DATING PETROLEUM SYSTEMS Rhenium-Osmium Isotope Geochronology – Dating the Finer Aspect Oil production at Foinaven field, of Petroleum SystemWest of Shs etland – 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 Initial 187Os/188Os = 0.526 ± 0.023 MSWD = 4.6 DS108-05 2.8 DS108-05.2 DS107-05 s 2.4 O 8 DS103-05 8 1 / s O DS1005-05 7 8 2.0 Organic-rich sedimentary units are the typical source for hydrocarbons 1 in many of the world’s sedimentary basins. By establishing the absolute 1.6 DS96-05 age of source rocks it is possible to provide important chronologic DS98-05,, DS102-05 400 600 800 1000 constraints for the evolution of a petroleum system and thus permit the 187 Re/ 188 Os data -point error ellipses are 2 V calibration of biostratigraphy and date geophysical markers. Gradstein et al., 2004 Re - Os age 150 Tithonianonian Tithonian 150 BY ALEX FINLAY, DAVID SELBY AND EDDIE MERRICK 150.8 ± 4.04 0 Ma 150.8 ± 4.0 Ma Presently, organic-rich rocks are the Re-Os geochronometer is an umn during sedimenta - 3.33 Ma Kimmeridgian typically constrained either ideal tool for dating the deposi - tion. Under oxygen rich Kimmeridgianridgian 5.6 Ma through relative age methods such tional age of petroleum source conditions both Re and 154.1 ± 2.2 Ma as biostratigraphy, chemostratigra - rock throughout the entire geolog - Os are soluble in sea 155 155 155.6 ± 4.04 0 Ma phy, and/or the extrapolation of ical record and, therefore, furthers water, however, under absolute ages produced from vol - our ability to understand global oxygen limited conditions canic ash layers. However, over the petroleum systems. Re and Os become insol - Oxfordianrdian Oxfordian last 20 years the rhenium-osmium uble and, therefore, 160 (Re-Os) isotope geochronometer What is Re-Os organic matter in sedi - 160 has been developed and accepted Geochronology? ment becomes enriched by the scientific community as a Rhenium-Osmium geochronology in hydrogenous Re and powerful tool to accurately and is based on the β decay of 187 Re to Os. This means that the precisely date the depositional age 187 Os. 187 Re has a half life of ~42.5 Os isotope composition of organic-rich rocks. thousand million years. The Re-Os (187 Os/ 188 Os) of organic- geochronometer is dissimilar to rich sediment reflects the The Re-Os systematics in source other lithophilic radioisotope sys - seawater osmium isotope Global reconstructionon - R Blakey units are not disturbed by burial tems (e.g. Rb-Sr, U-Pb, Sm-Nd, composition at the time A Re-Os isochron formed from sam - digenesis, hydrocarbon matura - Lu-Hf) in that both Re and Os are of deposition. ples from the Oxfordian- tion, and even low-grade meta - siderophilic. Furthermore, both Re Kimmeridgian boundary, Skye. morphism. Importantly, in addi - and Os have an organophilic affin - Assuming that the 187 Re/ 188 Os and 187 Os/ 188 Os values posi - tion to Phanerozoic marine organ - ity, meaning that the elements are 187 Os/ 188 Os composition at tively correlate and produce an age of ic-rich rocks, lacustrine and Pre- enriched in organic-rich sedimen - the time of deposition is 154.1 ±2.2 Ma. B) – a new geological Cambrian source rocks which are tary units relative to upper conti - constant and the sample time scale from the new Re-Os geochronology compared to the origi - typically bereft of any biostrati - nental crust. This enrichment like - Re-Os systematics are nal demonstrating the implications graphic controls can be utilised for ly occurs through the sequestration undisturbed, the of the new age to the Geological time Re-Os geochronology. As a result, of Re and Os from the water col - 187 Re/ 188 Os and 187 Os/ 188 Os scale (illustrations: Chemostrat) 18 SCANDINAVIAN OIL-GAS MAGAZINE NO. 3/4 2012 DATING PETROLEUM SYSTEMS 5° W 4° W 3° W 2° W variations, proportional scaling of the Early Tithonian, but much ammonite zones, and magnetic more radiogenic than the Late polarity (M-sequence) time scale Callovian likely as a result of Clair derived from estimates of Pacific increased crustal weathering. sea floor spreading. This is where Cuillin the advantage of the Re-Os Re-Os Dating of Petroleum rm chronometer becomes apparent; it The Re-Os method described here Schiehallion o tf la is possible to date the exact strati - is not only limited to organic-rich Foinaven P d graphic levels that act as stage sediments, it can also be applied to an tl e h Shetland boundaries. constrain the generation age of S 60° N st e Islands petroleum. Petroleum maturation W A study by Selby (2007) involved has been demonstrated to not the analysis of eight samples of observably disturb source rock Re- organic-rich