Oil and Water – Intimate Conversations

Rhenium–osmium (Re–Os) geochronology of marine petroleum systems has allowed the determination of the depositional age of source rocks as well as the timing of petroleum generation. In addition, Os isotopes have been applied as a fingerprinting tool to correlate oil to its source unit. To date, only classic marine petroleum systems have been studied. Here we present Re–Os geochronology and Os isotope fingerprinting of different petroleum phases (oils, tar sands and gilsonite) derived from the lacustrine Green River petroleum system in the Uinta Basin, USA. In addition we use an experimental approach, hydrous pyrolysis experiments, to compare to the Re–Os data of naturally generated petroleum in order to further understand the mechanisms of Re and Os transfer to petroleum.The Re–Os geochronology of petroleum from the lacustrine Green River petroleum system (19±14Ma – all petroleum phases) broadly agrees with previous petroleum generation basin models (∼25Ma) suggesting that Re–Os geochronology of variable petroleum phases derived from lacustrine Type I kerogen has similar systematics to Type II kerogen (e.g., Selby and Creaser, 2005a,b; Finlay et al., 2010). However, the large uncertainties (over 100% in some cases) produced for the petroleum Re–Os geochronology are a result of multiple generation events occurring through a ∼3000-m thick source unit that creates a mixture of initial Os isotope compositions in the produced petroleum phases. The 187Os/188Os values for the petroleum and source rocks at the time of oil generation vary from 1.4 to 1.9, with the mode at ∼1.6. Oil-to-source correlation using Os isotopes is consistent with previous correlation studies in the Green River petroleum system, and illustrates the potential utility of Os isotopes to characterize the spatial variations within a petroleum system.Hydrous pyrolysis experiments on the Green River Formation source rocks show that Re and Os transfer are mimicking the natural system. This transfer from source to bitumen to oil does not affect source rock Re–Os systematics or Os isotopic compositions. This confirms that Os isotope compositions are transferred intact from source to petroleum during petroleum generation and can be used as a powerful correlation tool. These experiments further confirm that Re–Os systematics in source rocks are not adversely affected by petroleum maturation. Overall this study illustrates that the Re–Os petroleum geochronometer and Os isotope fingerprinting tools can be used on a wide range of petroleum types sourced from variable kerogen types.

Provenance studies of produced water are essential to trace flow dynamics and reservoir compartmentalization in petroleum systems and to quantify fluid recovery rates from unconventional fracturing. Produced water from a hydraulically fractured well in the Qusaiba Hot Shale in the Northern Exploration Area, Saudi Arabia, was daily monitored and analyzed for water chemistry, and environmental (δ2H, δ13C, δ18OH2O, δ18OSO4, δ34SSO4, δ37Cl, 87Sr/86Sr) and cosmogenic isotopes (3H, 14C, 36Cl), to differentiate from reference fluids of supply water, fracturing fluids, and formation water from adjacent Paleozoic units. Initially, recovered water is composed of fracturing fluids and subsequently replaced by a homogeneous cut of pristine formation water. Formation water is composed of dominant meteoric water (approximately 84 vol%) and minor fossil evaporated seawater. The young 14C‐apparent age between 6000 and 6700 years BP and depleted δ18O/δ2H values for the meteoric component confirm the infiltration of surface water into the Qusaiba Hot Shale interval or adjacent units during the Early Holocene Pluvial Period under cooler and wetter climatic conditions than present, which suggest the presence of a very recent, dynamic hydraulic flow system. 36Cl/Cl ratios between 102 × 10−15 and 31 × 10−15 are ambiguous and can be attributed to atmospheric recharge close to the coast, mixing of 36Cl‐enriched Quaternary meteoric recharge with 36Cl‐depleted fossil seawater, and/or hypogene production by U‐Th‐enriched host rock. Produced waters from Qusaiba Hot Shale are within the compositional range of Na‐Cl‐type formation water from Paleozoic reservoir units in northern Saudi Arabia with salinities from 30 000 to 130 000 mg l−1. As a novel technological approach for exploration wells in Northern Saudi Arabia, multi‐isotopic methods were successfully implemented to quantify flowback volumes from hydraulic fracturing, and to fingerprint pristine formation water or pore water in Paleozoic systems on their provenance, residence time, migration pathways, and secondary alteration processes.

