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Unraveling petroleum degradation, maturity, and mixing and addressing impact on petroleum prospectivity: Insights from frontier exploration regions in New Zealand Zachary Burton, J. Michael Moldowan, Richard Sykes, and Stephan Graham Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03261 • Publication Date (Web): 15 Jan 2018 Downloaded from http://pubs.acs.org on January 22, 2018

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Unraveling petroleum degradation, maturity, and mixing and addressing impact on petroleum prospectivity: Insights from frontier exploration regions in New Zealand Zachary F. M. Burton1*, J. Michael Moldowan1,2, Richard Sykes3, Stephan A. Graham1 1

Department of Geological Sciences, Stanford University, Stanford, California 94305, U.S.A.

2

Biomarker Technologies Incorporated, Rohnert Park, California 94928, U.S.A.

3

GNS Science, Lower Hutt 5040, New Zealand

*Corresponding author: [email protected]

Abstract Determining oil quality is essential to identifying valuable resource accumulations. However, in new areas of exploration, little information is available on the processes affecting resource quality. Geochemical analyses of oil seeps from frontier regions of New Zealand’s east coast illustrate an application of underutilized resource quality assessment techniques. Distributions of n-alkanes and isoprenoids reveal biodegradation, and thus potentially lower oil quality in the “southern” versus the “northern” oil seeps. However, sterane and terpane compounds are unaltered, indicating overall biodegradation of these oils is low to moderate. Additionally, lack of 25-norhopane indicates degradation of southern oils may be solely aerobic. Therefore, any subsurface accumulations are potentially unaffected. Investigation of sterane and hopane isomerization ratios and additional sterane and terpane maturity parameters is paired with diamondoid analyses of oil-to-gas conversion and petroleum mixing. Three distinct petroleum mixtures are identified among the sampled seeps: 1) a seep composed of an early/peak oil window component and an intensely cracked condensate/wet gas component, 2) seeps solely containing a peak/late oil window component, and 3) seeps composed of a peak/late oil window component and an intensely cracked condensate/wet gas component.

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Identified components indicate at least three distinct charges or stages of petroleum generation. Black oil components might indicate actively producing source rock in all regions represented by the seeps. Intensely cracked components indicate petroleum mixing via thermogenic gas infiltration, and suggest an effect on oil quality. Important questions concerning migration pathways and timing, ties to New Zealand’s offshore basins, and potential for reservoir entrapment of these petroleum components remain.

Keywords: Petroleum mixing, biodegradation, biomarkers, diamondoids, maturity, seeps, East Coast Basin, Pegasus Basin, New Zealand

1. Introduction The key goal of the oil and gas industry’s upstream sector is to find and extract significant amounts of energy resources. Because there are high risks associated with this process, methods for identifying economically-attractive accumulations are critical. Geochemical analysis of oils is one technique available for assessing both the presence and producibility of petroleum accumulations1,2. Analytical geochemistry provides inexpensive methods to leverage the limited data available in frontier exploration areas. Geochemical technology can be successfully applied to determine impact of biodegradation on oil quality and to identify petroleum mixtures3-6. It can be used to assess contribution of shallow versus deep sources and to fingerprint conversion of oil to gas7-9. Analysis of geochemical data in conjunction with geological and geophysical data is critical for success in frontier exploration and production10.


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Oil seeps in areas of New Zealand’s east coast are an ideal target for demonstrating the power and utility of advanced geochemical techniques. This region contains hundreds of seeps, shows, and other petroleum indicators, and has attracted exploration interest for over a century11. However, petroleum systems are still poorly understood, and significant accumulations of oil and gas remain undiscovered11. The oils examined as a part of this study have been previously characterized as marine oils, however, the source rock units that generated these oils remain uncertain12-16. The east coast region was characterized by Gondwanan subduction lasting until ~85 Ma11. This period of subduction gave way to a prolonged period of passive margin sedimentation, lasting ~85 to 25 Ma11. It was during this period of passive margin sedimentation that the inferred source rock intervals were deposited in the area of interest, including within, it is inferred, the East Coast and Pegasus basins11. The Upper Cretaceous through Paleocene Whangai Formation and the Paleocene age Waipawa Formation black shale are the two most likely source rock candidates within the region, although, as mentioned above, it is uncertain which of these source rock candidates may be contributing to the oil seeps found up and down the east coast11-16. We present results from our studies of onshore oil seep samples from New Zealand’s North Island and South Island (Figure 1). We studied biomarker and diamondoid compounds to determine the degree of petroleum biodegradation, to assess thermal maturity, and to calculate the extent of cracking of these oils. We bring the results of these studies together to identify mixing of different charges within these oils.

