Petroleum source rock survey in the Solomon Islands and - SOPAC

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1 PETROLEUM SOURCE ROCK SURVEY IN THE SOLOMON ISLANDS AND VANUATU SOUTHWEST PACIFIC H. Johnson, B.D. Hackman, M.Hawkins, C.Mortimer, M.Stephen British Geological Survey June 1994 SOPAC Technical Report 207 Prepared for: South Pacific Applied Geoscience Commission (SOPAC)

2 [3] CONTENTS Page SUMMARY........................................................................................................................................... 5 ACKNOWLEDGEMENTS ........................................................................................................... 6 INTRODUCTION......................................................................................................................... 7 SURVEY PLANNING ................................................................................................................. 7 SAMPLING PROGRAM............................................................................................................ 7 Solomon Islands........................................................................................................ 8 Guadacanal................................................................................................................ 8 Florida Islands.......................................................................................................... 10 Choiseul....................................................................................................................... 10 Malaita..................................................................................................................... 10 Santa Isabel............................................................................................................. 11 Vanuatu.............................................................................................................................. 11 North-central Santo ................................................................................................. 11 Northwest Santo............................................................................................. 12 Maewo....................................................................................................................... 12 Malekula .............................................................................................................. 13 Pentecost..................................................................................................................... 13 GEOCHEMICAL ANALYSIS AND INTERPRETATION ............................................................. 13 Rock-Eval pyrolysis and TOC Analysis .............................................................................. 14 Extraction, GC-MS and Biomarker Analysis........................................................................ 15 Solomon Islands Samples........................................................................................ 16 Vanuatu Samples ........................................................................................................ 17 REFERENCES .......................................................................................................................... 49 APPENDIX 1 Experimental Procedure............................................................................................... 51 2 Gas Chromatograms of Saturated Hydrocarbon Fractions............................................ 55 [TR207- Johnson and others]

3 [4] LIST OF FIGURES & TABLES Page Figure 1 General map of sample locations in the Solomon Islands.........................................19 2 Location of outcrop samples in the Mbokokimbo Formation, Guadalcanal.................20 3 Location of outcrop samples in the Florida Islands......................................................21 4 Location of outcrop samples in Choiseul..................................................................... 22 5 Location of outcrop samples in Malaita.....................................................................23 6 Location of outcrop samples in north-central Santo...................................................24 7 Location of outcrop samples in northwest Santo........................................................25 8 Location of outcrop samples in Maewo....................................................................... 26 Table 1 Details of samples analysed from the Solomon Islands........................................27-35 2 Details of samples analysed from Vanuatu..........................................................36-40 3 Rock-Eva1 tests on selected samples before and after acid digestion.......................41 4 Extraction and detailed hydrocarbon analysis, Solomon samples............................ 42 5 Extraction and detailed hydrocarbon analysis, Vanuatu samples..............................43 6 GC-MS data, Solomons & Vanuatu samples: absolute areas of diagnostic peaks .44-45 7 GC-MS data, Solomons & Vanuatu samples: key ratios for maturity & diagnostic parameters of depositional environments...........................................46-47 8 Key to source-specific biomarker parameters...........................................................48 [TR207- Johnson and others]

4 [5] SUMMARY 1. A wide ranging survey for potential petroleum source rocks was carried out in the Solomon Islands and Vanuatu by BGS, on behalf of SOPAC. A total of 137 samples from the Solomon Islands and 68 from Vanuatu were subjected to preliminary screening analysis (see below). The principal targets were marginal marine facies, but deep water deposits were also investigated. 2. A portable source rock analyser was provided for the survey, so that the organically richest samples could be identified and sent for more accurate laboratory analysis. However, only 7 of the Solomons Islands samples tested, and 4 of the Vanuatu samples tested, gave fair to very good potential yields on the field analyser. The remainder of the samples tested gave poor potential yields. On Guadalcanal (Solomon Islands), twenty-seven field samples were tested from the Upper Miocene Mbokokimbo Formation, and a further twenty-nine archive samples were tested from the Mbokokimbo Formation, Lower Miocene Lake Lee Calcarenite, Miocene Tangareso Beds, Miocene Tina Calcarenite, Miocene Kavo Greywacke Beds and the Pliocene Lungga Beds. On the Florida Islands, ten field samples were tested from the Mio-Pliocene Mboli Beds and the Pliocene Anuha Calcarenite. On Chioseul, ten field samples were tested from the Pliocene Pemba and Mole formations. From Malaita, five field samples were tested from the Alite Formation, Haruta Calcisiltite and the Kwara'ae Mudstone, and fifty-one archive samples were tested, mainly from the Cretaceous to Pliocene Malaita Group (including the Rokera Limestone and the Suafa Calcisiltite). From Santa Isabel, five archive samples were tested from the Oligocene to Miocene Maringe Group and the Upper Miocene to Pliocene Sisibena Formation. On Santo (Vanuatu), thirty-eight field samples were tested from the Middle Miocene-Lower Pliocene Tawoli Formation and the Middle Miocene Peteao and Pelapa formations. Two archive samples from Santo, of unknown lithostratigraphy, were also tested. On Maewo, nine field samples were tested from the Middle to Upper Miocene Tafwutmuto and Wustorogha formations, and two archive samples of unknown lithostratigraphy were also tested. From Malekula, fifteen archive samples of unknown lithostratigraphy were tested. From Pentecost, two samples of unknown lithostratigraphy were tested. 3. Twenty-five of the richest samples from the Solomon Islands and twenty-five samples from Vanuatu were analysed for source rock potential in the laboratory, using Rock-Eva1 pyrolysis and Leco total organic carbon (TOC) determination. Although some samples showed high TOC contents, most had low pyrolysable carbon (S2) and hydrogen index (HI) values and, as a consequence, are considered to have poor potential for petroleum generation. [TR207 - Johnson and others]