Just a few years ago, the petroleum industry relied almost solely on lithostratigraphy, biostratigraphy, and U-Pb dating of volcanic tuffs and flows for correlations across basins in the sedimentary record. Re-Os technology was first applied to syn-sedimentary pyrite to obtain depositional ages for sedimentary rocks. Subsequently, organic-rich shales with their anomalous metal contents became the target for direct dating of source rock deposition. If organic material extracted from a source rock provides depositional ages, so too should the creation of new hydrocarbon be datable. Precise dating of organic-rich shales using Re-Os has immediate applications for hydrocarbon exploration by placing time pins at intervals of earth crises marked by anoxia and euxinia. These are the events that create rich source rock. In addition, unlike other geochronometers, Re-Os isochron ages carry key information on environmental conditions during source rock deposition through trace metal concentrations, stable isotope compositions, and the Os initial ratio. Trace metal abundances reflect the extent of anoxia and sedimentation rates, both of which impact concentration and preservation of organic material. Stable isotope compositions reflect the type of organic material. The Os initial ratio is a powerful proxy for marine and lacustrine conditions during shale deposition - for example: What was eroding and how fast? What are the depositional rates for accumulation of organic-rich material? What kinds of fluids were present and mixing in a basin and for how long? The nuances derived from this kind of information are far-reaching. For shales at maturation, with some or all of the generated hydrocarbons still residing in the shale (unconventional hydrocarbon), the timing of maturation can be teased from the Re-Os data. For conventional migrated hydrocarbon, good scientific interpretation requires the creativity to analyze players along migration paths before the reservoir is reached. We will present an accessible and understandable overview of Re-Os geochemistry and geochronology in the context of case studies. The field is new and the potential is enormous for petroleum exploration. Introduction The rhenium (Re) - osmium (Os) isotope system is applicable for geochronology and tracer studies of organic-rich shales because both Re and Os are fixed by reduction in organic matter at or below the sediment-water interface in anoxic environments. Parent isotope 187Re decays to daughter 187Os with a long half-life of 46.1 Gy (?187Re = 1.666×10-11 yr-1[1]); thus the Re-Os isotopic system can provide highly precise depositional ages for organic-rich shales from Archean to Cenozoic. In addition, shales acquire the 187Os/188Os ratio of contemporaneous seawater or lake water in lacustrine settings. Given the short residence time of Os in seawater (10 to 60 Ky[2]), short-term fluctuations in 187Os/188Os in the water column are captured in the sedimentary record. The 187Os/188Os ratio of the source rock at the time of deposition thus serves as a fingerprint of the paleoenvironment during deposition of organic matter, providing a starting point for tracking produced hydrocarbons. Caution is required in restricted environments, however, as the short residence time also captures local compositional variations in Os input where mixing with open oceans may be limited or episodic[3].

Evaluation of Re–Os geochronology and Os isotope fingerprinting of Late Cretaceous terrestrial oils in Taranaki Basin, New Zealand

