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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,
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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
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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
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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-
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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
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