A geochemical appraisal of oil seeps from the East Coast Basin, New Zealand

Organic Geochemistry 30 (1999) 593±605

A geochemical appraisal of oil seeps from the East Coast Basin, New Zealand Karyne M. Rogers a,*, John D. Collen b, Jim H. Johnston c, Nils E. Elgar b a Institute of Geological and Nuclear Sciences, PO Box 31312, Lower Hutt, New Zealand School of Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand c School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand b

Received 25 July 1997; accepted 22 February 1999 (Returned to author for revision 20 November 1997)

Abstract Oil seeps and stains from the East Coast Basin, New Zealand have been investigated using biomarker and stable carbon isotope analyses to determine oil-oil correlations. Oils sampled from the Raukumara Peninsula (northern East Coast Basin, North Island) and Marlborough (southern East Coast Basin, South Island) are derived from Late Cretaceous-Paleocene marine source rocks with a minor terrestrial content and are isotopically light. In contrast, oils sampled from Hawke's Bay and Wairarapa (central and southern East Coast Basin, North Island) are derived from Paleocene marine source rocks, which contain high abundances of C30 regular steranes and 28,30bisnorhopane, and are isotopically heavier than the other group. Biomarkers and bulk carbon isotopes show that there are at least two distinct sources of hydrocarbons in the basin. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Oil seeps; d 13C isotopes; New Zealand; East Coast Basin; Oil-oil correlation

1. Introduction The East Coast Basin of New Zealand (Fig. 1) has been of interest to petroleum explorers since 1865, with more than 45 wells drilled and more than 4 major oil seeps and 50 oil impregnations noted (McLernon, 1978; Francis, 1995). The basin comprises Early Cretaceous (110 Ma) to recent sediments (Fig. 2) and has an area of approximately 70,000 km2. It is up to 10 km deep in places and contains mostly ®ne-grained marine sediments. The geological history of the basin is complex as it is situated on an active plate margin

* Corresponding author.

boundary, with the Paci®c Plate being subducted under the Indo-Australian Plate. Structures formed prior to the end of the Cretaceous were mostly tilted fault blocks, later modi®ed by gentle compressional folding during subsequent tectonic activity. Subsidence continued during the Late Cretaceous, with marine deposition occurring in a transgressive sequence, which reached its maximum at the beginning of the Paleocene. Maximum transgression is coincident with deposition of the Waipawa Black Shale, an organicrich formation previously suggested as the only potential source rock (capable of generating commercial quantities of hydrocarbons) in the basin (Leckie et al., 1992). During the Oligocene to Miocene, further tectonic activity caused overthrusting of Cretaceous to

0146-6380/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 6 - 6 3 8 0 ( 9 9 ) 0 0 0 3 6 - 4

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K.M. Rogers et al. / Organic Geochemistry 30 (1999) 593±605

Fig. 1. Location of oil seeps and impregnations, East Coast Basin, New Zealand.

Oligocene strata to form the thick allochthonous complex present in the eastern margin of the East Coast Basin (Moore et al., 1986). To date, there has been no economic production of oil, however some gas production has recently been forthcoming from the basin in Kauhauroa-1 and Awatere-1 Wells, Hawke's Bay (Ministry of Commerce, 1998). Earlier wells were drilled on, or adjacent to, oil seeps with little regard to geology. More recent seismic re¯ection surveys and detailed surface mapping have not led to the discovery of a commercial accumulation, although the presence of three main seep areas in the north (Rotokautuku, Waitangi and Totangi; Fig. 1), numerous other smaller seeps (both oil and gas) and oil staining in rocks is evidence for petroleum generation regionally, on at least a moderate scale. Seeps and impregnations have been noted in the East Coast Basin in formations ranging in age from Late Cretaceous through to recent sediments, and determining the origin of the oil has therefore not been straightforward. This study has investigated 10 seep oils and stains, four of which are located in the northern part of the basin, and six in the south (Fig 1, Table 1).

