Occurrence and origin of olefins in crude oils. A critical review

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Pergamon

Org. Geochem. Vol. 29, No. 1 3, pp. 397 408, 1998 C©1998 ElsevierScienceLtd. All rights reserved Printed in Great Britain 0146-6380/98/$- see front matter

PII:S0146-6380(98)00058-8

Occurrence and origin of olefins in crude oils. A critical review J O S E P H A. C U R I A L E ~* and E U G E N E B. F R O L O V ~ 'Unocal Corporation, 14141 Southwest Freeway, Sugar Land, TX 77478, U.S.A. and 2Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninski Prospect 29, 117912 Moscow B-71, Russia Abstract--Unsaturated aliphatic hydrocarbons, or olefins, occur in many crude oils and condensates from numerous basins worldwide. Various types of olefins have been identified in the oils and condensates of North America, Africa, Europe and Asia. These compounds occur as homologous series and as characteristic compounds. This report compiles prior observations and conclusions, and presents new data on their identification, occurrence and origins in oils and condensates. We recognize one primary and three secondary sources for olefins in crude oils and condensates. As primary input, olefins may migrate with other soluble organic compounds directly from the source rock. One secondary origin occurs through the process of migration-contamination, wherein light oils act as solvents for syndepositional olefins that occur along the migration route or within the reservoir section. The other two sources for olefins in oil involve abiogenic alteration of the crude after it has entered the trap. The first such source involves "cold" radiolytic dehydrogenation of saturated hydrocarbons, introduced as a byproduct of decay of uranium, thorium and other radioactive elements among the reservoir minerals. The second source is pyrolysis due to thermal impact from igneous intrusions that occur close to the reservoired oil. From an exploration perspective, primary olefins (i.e., olefinic biomarkers) are useful as oil oil correlation tools and depositional indicators. Secondary olefins may be used as supporting evidence in reservoir geochemical studies, particularly those involving reservoir continuity determinations. The most intriguing application of secondary radiogenic olefins is their potential as indicators of the time an oil has spent in its present trap. © 1998 Elsevier Science Ltd. All rights reserved Key words--olefins in oil, migration-contamination, olefinic biomarkers, oleanene, ursene, radiolysis, unsaturated hydrocarbons, terminal olefins, trans-olefins

INTRODUCTION Unsaturated aliphatic hydrocarbons, or olefins, are found in many crude oils and condensates. Nevertheless, few petroleum geochemists study these compounds or their use as tools to solve geochemical problems; oils are seldom analyzed to determine the distribution and concentration of olefins. This is unfortunate, because olefins have a significant role to play in the determination of petroleum composition, both in the sense of biomarker input and as products of migration-induced or in-reservoir alteration. With the continuing interest in the development of new biomarkers, and the growing application of petroleum geochemistry as a tool to resolve reservoir geochemical issues, we feel it is time that the occurrence and origin of olefins be re-examined. Our initial objectives are to review the distribution of olefins in crude oil, and to present selected isolation and detection methods for these olefins. We then propose various origins to account for these distributions, and suggest applications of these compounds in exploration for oil and development of oil fields. *To whom correspondence should be addressed. Tel.: + 1281-287-5646; Fax: +1-281-287-5403; E-mail: [email protected],

Olefins in crude oil were recognized initially by petroleum chemists concerned with the effect of olefins on viscosity and other properties of refined petroleum. Infrared spectroscopic analysis was the early method of choice, and workers relied on the 10.3 t~m I R band as a non-exclusive indicator of trans-olefins (Fred and Putscher, 1949; Francis, 1956). Despite the introduction of more specific analytical procedures, the I R method of detection for trans-olefins is still used for rapid screening of these compounds (Mitzner et al., 1965; Curiale and Frolov, 1996). Later chemists concerned with the properties of unconventional petroleum products, including shale oil, added selective reagents (e.g., diborane) across the double bond to isolate olefins (Jackson et al., 1977; Poirier and George, 1982). With the widespread use of argentation chromatography and conventional gas chromatography over the past two decades, olefin determination has become straightforward. Despite the advent of modern accurate and high flow-through techniques, and the known presence of olefins in organic matter types that are capable of generating crude oil (Requejo and Quinn, 1983; Sinninghe Damst6 et al., 1995), petroleum geochemists do not commonly analyze for olefins in crude oils. In part, this could be due to the masking or

397

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J.A. Curiale and E. B. Frolov

loss of the compounds during standard laboratory workups. For example, Fenselau and Calvin (1966) note that zeolite molecular sieves, commonly used to remove n-alkanes from hydrocarbon mixtures, are also capable of occluding trans-olefins. We believe that the primary reason why olefins are not widely recognized and utilized by petroleum geochemists is because they are assumed to be thermally unstable over geologic time, and/or present in extremely low concentrations in crude oil. We will show that neither assumption is true.