sediment from the Os systematics, likely due to the Staffin Fm. (Skye, U.K.), the pro - minor amounts (low parts per tril - posed Global Stratotype Section lion) of Re and Os being trans - Orkney and Point (GSSP) for the ferred to the generated oil (Rooney Islands Oxfordian-Kimmeridgian bound - et al., 2012). Both Re and Os are ary. This produced a Re-Os age of enriched in the asphaltene fraction 59° N 154.1 ±2.2 Ma, the uncertainty relative to the whole oil and, reflecting a 45% (1.8 m.y.) importantly, both the Re and Os improvement in precision for the isotopic compositions of an basal Kimmeridgian. Furthermore, asphaltene are similar to those of this age together with a recently the oil it is separated from (Selby Scotland determined 40Ar/39Ar age for the et al., 2007). Therefore it is possi - West of Shetland oil fields from which oils were analysed for basal Berriasian suggests that the ble to analyse the asphaltene frac - Re-Os geochronology duration of the Kimmeridgian and tion of oil (and so have larger ratios of a given section of an fact, it is only the base of the Tithonian were nominally ~650 sample Re and Os) and be confi - organic-rich sedimentary rock will Hettangian that is constrained by a k.y. shorter than previously deter - dent that the Re-Os isotopic com - positively correlate. The slope of U-Pb zircon age (199.6 ±0.3 Ma, mined. As noted above it is possible positions reflect that of whole oil. this correlation together with the 2σ) from a tuff unit that lies to gather information about the To date all published work points decay constant of 187 Re will yield directly below the Triassic-Jurassic inputs to sea water from examin - towards the ages produced from the age of deposition of the sedi - boundary. Most of the stratigraphic ing the 187 Os/ 188 Os value, calculated Re-Os analysis of asphaltene to mentary unit. It is possible to boundaries within the Jurassic for the time of deposition. The Os record the timing of hydrocarbon analyse organic-rich sediment have been assigned an age isotope composition of the generation and migration. samples from both, outcrop and through the mathematical scaling analysed shales indicates that core material, of varying maturity. of radiometric dates, cyclic stratig - Oxfordian/Kimmeridgian seawater Case Study from the West For example, a study of overma - raphy, linear trends in Sr isotopic was radiogenic, similar to that for of Shetland Oil Fields ture samples of the Exshaw Fm., The West of Shetland Basin petro - Alberta, Canada produced a depo - leum system is a rifted fault block sitional age of 363 ±2 Ma, in Age = 68 ± 13 Ma G2762 and graben province of Tertiary to 1.8 Os = 1.05 ± 0.05 direct agreement with U-Pb i G2750 Mesozoic age running along the geochronology and biostratigraphy MSWD = 20 UK Atlantic Margin between the G5234 1.6 West Shetland platform and Faeroe constraints (Selby and Creaser, G5306 2005). Islands. A large body of geochemi - G5307 s cal and basin modelling research O 1.4 G2763 8 8 Case Study from the 1 indicate a significant Upper / G0124 G5271 Proposed Oxfordian- s Jurassic marine source with minor O 7 1.2 8 G2851 Kimmeridgian GSSP 1 G2749 Middle Jurassic terrestrial input for At present the uncertainties associ - G2075 UK Atlantic margin oils. Reservoirs ated with dating stratigraphic G0123, G5400, G5401, are found in fractured basement, G5402, G5403, G5404, boundaries within the Jurassic are G5399 Devonian (e.g. structural trapped large, typically 4 Ma. The reason 200 400 600 800 Clair field), Jurassic and for this is that it is rare for Jurassic Clair Cuillin Schiehallion Foinaven 187Re/188Os Palaeogene sediments (for exam - stratigraphic boundaries to be Re-Os generation and migration age for the West of Shetland ple, Stratigraphically trapped associated with traditionally iso - oil fields. Each sample is colour coded to demonstrate the Foinaven and Schiehallion fields). topically datable volcanic units. In field from where they are sourced At present oil generation is mod - SCANDINAVIAN OIL-GAS MAGAZINE NO. 3/4 2012 19 DATING PETROLEUM SYSTEMS elled to have occurred from the late only does it allow the depositional ages of The Authors: Cretaceous through the Tertiary. sediments to be constrained but also the generation age of hydrocarbons. n Alex Finlay graduated with a BSc in Geoscience from the university of To try and shed more light on which of St. Andrews in 2006. Dr. Finlay completed a PhD entitled “Re-Os these models is correct 18 oils from the Systematics and PGE Fingerprinting of Oils in Worldwide Petroleum West of Shetland petroleum system which Systems” at Durham University in 2010.