Other oil occurrences in the Maunga Structure, Cook's Tooth, Kerosene Blu€ and London Hill (McLernon, 1978) were also visited but there were no signs of activity. Several undocumented Waipawa Black Shale outcrops from the southern part of the basin (Taurekaitai and Otoro Streams) which smelt strongly of hydrocarbons (Moore, 1989; Elgar, 1997) were also found during the study, but are not discussed here. This study presents a geochemical investigation using traditional biomarker parameters as well as bulk stable carbon isotopes for oil-oil correlation. For comparative purposes, data have also been included for oil from the Kora-1 well from the north Taranaki Basin, as this oil has been associated with Paleocene black shales similar to those located in the East Coast Basin (Reed, 1992; Rogers et al., 1994; Killops et al., 1994) 2. Experimental Oil samples from seeps were collected and stored in glass jars prior to analysis. Oil-impregnated samples were hand picked to remove organic detritus, cleaned and coarsely broken. The oil was extracted from the

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Fig. 2. Overview of the stratigraphy of the East Coast Basin HG=Hapuku Group, WG=Wallow Group, CG=Coverham Group (Field et al., 1997).

rock with a rapid wash using dichloromethane. After deasphalting with n-pentane, the oils were separated into saturate and aromatic fractions using column chromatography (silica/alumina) with n-pentane, followed by a 1:1 pentane/dichloromethane mix as eluants. The saturate fraction was separated into nalkanes and branched-cyclic compounds using molecular sieve (5A) and iso-octane. The saturate fraction was analysed by gas chromatography, using a Hewlett Packard 5890 series II gas chromatograph a J&W DB1 fused silica column (30 m  0.2 mm I.D.  0.25 mm ®lm thickness). The gas chromatograph was programmed (after 1308C for 3 min), at 68C/min to 3008C, with a ®nal hold time of 9 min. Analysis by gas chromatography-mass spectrometry (GC-MS) of the triterpane (m/z 191) and sterane (m/z 217) biomarkers was performed using a Hewlett Packard 5995C GC-MS system with a J&W DB1 fused silica column (30 m  0.2 mm I.D.  0.25 mm ®lm

thickness). The gas chromatograph was programmed (after 508C for 15 min), at 48C/min to 2308C, then 28C/min to 2808C, with a ®nal hold time of 4 min. Identi®cation of appropriate m/z 191 and m/z 217 peaks are listed in Table 2 and were determined via comparison of relative retention time to previous studies on both Taranaki and East Coast oils (Collier and Johnston, 1991; Johnston et al., 1992). 3. Results and discussion 3.1. n-Alkane and isoprenoid biomarkers Oil seeps from the northern part of the basin (Waitangi, Rotokautuku, Totangi and Motu Valley) contain between 69±73% saturated alkanes, although Rotokautuku has a low saturate/aromatic ratio of 3.6 (according to Weston et al., 1988) compared to the

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Table 1 Location and geology of East Coast Basin oils Oil

Location

Location

Oil

Geology

Waitangi

Waitangi, Northern ECB

Y17/380 070

Seep

Totangi

Totangi, Northern ECB

X17/275 865

Seep

Rotokautuku

Rotokautuku, Northern ECB

Z15/766 573

Seep

Motu Valley

Motu Valley Road, Northern ECB

X17/062 076

Stain

Knights Stream

Knights Stream, Otane, Southern ECB North-eastern Malborough, Southern ECB

V22/163 386

Stain

P29/919 225

Seep

Kerosene Rock, north-eastern Wairarapa, Southern ECB Westcott Station, Wairarapa, Southern ECB Tiraumea Valley Road, Southern ECB Okau Stream, Castlepoint, Southern ECB

U25/918 517

Stain

U24/888 902

Stain

T25/661 561

Stain

U26/817 364

Stain

Seeping from a fault plane between Upper Cretaceous shales and Miocene mudstones (Hoolihan, 1980; Francis, 1995) Seeping from a Miocene-Pliocene angular unconformity visible in outcrop (Francis, 1995) Seep sampled from hand-dug shaft near other seeps Outcrop smelt strongly of hydrocarbons, Upper Calcareous Member, Whangai Formation Outcrop smelt strongly of hydrocarbons, Rakauroa Member, Whangai Formation Fine ®lm of oil on a small pond near the right hand side of the creek, associated with Amuri Limestone Located in glauconitic sandstone of Akitio Sst Mbr, Weber Formation Sandstone bed within a marine siltstone of Early Miocene age (Hutson, 1989) Marine silt/sandstone under ? Miocene sediments Greensand enclosed in Waipawa Black Shale (Paleocene)