The Re–Os geochronometer has been applied to many petroleum systems worldwide. However, it is debated if the Re–Os systematics in petroleum actually record the timing of oil generation. Here, we investigate the Re–Os isotope systematics of the Duvernay petroleum system in the Western Canada sedimentary basin, which has been shown to be a relatively simple petroleum system that is associated with oil generated during the Late Cretaceous–early Eocene Laramide orogeny from a single source. The organic geochemistry of the Duvernay oils (pristane/phytane ratios of ∼1.5, smooth homohopane profile, C29 > C27 > C28 regular sterane distribution, and predominance of diasteranes over regular steranes) strongly suggests the oil source to be that of type I–II marine organic matter of the Upper Devonian Duvernay Formation. The asphaltene fraction Re–Os isotope data of the Duvernay oil yield an age of 66 ± 31 Ma, which is in excellent agreement with the timing of the main-stage hydrocarbon generation of the Duvernay Formation based on basin models. Further, the similarity between the 187Os/188Os compositions of the Duvernay Formation source rock (0.46–1.48) and the oil (0.55–1.06) at the time of oil generation supports the hypothesis that the 187Os/188Os composition of an oil is inherited from the source unit at the time of oil generation and therefore shows no, or limited, influence from interaction with basin fluids. This study supports that the Re–Os isotope systematics of an oil can yield the timing of oil generation and be used to trace its source.

Sequence stratigraphy and petrographic characteristics of Oligocene sediments in the West trough of the Cuu Long Basin, Vietnam

A very large database of formation water geochemistry has been acquired for the Kuwait petroleum system. This database makes it possible to explore the geological history of Kuwait’s formation and ground waters in a way that is not possible by other means. Working at this regional scale spatial and formational variation in groundwater chemistry in Kuwait identifies lithostratigraphy and physical barriers to flow that can be explored in the context of Kuwait’s geological history. Cretaceous oil reservoirs in north Kuwait have formation water saturated with Na (>50%) that is anomalously abnormal in salinity for its depth. The most likely subsurface source of such saline water is the Upper Jurassic Gotnia Formation which comprises beds of evaporite halite, and/or pre-Gotnia formations of deeper reservoirs. To reach their current location, both parts of the Gotnia and Hith formations must have been breached. To date, this is the only evidence to show that the sealing capacity of the Gotnia Formation has been overcome, implying that the formation, and therefore units beneath, are capable of sourcing fluids to younger and shallower reservoirs. Given the ubiquity of samples of formation water in many other regions, applying formation water geochemistry to other petroleum systems would appear a good method for evaluating basin models and understanding of fluid migration in mature petroleum provenances.

Chemistry and isotopic composition of Rotliegend and Upper Carboniferous formation waters from the North German Basin

The syn-tectonic breccia-hosted Mount Isa Cu deposit in northwest Queensland is the largest sediment-hosted Cu deposit in Australia. Whole-rock samples of chalcopyrite-rich Cu ore form an isochron with a Re–Os age of 1,372 ± 41 Ma. This age is more than 100 Ma younger than the previously accepted age of Cu ore formation, an Ar–Ar mineral age for biotite separated from the host rocks within the alteration envelope to the Cu orebody. This discrepancy cannot be unequivocally resolved due to a lack of other absolute geochronological constraints for Cu mineralisation or the deformation event associated with Cu emplacement. The 1,372 ± 41 Ma date may reflect (a) the time of Cu deposition, (b) the time of a hydrothermal event that reset the Re–Os signature of the Cu ore or (c) mixing of the Re–Os isotope systematics between the host rocks and Cu-bearing fluids. However, a range of published Ar–Ar and Rb–Sr dates for potassic alteration associated with Cu mineralisation also records an event between 1,350 and 1,400 Ma and these are consistent with the 1,372 Ma Re–Os age. The 1.8 Ga Eastern Creek Volcanics are a series of tholeiitic basalts with a primary magmatic Cu enrichment which occur adjacent to the Mount Isa Cu deposit. The whole-rock Os isotopic signature of the Eastern Creek Volcanics ranges from mantle-like values for the upper Pickwick Member, to more radiogenic/crustal values for the lower Cromwell Member. The Re–Os isotope signature of the Cu ores overlaps with those calculated for the two volcanic members at 1,372 Ma; hence, the Os isotope data are supportive of the concept that the Os in the Cu ores was sourced from the Eastern Creek Volcanics. By inference, it is therefore postulated that the Eastern Creek Volcanics are the source of Cu in the Mount Isa deposit, as both Os and Cu are readily transported by oxidised hydrothermal fluids, such as those that are thought to have formed the Cu orebody. The Pickwick Member yields a Re–Os isochron age of 1,833 ± 51 Ma, which is within error of previously reported age constraints. The initial 187Os/188Os isotopic ratio of 0.114 ± 0.067 (γOs = −0.7) is slightly subchondritic, and together with other trace element geochemical constraints, is consistent with a subcontinental lithospheric mantle source. The Pickwick Member records a minimum age of ca. 1.95 Ga for melt depletion in the subcontinental lithospheric mantle beneath the Mount Isa Inlier prior to the extraction of the magmas which formed the Eastern Creek Volcanics. This corresponds with the end of subduction-related magmatism along the eastern margin of the Northern Australian Craton, which included the Mount Isa Inlier.

