Petroleum Source Rock Identification of United Kingdom Atlantic Margin Oil
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Earth and Planetary Science Letters 313–314 (2012) 95–104 Contents lists available at SciVerse ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl Petroleum source rock identification of United Kingdom Atlantic Margin oil fields and the Western Canadian Oil Sands using Platinum, Palladium, Osmium and Rhenium: Implications for global petroleum systems Alexander J. Finlay a,⁎, David Selby a, Mark J. Osborne b a Department of Earth Sciences, Durham University, Science Laboratories, Durham, DH1 3LE, UK b BP, Exploration & Production Technology, Building H, Sunbury-on-Thames, Middlesex, TW16 7LN, UK article info abstract Article history: This study demonstrates that petroleum and source rocks are enriched in Pt and Pd to the ppb level, and that Received 15 June 2011 the 187Os/188Os composition coupled with the Pt/Pd value permits the fingerprinting of petroleum to its Received in revised form 4 November 2011 source. Oils from the United Kingdom Atlantic Margin (sourced from the Upper Jurassic Kimmeridge Clay Accepted 5 November 2011 Fm.) as well as source rock samples have been analysed for Pt and Pd. When the Pt/Pd value is compared Available online xxxx 187 188 with Os/ Os (calculated at the time of oil generation; Osg) the values from both the known source and fi Editor: T.M. Harrison the oils are similar, demonstrating that they can be used as an oil to source ngerprinting tool. This inorganic petroleum fingerprinting tool is particularly important in heavily biodegraded petroleum systems where tra- Keywords: ditional fingerprinting techniques (e.g. biomarkers) are severely hampered, e.g. the world's largest oil sand Platinum deposit, the West Canadian Oil Sands (WCOS). This has caused the source of the WCOS to be hotly debated, Palladium with no present day consensus between inputs from potential source units e.g. Exshaw and Gordondale Fms. Rhenium 187Os/188Os and Pt/Pd fingerprinting of the oil sands shows that the majority of the petroleum have similar Osmium 187Os/188Os and Pt/Pd values, supporting the hypothesis of one principal source. Analysis of the potential Petroleum source rocks establishes that the principal source of the oil sands to be from the Jurassic Gordondale Fm., Source rock with a minor Exshaw Fm. input. Thus, the combination of previously pioneered Re–Os petroleum geochronology with 187Os/188Os and Pt/Pd values of petroleum permits both a temporal and spatial understanding of petroleum systems. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Zhou et al., 2008). However, mass balance calculations rule out a sin- gle Exshaw Fm. source because of its limited stratigraphic extent Biodegradation preferentially removes light hydrocarbons from (2–12 m thickness therefore only forming 8.6% of total reserves; petroleum and so can cause traditional oil to source fingerprinting Creaney and Allan, 1990). Furthermore, mass balance calculations techniques to be challenged (e.g. Biomarkers and Carbon isotopes; suggest up to four units from the Western Canadian Sedimentary Peters et al., 2005 and references therein). One such example is the Basin could have formed/contributed to the WCOS. These are, in West Canadian Oil Sands (WCOS), which is the world's largest oil order of importance, the Lower Jurassic Gordondale Fm. (previously sand reserve (27.6 billion m3; Alberta's Energy, 2005; Fig. 1). This the Nordegg Fm.; Asgar-Deen et al., 2004), the Devonian–Mississippian means that, despite decades of research, the source of the WCOS re- Exshaw Fm., and the Middle Triassic Doig and Upper Devonian mains controversial (Allan and Creaney, 1991; Barson et al., 2000; Duvernay Fms. (Allan and Creaney, 1991; Creaney and Allan, 1990, Creaney and Allan, 1990, 1992; Fowler et al., 2001; Higley et al., 1992; Higley et al., 2009). Although recent mathematical models sug- 2009; Riediger et al., 2000; Zhou et al., 2008). Although the WCOS gest the Gordondale Fm. to be the dominant source of the WCOS, it are biodegraded (biomarker biodegradation scale of between 4 has been debated whether oil volumes generated from the Gordondale [Peace River] and 8 [Athabasca]; Peters et al., 2005 and references Fm. could have been sufficient to have migrated ~500 km to the WCOS therein; Zhou et al., 2008; Fig. 1) it is hypothesised that biomarker reservoirs because migration would have been hampered by strati- preservation is sufficient to propose the Exshaw Fm. as the principal graphic barriers (Riediger, 1994). source (Barson et al., 2000; Fowler et al., 2001; Riediger et al., 2000; To identify the source of the WCOS this study utilises organophyllic Rhenium (Re), Osmium (Os), Platinum (Pt) and Palladium (Pd). We demonstrate that, like Re and Os, shales and asphaltene are enriched ⁎ Corresponding author. in Pt and Pd relative to upper continental crust (UCC). We then demon- 187 188 E-mail address: a.j.fi[email protected] (A.J. Finlay). strate that the Os/ Os composition