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DATING PETROLEUM SYSTEMS Dating the Finer Aspect of Petroleum Systems Rhenium-Osmium Isotope Geochronology – Oil production at Foinaven field, West of Shetland – the Petrojarl Foinaven is an FPSO specially designed for oil production in the ultra harsh environment of the north sea and north Atlantic margin (photo: Teekay Corporation) 3.2 Age = 154.1 ± 2.1 Maa In nitial 187Os/188Os = 0.526 ± 0.023 MSWD = 4.6 2.8 DS108-05 DS108-05.2 Organic-rich sedimentary units are the typical source for hydrocarbons in many of the world’s sedimentary basins. By establishing the absolute age of source rocks it is possible to provide important chronologic constraints for the evolution of a petroleum system and thus permit the calibration of biostratigraphy and date geophysical markers. BY ALEX FINLAY, DAVID SELBY AND EDDIE MERRICK Presently, organic-rich rocks are typically constrained either through relative age methods such as biostratigraphy, chemostratigraphy, and/or the extrapolation of absolute ages produced from volcanic ash layers. However, over the last 20 years the rhenium-osmium (Re-Os) isotope geochronometer has been developed and accepted by the scientific community as a powerful tool to accurately and precisely date the depositional age of organic-rich rocks. The Re-Os systematics in source units are not disturbed by burial digenesis, hydrocarbon maturation, and even low-grade metamorphism. Importantly, in addition to Phanerozoic marine organic-rich rocks, lacustrine and PreCambrian source rocks which are typically bereft of any biostratigraphic controls can be utilised for Re-Os geochronology. As a result, 18 the Re-Os geochronometer is an ideal tool for dating the depositional age of petroleum source rock throughout the entire geological record and, therefore, furthers our ability to understand global petroleum systems. What is Re-Os Geochronology? Rhenium-Osmium geochronology is based on the β decay of 187Re to 187 Os. 187Re has a half life of ~42.5 thousand million years. The Re-Os geochronometer is dissimilar to other lithophilic radioisotope systems (e.g. Rb-Sr, U-Pb, Sm-Nd, Lu-Hf) in that both Re and Os are siderophilic. Furthermore, both Re and Os have an organophilic affinity, meaning that the elements are enriched in organic-rich sedimentary units relative to upper continental crust. This enrichment likely occurs through the sequestration of Re and Os from the water col- S C A N D I N A V I A N O I L - G A S 187 Os / 188 Os DS107-05 2.4 DS103-05 DS10 05-05 2.0 1.6 DS96-05 DS98-05,, DS102-05 400 600 187 Re/ 188 Os Gradstein et al., 2004 800 1000 d ata -p o int erro r ellip s es are 2 V Re - Os age Tithonian onian Tithonian 150.8 ± 4.0 4 0 Ma 150.8 ± 4.0 Ma 150 umn during sedimentaKimmeridgian 3.3 3 Ma tion. Under oxygen rich 5.6 Ma Kimmeridgian ridgian 154.1 ± 2.2 Ma conditions both Re and 155 Os are soluble in sea 155.6 ± 4.0 4 0 Ma water, however, under oxygen limited conditions Oxfordian rdian Oxfordian Re and Os become insoluble and, therefore, 160 organic matter in sediment becomes enriched in hydrogenous Re and Os. This means that the Os isotope composition (187Os/188Os) of organicrich sediment reflects the seawater osmium isotope Global reconstruction on - R Blakey composition at the time A Re-Os isochron formed from samof deposition. ples from the OxfordianAssuming that the 187 Os/188Os composition at the time of deposition is constant and the sample Re-Os systematics are undisturbed, the 187 Re/188Os and 187Os/188Os M A G A Z I N E N O . Kimmeridgian boundary, Skye. Re/188Os and 187Os/188Os values positively correlate and produce an age of 154.1 ±2.2 Ma. B) – a new geological time scale from the new Re-Os geochronology compared to the original demonstrating the implications of the new age to the Geological time scale (illustrations: Chemostrat) 187 3 / 4 2 0 1 2 150 155 160 DATING PETROLEUM SYSTEMS 4° W 5° W 3° W 2° W Clair Cuillin Schiehallion Foinaven t es W nd la t e Sh rm fo at l P Shetland Islands 60° N Orkney Islands 59° N Scotland West of Shetland oil fields from which oils were analysed for Re-Os geochronology At present the uncertainties