Fault-charged mantle-fluid contamination of United Kingdom North Sea oils: Insights from Re-Os isotopes: Figure 1.

Abstract

187 Os/ 188 Os, as expected from a Kimmer- idge Clay Formation source. However, oils from the Viking Graben, with which the Brent fi eld is associated, and main East Shetland Basin display unradiogenic 187 Os/ 188 Os. In this study we seek to demonstrate that these anomalous results are due to interaction with a mantle-like fl uid that propagated through the main fault zone of the Viking Graben and East Shetland Basin. This study enhances our understanding of the Re-Os oil systematics, and supports cur- rent ideas regarding fault architecture, crustal thinning, and associated fl uid fl ow and interac- tion in the North Sea oil system. ABSTRACT United Kingdom North Sea oils sourced from the Kimmeridge Clay Formation contain unradiogenic (~0.17 to ~0.48) and radiogenic (~1.04 to ~3.34) 187 Os/ 188 Os values. The unra- diogenic 187 Os/ 188 Os values are spatially associated with the main basin-bounding faults of the Viking Graben and East Shetland Basin. In contrast, the radiogenic 187 Os/ 188 Os values are associated with North Sea basins located farther away from the basin boundary faults. We suggest that crustal thinning and strain localization within the Viking Graben and East Shetland Basin are suffi cient to have allowed basin-bounding faults to propagate to suffi cient depth to act as conduits for mantle-derived fl uids to interact with oil. This hypothesis is sup- ported by previous geochemical data for North Sea oil fi elds. Furthermore, we demonstrate the use of Os isotopes as an important tracker of crustal-scale fl uid dynamics and petroleum migration pathways in extensional basins.