2. Samples and methods 2.1. Sample selection


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Oil seeps provide key insight into petroleum system characteristics and can greatly reduce exploration risk by establishing active source rock presence, by constraining maturity and biodegradation, and by identifying mixed sources17,18. For this study, we selected five oil seep samples provided by GNS Science19. The three “northern” oil samples are from the Rotokautuku, Totangi, and Waitangi oil seeps, located most proximal to New Zealand’s East Coast and Raukumara basins (Figure 1). The “southern” oils are from the Isolation Creek and Kaikoura seeps, located closest to New Zealand’s deepwater Pegasus Basin (Figure 1). 2.2. Methods 2.2.1. GC-FID – Whole-oil analysis Whole-oil chromatograms for the five oil seep samples were provided by GNS Science from Sykes et al.19. These chromatograms were obtained via gas chromatography-flame ionization detection (GC-FID) at Applied Petroleum Technology (APT) in Kjeller, Norway using procedures detailed in Weiss et al.20. 2.2.2. Liquid chromatography – Sample preparation Crude oil samples (30-40 mg) were weighed and spiked with 5β-cholane and deuterated diamondoid internal standards for quantitation. Deuterated diamondoids in the spike include D15-1-methyladamantane, D16-adamantane, D3-1-methyldiamantane, D4-diamantane, and D4triamantane. Spiked samples were fractionated by sequential elution using a proprietary light hydrocarbon solvent and dichloromethane on silica gel columns to obtain saturate and aromatic fractions. Paraffins were removed from saturated hydrocarbon fractions using a proprietary light hydrocarbon solvent on zeolite columns. 2.2.3. GC-MS – Saturated hydrocarbon fraction analysis


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Saturated fractions were analyzed by gas chromatography-mass spectrometry (GC-MS) using an Agilent Technologies 7890A/5975C GC-MS system with a 7693A autosampler. The GC was equipped with a 60 m DB-1 column (0.25 mm i.d.; 0.25 µm phase thickness). Helium was used as carrier gas at a flow rate of 2.25 mL/min. The temperature program began at 80°C and was raised at a rate of 2°C/min to 320°C where it was held for 15 min. Calibrated external standards were run alongside the oil samples to account for response factors of different compounds. Biomarker analysis and quantitative diamondoid analysis was conducted by monitoring ions with specific mass-to-charge ratios (m/z). Biomarkers were quantified using m/z 191 for hopanes and m/z 217, 218, and 259 for steranes. Diamondoids were analyzed and quantified using m/z 135, 136, 149, 150, and 152 for adamantanes, m/z 187, 188, 192, and 201 for diamantanes, m/z 239, 240, and 244 for triamantanes, and m/z 292 for tetramantanes. Quantitation of biomarker and diamondoid compounds was achieved by integration of compound peak areas and/or measurement of peak heights relative to internal and external standards. 2.2.4. GC-MS-MS – Saturated hydrocarbon fraction analysis Gas chromatography-tandem mass spectrometry (GC-MS-MS) of the saturated hydrocarbon fractions was conducted using an Agilent Technologies 7890A GC interfaced to a 7000 Triple Quad GC-MS with a 7693A autosampler. The GC was equipped with a 60 m DB-1 column (0.25 mm i.d.; 0.25 µm phase thickness). Helium was used as carrier gas at a flow rate of of 2.25 mL/min. The temperature program began at 80°C and was raised at a rate of 2°C/min to 320°C where it was held for 15 min. Oil samples were analyzed alongside calibrated external standards in order to account for variability in chemical compound response differences.


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The mass spectrometer was run in parent-to-daughter mode using the first quadruple to focus parent ions and the third quadrupole to focus daughter ions. The transitions monitored were m/z 370→177, 318→191, 330→191, 332→191, 370→191, 384→191, 398→191, 412→191, 426→191, 440→191, 454→191, 468→191, 482→191, and 426→205 for hopane biomarkers and 288→217, 302→217, 330→217, 358→217, 372→217, 386→217, 400→217, and 414→217 for sterane biomarkers. Biomarkers were quantified by integrating compound peak areas and/or measuring peak heights relative to internal and external standards.

3. Results and discussion 3.1. Biodegradation Assessment of biodegradation is critical in exploration and production. Biodegradation can lead to removal of the most economically-desirable petroleum compounds and can significantly affect the value of a petroleum accumulation4,21. Biodegradation leads to lower API gravity (i.e., higher viscosity) and lower producibility (i.e., increased difficulty of extraction) of petroleum accumulations22,23. 3.1.1. n-Alkanes and isoprenoids Whole-oil chromatograms reveal key differences between samples (Figure 2). Gas chromatograms for the Totangi, Waitangi, and Rotokautuku oil seep samples show good preservation of both n-alkane and isoprenoid compounds (Figure 2). In contrast, Isolation Creek and Kaikoura samples show almost no preservation of n-alkanes and minimal preservation of isoprenoids (Figure 2).


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The variations in compound class preservation between 1) Totangi, Waitangi, and Rotokautuku crude oil samples and 2) the Isolation Creek and Kaikoura oil samples highlight distinctions in the degree of biodegradation to which each group of samples has been subjected. The n-alkanes and isoprenoids are the compound classes most susceptible to destruction via biodegradation24-26. Therefore, the high degree of both n-alkane and isoprenoid compound preservation in the Totangi, Waitangi, and Rotokautuku oil seep samples suggests no more than slight amounts of biodegradation. In contrast, the poor preservation of n-alkanes and isoprenoids in the Isolation Creek and Kaikoura oil samples suggests greater, although still moderate, biodegradation of these samples27. A more pronounced unresolved complex mixture of hydrocarbons in the Isolation Creek and Kaikoura oils also suggests greater biodegradation of the n-alkane and isoprenoid compounds28-30. 3.1.2. Steranes Despite the contrast in the degree of biodegradation identified via assessment of n-alkane and isoprenoid compounds (Figure 2), the GC-MS sterane (m/z 217) distributions of all five oil samples show clean peaks without identifiable evidence for degradation of steranes in any of the samples (Figure 3; for peak assignment see Table 1). In all samples, both regular steranes and rearranged steranes (diasteranes) are well-preserved. After n-alkanes and isoprenoids, steranes are typically the compounds most susceptible to biodegradation and can be completely removed under heavy biodegradation, whereas diasteranes are much more resistant to destruction3,6. Comparison of sterane and diasterane fingerprints shows that sterane peaks in the Kaikoura oil sample are just as well-preserved relative to diasterane peaks as the steranes in the Waitangi sample (Figure 3; for peak assignment see Table 1). This suggests absence or only minimal amounts of sterane biodegradation in all samples and


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provides a contrast to results from examination of n-alkane and isoprenoid biodegradation discussed above. Sterane and diasterane concentrations obtained via GC-MS-MS further support the finding of negligible biodegradation of steranes in any of the crude oil samples. Ratios of total C27, C28, and C29 steranes to total C27 + C28 + C29 steranes are nearly identical to respective ratios of total C27, C28, and C29 βα-diasteranes to total C27 + C28 + C29 βα-diasteranes (i.e., C27 sterane / total C27 + C28 + C29 steranes is nearly identical to C27 βα-diasteranes / total C27 + C28 + C29 βα-diasteranes) (Table 2) for all samples except Rotokautuku, which shows a lower C27 sterane ratio versus C27 diasterane ratio, possibly due to thermal maturity effects or the relative activity of clay31. Comparison of ternary diagrams clearly illustrates the corroboration of sterane and diasterane data (Figure 4). This strongly supports the finding that steranes have not been altered by biodegradation and also suggests that maturity is not playing a role in affecting sterane distributions, with the possible exception of slight differences in the C27 steranes of the Rotokautuku oil sample3. 3.1.3. Hopanes Hopanes are often more robust to biodegradation than steranes, however, in some instances hopanes may be removed before steranes32,33. GC-MS m/z 191 hopane distributions of the five oil samples show no indication of hopane destruction (Figure 5). As with sterane distributions, the hopane peaks are qualitatively clean. Detailed examination of the C31-C35 17α(H),21β(H)-Homohopanes reveals good preservation of compound peaks in all samples (Figure 5), and in particular, good preservation of the C35 homohopane peaks, which in some cases are more susceptible to biodegradation34,35. The lack of destruction of hopane compounds, as with the lack of destruction of steranes, suggests relatively low degrees of biodegradation in these oil samples.


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Biodegradation of hopanes often leads to observation of 25-norhopanes in abundances corresponding to the severity of biodegradation36-38. Measurement of the m/z 398→191 transition reveals the absence of 25-norhopanes in all of these oils (Figure 6). This provides further indication of little to no biodegradation of hopane compounds. 3.1.4. Discussion of biodegradation Preservation of steranes and hopanes suggests relatively low levels of biodegradation in these oils, or, at least suggests that the seep samples contain a component of relatively nonbiodegraded oil. The distributions of n-alkanes and isoprenoids in the northern samples (Rotokautuku, Totangi, and Waitangi seep samples) indicate good preservation of these compound classes, and is likely indicative of essentially fresh seep oil for these particular samples i.e., active oil seeps6. In contrast, the extensive destruction of n-alkanes and isoprenoids seen in the southern oils (Isolation Creek and Kaikoura samples) suggests greater biodegradation of these compounds. Biodegradation of these two southern oil samples might be related to aerobic microbial activity39-41. Aerobic degradation is more common in less active oil seeps i.e., seeps with less frequent fresh oil input. The interpretation of subaerial biodegradation in the southern oils is supported by absence of 25-norhopanes in all oil samples. 25-norhopane is generally associated with anaerobic degradation of petroleum in subsurface reservoirs and is therefore not an expected product of aerobic degradation of subaerial seeps42. 3.2. Thermal maturity Thermal maturity of oils directly affects oil quality. Increasing maturity leads to increasing API gravities and changes in other bulk oil properties43. However, traditional bulk properties such as API gravity, sulfur, and saturated hydrocarbon content are not only affected by maturity but also by differences in source-rock facies, migration, and reservoir


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characteristics17,33. Therefore, examination of bulk properties alone tends to yield a less robust measure of maturity44. Sterane and hopane biomarker compounds provide a much more robust means of assessing thermal maturity of oils. Ratios of various biomarker compounds can be utilized to bracket the maturity of liquid hydrocarbon samples in the early, peak, and late oil windows, and into the condensate/wet gas stage44,45. Concentrations of diamondoid compounds are especially effective in assessing the maturity of late oil window and condensate/wet gas window hydrocarbons9,46-48. Diamondoid analysis can indicate the contribution of a condensate/wet gas-generating source to both determine thermal maturity as well as fingerprint previously unidentified components of petroleum mixtures9,49,50. 3.2.1. Biomarker parameters Biomarker isomerization ratios are some of the most commonly used and most robust parameters for indicating thermal maturity51-53. It should be noted that biomarker maturity proxies are only effective in the main phase of the oil window. Maturity of late- or postmature oils will not be characterized by biomarkers. Diamondoids show clearly that some of these oils are high-low maturity mixtures (see ahead). Due to C31–C35 hopane equilibration as early as equivalent vitrinite reflectance of ~0.5%, isomerization ratios of C31–C35 17α-hopanes are particularly useful in assessing immature to early oil generation54. Hopane 22S/(22S+22R) ratios of ~0.6 indicate equilibration in the early oil window55, and do not increase from this ~0.6 endpoint value as thermal maturity increases beyond the early oil window. Each of the C31, C32, C33, C34, and C35 22S/(22S+22R) ratios for all oil samples examined in this study have reached the ~0.6 endpoint value (Table


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3). This suggests equilibration in these oil samples and indicates maturities at least as high as the early oil window. Both 20S/(20S+20R) and ββ/(ββ+αα) isomerization ratios of the C29 steranes increase with increasing thermal maturity53, and are effective in assessing maturities ranging from immature to peak oil window maturity44. These C29 isomerization ratios suggest similar maturities for the Totangi, Waitangi, Isolation Creek, and Kaikoura oils, whereas both isomerization parameters suggest a somewhat lower maturity in the Rotokautuku oil (Figure 7). The 20S/(20S+20R) isomerization ratios have all reached equilibrium in the Totangi, Waitangi, Isolation Creek, and Kaikoura oils, but none of these oils show equilibration of the ββ/(ββ+αα) isomerization ratios (Figure 7). This suggests early to peak oil window generation of at least some component of these oils. In contrast, the Rotokautuku oil has reached neither isomerization endpoint, indicating a lower maturity pulse of oil generated in the early oil window. Slightly higher ββ/(ββ+αα) isomerization ratios in the Isolation Creek and Kaikoura oils may suggest slightly higher thermal maturity than in the Totangi and Waitangi oils. In addition to biomarker isomerization ratios, other sterane and hopane compound parameters such as trisnorhopane and norcholestane ratios can provide corroborating information on thermal maturity44. The ratio of C27 18α-trisnorneohopane to C27 17α-trisnorhopane, here expressed as Ts/(Ts+Tm), shows a strong dependence on maturity and can be used to assess maturities ranging from immature into the condensate/wet gas window17,56. Because Ts/Tm ratios can be highly influenced by source-rock facies, these ratios are best interpreted alongside other biomarker parameters57. Maturities indicated by the Ts/(Ts+Tm) hopane ratios of these oils show agreement with the maturities indicated by sterane isomerization ratios (Figure 7).


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Isolation Creek and Kaikoura oils appear to be at least slightly more mature than Totangi and Waitangi oils, and the Rotokautuku oil is less mature than all other oils (Figure 7). Relative concentrations of C26 steranes can also serve as maturity parameters. The abundance of C26 21-norcholestane relative to the total amount of 21-, 24-, and 27-norcholestanes increases with increasing thermal maturity58, although source rock lithology can also have an impact on this ratio. The ratio of 21-norcholestane relative to total norcholestanes is more than twice as high in the Isolation Creek and Kaikoura oils as it is in the Totangi, Waitangi, and Rotokautuku oils (Figure 7). This suggests a higher thermal maturity in the Isolation Creek and Kaikoura oils. This finding is consistent with sterane ββ/(ββ+αα) isomerization ratios and with Ts/(Ts+Tm) hopane ratios. The 21-norcholestane ratios do not suggest significant difference in the maturities of the Totangi, Waitangi, and Rotokautuku oils (Figure 7). 3.2.2 Diamondoid parameters Analysis of diamondoid concentrations in conjunction with biomarker concentrations allows for both assessment of the degree of oil-to-gas cracking and direct identification of mixed oils9. Utilizing a high diamondoid baseline of 15 ppm59, the southern oils (Isolation Creek and Kaikoura samples) are shown to have extremely high concentrations of 3- + 4methyldiamantanes at 202 ppm and 163 ppm, respectively. The Rotokautuku oil sample has a high concentration of 36 ppm, while the Totangi and Waitangi oil samples have lower concentrations (i.e., below the baseline of 15 ppm) of 13 ppm and 9 ppm, respectively. The high diamondoid concentrations of the Isolation Creek, Kaikoura, and Rotokautuku samples indicate intensely cracked oils (Figure 8), whereas the low concentrations of the Totangi and Waitangi oils likely represent end-member uncracked oil values. Figure 8 shows the 3- + 4methyldiamantane concentrations plotted versus a biomarker concentration, C29-sterane


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stigmastane (5α,14α,17α(H),20R-24-ethylcholestane). Stigmastane decreases with increasing maturity and is effectively completely destroyed by the time oil begins to crack. Therefore, any appreciable amount of stigmastane in a sample containing high amounts of diamondoids indicates a mixed oil9,59,60. The Isolation Creek and Kaikoura oil samples have lower concentrations of stigmastane than the northern oils; however, both oils still contain over 10 ppm stigmastane, indicating a widely mixed maturity range9. The pairing of both high biomarker (stigmastane) concentrations and high diamondoid (3- + 4-methyldiamantane) concentrations strongly indicates that Isolation Creek, Kaikoura, and Rotokautuku all represent a mixture of black oil (high biomarkers, negligible diamondoids) and cracked oil (negligible biomarkers, high diamondoids). The ubiquity of well-preserved sterane and hopane biomarkers, discussed in the sections above, similarly supports the finding that these three oil samples represent mixtures. This is because if samples solely represented unmixed, thermally-cracked hydrocarbons, the sterane and hopane biomarkers observed here would not be preserved. The extent of cracking of the Rotokautuku, Isolation Creek, and Kaikoura samples can be quantitatively determined using the equation %Cracking = [1-(Co/Cc)] X 100, whereby Co represents the methyldiamantane concentration in the end-member uncracked oil samples and Cc is the methyldiamantane concentration of any cracked sample9. It should be noted that this assumes that any cracked sample has been derived from the same source rock and source facies. It is also possible that, due to potential evaporative loss of light ends, calculation of %Cracking can cause an overestimate of the degree to which an oil sample has been cracked9,47. To account for these potential uncertainties, we calculate the percentage of cracking based on a Co (diamondoid baseline) of 4 ppm, or about the global average59, and based on a very high (i.e., quite conservative) Co of 10 ppm. Even utilizing the less-realistic conservative baseline value of 10 ppm, both Isolation Creek and Kaikoura samples show over


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94% conversion of liquid hydrocarbons to gas and the Rotokautuku sample shows over 70% cracking (Table 4). Using the more realistic diamondoid baseline value of 4 ppm, Isolation Creek and Kaikoura both show about 98% cracking and Rotokautuku shows nearly 90% cracking (Table 4). Destruction of oil via biodegradation may account for some elevation of diamondoid concentrations, however, biodegradation does not realistically explain 70+% cracking in these oils. Larter et al.61 demonstrated at most 60% cracking at biodegradation levels of PM 8 based on the Peters & Moldowan6 biodegradation scale. A biodegradation level of PM 8 is quite severe, and would surely lead to alteration of sterane and hopane parameters, which we see no evidence for. Therefore, although biodegradation may account for some elevation of diamondoid concentrations, it cannot fully account for the high concentrations of diamondoids observed here. Diamondoid results indicate mixed oils containing very intensely cracked components in the southern (Isolation Creek and Kaikoura) oil samples and in the Rotokautuku oil seep sample, while the Totangi and Waitangi oil samples represent uncracked (end-member) oils. 3.3. Implications for petroleum mixing and charge history Hopane and sterane biomarker maturity parameters reveal a contrast between more mature Totangi, Waitangi, Isolation Creek, and Kaikoura black oil components (i.e., components rich in heavy, large, non-volatile hydrocarbons) and a less mature Rotokautuku black oil component. The lower maturity of the Rotokautuku oil indicates that this oil was generated by a source rock at an earlier oil window maturity than the other four oils. This suggests presence of a lower-maturity black oil charge in the Rotokautuku oil versus higher-maturity black oil charges in the Totangi, Waitangi, Isolation Creek, and Kaikoura oils.


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Diamondoid concentrations indicate high-maturity condensate/wet gas window components in the Isolation Creek, Kaikoura, and Rotokautuku oils, whereas the Totangi and Waitangi oils are missing the high-maturity charge. Both biomarker maturity parameters and diamondoid cracking parameters reveal three distinct charge histories represented by the five oil samples studied here. The Rotokautuku oil contains evidence for an early to peak oil window charge as well as a condensate/wet gas charge, the Totangi and Waitangi oils reveal evidence for just a peak to late oil window charge, and the Isolation Creek and Kaikoura oils indicate the presence of a peak to late oil window charge and a condensate/wet gas charge.

4. Conclusions We have investigated both biomarker compounds and diamondoid compounds to assess oil seep biodegradation, maturity, and oil-to-gas cracking. We used these results to understand petroleum mixing and charge history for petroleum systems in this area. 1. Biodegradation) The northern oils (Rotokautuku, Totangi, and Waitangi) show preservation of n-alkane and isoprenoids, suggesting very slight biodegradation. This is uncommon in subaerial oil seeps, and likely indicates highly active seeps. In contrast, the southern oils (Isolation Creek and Kaikoura) show extensive destruction of these compounds, indicating heavy biodegradation and perhaps less active seeps. All oils show good preservation of sterane and hopane compounds and show absence of 25norhopane. A reasonable explanation for the destruction of n-alkanes and isoprenoids in the southern oils could be aerobic degradation of the oils once they were exposed subaerially.


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This is further supported by the lack of 25-norhopane, which tends to be generated through subsurface anaerobic degradation (e.g., in-reservoir biodegradation). 2. Maturity) Both sterane and hopane maturity parameters identify a component of relatively high-maturity (peak to late oil window) black oil in the Totangi, Waitangi, Isolation Creek, and Kaikoura oil seeps. In contrast, these parameters suggest a lower-maturity (early to peak oil window) black oil component in the Rotokautuku oil seep. 3. Oil-to-gas cracking) Diamondoid compounds indicate the contribution of intensely cracked components to the Isolation Creek, Kaikoura, and Rotokautuku oil seeps. The conflicting maturities suggested by the coexistence of the lower-maturity component (identified via biomarker assessment) and the highly cracked component (identified via diamondoid assessment) indicate multiple charges to the Rotokautuku oil. Furthermore, Isolation Creek, Kaikoura, and Rotokautuku oils all indicate multiple charges due to the coexistence of wellpreserved steranes and hopanes with very high diamondoid concentrations. In other words, if any of these oils were solely derived from a charge of intensely cracked oil, sterane and hopane biomarkers would not be present. 4. Implications for oil mixing) The northern Totangi and Waitangi oil seeps contain no evidence of a deep thermal source, and likely represent unmixed (end-member) black oils. The northern Rotokautuku oil and the southern Isolation Creek and Kaikoura oils are mixed oils containing evidence for at least two charges: one of black oil, and one of intensely cracked hydrocarbons. The identification of this cracked component suggests mixing via gas infiltration from a deep source, and impacts prospectivity of these mixed oils relative to the uncracked Totangi and Waitangi oils. 5. Implications for depth of different charges) Maturity parameters reveal that the endmember Totangi and Waitangi oils and the black oil components of the mixed Isolation Creek


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and Kaikoura oils were derived from higher-maturity charges generated in the peak to late oil window. In contrast, the black oil component of the Rotokautuku oil was derived from a lower-maturity charge generated in the early to peak oil window. This suggests that the black oil charge of the Rotokautuku seep likely originated from a shallower depth than that of all other oils. 6. Importance of utilizing advanced geochemical technologies) This study emphasizes the power of applying advanced geochemical technologies and, in particular, diamondoid analysis, in improving understanding of petroleum systems. The high maturity contribution identified in the mixed Isolation Creek, Kaikoura, and Rotokautuku oils would have been overlooked if only conventional biomarker parameters had been investigated. 7. Future work) The findings of this work raise a number of important questions to be addressed through the assessment of petroleum migration pathways in New Zealand’s east coast frontier regions. The prevalence of faulting in the regions where the studied oil seeps are found implies some degree of structural control on the migration of hydrocarbons. However, as of yet unexamined are questions related to whether these oil seeps originate directly from active source rocks or from leaking reservoirs. Also unexamined would be whether the multiple charges contributing to some seeps represent charges from the same source-rock organofacies or whether they represent charges from different organofacies. Assessment of the relative volumetric contribution of deep cracked sources versus normal oil window sources to any existing petroleum accumulations will be important. A valuable contribution will be characterizing burial history within each seep’s geological setting to assess more absolute timing of the generation and expulsion of the various charges identified in this study.


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Acknowledgements ZB thanks the Stanford University Basin and Petroleum System Modeling (BPSM) research group, the U.S. Department of Energy National Energy Technology Laboratory (DOE NETL), and the Stanford McGee/Levorsen Research Grant Program for funding. ZB also thanks Allegra Hosford Scheirer, Inessa Yurchenko, William Thompson-Butler, Jeremy Dahl, and the staff at Biomarker Technologies Incorporated and GNS Science for helpful comments and assistance. RS thanks the Ministry of Business, Innovation and Employment for funding through the GNS Science-led research program on New Zealand petroleum source rocks, fluids, and plumbing systems (contract C05X1507).

The authors declare no competing financial interests.

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Figure 1: Area of interest and general setting; white circles indicate oil seep locations labeled as follows: R for Rotokautuku, W for Waitangi, T for Totangi, IC for Isolation Creek, and K for Kaikoura; black lines indicate active plate boundary; frontier basins most relevant to this study, as well as the established Taranaki Basin, are labeled.


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Figure 2: Whole-oil gas chromatograms show n-alkane and isoprenoid distributions and illustrate the contrast between 1) the very high preservation of n-alkane and isoprenoid compounds in the Totangi, Waitangi, and Rotokautuku oil seep samples and 2) the poor preservation, i.e., nearly complete destruction of n-alkanes and isoprenoids in the Isolation Creek and Kaikoura oil samples; GC-FID chromatograms from GNS Science public report19.


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Figure 3: Representative m/z 217 fragmentograms of selected sterane distributions for the Waitangi and Kaikoura samples illustrate the preservation of sterane biomarkers and the similarity of sterane:diasterane ratios for all samples; Obtained via GC-MS of saturate hydrocarbon fractions.


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Figure 4: Ternary diagrams show high degree of similarity in the relative abundances of C27, C28, and C29 regular steranes versus βα-diasteranes (rearranged steranes); For steranes, C27 = Total C27 / Total (C27 + C28 + C29) and so on for C28 and C29, for βα-diasteranes, C27 = C27/(C27 + C28 + C29) βα-diasteranes; Determined by GC-MS-MS of oil sample saturate hydrocarbon fractions.


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Figure 5: Representative m/z 191 fragmentograms of selected hopane distributions from the Waitangi oil and the Kaikoura oil illustrate the good preservation of biomarkers in all five samples. Enlarged region highlights the preservation of the C31-C35 17α(H),21β(H)Homohopanes.


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Figure 6: GC-MS-MS 398→191 transitions showing the C29 25-Nor-17α(H)-Hopane peak in a reference standard as a contrast to the lack of a discernible 25-norhopane peak in any of the samples analyzed in this study.


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Figure 7: (a) C29 sterane isomerization and (b) C26 norcholestane versus C27 hopane maturity parameters. “Endpoint” areas indicate equilibration of isomerization ratios. All ratios calculated from GC-MS-MS measurements (C29 steranes monitored with m/z 400→217, C26 norcholestanes with m/z 358→217, and C27 hopanes with 370→191).


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Figure 8: Concentrations of thermally stable diamondoids (methyldiamantanes) versus much less stable biomarkers (stigmastane) indicate thermal maturity and degree of cracking of oils, and reveal petroleum mixtures9.


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Table 1: Diasterane and sterane peak assignment for Figure 3. 1

C27 βα-diacholestane 20S

2

C27 βα-diacholestane 20R

3

C28 βα-diasterane 20S

4

C28 βα-diasterane 20S

5

C28 βα-diasterane 20R

6

C28 βα-diasterane 20R

7

C27 αα-cholestane 20S

8

C27 ββ-cholestane 20R & C29 βα-diastigmastane 20S

9

C27 ββ-cholestane 20S

10 C27 αα-cholestane 20R 11 C29 βα-diastigmastane 20R 12 C28 αα-ergostane 20S 13 C28 αα-ergostane 20S 14 C28 ββ-ergostane 20R 15 C28 ββ-ergostane 20S 16 C28 αα-ergostane 20R


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17 C29 αα-stigmastane 20S 18 C29 ββ-stigmastane 20R 19 C29 ββ-stigmastane 20S 20 C29 αα-stigmastane 20R


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Table 2: Sterane and diasterane %C27-29 for all samples, illustrating similarity of the sterane and diasterane ratios. Rotokautuku Totangi Waitangi Isolation

Kaikoura

Creek Total C27 / Total (C27 + C28

0.25

0.33

0.28

0.44

0.44

0.30

0.35

0.31

0.42

0.44

0.44

0.47

0.48

0.40

0.40

0.47

0.48

0.49

0.40

0.41

0.31

0.21

0.24

0.16

0.16

0.23

0.17

0.20

0.17

0.16

+ C29)

C27/(C27 + C28 + C29) βαdiasteranes

Total C28 / Total (C27 + C28 + C29)

C28/(C27 + C28 + C29) βαdiasteranes Total C29 / Total (C27 + C28 + C29) C29/(C27 + C28 + C29) βαdiasteranes


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Table 3: C31–C35 17α-hopane 22S/(22S+22R) isomerization ratios; Calculated from GC-MSMS.

C31 αβ-hopane (22S) / (C31

Rotokautuku

Totangi

Waitangi Isolation Creek

Kaikoura

0.59

0.60

0.61

0.61

0.59

0.58

0.60

0.61

0.59

0.58

0.63

0.61

0.62

0.60

0.60

0.64

0.62

0.62

0.61

0.61

0.66

0.62

0.65

0.62

0.62

αβ-hopane (22S) + C31 αβhopane (22R))

C32 αβ-hopane (22S) / (C32 αβ-hopane (22S) + C32 αβhopane (22R))

C33 αβ-hopane (22S) / (C33 αβ-hopane (22S) + C33 αβhopane (22R)) C34 αβ-hopane (22S) / (C34 αβ-hopane (22S) + C34 αβhopane (22R))

C35 αβ-hopane (22S) / (C35 αβ-hopane (22S) + C35 αβhopane (22R))


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 4: Extent of cracking for the mixed-source samples; Calculated using %cracking = [1(Co/Cc)] × 100 (after Dahl et al.9) Rotokautuku

Isolation

Kaikoura

Creek Diamondoid

%cracking

%cracking %cracking

4 ppm

89

98

98

10 ppm

72

95

94

baseline (C0)


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