5 [6] 4. One field sample of carbonaceous mudstone from the Mbokokimbo Formation, Guadalcanal, Solomon Islands (5: 14/8, #7067) had apparently "good" source potential. It contained a high content of immature and very labile (i.e. readily reactive on heating) kerogen that contributed, in Rock-Eva1 testing, to both the free hydrocarbons in the rock (Si) and the hydrocarbons generated during pyrolysis (S2). However, this sample was taken from a thin, lenticular bed of limited geographical extent. One archive sample of breccia (of unknown lithostratigraphy) from Malekula, Vanuatu (MM 443, #7446) had "fair" source potential. Hydrogen Index values suggest that these two samples have potential to generate only gas. 5. The Solomon Islands sediments examined for diagnostic biomarkers were all deposited in anoxic marine settings with variable contents of marine and terrestrial organic matter. Type ll or Type ll/lll organic matter predominates and the mainly gas-prone sediments are below the maturity threshold for hydrocarbon generation. 6. The Vanuatu samples predominantly contain immature, terrestrial, Type lll organic matter. Despite the high organic carbon content of three of the samples, they are only likely to be gas prone and are not yet near the maturity window for hydrocarbon generation. ACKNOWLEDGEMENTS The Commission of the European Community is thanked for funding this project. The substantial help and support of the Department of Geology, Mines and Water Resources, Vanuatu and the Solomon Islands Ministry of Natural Resources in the field survey is gratefully acknowledged. In particular we thank: Mr Don Tolia (Chief Geologist, Solomon Islands Geological Survey), Mr Patrick Nanau (Economic Geologist, Solomon Islands Geological Survey), Mr M. Petterson (BGS/Solomon Islands Geological Survey), Dr B. McGrail (BGS/Solomon Islands Geological Survey). The help of Mr N. Legua, Mr I. Poloso, Mr I. Leanamae (Lands Officer), Mr A. Mason (Geological Assistant), and the Premier of Choiseul Province, is also acknowledged. [TR207- Johnson and others]

6 [7] INTRODUCTION In 1993/94, the British Geological Survey (BGS) conducted a survey in the Solomon Islands and Vanuatu to evaluate petroleum source rock potential, on behalf of the South Pacific Applied Geoscience Commission (SOPAC). The project was initiated by the Governments of the islands and was managed by SOPAC, using funds provided by the Commission of the European Community. The project was divided into three phases: survey planning, sampling program, and geochemical analysis and interpretation. Local support for the project was provided by the Solomon Islands Ministry of Natural Resources and the Department of Geology, Mines and Water Resources, Vanuatu. SURVEY PLANNING Survey planning involved research of previous work and discussions with experts in the local geology. Models for source rock genesis in Southwest Pacific island arcs have been developed by, for example, Eden & Smith (1984), Coleman (1989) and Falvey et a/. (1991). These models suggest that marginal marine lithofacies probably constitute the best potential source rocks. Consequently, such rocks were the primary targets for further investigation in this survey. However, testing of deeper water facies for source beds was also carried out, as had been proposed by Katz (1 986). The local occurrence of fine grained laminated units in these basinal sediments was taken as an encouraging indication. A detailed plan for sampling potential source rocks exposed on the islands (Johnson 1993) was submitted to, and approved by, SOPAC before commencing the survey. SAMPLING PROGRAM To enable semi-quantitative analysis and screening of organically lean samples to be carried out on-site, BGS supplied a portable Source Rock Analyser for the field survey. This instrument was developed by British Petroleum Research and comprises a portable pyrolysis and detector system. After heating a small, sieved, sample, the detector measures the amount of volatile hydrocarbons expelled from the contained organic matter. The instrument has been calibrated to give approximate total potential yield (PY) values (i.e. S1 + S2) in kilograms of hydrocarbons per tonne of rock (see Appendix 1 for a brief explanation of source rock parameters referred to in this report). The following rough classification for source rocks based on potential yield was suggested by Tissot & Welte (1 984) and Peters (1 986): [TR207- Johnson and others]

7 [8] 0- 3 kg pertonne poor 3- 6 kg pertonne fair 6-12 kg pertonne good > 12 kg per tonne very good However, results from the field analyser are only approximate and laboratory testing is necessary for accurate assessments of source potential. The results of the field survey in the Solomon Islands and Vanuatu are summarised in Tables 1 &2*, respectively. Solomon Islands A wide-ranging and detailed sampling program was carried out in the Solomon Islands by Dr B. Hackman (Figure 1)*. The survey included aircraft flights to access marginal marine facies of the Mbokokimbo Formation on Guadalcanal and the Mole Formation on Choiseul, and boat journeys to investigate outcrops on the Florida Islands and deep water facies on Malaita. The sampling program took account of "petroleum play concepts" in both the "Central" and "Pacific Provinces" as defined by Coleman (1965). A total of 136 samples were selected for screening analysis of which 52 were from outcrop and 84 were representative and well-documented samples from the archives of the Solomon Islands Geological Survey. The field analyser was used to broadly assess the organic richness of the sediments and to assist in the selection of twenty-five samples for more accurate laboratory analysis. Guadalcanal The sampling program of Buchbinder & Halley (1986) was limited to outcrops along the road network on the north coast. Buchbinder & Halley (1986) acknowledged the inadequacy of their coverage of Guadalcanal for the purposes of source rock evaluation, and recommended more extensive sampling. In this survey, twenty-seven samples from the interior of the island were collected at outcrop, and 26 others from this area were selected from the Geological Survey archives for testing in Honiara (Table 1). The primary stratigraphic target for potential source beds on Guadalcanal was the Mbokokimbo Formation in east-central Guadalcanal (Figure 2), as it is the most extensive onshore outcrop of For all tables and figures see pages19 to 48 [TR207 - Johnson and others]