Isolation Creek Kerosene Rock Westcott Tiraumea Okau Stream

Waitangi and Totangi oils (around 6). The saturate fractions of these oils have unimodal n-alkane distributions (Fig. 3) which lie predominantly between C8 and C30, with a maximum at C15. The Carbon Preference Indices (CPI) are approximately 1, Table 3). Slight biodegradation of the Waitangi, Rotokautuku and Motu Valley oils is indicated by the moderately high abundance of acyclic isoprenoids (e.g. Pr/nC17 0 2±3) relative to n-alkanes, as the latter are one of the ®rst groups of biomarkers to be a€ected by biodegradation (Volkman et al., 1983). The Totangi seep oil has su€ered a higher degree of biodegradation than the other northern oils (Fig. 3), characterised by an unresolved complex mixture of hydrocarbons with only isoprenoids (predominantly pristane and phytane) present as discrete peaks. Oils from the southern part of the basin show higher levels of biodegradation than those from the north. Knights Stream and Kerosene Rock oils contain abundant acyclic isoprenoids relative to n-alkanes (Pr/ nC17>3), suggesting light biodegradation (Table 3). Isolation Creek oil shows low isoprenoid/n-alkane ratios; however, it contains very low levels of lower molecular weight hydrocarbons and may have been a€ected by biodegradation. The southern oils are unimodal with a carbon preference index (CPI) around 1. Knight's Stream oil has a n-alkane maximum at 16,

while Isolation Creek and Kerosene Rock contain higher molecular weight hydrocarbons and have nalkane maxima at 24 and 27 respectively. Westcott, Tiraumea and Okau Stream oils showed an unresolved complex mixture, suggesting extensive biodegradation of both the n-alkanes and acyclic isoprenoids. On the basis of gas chromatography, the northern oils and the southern Knights Stream oil could be marine derived as they show a unimodal n-alkane distribution centred around the most abundant n-alkane (Cmax=16) and a predominance of lower molecular weight hydrocarbons. However the southern Isolation Creek and Kerosene Rock oils contain more abundant higher molecular weight hydrocarbons, which are unimodal and centred around C24 and C27 respectively, and are likely to be derived from source rocks with some terrigenous input. The oils have all experienced some degree of biodegradation, usually associated with weathering of exposed seep oils and impregnations, and therefore these results cannot be conclusive evidence for source input. 3.2. Triterpane biomarkers The triterpane (m/z 191) distributions of the four northern oils and the southern Knights Stream and Isolation Creek oils display similar biomarker charac-

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Table 2 Triterpane and sterane peak identi®cation Peak

Steranes

Carbon No.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

13b(H), 17a(H)-Diacholestane (20 S) 13b(H), 17a(H)-Diacholestane (20R) 24-Methyl-13b(H), 17a(H)-diacholestane (20 S) (24 S+R) 24-Methyl-13b(H), 17a(H)-diacholestane (20R) (24 S+R) 24-Ethyl-13b(H), 17a(H)-diacholestane (20 S) (co-eluting) 24-Ethyl-13b(H), 17a(H)-diacholestane (20R) (co-eluting) 5a(H), 14a(H) 17a(H)-Cholestane (20 S) 5a(H), 14b(H) 17b(H)-Cholestane (20R) 5a(H), 14b(H) 17b(H)-Cholestane (20 S) 5a(H), 14a(H) 17a(H)-Cholestane (20R) 24-Methyl-5a(H), 14a(H) 17a(H)-cholestane (20 S) 24-Methyl-5a(H), 14b(H) 17b(H)-cholestane (20R) 24-Methyl-5a(H), 14b(H) 17b(H)-cholestane (20 S) 24-Methyl-5a(H), 14a(H) 17a(H)-cholestane (20R) 24-Ethyl-5a(H), 14a(H), 17a(H)-cholestane (20 S) 24-Ethyl-5a(H), 14b(H), 17b(H)-cholestane (20R) 24-Ethyl-5a(H), 14b(H), 17b(H)-cholestane (20 S) 24-Ethyl-5a(H), 14a(H), 17a(H)-cholestane (20R) 24-Ethyl-5b(H), 14a(H), 17a(H)-cholestane (20R) C30 Steranes