associated with dating stratigraphic boundaries within the Jurassic are large, typically 4 Ma. The reason for this is that it is rare for Jurassic stratigraphic boundaries to be associated with traditionally isotopically datable volcanic units. In A study by Selby (2007) involved the analysis of eight samples of organic-rich sediment from the Staffin Fm. (Skye, U.K.), the proposed Global Stratotype Section and Point (GSSP) for the Oxfordian-Kimmeridgian boundary. This produced a Re-Os age of 154.1 ±2.2 Ma, the uncertainty reflecting a 45% (1.8 m.y.) improvement in precision for the basal Kimmeridgian. Furthermore, this age together with a recently determined 40Ar/39Ar age for the basal Berriasian suggests that the duration of the Kimmeridgian and Tithonian were nominally ~650 k.y. shorter than previously determined. As noted above it is possible to gather information about the inputs to sea water from examining the 187Os/188Os value, calculated for the time of deposition. The Os isotope composition of the analysed shales indicates that Oxfordian/Kimmeridgian seawater was radiogenic, similar to that for Age = 68 ± 13 Ma Osi = 1.05 ± 0.05 MSWD = 20 1.8 G2762 G2750 G5234 1.6 Os/188Os Case Study from the Proposed OxfordianKimmeridgian GSSP fact, it is only the base of the Hettangian that is constrained by a U-Pb zircon age (199.6 ±0.3 Ma, 2σ) from a tuff unit that lies directly below the Triassic-Jurassic boundary. Most of the stratigraphic boundaries within the Jurassic have been assigned an age through the mathematical scaling of radiometric dates, cyclic stratigraphy, linear trends in Sr isotopic 187 ratios of a given section of an organic-rich sedimentary rock will positively correlate. The slope of this correlation together with the decay constant of 187Re will yield the age of deposition of the sedimentary unit. It is possible to analyse organic-rich sediment samples from both, outcrop and core material, of varying maturity. For example, a study of overmature samples of the Exshaw Fm., Alberta, Canada produced a depositional age of 363 ±2 Ma, in direct agreement with U-Pb geochronology and biostratigraphy constraints (Selby and Creaser, 2005). variations, proportional scaling of ammonite zones, and magnetic polarity (M-sequence) time scale derived from estimates of Pacific sea floor spreading. This is where the advantage of the Re-Os chronometer becomes apparent; it is possible to date the exact stratigraphic levels that act as stage boundaries. G5306 G5307 1.4 G2763 G5271 G0124 1.2 G2851 G2749 G2075 G0123, G5400, G5401, G5402, G5403, G5404, G5399 200 Clair Cuillin 400 Schiehallion 600 Foinaven 800 187 188 Re/ Os Re-Os generation and migration age for the West of Shetland oil fields. Each sample is colour coded to demonstrate the field from where they are sourced S C A N D I N A V I A N O I L - G A S M A G A Z I N E N O . the Early Tithonian, but much more radiogenic than the Late Callovian likely as a result of increased crustal weathering. Re-Os Dating of Petroleum The Re-Os method described here is not only limited to organic-rich sediments, it can also be applied to constrain the generation age of petroleum. Petroleum maturation has been demonstrated to not observably disturb source rock ReOs systematics, likely due to the minor amounts (low parts per trillion) of Re and Os being transferred to the generated oil (Rooney et al., 2012). Both Re and Os are enriched in the asphaltene fraction relative to the whole oil and, importantly, both the Re and Os isotopic compositions of an asphaltene are similar to those of the oil it is separated from (Selby et al., 2007). Therefore it is possible to analyse the asphaltene fraction of oil (and so have larger sample Re and Os) and be confident that the Re-Os isotopic compositions reflect that of whole oil. To date all published work points towards the ages produced from Re-Os analysis of asphaltene to record the timing of hydrocarbon generation and migration. Case Study from the West of Shetland Oil Fields The West of Shetland Basin petroleum system is a rifted fault block and graben province of Tertiary to Mesozoic age running along the UK Atlantic Margin between