8 [9] fine grained sediments where some indications of organic debris have been recorded. It comprises Upper Miocene and Pliocene fine grained sandstones, siltstones and mudstones which crop out over an area of about 550 km (Hackman 1980). It reaches a stratigraphic thickness of about 4000 m, but wedges out rapidly to the west of the Mbokokimbo River, and to the east of the Mongga River. The siltstones in the basal 300 m of the Mongga River section tend to be rich in carbonised wood fragments, but lenses of carbonaceous mudstone occur only intermittently and locally can be seen to wedge out rapidly. The Mbokokimbo Formation was probably deposited in an estuarine environment which received large volumes of sediment from the mafic volcanic terrain to the south. The organic-rich lenses may represent local vegetation mats that survived burial (Hackman 1980). Field tests gave good to very good values in just two cases (12/4, 17.6 kg/tonne & 14/8, 13.7 kg/tonne), on samples which were megascopically carbonaceous (Table 1). However, these samples were taken from lenses up to about 5m thick for which there is no evidence of significant lateral persistence. Another sample, of dark brown mudstone from the lower Manuhoho River section, yielded a fair value (12/3, 4.0 kg/tonne). The other 24 field samples were more representative of the bulk of the Mbokokimbo Formation. Although they all gave some indication of volatile organic matter, the analyser values were all in the "poor" range. A total of twenty-nine Guadalcanal samples were tested from the archives (Table 1). Of these, fifteen were from the Mbokokimbo Formation. One sample (7151), a selected black clast within siltstone of the Mbalanga Shale from the Tuatolu headwater of the Tina River, gave a good yield of 8.6 kg/tonne (Table 1). The remaining fourteen Mbokokimbo archive samples gave poor potential yields. The Mbokokimbo Formation dips at low angles to the north, and overlies the Lower Miocene Lake Lee Calcarenite (1 00-350 m), which largely comprises reworked reef material. Two archive samples from the Lower Miocene Lake Lee Calcarenite gave poor potential yields. Archive material tested from Guadalcanal also included two samples from the Tina Calcarenite and seven samples of mudstone and siltstone from the Tangareso Beds. The Lower Miocene Tina Calcarenite (up to 300 m) and the overlying Mid Miocene Tangareso Beds (800 m) crops out to the west of the Mbokokimbo Basin and correlate, respectively, with the Lake Lee Calcarenite and the Mbokokimbo Formation. However, all these samples gave poor potential yields. Two archive samples were tested from the Kavo Greywacke Beds from southwest Guadalcanal. One gave a fair potential yield (5.7: 1b, 5.0 kg/tonne), the other was poor. One sample from the Pliocene Lungga Beds gave a poor potential yield. [TR207 - Johnson and others]

9 [10] Florida Islands The ten outcrop samples from the Florida Islands were collected from the Neogene Mboli Beds (Thompson 1958) and Anuha Calcarenite (Table 1), at sites that did not duplicate the work of Buchbinder & Halley (1986) (Figure 3). The Mboli Beds are lithologically similar to the Lungga Beds of western Guadalcanal and essentially comprise muddy sandstones and rudities with a higher volcaniclastic content than the Mbokokimbo Formation. The field analyser results were generally disappointing and all the tests gave poor potential yields. Choiseul Sampling was undertaken in the Choiseul Bay and Voza areas (Figure 4), which were geologically mapped on the scale of 1:50 000 by Strange (1981) and Strange & Danitofea (1979), respectively. Three samples from interbedded mudstones and siltstones of the mid-Upper Miocene Mole Formation gave poor potential yields (Table 1). Seven samples were tested from the Lower-Upper Pliocene Pemba Formation (from both the Mbani Calcisiltite and Sui Calcarenite members), but all gave poor values (Table 1). Malaita Five outcrop and fifty-one archive samples were tested from Malaita (Table 1), but the analyser results are not encouraging. This is unfortunate, as the folded structure of Malaita and adjacent areas offers many potential subsurface traps. The Cretaceous-Pliocene oceanic sediments of Malaita are laterally more continuous, and less susceptible to rapid facies changes, than are the post-Eocene sediments of the other major islands in the Solomons group. Thus, the five outcrop samples tested (Figure 5) are considered to be representative of the most prominent stratigraphic units in the Malaita Group. The field analyser results were poor, however, as none of the samples yielded detectable gas. For north Malaita, A. J. M. Barron provided twenty-three samples representative of various levels of the Neogene Suafa Limestone Formation, a pale grey chalk with variable terrigenous content. All of these gave poor potential yields (Table 1). A comprehensive suite of samples covering the complete range of stratigraphic units on southern Malaita and Maramasike (Hughes & Turner 1976) was available for testing in the Honiara archives. All of these samples gave poor potential yields (Table 1). [TR207 - Johnson and others]