27 27 28 28 29 29 27 27 27 27 28 28 28 28 29 29 29 29 29 30

Peak A B C Ns D E F G,H I,J K,L M N P,Q X O

Triterpanes 18a(H)-22,29,30-Trisnorneohopane (Ts) 17a(H)-22,29,30-Trisnorhopane (Tm) 17a(H), 21b(H)-30-Norhopane 18a(H)-30-Norneohopane (C29 Ts) 17b(H), 21a(H)-30-Norhopane (Normoretane) 17a(H), 21b(H)-Hopane 17b(H), 21a(H)-Moretane 17a(H), 21b(H)-30-Homohopane (22 S,22R) 17a(H), 21b(H)-30,31-Biohomohopane (22 S,22R) 17a(H), 21b(H)-30,31,32-Trishomohopane (22 S,22R) 17a(H), 21b(H)-28,30-Bisnorhopane 17b(H)-22,29,30-Trisnorhopane 17b(H), 21a(H)-30-Homohopane (22 S,22R) C30 17a(H)-Diahopane (Compound X) 18a-(H) Oleanane

Carbon No. 27 27 29 29 29 30 30 31 32 33 28 27 31 30 30



teristics (Fig. 4, Table 4). Their chromatograms are dominated by 17a(H), 21b(H) hopane, and show a decreasing abundance of C31 to C35 homohopanes, with relatively low abundances of Ts and Tm. Other rearranged hopanes, especially C29Ts and the C30 diahopane are also present in minor amounts. Based on conventional homohopanes and hopane:moretane ratios these oils have attained near maximum equilibration levels, suggesting a reasonable level of thermal maturity. Source speci®c biomarkers such as 18a(H)-oleanane (O), an angiosperm plant indicator, 25,28,30-trisnorhopane (N) and 28,30-bisnorhopane (M) (found in immature samples and/or in those derived from an anoxic

environment; Moldowan et al., 1984, 1994), are present only in trace quantities in the oils. Very low levels of 28,30-bisnorhopane are consistent with the higher degree of maturity of these oils, but may have also been absent from the source. Ts/Tm ratios between 0.5 and 1.0 suggest that these oils may be of moderate maturity, but could also indicate an oxic environment of deposition (Moldowan et al., 1986), consistent with the low abundance of 28,30-bisnorhopane. A lack of diterpanes in these oils suggests they do not contain organic matter derived from gymnosperms (prevalent from mid Paleozoic until Early Cretaceous, Weston et al., 1989). However, minor abundances of oleanane (an angiosperm-derived marker) suggest the

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K.M. Rogers et al. / Organic Geochemistry 30 (1999) 593±605

Fig. 3. Saturate fraction gas chromatograms of East Coast Basin oils: 1. Waitangi 2. Totangi 3. Rotokautuku 4. Motu Valley 5. Isolation Creek 6. Knights Stream. Pr=pristane, Ph=phytane, numbers refer to n-alkane chain length.

possibility of a slight terrestrial contribution and relates these oils to a Cretaceous or younger source (Moldowan et al., 1994). The four other southern oils (Kerosene Rock, Westcott, Tiraumea and Okau Stream), are also dominated by 17a(H), 21b(H) hopanes, and show a decreasing abundance of C31 to C35 homohopanes; however, they contain a higher abundance of 28,30-bisnorhopane (M) than the other group of oils. They also contain higher abundances of oleanane and rearranged

hopanes, especially C29Ts and the C30 diahopane, than the other oils. On the basis of GC-MS data, the 10 oils can be divided into two main groups. Hopane and homohopane abundance and distribution are similar for all oils, however four southern oils (Kerosene Rock, Westcott, Tiraumea and Okau Stream) show much higher abundances of 28,30-bisnorhopane, suggesting that these oils were derived predominantly from reduced oxygen or anoxic source rock. Higher abun-

K.M. Rogers et al. / Organic Geochemistry 30 (1999) 593±605 Table 3 Saturate ratios East Coast oils Oil

Pr Ph

Pr nC17

Ph nC18

CPI n-Alkane max

C21 ‡C22 C28 ‡C29

Waitangi Totangia Rotokautuku Motu Valley Knights Stream Isolation Creek Kerosene Rock Westcott Tiraumea Okau Stream

0.95 2.95 0.58 1.93 2.57 0.99 2.17

2.63 1.52 2.97 2.02 3.66 0.38 6.12

1.83 0.64 0.83 1.15 1.57 0.42 2.11

1.10 1.06 0.96 0.99 1.02 1.05 0.97

1.19 n.d.b n.d. 1.48 2.73 2.14 0.38

a b

15 16 15 15 16 24 27

No data due to biodegradation

Data from Weston et al. (1998). n.d.=not detected.

