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

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 unradiogenic 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 supported 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.

Evaluating Re–Os systematics in organic-rich sedimentary rocks in response to petroleum generation using hydrous pyrolysis experiments

“…Crude oil generated from ORS through the maturation of organic matter is enriched in Re and Os, which has been transferred from source rocks (Barre et al, 1995;Woodland et al, 2001;Selby and Creaser, 2005b;Selby et al, 2007;Finlay et al, 2010). Applying Re-Os geochronology directly to crude oil has yielded ages that are in excellent agreement with basin models and are suggested to constrain petroleum generation events (Selby et al, 2005;Selby and Creaser, 2005a;Finlay et al, 2011a).…”

“…This assumption can be considered to be valid as Re and Os in ORS have been shown to be hydrogenous in origin and are complexed into organic matter under reducing conditions (Ravizza and Turekian, 1989;Ravizza et al, 1991;Colodner et al, 1993;Ravizza and Esser, 1993;Cohen et al, 1999;Morford and Emerson, 1999;Morford et al, 2005). Studies working on source rocks and crude oil products have also recorded enrichments of Re and Os in organic matter (Selby andCreaser, 2003, 2005a;Selby et al, 2005Selby et al, , 2007Finlay et al, 2010Finlay et al, , 2011a. This assumption also holds for the bitumen extracted from the whole rocks as chloroform will only extract the soluble organic matter upon which the Re and Os determinations of bitumen are conducted.…”

“…As such, the Re-Os ORS geochronometer records the timing of sediment deposition and not the age of post-depositional events. Although current evidence suggests that the ReOs system is not adversely affected by these events, we do know that natural crude oils are enriched in Re and Os that have been transferred from the source rock (Selby et al, 2005Selby and Creaser 2005b;Finlay et al, 2010Finlay et al, , 2011b. However, we have a very limited understanding of the mechanisms and magnitude of elemental transfer from a source rock to the generated crude oil.…”

“…The lack of Re and Os in expelled oil from the hydrous pyrolysis experiments is unexpected from the standpoint that these metals are observed in natural crude oils (Woodland et al, 2001;Selby and Creaser, 2005a;Selby et al, 2007;Finlay et al, 2010Finlay et al, , 2011a and have been measured in crude oils from Phosphoria source rocks (Lillis and Selby, 2010). Other experimental pyrolysis results using Woodford Shale with Type II kerogen also report a lack of Re and Os partitioning into oils from source rocks (Reisberg et al, 2008).…”

“…This work was carried out concomitantly with work by Rooney et al (2010) and Finlay et al (2010) with the standard data reported below identical, within uncertainty. The Re standard analysed during the course of this study is made from 99.999% zone-refined Re ribbon and is considered to be identical to that of AB-1 (Creaser et al, 2002;Selby and Creaser, 2003;Kendall et al, 2004).…”