at the time of generation (Osg) 0012-821X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2011.11.003 96 A.J. Finlay et al. / Earth and Planetary Science Letters 313 (2012) 95–104 Alberta 120° 118° 116° 114° 112° 110° 5° W 4° W 3° W 2° W 58° Flett Ridge Athabasca Westray Clair 57° Peace Ridge River Grosmont Cuillin Rona Ridge 56° Wabasca Foinaven Schiehallion tform 55° Shetland Islands 60° N Cold 54° Lake West Shetland Pla Edge of foothills Lloydminster Orkney belt Islands Provost Tar Sand 59° N Heavy Oil Black = Overmature Gordondale sample Grey = Mature Exshaw sample White = Immature Period Geological event Tectonic event Period Geological event Tectonic event North Oil Generation Cordilleran Atlantic (~112 Ma Re-Os rifting Cret age) Orogenic Event Foreland Palaeogene Neogene Basin Jurassic source become mature Central Laramide (~68 Ma Re-Os age) Jur. Orogenic Atlantic Gordondale Fm. Event rifting deposition Tri. Perm. Cretaceous Carb. Exshaw Fm. “Pacific” U. deposition Margin Dev. Duvernay Fm. Deep marine source deposition M. U. Jur. unit deposition Dev. (Kimmeridge Clay Fm.) Fig. 1. Location map and geochronological summary of the West Canadian Oil Sands and United Kingdom Atlantic Margin oil fields. Abbreviations; Sk = Saskatchewan; AB = Al- berta; BC = British Columbia (modified from Finlay et al., 2010 and references therein; Selby and Creaser 2005; Creaser et al., 2002; Riediger et al., 1994). Re–Os oil generation age for West Canadian Oil Sands from Selby and Creaser 2005,Re–Os oil generation age of United Kingdom Atlantic Margin oil from Finlay et al. (2010). A.J. Finlay et al. / Earth and Planetary Science Letters 313–314 (2012) 95–104 97 and Pt/Pd values can be used to fingerprint petroleum to its source unit by Finlay et al., 2010). Furthermore 3 samples of the proven UKAM through the analysis of United Kingdom Atlantic Margin (UKAM) oils Kimmeridge Clay Fm. equivalent shale (Clair oil field, well 205/22-1A; and samples of the proven Upper Jurassic source (Kimmeridge Clay Fig. 1; Finlay et al., 2011) were analysed for PGE and Re–Os abundances Fm.; Finlay et al., 2011 and references therein). Furthermore we demon- as well as Re–Os isotopic compositions (Table 1). strate that this method is unaffected by biodegradation. As a result, this novel inorganic fingerprinting method has widespread application to 2.2. West Canadian Oil Sands global petroleum systems. At present there is no consensus on the source of the WCOS, 2. Materials and methods however, the Upper Devonian Duvernay, Devonian/Mississippian Exshaw and Lower Jurassic Gordondale Fms. have all been proposed 2.1. United Kingdom Atlantic Margin as possible source units (e.g., Higley et al., 2009 and references therein). Therefore, fifteen samples of these units were analysed as part of this The UKAM oils are found in a series Devonian to Miocene rift basins study (Table 1). situated between the Shetland Platform and Faroe Islands (Spencer et Two core samples of Upper Devonian Duvernay Fm. were analysed al., 1999; Carruth, 2003; Fig. 1). Two main phases of rifting occurred for PGE and Re abundances as well as Re–Os isotopic composition during the Early Cretaceous and Late Cretaceous/Paleocene forming (Table 2; Fig. 1). The Duvernay Fm. is comprised of two interbedded local depocentres. Reservoirs are found in Devonian/Carboniferous sed- lithofacies controlled by bottom water oxic/anoxia: bioturbated, iments and fractured, weathered basement (e.g., structural trapped nodular, lime mudstones, deposited under oxic conditions and Clair field; Carruth, 2003) and Jurassic and Paleogene sediments (e.g., unbioturbated bituminous mudstones, deposited under deep water stratigraphically trapped Foinaven and Schiehallion fields; Spencer et anoxic conditions. The bituminous mudstones form the oil source in- al., 1999). The petroleum samples selected for this study are sourced tervals and vary from being mature to the south-west to immature in from the Upper Jurassic Kimmeridge Clay Fm. (Finlay et al., 2011). The the north-east (cf. Creaney and Allan 1990; Cioppa et al., 2002). δ13C values of whole oil, saturate and aromatic fractions are almost The PGE abundances of seven samples of regional Exshaw Fm. pre- identical (0.1–0.2‰ Standard Deviation [S.D.]) as well as being within viously utilised for Re–Os analysis were determined (Creaser et al., the range of North Sea oils sourced from the Kimmeridge Clay Fm. (cf. 2002; Selby and Creaser, 2003; Table 2). The Exshaw Fm. formed Fig. 2 of Finlay et al., 2011). In the same figure it is shown that the iso- during a Devonian/Mississippian marine transgression and varies in prenoid and C27,C28,andC29 sterane values both suggest that the oils thickness from 2 to 16 m (Creaney and Allan 1990; Creaser et al., were sourced from 1 single marine planktonic-bacterial source 2002). The Exshaw Fm. comprises two lithofacies, a stratigraphically with some terrigenous material, as expected from the Kimmeridge lower Devonian/Mississippian organic-rich member (TOCb20%) and Clay Fm.