dances of 17a(H)-diahopane, which has previously been suggested as a terrestrial marker (Philp and Gilbert, 1986), are also present in these four oils. 3.3. Sterane biomarkers The four northern oils and two southern oils (Knights Stream and Isolation Creek) can be grouped together as they display similar sterane distributions to each other (Fig. 4, Table 4). The regular sterane distribution pattern of the group is C27 r C28>C29>C30, which is indicative of a marine-source (Huang and Meinschein, 1979) and is consistent with the n-alkane data. These oils are characterised by low levels of C30 regular steranes (between 7±15%, con®rmed by GCMS-MS; Murray et al., 1994; Rogers, 1995) relative to other regular steranes, and a similar abundance of C27, C28 and C29 regular steranes. The abundance of diasteranes relative to regular steranes of the oils is moderate to high, and indicates a relatively high level of thermal maturity, as biodegradation (which would increase these values) has not signi®cantly a€ected the sterane distributions. The Totangi and Knights Stream oils have lower C27 diasterane contents (Fig. 4, peaks 1 and 2) than the other oils in this group, consistent with a slightly lower thermal maturity. Based on %bb steranes and S/(S+R) steranes (Fig. 5, Table 4), Isolation Creek, Waitangi and Motu Valley oils display the highest maturity characteristics in this group, which appear to have formed from source rocks at or near peak oil generation. The Rotokautuku oil shows lower maturity, but contains the highest level of C30 steranes of the group. Totangi and Knights Stream oils are the least mature of the group, with prominent aaaR steranes, low C27 and C29 diasterane abundance and the lowest levels of C30 regular steranes in the group.

599

In contrast, Kerosene Rock, Westcott, Tiraumea and Okau Stream oils are dominated by C30 regular steranes (up to 45% of total sterane content), showing sterane distribution patterns of C30 r C29 r C27>C28. This suggests a marine source dominated by organisms generating predominantly C30 regular steranes rather than C27 regular steranes. C30 regular steranes (24-npropylcholestanes) are marine algal-derived compounds of phytoplanktonic origin which are absent in oils derived solely from lacustrine or nonmarine rocks (Moldowan et al., 1985, 1990; McCa€rey et al., 1994). The level of C29 regular steranes (around 30%) is similar to oils from the ®rst group, suggesting a similar small terrestrial in¯uence in both groups. Oils with a slightly lower C30 regular sterane content (Kerosene Rock and Tiraumea) suggest dilution by oil derived from a di€erent source rock, or oil derived from the same source rock that contains lower C30 sterane abundances. Okau Stream oil is the only sample taken adjacent to the Waiapawa Black Shale (a stained greensand enclosed within the shale) and contains the highest C30 regular sterane content (42%). Westcott, Tiraumea and Kerosene Rock oils are located in Oligocene/Miocene age rocks and contain lower abundances of C30 regular steranes than the Okau Stream oil, consistent with either mixing/dilution by another oil, or a lower C30 sterane organic input to the source. An in-depth source rock study conducted by Rogers (1995, 1996) and Elgar (1997) has shown that the C30 regular sterane content of the Waipawa Black Shale is variable throughout the basin. This suggests that the abundance of C30 steranes in the oils may be dependent on variations within the source, rather than a more complex mixing regime. A sterane ternary diagram (Fig. 6) shows that the C29 sterane levels are fairly consistent between the studied oils and comprise 20±40% of the total sterane content. The main variation occurs in the abundance of C27 and C28 steranes, which range from 29% (for the Okau Stream oil) up to 70% (for the Motu Valley oil). Lower abundances of C27 and C28 steranes are compensated by a higher C30 sterane abundance, and serve to distinguish the oils from their di€erent sources. The C30 steranes present in these oils (<8% of the total aaaR sterane content, Table 4) supports a slight marine algal contribution in their corresponding source rocks. 3.4. Other geochemical data The organic sulfur content of the Waitangi and Rotokautuku oil seeps is 0.4% (0.21%; Lowe and O'Reilly, 1980) and 0.12% respectively (Frankenberger, 1994), which suggests that these oils are generated from source rocks with a comparably low organic sulfur content.

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Fig. 4. Triterpane (m/z 191) and sterane (m/z 217) chromatograms of East Coast Basin oils: 1. Waitangi 2. Totangi 3. Rotokautuku 4. Motu Valley 5. Isolation Creek 6. Westcot. Refer to Table 2 for peak identi®cation.