Summary In the saline formation water environment of the Middle East, TDT* thermal decay time logging has proved very reliable for the detection of formation water level movement. This technique has also been widely used to monitor injection water front, in cases of saline injection water. However, many injection schemes in the area are now using relatively fresh sea water, rather than saline formation water, as the displacing fluid. In these cases, thermal decay time logs are very difficult to interpret due to the lack of contrast in the capture cross section between oil and injection water. TDT logs have also been widely used for gas detection behind casing, and in some favorable situations, a reliable gas saturation can be obtained from the TDT porosity. A new type of slim induced gamma ray tool, which measures carbon and oxygen and can be run through completion tubing, was introduced recently. The RST* Reservoir Saturation Tool, which is run inside the casing below the wireline entry guide, does not require killing the well ad removing the completion before logging. This means that data can be acquired with the well flowing. The main measurement is the carbon/oxygen ratio (COR), which can be used to compute oil and water saturation, independent of water salinity. The tool also provides a measurement of capture cross section, and therefore waiter salinity analysis can be performed in case of complex mixing of formation and injection water. Finally, the RST tool also provides a cased-hole neutron porosity, and can therefore be used for gas evaluation. This paper presents a new technique to simultaneously use carbon/oxygen ratio, capture cross section and TDT/RST porosity for evaluating the respective saturations of oil, gas, fresh water and saline water when they are found together in the reservoir. An example is used to illustrate the technique and its successful application in a typical reservoir from the Middle East. 1. Introduction TDT campaigns are conducted yearly in the Middle East, to monitor the year-by-year changes in fluid saturation in the reservoirs. More widespread use of this measurement has been hampered by difficulties in interpreting the results in some specific conditions. Well-known adverse effects are the so-called "filtrate effect" and "acid effect". Recently, however, one additional difficulty has further complicated TDT interpretation. Many water injection schemes use sea water as the displacing fluid. Sea water has a capture cross section of about 35 c.u., which is much closer to the capture cross section of oil than to that of formation water. Therefore, if oil is displaced by injection water, the change in sigma is very small. Very often, injection water displaces both oil and formation water, possibly leading to a reduction in sigma reading and an apparent decrease in water saturation. Even when sigma increases, only a very qualitative evaluation can be performed. An algorithm that accounts for the presence of both formation water and injection water has been used with some success, but it is based on the assumption, which is not always valid, that the original irreducible formation water has not been displaced by invasion water. The TDT tool is also used for gas monitoring. This is achieved by simultaneously using TDT porosity and sigma measurement. Although porosity measured by the TDT tool is affected by many parameters related to the surrounding environment, tubing packer, casing, cement, etc., a reliable gas detection and acceptable gas saturation computation can be performed whenever gas-free zones are available for calibration in the same environment.

We demonstrate that the Re-Os system can be used to understand the temporal evolution of a petroleum system as well as aid in the identification of source units. Traditional geochemical methods (high-performance liquid chromatography, carbon isotope, gas chromatography, and biomarker analysis) indicate that the main source of 18 oils analyzed for Re-Os geochronology from the United Kingdom Atlantic margin (Clair, Schiehallion, Cuillin, and Foinaven fields) is Late Jurassic marine shales. This is supported by the Os isotopic composition ( 187 Os/ 188 Os) of the Late Jurassic source (∼0.9–2.4) at the timing of oil generation being similar to that of the oil (68 Ma; 0.92–1.12), indicating that the Re-Os system can potentially be used to fingerprint the source of an oil. The Re-Os data for the 18 oils yield an age (68 ± 13 Ma) that agrees with both the relative (basin models) and absolute (Ar-Ar geochronology) timing of oil generation, indicating that that Re-Os oil geochronology records oil generation events.

Re-Os geochronology and isotope systematics, and organic and sulfur geochemistry of the middle–late Paleocene Waipawa Formation, New Zealand: Insights into early Paleogene seawater Os isotope composition

<p>The Re-Os isotope system is an effective tool in geological studies, especially in radiometric dating. Since both Re and Os are highly siderophile and chalcophile elements, they tend to be concentrated in various sulfide minerals. Therefore, the Re-Os geochronology has been employed for direct age determination of sulfide mineralization [1, 2]. However, conventional analytical methods for the Re-Os dating are complex and consume much time and cost.</p><p>Here we present an improved analytical method for Re-Os in sulfides combined with acid digestion using HClO<sub>4</sub> [3] and sparging introduction of Os [4]. In our method, 0.4 g of powdered sulfide was digested by 1 mL of HClO<sub>4</sub> in addition to 4 mL of inverse aqua regia in Carius tube, and then the Re and Os isotope ratios were measured by MC-ICP-MS. We applied this method to the GSJ geochemical reference materials JCu-1 (copper ore from Kamaishi mine, northeastern Japan) and JZn-1 (zinc ore from Kamioka mine, central Japan). The Re-Os concentrations of JCu-1 and JZn-1 were 255-280 ppt and 4622-4828 ppt for Re, and 39.7-41.7 ppt and 21.7-30.0 ppt for Os, respectively. Furthermore, the analytical results (Re-Os concentrations, <sup>187</sup>Os/<sup>188</sup>Os, and <sup>187</sup>Re/<sup>188</sup>Os) of separated chalcopyrite from Kamaishi mine showed good agreements with those by the conventional method digesting 0.5 g of sample by 10 mL of inverse aqua regia and measured with N-TIMS.</p><p>The new method, using less total volume of acids for sample digestion, enables MC-ICP-MS analysis of sulfides with relatively lower Re and Os concentrations. In addition, for Os isotopes, a sparging method using MC-ICP-MS [4] can be utilized as a simplified analytical procedure. This simplified and improved method may be useful for dating a wider range of sulfide deposits efficiently.</p><p> </p><p>1: Nozaki, T. et al. (2013) Sci. Rep. 3, 1889.</p><p>2: Kato, Y. et al. (2009) Earth Planet. Sci. Lett., 278, 40-49.</p><p>3: Gao, B. et al. (2019) Microchem. J. 150, 104165.</p><p>4: Nozaki, T. et al. (2012) Geostand. Geoanal. Res. 36, 131-148.</p>

The largest negative carbon-isotope excursion in geological history has been reported by several studies of the upper Doushantuo Formation of South China, which has been correlated to the middle Ediacaran-Shuram excursion (SE). Due to a scarcity of radiometric age constraints on the excursion in South China, however, global correlations and comparisons of this event remain a debate. Here, we present Re-Os and carbon isotope data on organic-rich sediments obtained from a drill-core sample in the Chengkou area, the northeastern margin of the Yangtze Platform, and South China. The Re-Os geochronology yields a depositional age of 568 ± 15 Ma (Model 3, MSWD = 1.9, n = 13; 2σ), indicating a middle-late Ediacaran age for the upper Doushantuo Formation. This is supported by a negative δ13Ccarb excursion, which can be reliably correlated to the SE sequences. This age is consistent with the Re-Os radioisotopic dates bracketing the Shuram peaks in Northwest Canada and Oman. A compilation of 187Os/188Os and 87Sr/86Sr isotope ratios as well as the contents of redox-sensitive elements (RSE) from organic-rich sediments deposited between 635 and 540 Ma shows that the radiogenic 187Os/188Os ratios (>1.0) associated with enhanced oxidative weathering occurred at ca. 635, 580, and 560 Ma. As a result, accelerated influxes of nutrients stimulated primary productivity, promoting organic carbon burial and leading to ocean oxygenation. Additionally, elevated continental weathering could have delivered high fluxes of oxidants (e.g., sulfates) to oceans, resulting in transient ocean oxygenation. Corresponding to elevated radiogenic Os and Sr isotope ratios, the significant RSE enrichments at these three times indicate the presence of large marine RSE reservoirs and an oxygenated ocean. Therefore, the Re-Os age and initial Os isotope composition of organic-rich shale can be a sensitive tool for constraining the time interval of enhanced continental weathering and resulting pulses of ocean oxygenation during the Neoproterozoic era.

<p>Re and Os (rhenium and osmium) are chalcophile-siderophile elements (transition metals) with a unique position in isotope geochemistry.  Unlike other commonly used decay schemes for radiometric dating, these metals take residency in resource-related media, for example, sulfide minerals, the organic component in black shales, coals, and bitumens and oils.  In sum, the reducing environment is their haven whereas under oxidizing conditions, Re and Os become unmoored and the radiometric clock becomes compromised.  The clock is not temperature sensitive, and its applicability spans Early Archean to Pleistocene. </p><p>This Bunsen Medal lecture will explore and review the challenges in bringing Re-Os from the meteorite-mantle community into the crustal environment.  At the center of it all is our ability to turn geologic observation into a thoughtful sampling strategy.  The potential to date ore deposits was an obvious application and molybdenite [Mo(Re)S<sub>2</sub>], rarely with significant common Os and rarely with overgrowths, became an overnight superstar, yielding highly precise, accurate, and reproducible ages.  Yet, molybdenite presented our first sampling challenge with recognition of a puzzling parent-daughter (<sup>187</sup>Re-<sup>187</sup>Os) decoupling in certain occurrences.  A strategic sampling procedure was employed.  From there, the diversity of applications spread, as molybdenite is also an accessory mineral in many granitoids, and can be a common trace sulfide in metamorphic rocks.  Whether conformable with and/or crosscutting foliation, molybdenite ages define the timing of deformational events.  Pyrite and arsenopyrite can also be readily dated. </p><p>Applications jumped from sulfides to organic matter.  The hydrogenous component from organic matter in black shales gives us Re-Os ages in the sedimentary record for the Geologic Time Scale.  This led to construction of an Os isotope seawater curve – an ongoing process.  Unlike the well-known Sr seawater curve, the short residence time of Os in the oceans creates a high-definition time record with unambiguous high-amplitude swings in <sup>187</sup>Os/<sup>188</sup>Os.  Re-Os puts time pins into the biostratigraphic record, and we have even directly dated fossils.  Re-Os opened the door for a new generation of paleoclimate studies to evaluate seawater conditions at the time of organic blooms and organic sequestration in bottom mud.  Uplift and continental erosion can be balanced with hydrothermal input into oceans based on changes in the Os isotope composition of seawater.  The timing and connectivity of opening seaways can be determined, and the timing of glaciation and deglaciation events can be globally correlated.  The timing and instigators of mass extinctions are carried in the Re-Os record.  A major meteorite impact places an enormous scar in the Os isotope record as seawater drops toward mantle values but recovers in just a few thousand years.  Most recently, Re-Os has transformed our understanding of the events and fluids involved in construction of whole petroleum systems. </p><p>Looking to the future, what kinds of data sets will be explored and what are the interdisciplinary skill sets needed to interpret those data?  Re-Os will continue to provide us with new ways to dismantle geologic media for new scientific understanding of processes that have shaped our lithosphere, biosphere and hydrosphere, recording their intersection and exchange. </p>