the West Shetland platform and Faeroe Islands. A large body of geochemical and basin modelling research indicate a significant Upper Jurassic marine source with minor Middle Jurassic terrestrial input for UK Atlantic margin oils. Reservoirs are found in fractured basement, Devonian (e.g. structural trapped Clair field), Jurassic and Palaeogene sediments (for example, Stratigraphically trapped Foinaven and Schiehallion fields). At present oil generation is mod- 3 / 4 2 0 1 2 19 DATING PETROLEUM SYSTEMS elled to have occurred from the late Cretaceous through the Tertiary. To try and shed more light on which of these models is correct 18 oils from the West of Shetland petroleum system which had previously been shown to be from the same Upper Jurassic source were investigated by Re-Os geochronology (Finlay et al., 2011). This study produced a Re-Os age of 68 ±13 Ma, which is similar to that proposed from basin modelling. Importantly, this age is in agreement with 40 Ar/39Ar K-feldspar cement geochronology of the Victory Field (~24 km north east of the Clair field; Mark et al., 2005). The dated K-feldspar cements contain mixed aqueous and oil fluid inclusions. The oil from these inclusions was sourced from Late Jurassic shales, therefore, the Re-Os dates define the youngest possible timing of oil generation in the UKAM. Re-Os Geochronology: A Summary Re-Os geochronology is a powerful tool for understanding petroleum systems. Not only does it allow the depositional ages of sediments to be constrained but also the generation age of hydrocarbons. n Key References Finlay, A.J., Selby, D. and Osborne, M.J. 2011. “ReOs geochronology and fingerprinting of United Kingdom Atlantic Margin oil: Temporal implications for regional petroleum systems”. Geology, 39, 475–478. Rooney, A.D., Selby, D, Lewan, M.D., Lillis, P.G. & Houzay, J-P. (2012). “Evaluating Re-Os systematics in organic-rich sedimentary rocks in response to petroleum generation using hydrous pyrolysis experiments”. Geochimica et Cosmochimica Acta, 77: 275-291. Selby, D & Creaser, R.A 2005. “Direct radiometric dating of the Devonian-Mississippian time-scale boundary using the Re-Os black shale geochronometer”. Geology, 33(7): 545-548. Selby, D. 2007. “Direct Rhenium-Osmium age of the Oxfordian-Kimmeridgian boundary, Staffin bay, Isle of Skye, U.K., and the Late Jurassic time scale”. Norwegian Journal of Geology, 87, 291–299. Selby, D., Creaser. R.A. and Fowler, MG. (2007). “ReOs elemental and isotopic systematics in crude oils”. Geochimica et Cosmochimica Acta, 71, 378 – 386. The Authors: Alex Finlay graduated with a BSc in Geoscience from the university of St. Andrews in 2006. Dr. Finlay completed a PhD entitled “Re-Os Systematics and PGE Fingerprinting of Oils in Worldwide Petroleum Systems” at Durham University in 2010. Since then he has been conducting postdoctoral research at Durham University on Re-Os geochronology and PGE fingerprinting of oils concentrating on the Faroe-Shetland basin. David Selby has been involved in the development and application of the Re-Os isotope system since 1999. Dr. Selby’s work initially focused on the systematic development of the Re-Os molybdenite chronometer, research that now allows routine age determinations for molybdenite, which is critical for evaluating the age and genesis of metallic ore deposits worldwide and the timing of fluid flow in terranes. Further, Selby has been a pioneer in developing the Re-Os chronometer to obtain depositional ages for sedimentary rocks and the timing of petroleum generation in sedimentary basins. He has coupled this work with PGEs geochemistry to aid in the fingerprinting of an oil to its source. Eddie Merrick is currently Chemostrat’s director and CEO for the MENA Region. Previously Mr. Merrick ran his own business for 20 years before joining Chemostrat in 2004 as Business Manager. Continuing in this role, he is now responsible for the portfolio representing the MENA region. He is based in the head office in Mid Wales and coordinates travel and business development across North Africa and the Middle East. ANNONSE 20 S C A N D I N A V I A N O I L - G A S M A G A Z I N E N O . 3 / 4 2 0 1 2