ÿ 28.0 ÿ 27.5 ÿ 27.4 ÿ 27.9 ÿ 28.2 ÿ 26.2 ÿ 25.2 ÿ 22.4 ÿ 23.5 ÿ 22.0 ÿ 29.0 ÿ 28.5 ÿ 28.4 ÿ 28.9 ÿ 29.3 ÿ 28.5 ÿ 26.3 ÿ 23.1 ÿ 25.4 ÿ 23.6 54 40 47 51 24 48 52 57 48 51 57 52 62 59 35 53 62 58 51 44 44 36 37 42 28 51 51 55 39 40 35 20 31 40 34 64 53 62 37 35 29:31:29:1 32:35:25:8 31:27:27:15 28:42:21:9 35:24:34:7 31:24:34:11 26:19:31:24 19:20:33:37 29:16:26:29 17:13:28:42 15 17 16 16 22 18 14 15 21 38 5 3 4 4 5 6 26 26 9 18 55 53 57 54 55 50 55 52 59 56 2 2 2 2 2 3 8 45 18 32 Waitangi 2 Totangi 2 Rotokautuku 2 Motu Valley 2 Knights Stream 3 Isolation Creek 5 Kerosine Rock 10 Westcott 9 Tiraumea 7 Okau Stream 15

C27, S/(S+R) C29, S/(S+R) d 13C, d 13C, Saturate Aromatic %bbC29, Steranes 27:28:29:30, aaaR Steranes %bbC27, Steranes Oil

Oleanane C30 Hopane

BNH C30 Hopane

C31, S/(S+R)

C30 Diahopane C30 Hopane

C30 Moretane C30 Hopane

Steranes (%) Triterpanes (%)

Table 4 Biomarker and isotope data for East Coast oils. Ratios calculated from integrated peak areas (m/z 191 and m/z 217 chromatograms)

Bulk isotopes (-)

carbon

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601

The relationship between the East Coast Basin oils was investigated using bulk carbon isotope analysis. Results were compared with additional data from Reed (1992) on the, Kora oil, derived from Paleocene Rocks in the Taranaki Basin (Fig. 7, Table 4). Bulk stable carbon isotope analyses can support (but not prove) an oil-oil correlation, if oils of similar maturity di€er by less than 1- (Peters and Moldowan, 1993). Isotopic data of the saturate and aromatic hydrocarbon fractions of oils from the northern East Coast Basin (Waitangi, Totangi, Rotokautuku and Motu Valley), Knights Stream and Isolation Creek indicate that they are isotopically light and suggests that they were generated from the same or a very similar Upper Cretaceous marine source. However, subtle isotopic di€erences exist between this group of oils. Isotopic values for the Totangi, Rotokautuku and Isolation Creek oils are slightly heavier than for the Waitangi, Motu Valley and Knights Stream oils. These slight di€erences are probably due to slight source rock di€erences (possibly associated with variances in C30 regular sterane abundance, as already demonstrated in the Waipawa Black Shale) and/or biodegradation which could enrich the d 13C isotopic values (Hirner and Lyon, 1989). Oils from the central and southern East Coast Basin, North Island (Kerosene Rock, Westcott, Tiraumea and Okau Stream) are isotopically heavier (up to 6-) than the northern oils (Fig. 7). The Okau Stream and Westcott oils have the heaviest isotopic values. Tiraumea and Kerosene Rock oils lie in between the lighter northern oils and the heavier Okau Stream and Westcott oils. The main di€erences in biomarkers between all these oils is the C30 regular sterane contribution, which ranges from 7% in the northern oils, up to 37 and 42% in the Westcott and Okau Stream oils respectively. The Tiraumea oil contains a slightly lower level of C30 regular steranes (29%) and is isotopically lighter than the Okau Stream and Westcott oils. Kerosene Rock oil has even lower C30 regular steranes (24%) than Tiraumea oil and lies even further toward the isotopically light northern oils. Kora oil has been included as it is isotopically enriched (similar to Okau Stream and Westcott oils) and linked to Paleocene black shales in the Taranaki Basin (Reed, 1992; Rogers et al., 1994; Killops et al., 1994). Reed (1992) associated the heavy carbon isotope values of the Kora oil with algal blooms (which contribute C30 regular steranes). The carbon isotopic data can therefore distinguish isotopic di€erences between the northern oils (1st group), Kerosene Rock stain (2nd group) and the Okau stain (possibly a 3rd group). The Kora oil plots in the same region as the Okau Stream and Westcott oils, suggesting a likely correlation to the equivalent black shales in the East Coast Basin. The Kerosene Rock and Tiraumea oils

