Surya N. Khanna
Amatec
Victoria, Mahe, Seychelles
Saimone P. Helu
Ministry of Lands, Survey & Natural Resources
Nuku 'alofa, Tonga
The Kingdom of Tonga in the southwestern Pacific Ocean comprises 171 islands (Fig. 1), of which only 37 are inhabited.
There are four main groups of islands: Tongatapu, H'apai, Vava'u, and the Niuas. The total land area is 700 sq km, and the territorial waters extend to 700,000 sq km.
The kingdom is bordered by New Zealand, Fiji, Walhis and Fortuna, the Samoas, and Niue.
TECTONIC SETTING
Tonga lies at the eastern-most edge of the Australia India plate (Fig. 2), representing oceanic islands which have been formed due to subduction of the Pacific plate beneath the Australia-India plate for at least the past 45 million years.
The rate of subduction is estimated at 17 cm/year. This has been defined by earthquake foci beneath the trench 100-300 km at the Tonga ridge.
The numerous islands are the surface expressions of sediments of variable thickness up to 5 km in an elongate forearc basin with Tonga ridge up to 70 km wide and about 1,000 km long.
Along the ridge, the water depths are variable, but in the area north of Tongatapu the depth is generally less than 200 m. The depth of 400-600 m is quite common further south.
The forearc ridge is separated from the active and extinct volcanoes of Tofua arc (chain of andesitic volcanoes) to the west (Figs. 2 & 3) by the Tofua trough with water depths approaching 200 m. Tonga trench lies to the east, where the water depths reach 10,000 m.
According to plate tectonic concept the lateral movement during the development of the Tonga ridge is important for the understanding of petroleum potential of the forearc. Kroenke1 attempted reconstruction of the plate movement through Cenozoic (Fig. 4).
Tonga was close to New Caledonia-Norfolk ridge during middle Eocene times. During Oligocene, the south Fiji basin was opening and the Tonga and Kermadec section of the Outer Melanesian arc migrated east away from the New Caledonia, Norfolk, and Loyalty ridges.
The northern section of the Outer Melanesian arc may have been separated from New Caledonia and Tonga by oceanic crust to the north. Reconstructions prior to opening of Fiji basin are not clearly understood.
It is, however, clear that Tonga was close to the New Caledonia-Norfolk ridge, where either the Tonga ridge originated as a newly formed oceanic island arc or possibly broke away from the older Melanesian arc.
It is possible that a part of the Tonga ridge might have an older pre-Eocene basement, and it is anticipated that the sedimentation during Eocene was affected by the larger landmass of the New Caledonia-Norfolk ridge situated nearby to the west.
The migration pattern of Tonga moving eastward continued until Pliocene, separating the Lau arc from Tonga ridge and developing the present day geography. Quaternary movements are mainly restricted to regional tilting and warping, although some major transverse faults might have been active.
PREVIOUS WORK
The first and foremost known geologic records appeared to be by Hoffmeister2 for the island of 'Eua, wherein the stratigraphic sequence of the island was mapped with early Eocene volcanic basement overlain by Eocene limestone and Plio-Pleistocene limestones.
Ladd3 and Sterns4 described fossil contents.
Mulder and Nieuwenhuinzen5 and Tongilava and Kroenka6 are among the earlier comprehensive records of oil prospecting on Tonga, where the nature of main seepage on Tongatapu was confirmed to be crude oil with a proposal for seismic work.
Paleobathymetric interpretation of stratigraphic sections with subsurface was carried out by Robertson Research.7 Other important workers are Tappin et al.,8 Barney,9 and Maung et al.10
Cunningham and Anscombe11 outlined the thick sequence of clastic rocks of mid-Eocene onwards on 'Eua, emphasizing sufficient thickness of sedimentary rocks with potential maturation for petroleum.
Alexander12 indicated reefal or buried igneous mass on the basis of S.P. Lee 82 cruise data.
Buchbinder and Halley13 could not find much organic content (TOC) in 2 Kumimonu at 2,370 ft. But a study of the geochemical characteristics of the oil seeps on Tongatapu indicated an advanced stage of thermal maturity.