OCCURRENCE OF OLEFINSIN CRUDEOIL AND CONDENSATE Recent studies have established that olefins occur in numerous worldwide crude oils and condensates in reservoir rocks ranging from pre-Cambrian through Tertiary (Frolov and Smirnov, 1990, 1994; Frolov, 1995). Concentrations of these compounds in whole oil have been measured from less than 0.3% to greater than 10.0% (by weight; Frolov et al., 1996a,b). Substantiated worldwide occurrences of olefins in crude oil are shown in Fig. 1, and are reviewed below from oldest to youngest. The oldest reservoir rock known to contain petroleum with olefins is that described by McKirdy et al. (1983). These authors report an oil reservoired in Cambrian/Precambrian carbonates of Namibia that contains Cll 2i n-alkenes, accompanied by

minor amounts of n-alkanes, a high concentration of sulfur and a low pristane/phytane ratio. The oil is likely sourced from carbonates similar in age to the reservoir section. Frolov (1995) reports the occurrence of olefins in Ordovician-sourced (and late Paleozoic-reservoired) oils of the Ouachita Mountains thrustbelt of southeastern Oklahoma and the structurally-analogous Marathon thrustbelt of west Texas. Although having been sourced as long ago as the Cretaceous, and having migrated great vertical distances along listric faults (Curiale, 1992), the olefin concentration in one of these oils was reported to be 7.2% (by weight). Haak and van Nes (1951), Putscher (1952) and Hoering (1977) have identified trans-olefins in Devonian-reservoired oil of the Bradford Field (Pennsylvania) at concentrations up to 9% (Fig. 2). Other studies by Frolov and co-workers (e.g., Frolov and Smirnov, 1994, and references therein) have identified olefins in Paleozoic oils of various reservoir ages in several basins of the former Soviet Union. Concentrations show a general increase with increasing reservoir age (Fig. 3). Unpublished studies by G. H. Smith (proprietary Unocal data, 1971-1981) have identified the occurrence of terminal olefins (i.e., those with largely CH=CH2 double bonds) in oils of the Dannhauser area of South Africa (Rowsell and Connan, 1979). Recent work by Li et al. (1997) has also established the presence of terminal olefins in

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Fig. 1. Distribution of olefins in crude oils and condensates worldwide. The locations shown contain one or more of the compounds types in at least one oil or condensate in the basin: trans-olefins, terminal olefins, oleanenes, ursenes, diasterenes, hopenes, dammarenes. Details on each location are given in the text.

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Devonian-reservoired oils of the Canadian Williston Basin. Olefins in oils reservoired in Mesozoic rocks have been reported by Frolov and Smirnov (1994) from several basins of the former Soviet Union. Mesozoic oils of the Dineh-bi-kayeh Field of Arizona (McKenny and Masters, 1968) are rich in terminal olefins (G. H. Smith, personal communication), and Cretaceous oils containing trans-olefins occur in oils on the North Slope of Alaska (Fig. 4; J. A. Curiale, unpublished data, 1984). The oils of Azua Basin in the Dominican Republic, likely to be Mesozoic in age, are known to contain hopenes (C30 35 A 17'21) (Walters et al., 1995). Several olefin-containing Tertiary oils have been identified within the past decade, including various trans-olefin isomers in several Russian oils (Frolov and Smirnov, 1994); trans-olefins in a Tertiary condensate of the Cook Inlet Basin, Alaska (Fig. 4; J. A. Curiale, unpublished data, 1986); and diasterenes and dammarenes in Miocene oils of the San Joaquin Basin, California (Bac and Schulein, 1990; Bac et al., 1990) and the Gulf Coast, U.S.A. (Curiale and Bromley, 1996). Ukpabio et al. (1994) have identified oils in the Tertiary of the Niger Delta, Nigeria, that contain oleanenes and ursenes;

olefins of these same families have been identifed in Tertiary oils of the Beaufort-Mackenzie Basin of Canada (Curiale, 1991; Peakman et al., 1991) and the North Slope of Alaska (Curiale, 1995). Bazhenova et al. (1997) have recently isolated olefins from Tertiary-reservoired oils of the Eastern Kamchatka Basin, Russia. It is noteworthy that instances of specific molecular identifcation of olefins in Tertiary oils are related to biomarker configurations (e.g., oleanenes, ursenes, dammarenes, hopenes), whereas olefins identified in pre-Tertiary oils usually comprise suites of terminal olefins or trans-olefins. This observation may arise from the limitation of certain angiosperm contributors in pre-Tertiary sources (e.g., oleanoids; ursanoids). Alternatively, and more likely, our knowledge of the distribution of olefins as a function of geologic time is limited by the scattered and uneven nature of the dataset (Fig. 1).

ISOLATION AND DETECTION M E T H O D S FOR OLEFINS IN C R U D E O I L

Preferred methods for the isolation and detection of olefins in crude oils will be discussed in this section. Frolov and Smirnov (1990, 1994) present the

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details of their technique for isolation of olefinic fractions from crude oils using liquid chromatography. Olefin fractions isolated by this method are free of saturated hydrocarbons, but can contain 36 wt% monoaromatic hydrocarbons (as shown by 1H N M R spectroscopy). Olefins can be detected in 0.3-0.5 mg (starting mass) of crude using thin layer chromatography (TLC), by means of the fluorescein-bromine reaction (Kirchner, 1981), following separation of the sample on Silufol plates (Kavalier, Czechoslovakia). Full details are presented by Frolov and Smirnov (1990, 1994). The detection limit in this reaction is 0.5 wt% olefins per crude. A new method for olefin quantification has been reported recently by Frolov et al. (1996a,b), in which the n-octane eluent (non-aromatic hydrocarbons) from an HPLC procedure is monitored by micro-ozonolysis of the aliphatic fraction (containing both the saturates and alkenes, but free of aromatic hydrocarbons). Alkene detection is based on the addition reaction of ozone across olefinic double bonds, and saturated hydrocarbons are unaffected (Frolov et al., 1997). The detection limit by this method is as low as 0.25 #mol of olefinic double bonds per 1 g of oil (or 0.005 wt% olefins, assuming that all the alkene molecules are monoenes and the average number of C-atoms per olefin molecule is about 20).

401

Olefinic hydrocarbons may also be detected in crude oils by means of 1H N M R spectroscopy (4.35.8 ppm; Frolov and Smirnov, 1990, 1994; Smirnov et al., 1992, 1993; Melikhov et al., 1995). Examples in Fig. 5(a)-(d) show variations of the olefinic Hatom (Ho0 signal suite observed in various crude oils. The detection limit is estimated to be as low as 0.5 0.8 wt% alkenes per crude. Infrared spectroscopy (IR) may also be used to identify the presence of olefins in crude oils and their fractions, using the intense absorption band for trans-disubstituted double bonds at 970 cm -I, as well as bands for other olefin groups (e.g., CH2=CH and CH2=C~ / at 915 and 890cm 1) (Hoering, 1977; Frolov and Smirnov, 1994). The detection limit for total trans-olefins by IR analysis is > 2-3 wt%.

SOURCES OF OLEFINSIN OIL We have identified one primary and three secondary origins for olefins in crude oils and condensates. These compounds could be primary, in that they are generated initially in the source rock, and migrate with saturated and aromatic hydrocarbons to the reservoir. Alternatively, olefins could have entered the oil after it left the source rock, either enroute during migration or after it entered the trap. In this section we will examine these possibilities. Primary origin o f olefins in o i ~ o l e f i n i c biomarkers

The identification of olefins in crude oil as biomarkers requires (a) that their biochemical analogues exist in the original source rock organic matter, and (b) that they are thermally stable at temperatures associated with oil generation (i.e., 70-90°C and higher). These conditions have been observed in instances where source rock organic matter is comprised of land plant components (mainly angiospermous debris), and where these source rocks have generated and expelled oil at unusually low temperatures (e.g., less than 100°C). Examples are reported by Curiale (1991, 1995), Peakman et al. (1991) and Rullk6tter et al. (1994) for oils of northwestern Canada and northeastern Alaska. Beaufort Mackenzie Basin oils of northwestern Canada contain indigenous noroleanenes and at least one norursene (24-nor-urs-12-ene), whereas the H a m m e r h e a d oil of northeastern Alaska contains olean-13-ene, 18c~-olean-12-ene and at least one ursene (urs-12-ene; Curiale, 1995). The absence in these oils of other olefins (e.g., hopenes) that are restricted to immature, syndepositional organic matter suggests that the oleanoid and ursanoid olefins are indigenous and, therefore, that they may be considered as biomarkers. Independent geological evidence also indicates a relatively low generation temperature from the source rock

402

J.A. Curiale and E. B. Frolov

(O'Sullivan et al., 1993; Curiale, 1995), providing further support that the olefins in question are inherited as a primary contribution directly from the source. It is likely that many occurrences of individual olefinic biomarkers in crude oil can be accurately attributed to migration-contamination (see below). However, when these components are not accompanied by immature syndepositional olefins such as hopenes, sterenes and diasterenes, normally expected when migration-contamination occurs in a thermally immature sequence, the possibility of fully-migrated olefinic biomarkers must be considered. This possibility becomes significantly greater if the oils in question occur within rapidly subsiding Tertiary depositional basins which have source rocks that contain angiospermous organic debris. Inasmuch as indigenous olefinic biomarkers in such settings usually include one or more oleanenes, the occurrence of primary olefins in an oil can be easily recognized by monitoring the m/z 218 mass fragment in conventional GCMS output (cf. Figure 8 of Rullk6tter et al., 1994; Fig. 3 of Curiale, 1995). Secondary origin of olefins in oil--.en route and inreservoir rnigration-contamination The process of migration-contamination occurs when migrating or trapped petroleum acts as a solvent for syndepositional organic matter occurring along the secondary migration pathway or within the trap. Migration-contamination can impart a maturity "signature" to the migrating or trapped hydrocarbon fluid that is inconsistent with the maturity and source character of that fluid at the time it left the source rock. Because petroleum usually migrates from high-maturity to low-maturity areas, presence of the dissolved components usually decreases the net maturity level '(as measured by molecular maturity indicators) in the migrating oil or gas. In situations where substantial maturity differences are present between the effective source rock (at the time of expulsion) and the organic matter of the reservoir rock in the trap, it is possible for syndepositional olefins, i.e., those that are indigenous to the immature reservoir rock, to dissolve in the migrating hydrocarbon fluid. In such instance, we refer to these olefins as being of secondary origin. Secondary olefins in crude oils and condensates that arise from migration-contamination are generally easy to recognize if the compounds in question are relatively unstable at temperatures considered reasonable for hydrocarbon generation. For example, Curiale and Bromley (1996) have identified oleanenes, dammarenes and diasterenes in oils of the Vermilion 14 Field, offshore Louisiana (Gulf Coast Basin, US). Although the oleanenes could conceivably be derived directly from the source

rock, it is unlikely that dammarenes and diasterenes could have sustained the temperatures necessary for oil generation. The very low molecular maturities of the oils in question (e.g., 5e(H),14c~(H),17c~(H),2OR/ (20R + 20S)-24-ethylcholestane ratios as low as 0.05) provide further evidence for migration-contamination in this instance. It is also likely that this process is responsible for the occurrence of diasterenes and dammarenes observed in oils reservoired in the Paleocene Lodo Formation of the San Joaquin Basin, California (Bac and Schulein, 1990: Bac et al., 1990). Other olefin-containing oils m~L\ also originate via migration-contamination, including those of the Azua Basin (Dominican Republic) that contain a series of extended hopenes (Walters et al., 1995), and those of the Niger Delta (Nigeria) that contain several distinctive olefinic biomarkers (Ukpabio et al., 1994). Secondary origin ~?)cole[ins in oil--radiogenic alterations in the trap It has been hypothesized (Frolov and Smirnov, 1990, 1994) that olefins from old (Paleozoic and Precambrian) deposits can be generated by natural radiolytic dehydrogenation of the saturated hydrocarbons in crude oil, and supporting evidence for this concept is provided by both experimental data and modeling results. Radiolysis of alkanes, which can readily occur at low temperatures, produces predominantly monoolefins of the same skeleton as the starting alkane, with a random distribution of double bonds (see review by Cserep et al., 1981). Molecules with this structure comprise 50-80% of the unsaturated products, and 30-60% (excluding CI Ca components) of all the hydrocarbon products. Suites of olefin molecules created by irradiation of n-heptane, ethylcyclopentane and ethylcyclohexane are shown in Fig. 6. Continued dehydrogenation of monoolefins to diolefins during radiolysis is unusual. The mechanism of radiolytic dehydrogenation can generally be described as follows. A high-energy (103-107 eV) ~-, fl- or ?-particle entering the hydrocarbon phase generates an avalanche of low-energy (20-80eV) electrons. Each of these excites an alkane molecule, and alkenes are formed primarily through non-selective monomolecular elimination of H2 from the excited alkane molecules. This hypothesized mechanism suggests that distribution and structure of the radiogenic olefins are not influenced strongly by such factors as (i) type and energy of bombardment and (ii) the presence of water, salts, minerals, etc. Distribution and structure of ol~fins in oils Jkom Paleozoic and Precambrian rocks. Olefins isolated from Paleozoic and Precambrian oils are monoenes with double bonds scattered along the chains at random. A typical example was reported by Hoering (1977) for n-alkenes isolated from the Bradford

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alkane ratio within an oil should increase with increasing molecular weight, because an exciting particle has a greater probability of encountering a larger, rather than a smaller, alkane, n-Alkene/nalkane carbon number distributions have been reported for several oils from U.S.A. and FSU Paleozoic and Precambrian basins (Melikhov, 1993; Frolov, 1995; Frolov et al., 1996a), and typical correlations observed between n-alkenes and n-alkanes crude oil n-alkenes

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(sum of all isomers) is almost directly proportional to the content of the co-existing n-alkane Cm. This is consistent with modeling studies showing that all the C - C bonds of n-alkanes, regardless of the molecular weight of the compound, can be considered as equivalent in the radiolytic dehydrogenation process (Bishop and Firestone, 1970; Falconer et al., 1971; Rappoport and Gaumann, 1973). The hypothetical radiolytic n-alkene/n-alkane distribution calculated using the modeling results is shown in Fig. 9(b). It can be seen that the modeling results closely mimic the natural data. Importantly, we also note that the ratio of alkenes (created during radiolytic dehydrogenation) to alkanes in an oil appears to be independent of radiation dose. Concentrations of olefinic compounds within narrow vacuum distillate fractions increase with molecular weight (as linear functions) in three Russian oils that were examined by XH NMR (Melikhov, 1993; Smirnov et al., 1992; Melikhov et al., 1995). Correlations between the olefins and the co-existing saturated hydrocarbons within the distillate and superheavy oil fractions (> C50, prepared using gel permeation chromatography on Sephadex LH-20) have been studied, and found to be linear functions of the molecular weight, as shown for two oils in Fig. 10. This reinforces the suggestion that molecular weight distributions of olefinic compounds in oils are consistent with the concept of olefin generation by non-selective radiolytic dehydrogenation of the co-existing saturates. Natural radiolytic dehydrogenation can also create olefinic double bonds in the hydrocarbon substituents belonging to the aromatic and polar petroleum molecules, as shown by recent nH NMR spectroscopy studies (Melikhov et al., 1995; Frolov et al., 1996a,b; Frolov et al., 1997). The fractions

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Fig. 10. The olefins/(olefins+ saturates) weight ratio distributions for oil fractions determined by chromatographic separation (Ag-chromatography) vs carbon number fraction. Oil 1 is from Precambrian Verkhne-Chonsk production (olefins - - 6.5 wt% in whole oil), and oil 2 is from Carboniferous Kamenologsk production (olefins - 2.5wt% in whole oil) (both from the Former Soviet Union).