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Fig. 5. Sterane maturity parameters of East Coast Basin oils.

plot in between the northern oils and the Okau Stream and Westcott oils, suggesting either a mixing regime between oils from two di€erent sources, or lateral organic variations within the Waipawa Black Shale. On the basis of isotopic and biomarker data, the East Coast Basin contains at least two di€erent source

rocks, individually generating isotopically di€erent oils. One group of oils, including the northern East Coast Basin oils (Totangi, Waitangi, Rotokautuku and Motu Valley), the southern Marlborough Isolation Creek and central Hawke's Bay Knight Stream oils which are isotopically light, and another group in the southern Wairarapa (Okau Stream and Westcott) which is isotopically enriched. Two other oils (Kerosene Rock and Tiraumea) fall between these two extremes and either contain a mixture of these two end members (suggesting a complex mixing regime), or re¯ect lateral variations within the Waipawa Black Shale. This is supported by their intermediate C30 regular steranes, 28,30-bisnorhopane, and oleanane abundances, with Kerosene Rock oil containing lower relative quantities (more similar to the northern oils) than the Tiraumea oil which is closer to Okau Stream and Westcott oils.

4. Conclusions A geochemical study of 10 oils and stains from the East Coast Basin, New Zealand, has shown that two

Fig. 6. Ternary diagram showing regular sterane distribution of East Coast Basin oils and Kora-1 oil (Taranaki Basin).

K.M. Rogers et al. / Organic Geochemistry 30 (1999) 593±605

603

Fig. 7. Stable carbon isotope data of the aromatic and saturate hydrocarbon fractions from East Coast oils (additional Kora oil data after Reed, 1992).

distinct groups of oils exist. Biomarker and isotopic studies show the northern oils (Waitangi, Totangi, Rotokautuku and Motu Valley) as well as southern Marlborough Isolation Creek, and central Hawke's Bay Knight Stream oils are generated from a Late Cretaceous clastic marine source rock with a minor terrestrial in¯uence deposited in a sub-oxic environment, most likely the Whangai Formation. This is supported by abundant C27 and C28 regular steranes and by the presence of C30 regular steranes, but they also contain minor traces of oleanane, a CretaceousTertiary angiosperm source indicator. However, the Waitangi, Motu Valley and Knights Stream oils appear to have a closer correlation to each other, both isotopically and from biomarker ratios, than to the Rotokautuku, Totangi and Isolation Creek oils. Variations within these oils such as C30 steranes and isotopic variations are attributed to regional and lateral variations in organic matter type (the Rakauroa Member of the Whangai Shale has a slightly higher C30 sterane content than the Upper Calcareous Member; Rogers, 1995, 1996), biodegradation, and possibly to small di€erences in thermal histories. The second group of oils (Okau Stream, Westcott, Tiraumea and Kerosene Rock) are likely to be derived from the Paleocene Waipawa Black Shale because of biomarker similarities (high C30 regular sterane abundances, Rogers et al., 1994) and isotopic correlation to

the enriched Paleocene derived Kora oil (Reed, 1992). Either a mixing regime or a lateral and/or stratigraphical source variation controls the biomarker and isotopic characteristics of the Kerosene Rock and Tiraumea oils. If a mixing regime between the two oil sources is considered, an estimated 50:50 ratio (based on isotopic data and C30 regular sterane abundance) is contributed by both the Whangai Formation and Waipawa Black Shale to the Kerosene Rock oil. The Tiraumea oil is estimated to contain a 30:70 ratio of oil derived from the Whangai Formation and Waipawa Black Shale. This study has shown that East Coast Basin oils fall into at least two distinct families, suggesting at least two independent hydrocarbon source rocks in this basin, and has proposed a mixing regime or lateral variation in source to explain isotopic and biomarker di€erences between these groups. Acknowledgements Thanks are due to Prof. Paul Philp (University of Oklahoma) for GC-MS-MS analyses, Robert Pallasser (CSIRO, Sydney) and Prof. M. Engel (University of Oklahoma) for bulk carbon isotope analyses, and to Drs Martin Fowler, Andrew Murray and Simon George for helpful comments and reviews. This work was ®nanced by Petrocorp Exploration Limited,

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