Kroenka & Yan14 carried out reconstruction of Cenozoic plate boundaries in the Southwest Pacific. Scholl & Vallier15 edited geology and offshore resources of Pacific Islands arcs Tonga region for Circum-Pacific Council for Energy & Mineral Resources, Earth Science series, indicating thick sediments with potential for petroleum generation.
Gatliff et al.16 gave seismic stratigraphy and petroleum potential of the Tongatapu-'Eua area. Herzer and Exon17 referred on the block faulting as well as volcanic activity.
Sandstrom18 indicated the presence of wood and pollen grains pointing to a land source of sediments during late Tertiary deposition.
Pflueger and Havard19 indicated presence of suitable structures on Tonga Ridge for hydrocarbon entrapment. Gatliff20 prepared a promotion brochure on the petroleum prospects of Tonga for the government.
EXPLORATION HISTORY
The presence of oil seeps attracted Tonga as a possible place of exploration in the late 1960s.
The seeps occur in vuggy coral limestones a few feet below the ground surface on Tongatapu and in the sea off Ohonua harbor in 'Eua. Samples of the oil have been analyzed by many oil companies, and results show it to be genuine weathered crude oil.
In 1970, a petroleum agreement was concluded under the provisions of the Petroleum Mining Act 1969 and Petroleum (Income Tax) Act 1969-70, with a consortium of oil companies with Shell as operator.
The concession covered 15,540 sq km of the Tonga ridge from Vava'u in the north to Tongatapu in the south. As a result, a geological field survey was carried out followed by about 2,200 km of seismic along with magnetic and gravity surveys.
Based on above data, Mulder and Nieuwenhuizen5 proposed further work recommending special attention on Tongatapu. Consequently, a land gravity and magnetic survey was carried out.
The consortium concentrated on Tongatapu and found a basin extending north-south across the island. Two wells were drilled, but no hydrocarbons were encountered (Fig. 6).
The first well 1 Kumifonua, failed to reach the targeted limestone due to limited rig capacity and was abandoned at 1,684 m still in volcaniclastics.
The 2 Kumifonua drilled in an updip position, on the western flank of the basin near the seepage of Hofoa. It also failed to reach the target due to rig capacity and had to be abandoned in Miocene volcaniclastics at a depth of 1,685 m, without any significant hydrocarbon shows.
In spite of the inconclusive results, Shell withdrew in June 1972 and was soon followed by British Petroleum, which also terminated its agreement in late 1974 without much work.
The following year witnessed interest by many oil companies and finally a license was concluded with Webb Tonga Inc. in February 1976 for 7,770 sq km, the southern half of Shell's original concession area. The area was later increased to 15,540 sq km.
Webb Tonga re-evaluated all the available data and reprocessed the Shell seismic lines. Warters21 summarized the results.
Further land seismic (283 km) was carried out to map potential oil bearing Mio-Oligocene carbonate reefs. This was followed by drilling of three wells.
The first two wells, 1 & 2 Kumimonu, did not encounter any reef section and were drilled to 2,555 m & 2,295 m, respectively, ending in volcanic basement.
The 3 Kumimonu was drilled to 2,635 m encountering some reefal limestone, but without any significant hydrocarbon shows.
After discouraging results from three onshore wells, Webb directed its exploration program offshore. This included offshore seismic with gravity and magnetics in 1979 in the region between the islands of Tongatapu and 'Eua referred to as 'Eua channel for Eocene/Oligocene reefs.
The interests were passed on to Samuel Gary Oil Producer, Denver, as a result of takeover of Webb Resources. The new operator prepared a drilling proposal but could not raise enough funds. This license was terminated in 1984.
More data were collected through various cruises and the results are available in two volumes of the Circum Pacific Council for Energy & Mineral Resources, Houston, published in 1985.
Pflueger and Havard19 carried out prospectivity of Tonga, based on extensive reprocessing of seismic. This was followed by Gatliff et al.16 for the petroleum prospects of "Eua Channel."
In December 1990, Bird Oil Corp., Denver, Colo., concluded an exploration license and is presently evaluating the prospects. Advantage Resources Inc., Denver, is attempting to secure partners on Bird's behalf. