analysis. Emphasis was placed on identification of “bona fide” velocity coherences and associated inflection points within stacking velocities that might indicate “bulk” changes to true interval velocities. The geology of the Late Cretaceous and Early Tertiary in the vicinity of the Faroe Islands is dominated by incipient continental break-up and associated rifting between Greenland and northwest Europe. This break-up, and its association with a major mantle plume, has created one of the world's major basalt provinces. The Faroe Islands are predominantly made of these basaltic lavas, intercalated with tuff and relatively minor sedimentary rocks. Faroese lavas are broadly equivalent in age to those encountered in the British Isles, Greenland, and offshore Norway, and have …