10 [11] Santa Isabel Maps and reports on this area await publication, but the sedimentary lithofacies are similar to those of northeast Guadalcanal. Five archive samples were tested from the northwestern end of Santa Isabel (Figure 1 & Table 1). One of these, a siltstone from the Oligocene-Miocene Kuakula Sandstone Formation (at least 1000 m thick) of the Maringe Group (Barron & Hawkins, personal communication 1993), gave a fair potential yield (12645, 4.5 kg/tonne). From the same area, a calcareous siltstone from the Sisibena Formation (200m thick, Upper Miocene-Pliocene) gave a fair potential yield (12769, 3.00 kg/tonne). Vanuatu The Vanuatu field survey covered the north-central and northwest areas of Santo (Figures 6 & 7, respectively), and stratigraphic targets in Maewo (Figure 8). On Santo, further sampling of Middle Miocene fore-reef greywackes of the Pelapa and Peteao formations was conducted, as these strata have so far provided the most encouraging evidence for source rock potential in Vanuatu (Glikson 1988). Restricted back-reef facies of the Pliocene Tawoli Calcilutite Formation (Robinson 1969, Mallick & Greenbaum 1977) on Santo were also tested. On northern Maewo, the basinal Tafwutmuto and Wustorogha formations were tested, as proposed by Katz (1986 & 1988). North-central Santo Sampling of the Tawoli Formation in north-central Santo (Figure 6) was carried out by Mr. Hawkins and Mr. Morris. Access to the area was by air from Port Vila to Luganville, then by four- wheel-drive vehicle, and finally on foot. Traverses were made along the Ora and Raovi rivers, across the width of the Tawoli Formation outcrop. Foraminifera indicate that the Tawoli Formation (Mallick & Greenbaum 1977) is mainly of Mid to Late Miocene age, but may locally extend up into the Pliocene. Typically, it comprises white to buff or pale grey, soft to hard and recrystallised, well-bedded fine calcarenites, calcisiltites and calcilutites, and reaches up to 1600 m thick in the Tawoli River valley (Figure 6). Mallick & Greenbaum (1 977) considered the Tawoli Formation to have accumulated in a north-northwest trending trough which lay between volcanic highs, and was most probably bordered by marinal reefs. The Tawoli sediments were probably input into the deeper parts of the basin by gravity flow mechanisms. Small amounts of concretionary pyrite in the Tawoli Formation of the Apuna valley (Mallick & Greenbaum 1977), suggest restricted basin conditions, but its association with burrowed sediments indicates that anoxic bottom conditions were not attained. [TR207 - Johnson and others]

11 [12] Sections along the Raovi and Ora rivers are dominated by well-bedded limestones with some fine grained volcaniclastic detritus. Beds are typically 10-50 cm thick, have sharply defined bases and fine upwards. The bedding displays conspicuous lateral continuity, with no apparent changes in bed thickness, except where slumping has occurred. Asymmetric slump folds and convolute bedding are not uncommon and occasional load structures are also present. Burrow structures are common within the sediments and there were no field indications of any carbonaceous material. Tests on ten field samples from the Tawoli Formation gave poor potential yields (Table 2). Northwest Santo Sampling of the Pelapa and Peteao formations (Figure 7) was carried out by Dr C. Mortimer and Mr Morris. Access to the survey area was via the Vanuatu Government survey yacht, the Spia Laen. Traverses along the valleys of the Pelapa, Penaoru, Petawata, Pelovu, Peteao and Wosaeriki rivers were made on foot, using the ship as base. The Petawata River was revisited by helicopter and sampling was extended further upstream than had previously been possible. The Pelapa and Peteao Formations are of early Mid Miocene age and comprise volacaniclastic sandstones, siltstones and mudstones with interbedded limestones, breccias and conglomerates (Robinson 1969). Buchbinder & Haley (1988) recorded mudstone and siltstone samples from the Petawata River with up to 1.44% total organic carbon. A single 10 cm clast of black, carbonised fossil wood, containing oxidised pyrite, was found as a "float" in the Petawata valley. Its provenance is uncertain, but it is most likely to have been derived from the Peteao Formation. On testing with the field analyser, this sample gave a potential yield of 10.5 kg/tonne (sample number WS 107, Table 2). Twenty-five other field samples from the Peteao Formation together with two samples from the Pelapa Formation gave poor potential yields (Table 2). Three Santo archive samples, of unknown lithostratigraphy, gave poor potential yields (Table 2). Maewo Sampling of the Tafwutmuto and Wustorogha formations of northern Maewo (Figure 8) was carried out by Mr M.P. Hawkins, Dr C. Mortimer and Mr Stephen. A helicopter was chartered to provide access from Port Vila to the survey area and local boats were used to sample specified sites. [TR207 - Johnson and others]

12 [13] The mid-Upper Miocene Wustorogha Formation typically comprises reworked tuffs and volcaniclastic breccias, tuffaceous mudstones and siltstones and limestones (Carney 1986). Outcrops of the Wustorogha Formation examined for this survey were mainly of pale grey tuffaceous siltstones and fine grained sandstones. Five outcrop samples from the Wustorogha Formation gave poor potential yields (Table 2). The Upper Miocene Tafwutmuto conformably overlies the Wustorogha Formation and comprises foraminiferal mudstones and some limestones with rare pumiceous tuffs (Carney 1986). Foraminifera within the Tafwutmuto Formation indicate a depositional palaeodepth of 2000-3000 m. Outcrops of the Tafwutmuto Formation examined for this survey were mainly of pale grey, fine grained volcaniclastic sandstones. Results from four field samples gave poor potential yields (Table 2). Two Maewo archive samples, of unknown lithostratigraphy, gave poor potential yields (Table 2). Malekula Two Malekula archive samples of breccia, of unknown lithostratigraphy, gave good or very good potential yields (MM 233, 14.5 kg/tonne & MM 443, 6.4 kg/tonne; Table 2). Thirteen other Malekula archive samples of unknown lithostratigraphy gave poor potential yields. Pentecost One Pentecost archive sample of fossil wood, of unknown lithostratigraphy, gave a good potential yield of 10.7 kg/tonne (Pel 22/10, Table 2). A Pentecost archive sample of bedded tuff gave a poor potential yield. GEOCHEMICAL ANALYSIS AND INTERPRETATION Twenty-five samples from the Solomon Islands and twenty-five samples from Vanuatu were selected for laboratory screening analysis of total organic carbon (TOC) and Rock-Eva1 pyrolysis. The organically richest samples were then the subject of gas chromatography (GC). Gas chromatography-mass spectrometry (GC-MS) was also carried out, primarily to isolate hydrocarbons for independent evaluation of relative maturities and to make an evaluation of biomarkers diagnostic for palaeoenvironment of deposition. A brief explanation of key geochemical parameters referred to in this section is given in Appendix 1. [TR207 - Johnson and others]

13 [14] Rock-Eval Pyrolysis and TOC Analysis Sediment samples collected from the Solomon Islands (Table 1) and Vanuatu (Table 2) were analysed for petroleum source rock potential using Rock-Eva1 pyrolysis and Leco total organic carbon determination (see Appendix 1). Although some samples showed high TOC contents, most had low pyrolysable carbon (S2) and hydrogen index (HI) values and, as a consequence, are considered to have poor potential for petroleum generation. One field sample of carbonaceous mudstone from the Mbokokimbo Formation, Guadalcanal, Solomon Islands (5: 14/8, #7067) had apparently "good" source potential (but see below), This sample was taken from a thin lenticular bed of limited distribution. One archive sample of breccia of unknown lithostratigraphy from Malekula, Vanuatu (MM 443, #7446) had "fair" source potential. Hydrogen Index values suggest that these two relatively good samples have potential to generate only gas. Several samples from Vanuatu had high S3 (and Oxygen Index) values indicating the presence of either unstable carbonates or immature organic matter with considerable abundance of labile CO. Digestion of these samples with hydrochloric acid and repeats of the Rock-Eva1 tests (Table 3) resulted in only modest loss of weight and no significant change of S3. It is concluded that the organic matter in samples contain significant amounts of labile CO as a consequence of oxidation/reworking and their low maturity. Type lll organic matter seems to be present throughout. Tmax values provide information on the thermal maturation of the samples. However, they are only meaningful if there is sufficient S2 (20.2 kg/tonne) to obtain an accurate reading. The seven Solomon Island samples that met this criterion were: 5: 12/6, 7060 (shelly & muddy sandstone, Mbokokimbo FM., Guadalcanal) 5: 13/8, 7062 (grey calcareous siltstone, Mbokokimbo Fm., Guadalcanal) 5: 14/3, 7064 (grey siltstone, Mbokokimbo FM., Guadalcanal) 5: 14/5, 7065 ((poorly sorted sandstone, Mbokokimbo Fm., Guadalcanal) 5: 1418, 7067 (carbonaceous mudstone, Mbokokimbo Fm., Guadalcanal) 8: 26/2, 7069 (calcareous siltstone, Pemba FM., Choiseul) 8: 26/3, 7070 (fine grained muddy sandstone, Pemba Fm., Choiseul) Three samples from Vanuatu had S2 values over 0.2 kg/tonne and thus meaningful Tmax values: WS 107, #7439 (fossil wood, ?Peteao Fm., Santo) MM 443, 7446 (breccia, unknown lithostratigraphy, Malekula) Pel 22/10, 7447 (fossil wood, unknown lithostratigraphy, Pentecost) [TR207 - Johnson and others]

14 [15] These samples (with one anomaly, see below) had Tmax values in the range 410-433 degrees Celsius and it is concluded that all the samples are immature or marginally mature. This is supported by the low ratio of free hydrocarbons to free plus pyrolysable hydrocarbon (i.e. the Production Index, PI=S1+S2). Sample 5: 14/8, 7067 (carbonaceous mudstone, Mbokokimbo Formation, Guadalcanal) is anomalous in that it has an extremely low Tmax value (356C) and a high Production Index (0.21). This is probably due to a high content of immature and a very labile (i.e. readily reactive on heating) kerogen ("protokerogen") that contributed to both the S1 and S2 Rock-Eva1 peaks. Extraction, GC-MS & Biomarker Analysis On the basis of their higher S2 values, the following six samples from the Solomon Islands, and three from Vanuatu, were selected for solvent extraction and detailed hydrocarbon analysis for evaluation of relative maturities and to determine palaeoenvironment of deposition: 5: 12/6, #7060 (shelly & muddy sandstone, Mbokokimbo Fm., Guadalcanal) 5: 13/8, 7062 (greycalcareoussiltstone, MbokokimboFm., Guadalcanal) 5: 14/5, 7065 (poorlysortedsandstone, MbokokimboFm.,Guadalcanal) 5: 14/8,7067(carbonaceousmudstone, MbokokimboFm.,Guadalcanal) 8: 26/2, 7069 (calcareoussiltstone, Pemba Fm., Choiseul) 8: 26/3, 7070 (fine grained muddy sandstone, Pemba Fm., Choiseul) WS 107, #7439 (fossil wood, ?Peteao Fm., Santo) MM 443, 7446 (breccia, unknown lithostratigraphy, Malekula) Pel 22/10, 7447 (fossil wood, unknown lithostratigraphy, Pentecost) Detailed analyses for these samples are summarised in Tables 4 & 5. The generally low extractable organic matter (EOM; 280-1000 ppm) values for the extracts of Solomon Islands samples were in accordance with the Rock-Eva1 result and suggest low thermal maturity. The Vanuatu samples showed somewhat higher EOM contents. Separation of the polar and asphaltene fractions from the remainder of the bitumen confirmed that the bulk of the EOM was in the heavy fraction (asphaltenes & polars), also in accord with the low maturity. Vanuatu samples had particularly high percentages of EOM present as polars and asphaltenes (87 to 92%). Yields of n-alkanes were low and reduced further after treatment to remove the high content of elemental sulphur present in most of the samples. Gas chromatography was carried out on the saturated hydrocarbons, and GC traces are appended (Appendix 2). The generally low n-alkane contents can be gauged with reference to the [TR207 - Johnson and others]

15 [16] low heights compared to the internal standard peak (anteiso-C22) which was added to the sample. Patterns for the n-alkanes from Solomon Islands samples showed a strong odd carbon number predominance over the C25-C33 suggesting a signal from terrestrial plant waxes and consistent with the low maturity of the samples. Pristane/Phytane values are generally not meaningful at low transformation ratios (i.e. low conversion of kerogen to bitumen) and therefore have not been computed. Diagnostic hydrocarbons were determined from the GC-MS traces which are attached (Appendix 2). Peaks were identified with reference to standard samples including a conventional Middle East crude oil (#897 reference oil) which was run under identical conditions. Interpretations are based on guidelines given in Peters and Moldowan (1993), Moldowan et a/. (1 985) and Seifert and Moldowan (1978). Absolute areas for diagnostic peaks are given in Table 6 and key ratios for maturity and environment diagnostic parameters are given in Table 7. A key to the use of different parameters is given in Table 8, although it should be noted that not all parameters apply all the time. For example, the methylphenanhrene index of maturity only applies for maturity levels in excess of Vitrinite Reflectance (Ro) greater than 0.5%, and there is no point doing this measurement in the case of the present sample set. Solomon Islands Samples All samples had similar distributions of diagnostic biomarkers indicating similar depositional environments. Differences were primarily in the ratios of "biological" stereoisomers to "geological" stereoismers, which means that there were slight differences in the thermal histories experienced by the samples. The least mature sample was 8: 26/2, 7069 (calcareous siltstone, Pemba Fm., Choiseul), based on the very low S/R ratio of the C29 steranes (parameter 2) and consistent with the other indicators such Ba/Ba+aB triterpanes, the presence of BB triterpanes (not tabulated) and the presence of 17B(H) trisnorhopane (not tabulated). The most mature sample was 5: 14/51, 7065 (poorly sorted sandstone, Mbokokimbo Fm., Guadalcanal), based on similar criteria. Biomarkers diagnostic for depositional environment include the C30 desmethylsterane (24-n- propylcholestane in 414-->217) and a mixture of C30 4- (in 414-->231) including dinosterane. Both of these compound classes are related to sterols of marine algae, and restricted to sediments with inputs of marine organic matter (Moldowan et al., 1985, 1990). Moreover, the presence of dinosterane is restricted to sediments and oils of Mesozoic or younger age (Summons et al., 1987). Oleanane a biomarker produced by flowering plants, was also present in significant abundance indicating the contribution of terrestrial organic matter to the sediments. [TR207- Johnsonand other]

16 [17] There is some variation in the ratio of C30/C29 steranes and in the relative proportions of C27:C28:C29 steranes. Samples with the highest proportion of C29 steranes also have the highest abundance of oleanane (5: 12/6, #7060, shelly and muddy sandstone, Mbokokimbo Fm., Guadalcanal & 5: 1418, 7067, carbonaceous mudstone, Mbokokimbo Fm., Guadalcanal). Since both compounds have terrestrial sources the primary control on source biomarker patterns is the relative contents of marine and terrestrial organic carbon. The sediments were deposited in an anoxic marine setting as shown by the presence of marine steroid markers but also by the presence of 28, 30 bisnorhopanes. Parameter "l" (28bis/hop) is high in all the samples. The origin of this marker is not known, but it appears to arise from bacteria involved in either sulfate reduction or sulfide oxidation (i.e. marine processes). It is a distinctive marker on oils from anoxic marine basins (e.g. North Sea and California Borderland oils). Diasteranes, otherwise known as Ba-rearranged steranes are produced by rearrangement of ordinary steranes on acidic catalytic surfaces of clay minerals. The abundance of diasteranes is affected by maturity with more mature samples having higher relative abundances. In summary, the Solomon Islands sediments examined for diagnostic biomarkers were all deposited in anoxic marine settings with variable contents of marine and terrestrial organic matter. Type ll or Type II/III organic matter predominates and the mainly gas-prone sediments are below the maturity threshold for hydrocarbon generation. Vanuatu Samples The three samples from Vanuatu selected for GC-MS (Appendix 2) and biomarker analysis also had very similar distributions of biomarkers and characteristics indicative of Type lll organic matter. The samples were generally less mature than those from the Solomon Islands as shown by very low S/R ratio of the C29 steranes (parameter 2) and consistent with the other indicators such Ba/B a+aB triterpanes and the presence of BB triterpanes. This is consistent with the bulk geochemical parameters and the asphaltic nature of the EOM. All three samples were devoid of marine algal sterane markers. They also have very high contents of C29 steranes relative to the other steranes suggesting a predominant higher plant component. Interestingly, oleanane was not identified but this could be the result of the very low maturity of the samples. Diagenetic reactions are required to produce the oleanane from its precursor lipid and it is possible that the thermal regime has not been sufficiently intense to cause these to proceed. 28, 30 Bisnorhopane was identified in sample MM 443, 7446 (breccia, [TR207 - Johnson and others]

17 [18] uncertain lithostratigraphy, Malekula) so it is possible that the sediment was deposited under brackish water conditions but with an overwhelming predominance of land-plant matter. In summary, the Vanuatu samples are predominantly of terrestrial origin with immature Type lll organic matter present. Despite the high organic carbon content of three of the samples, they are only likely to be gas prone and are certainly not yet near the maturity window for hydrocarbon generation. [TR207- Johnson and others]

18 [49] REFERENCES Alexander, R., Kagi, R.l., Woodhouse, G.W. & Volkman, J.K. 1983. The geochemistry of some biodegraded Australian oils. APEA Journal 23, 53-63. Allen, P.A. & Allen, J.R. 1990. Basin analysis: principles and applications. Blackwell Scientific Publications, 451pp. Barron, A.J.M. 1993. The geology of northernmost Malaita. A description of sheets 8/160/7 & 8/160/8. Ministry of Natural Resources, Honiara. 40pp (unpublished). Boreham, C.J., Crick, I.H. & Powell, T.G. 1983. Alternative calibration of the methylphenanthrene index against vitrinite reflectance: application to maturation measurement of oils and sediments. Organic Geochemistry 12, 289-294. Buchbinder, B. & Halley, R.B. 1986. Source-rock evaluation of outcrop samples from Guadalcanal, Malaita, and the Florida Island Group, Solomon Islands. In: Vedder, J.G., Pound, K.S. and Boundy, S.Q. (eds). Geology and offshore resources of Pacific island arcs - central and western Solomon Islands. Circum- Pacific Council for Energy and Mineral Resources Earth Science Series, Houston, Texas, 4, 267-276. Coleman, P.J. 1965. Stratigraphical and structural notes on the British Solomon Islands with reference to the first geological map: British Solomon Islands Geological Record (1959-1962), volume 2, report No 29, 17-31. Coleman, P.J. 1989. Petroleum potential of Solomon Islands, SW Pacific. Bureau of Mineral Resources, Canberra, 24pp. Eden, R.A. & Smith, R. 1984. Fiji as a petroleumprospect. Fiji Mineral Resources Department, Suva, Fiji, 34pp. Ensminger, A., Joly, G. & Albrecht, P. 1978. Rearranged steranes in sediments and crude oils. Tetrahedron Letters, 1575-1578. Falvey, D.A., Colwell, J.B., Coleman, P.J., Greence, H.G., Vedder, J.G. & Bruns, T.R. 1991. Petroleum prospectivity of Pacific island arcs: Solomon Islands and Vanuatu. APEA Journal, 191-212. Glikson, M. 1988. Miocene algal reef-derived deposits in Vanuatu - possible petroleum source rocks. In: Greene, H.G. & Wong, F.L. (eds) Geology and offshore resources of Pacific island arcs - Vanuatu region. Circum-Pacific Council for Energy and Mineral Resources, Houston, Texas, Earth Science Series, 8, 267-274. Hackman, B.D. 1980. The geology of Guadalcanal, Solomon Islands. Institute of Geological Sciences Overseas Memoir 6, 115pp. Hine, N. 1991. Calcareous nannofossil study of Upper Cretaceous to Pliocene sediments from northern Malaita, Solomon Islands. British Geological Survey Technical Report, Stratigraphy Series WH/91/298 R. Biostratigraphy & Sedimentology Group Report PD/91/278, 11 pp. Hughes, G.W. &Turner, C.C. 1976. Geology of southern Malaita. Solomon Islands Geological Survey Bulletin 2, 80pp. Hughes, G.W. & Turner, C.C. 1977. Upraised Pacific Ocean floor, southern Malaita, Solomon Islands. Geological Society of America Bulletin, 83 41 2-424. Kaatz, H.R. 1986. Hydrocarbon potential in the SW Pacific. In: Cronan, D.S. (ed), Sedimentation and Mineral Deposits in the Southwestern Pacific Ocean. Academic Press, London. 83-1 16. Katz, H.R. 1988. Offshore geology of Vanuatu - previous work. In: Greene, H.G. & Wong, F.L. (eds) Geology and offshore resources of Pacific island arcs - Vanuatu region. Circum-Pacific Council for Energy and Mineral Resources, Houston, Texas, Earth Science Series, 8, 93-122. Kvaldheim, O.M., Christy, A.A., Telnaes, N. & Bjorseth, A. 1987. Maturity determination of organic matter in coals using the methylphenanthrene distribution. Geochimica et Cosmochimica Acta, 51, 1883-1 888. Mackenzie, A.S., Patience, R.L., Maxwell, J.R., Vandenbroucke, M. & Durand, B. 1980. Molecular parameters of maturation in the Toarcian shales, Paris Basin, France-I: Changes in the configuration of acyclic isoprenoid alkanes, steranes and triterpanes. Geochimica et Cosmochimica Acta, 44, 1709-1 721. Mallick, D.I.J. & Greenbaum, D. 1977. Geology of Southern Santo. Regional Report of the New Hebrides Condominium Geological Survey, 84pp. Moldowan, J.M., Seeifert, W.K. and Gallegos, E.J. 1985. Relationship between petroleum composition and depositional environment of petroleum source rocks. AAPG Bulletin 69, 1225-1 268. 247, 309-312. Neef, G. 1979. Cenozoic stratigraphy of Small Nggela island, Solomon Islands-early Miocene deposition in a forearc basin followed by Pliocene patch reef deposition. New Zealand Journal of Geology and Geophysics 22, 53-70. Radke, M. & Welte, D.H. 1983. Advances in Organic Geochemistry. John Wiley & Sons Limited. [TR207 - Johnson and others]

19 [50] Robinson, G.P. 1969. The Geology of North Santo. Regional Report of the New Hebrides Condominium Geological Survey, 77pp. Seifert, W.K. and Moldowan, J.M. 1978. Application of steranes, terpanes and monoaromatics to the maturation, migration and source of crude oils. Geochimica et Cosmochimica Acta, 42, 77-95. Strange, P.J. 1981. The geology of the Choiseul Bay and Chorovanga areas, Choiseul. British Technical Cooperation Project Western Solomons mapping project Report No. 1/2. Ministry of Natural Resources, Honiara, 95pp. Strange, P.J. & Danitofea, S. 1979. Voza: Choiseul geological map sheet CH3, 1:50000, with brief explanation of the geology. Ministry of Natural Resources, Honiara. Summons, R.E., Volkman, J.K. & Boreham, C.J. 1987. Dinosterane and other steroidal hydrocarbons of dinoflagellate origin in sediments and petroleum. Geochimica et Cosmochimica Acta 51, 3075-3082. Taylor, G.R. 1977. West Florida Islands: Florida islands geological map sheet FL1, 1:50000, with brief explanation of the geology. Ministry of Natural Resources Honiara. Thompson, R.B.M. 1958. The geology of the Florida Group, 1956. British Solomon Islands Geological Survey Memoir 2, pp.97-101. Tissot, B.P. & Welte, D.H. 1984. Petroleum Formation and Occurrence. Springer Verlag, Berlin. 538 p. Volkman, J.K., Alexander, R., Kagi, R. and Sheppard, P.N. (1983) Demethylated hopanes in crude oils and their applications in petroleum geochemistry. Geochim. Cosmochim. Acta 47, 785-794. Zumberge, J.E. (1987). Prediction of source rock characteristics based on terpane biomarkers in crude oils: A multivariate statistical approach. Geochimica et Cosmochimica Acta 51, 1625-1 637. [TR207 - Johnson and others]

20 [51] APPENDIX 1 Experimental Procedure Sample Preparation Outcrop samples were rinsed in dichloromethane or chloroform, dried and crushed. TOC Determination and Rock-Eva1 Pyrolysis Total Organic Carbon determination was carried out using a Leco carbon analyser. Crushed samples (ca. 100 mg) were analysed in a Rock-Eva1 pyrolysis apparatus (Rock Eval III) to give results for Tmax, S1, S2, S3, P1 and PY (see below). The analysis involves progressive heating of a small amount of material (ca. 100 mg) to 550'C. During the assay, the hydrocarbons already present in the rock in a free or adsorbed state are volatilized at low temperature. The amount of these hydrocarbons (S1, mg/g of rock) is measured by a flame ionization detector. Continued heating of the sample results in the pyrolysis of kerogen, producing hydrocarbons and hydrocarbon-like compounds (S2, mg/g of rock) and, finally, oxygen- containing volatiles which are predominantly CO2 (S3, mg/g of rock). A fourth parameter measured is the temperature during pyrolysis at which the maximum hydrocarbon generation occurs. This Tmax measurement is related to both the type of the contained organic matter and its thermal maturity. The potential yield (PY) of hydrocarbons, or genetic potential as it is also known (Tissot and Welte, 1984), is simply the total amount of hydrocarbons in the rock i.e. S1 + S2. The Production Index (PI) is a measure of the maturity of the sample and reflects how much of the kerogen has been converted to free hydrocarbons. It is defined by S1/S1+S2. Tissot and Welte (1984) and Peters (1986) suggested the following rough guideline for source rock classification based on potential yield: 0 - 3 kg tonne-' = poor 3- 6 kg tonne-' =fair 6 - 12 kg tonne-' = good > 12 kg tonne-' = very good [TR207- Johnson and others]

21 [52] Source rock classification based on TOC is: 0 - 0.5% = low 0.5 - 1.0% = average 1 -2% = high >2% = very high Source rock maturity based on Production Index values for Type II kerogen are as follows: 300 >5 The S2/S3 ratio assumes that the level of thermal maturation is equivalent to a Vitrinite Reflectance (Ro) value of about 0.6-0.7%. The table below gives a rough guide to maturity based on the Tmax parameter. 410 - 440 = immature = 0.0 - 0.6 %Ro 440-470 = oil = 0.6 - 1.35 %Ro 470 - 500 = wetgas = 1.35 - 2.0 %Ro Sediment Extraction The crushed sediments were extracted using pre-washed soxhlets and thimbles, using 87:13 CHCI3:MeOH as solvent and continuing the process for 48 hours. The extracts were filtered using a 0.2 pm filter and then evaporated to near dryness (EOM, Tables 4 & 5). Saturated Hydrocarbon Analysis Approx 12 mg of the Solomon Islands EOM was placed on a Sep-Pak to separate into two parts, namely a combined asphaltene and polar fraction and one containing predominantly saturates plus aromatics. The latter fraction was separated by HPLC into saturates and aromatics (Table 4). Samples from Vanuatu (Table 5) were separated by conventional column chromatography on silica gel. Each extract was placed on a 12 g silica gel column (pre-washed with petroleum spirit) and three fractions were collected (in 100 ml round bottom flask) by eluting the column with: 1. 40 ml petroleum spirit SATURATES 2. 50 ml petroleum spirit/dichloromethane (1;1) AROMATICS 3. 40 ml chloroform/methanol (1:1) POLARS Each fraction was reduced in volume on a rotary evaporator to approx 1ml and transferred to a pre-weighed vial with dichloromethane (0.5 ml 3X). The solvent was carefully removed by gentle exposure to a stream of dry nitrogen. Each fraction as weighed and labelled. Gas Chromatography GC analysis of saturated hydrocarbon fractions was carried out using a HP 5890 Series II GC equipped with a fused silica capillary column (25 M x 0.2 mm) coated with cross-linked methylsilicone (HP Ultra-1). The samples, in hexane, were injected on-column at 60C and held isothermal for 2 min using an HP7363A autosampler. The oven was programd to 300C at 4'C/min with a hold period of 30 min. The carrier gas was hydrogen at a linear flow of 30 cm/sec. Data was collected, integrated and manipulated using DAPA GC software. GC-MS Analysis GC-MS analysis was carried out using a VG 70E instrument fitted with an HP 5790 GC and controlled by a VG 11-250 data system. The GC was equipped with an HP Ultra-1 capillary column (50 m x 0.22 mm) and a retention gap of uncoated fused silica (0.5 m x .53 mm). The samples, in hexane were injected on-column at 50'C and the oven programd to 150C at 10'C/min then to 300C at 3'C/min with a hold period of 30 min. [TR207 - Johnson and other]

22 [53] The carrier gas was hydrogen at a linear flow of 30 cm/sec. The mass spectrometer was operated with a source temperature of 240'C, ionisation energy of 70eV and interface line and re-entrant at 310C. In the multiple reaction monitoring (MRM) mode, the magnet current and ESA voltage were switched to sequentially sample for 26 selected parent -daughter pairs. The sampling time was 40 ms per reaction with 10 ms delay giving a total cycle time of 1.3s. The chromatograms are annotated with the diagnostic masses for parent and daughter and a five digit number indicating relative intensity. The identities of specific molecules are also given where they have been established. Data were interpreted according to guidelines laid out in Peters & Moldowan (1993). [TR207 - Johnson and others]

23 [54] References PETERS K.E. 1986. Guidelines for evaluating petroleum source rock using programd pyrolysis. AAPG Bulletin 70, 318- 329. PETERS K.E. & MOLDOWAN J.M. 1993. The Biomaker Guide. Interpreting Molecular Fossils in petroleum and Ancient Sediments. Prentice Hall. 363pp. TISSOT, B.P. & WELTE, D.H. 1984. Petroleum Formation and Occurrence. Springer Verlag, Berlin. 538 p. [TR207 - Johnson and others]

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