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Fig. 11. Distribution of olefinic H-atoms (6, 4.3-5.8 ppm) within the aliphatic, aromatic and polar petroleum fractions of Nizne-Omrinsk oil (FSU). The Hol values (measured by ]H NMR) in the chromatographic fractions are normalized to those measured for the whole oil. The area integrated for the detection of aromatic H-atoms (Har) encompasses the 6.2-8.8 ppm region. analyzed were prepared by separating olefin-containing oils on silica gel by elution with hexane, hexane-diethylether, and benzene-methanol. The olefin H-atoms (6, 4.3-5.8 ppm) in the fractions are represented as Hol/Htot ratios normalized to the corresponding whole oil Hol/Htot value (Har/Hto t ratios are also shown). A typical distribution (Fig. 11) indicates that not only aliphatic (Rf 0.85-1.0), but also monoaromatic (Rf 0.65-0.85), polyaromatic (Rf 0.1-0.65) and even resinous petroleum compounds contain significant levels of olefinic double bonds in their structures. Modeling the irradiation of petroleums. Experiments involving 7-irradiation of petroleums (samples discussed by Frolov et al., 1996a,b, 1997) have shown that crude oils initially free of appreciable amounts of olefins (<0.1 wt%) contained olefins in considerable quantities (about 0.6-1.0 wt% per 60 Mrad of the absorbed energy) after 6°Co 7bombardment (1.25 MeV; 30-70°C; in oxygen free conditions). Olefin creation occurred at dosages as low as 7 M r a d . The yield of olefins was proportional to the dose value and to the amount of saturated hydrocarbons in the oil sample. As noted previously, compositional and structural comparison of laboratory-prepared radiogenic olefins with olefins isolated from crude oil (using chromatographic and ]H and 13C N M R measurements) indicate close structural similarities. One of the few exceptions is that natural crude oil olefins contain lower concentrations of the most reactive terminal double bonds (CHz=CH-). This may be due to a greater rate of the secondary thermocatalytic and/or addition reactions operative in the earth's crust. In general, however, most studies indicate that 7-irradiation can be used successfully to simulate the natural radiolysis of crude oil.

405

Occurrence of radiogenic olefins in crude oils. Radiogenic olefins have been observed in over one thousand oils and condensates sampled under primary recovery conditions from about 450 fields in various FSU provinces (Frolov and Smirnov, 1994). Concentrations were summarized earlier in Fig. 3. As indicated, olefin concentrations in oils normally range from 0.02wt% up to 5 - 1 0 w t % (Frolov et al., 1997), and there is a tendency for olefins to occur in higher concentration in oils from Paleozoic and Precambriarr reservoirs. We also observe a strong tendency toward decreasing olefin concentration with increasing reservoir depth (Fig. 12) and temperature (Fig. 13). Furthermore, no differences in olefin concentrations were observed between oils and condensates. The reasons for these relationships are presently unclear, although it is intriguing that some of the highest olefin concentrations are present in oils reservoired at shallow depths and low temperatures, adjacent to basement rocks exhibiting a high level of natural radioactivity (e.g., the crude oils of the Riphean-Vendsk and Lower Cambrian formations of the Siberian platform; see Kontorovich et al., 1986, and Drobot, 1988). Experiments are continuing to assess these trends. Natural radiolysis of petroleum on a geological time scale. The natural radiolysis of petroleum in the earth crust is likely caused by irradiation from elements of the U and Th disintegration series (cf. Berkovskaya, 1996). For example, increased concentrations of Ra and Rn are often associated with oil fields (Vovk, 1979), and Baranov and Titaeva (1973) report that the natural radioactivity of crudes immediately after sampling ranges from 10-l° up to 10-7 C/1 (attributed mostly to concomitant Rn, which is readily soluble in the hydrocarbon phase). This radioactivity is of extremely low intensity compared to typical laboratory dosages;nevertheless, radiolytic generation of olefins is certainly reasonable over geologic time, as shown by Frolov and Smirnov (1994) using published yield values for radiogenic olefins measured in experiments on radiolysis of pure, individual alkanes (Bishop and Firestone, 1970; Falconer et al., 1971; Rappoport and Gaumann, 1973). Refined calculations now show that 5 wt% olefins can be accumulated in about 5 × 107 yr (at a permanent Rn content of 10-8 C/l). The reactive nature of the double bonds created in this process could also subsequently lead to the formation of further secondary compounds in crude oils, including aromatic hydrocarbons, cyclanes, sulfur compounds, etc. Secondary origin of olefins in oil--intrusion-induced thermal alterations in the trap Terminal olefins can be generated in trapped oils that are exposed to temporal heating events, including combustion (Ryabov et al., 1996) and the emplacement of intrusions. Analogous to the for-

406

J.A. Curiale and E. B. Frolov

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to limit situations such as these (McKenny and Masters, 1968). APPLICATIONS OF OLEFIN ANALYSIS TO PETROLEUM EXPLORATION AND DEVELOPMENT

The recognition of olefins as potentially significant components of crude oils and condensates provides opportunities to use these molecules as tools in petroleum exploration and development efforts. Because of their distinctive molecular character, olefinic biomarkers are useful as oil-oil and oil-source rock correlation parameters, and have been applied for this purpose in Alaska and Canada (Curiale, 1991). These primary olefins also have utility as indicators of age and depositional setting of an oil's source rock, particularly when olefinic oleanoid and ursanoid hydrocarbons are available to infer a late Cretaceous/Tertiary source rock rich in angiospermous organic debris (Ukpabio et al., 1994). Although secondary olefins tend to overprint the original molecular input from the source, they can also serve as indicators for non-genetic processes that operate during petroleum accumulation. In the event of migration-contamination (e.g., Curiale and Bromley, 1996, and references therein), the olefinic input to a migrating oil will represent a time-andspace composite of the oil's migration history through thermally immature sediments. It follows that secondary olefins may have utility as indicators of migration distance, and this possibility is currently being studied. The recognition of olefins as distinct molecular markers in oil also has applications in reservoir geochemistry. Olefinic biomarkers, as with many other distinctive biomarkers, have potential in reservoir continuity studies and in efforts to assess filling directions as determined by iso-maturity trends.

Olefins in crude oil Secondary terminal olefins, created due to the presence of intrusives, can provide an indicator of proximity to the intrusive, and such knowledge can be helpful in planning field development. Recognition of olefins as a time-since-trapping indicator provides another intriguing potential application of secondary olefins. Because "radiogenic" olefins act as an integrator of the effects of alpha bombardment over time, absolute concentrations of trans-olefins in oil could provide information about the absolute time that an oil has been in a trap. Such knowledge would have obvious implications for our ability to assess the timing of generation and entrapment.

SUMMARY

This review has emphasized that olefins occur widely in crude oils and condensates. Several processes are responsible for olefin incorporation into crude oil. Olefins may be primary, in the sense that they derived directly from the oil's source rock. Secondary olefins could arise from the process of migration-contamination, from alpha bombardment of the oil post-trapping, or from high-level, shortterm heat input from nearby intrusives. Each of these origins provides tools for the petroleum geochemist to use in exploration for and development of oil and gas. Infrared and N M R spectroscopy, G C M S and liquid chromatography all provide c o m m o n and rapid means of recognizing the occurrence of olefins in oils, and it is anticipated that the identification of these compounds will become increasingly common. We recommend that these techniques be applied routinely, and that petroleum geochemists utilize olefins as a supplementary tool for defining the genetic and non-genetic aspects of petroleum composition. We also suggest that refiners consider that the olefin content in crude oils can vary over a wide range (from 0.02% up to 5-10%), which may have a considerable effect on refining characteristics.

Acknowledgements--We are indebted to our co-workers in Unocal Corporation and the Institute of Petrochemical Synthesis, who served as sounding boards for these ideas. JAC acknowledges Michael Jacob for assistance with olefin analysis on Alaskan oils, and Jerry Smith for permission to report his unpublished conclusions. The research described in this publication and carried out by EBF was made possible in part by Grant NG 2000 and Grant NG 2300 from the International Science Foundation and Russian Government, and also by financial support from the Russian Foundation for Basin Research (Project No. 97-05-64388a). Dr. M. B. Smirnov, Dr. V. A. Melikhov and Dr. N. A. Vanyukova are acknowledged for help and fruitful discussions. Dr. T. M. Peakman provided helpful review comments. Unocal Corporation is acknowledged and thanked for permission to publish.

407 REFERENCES

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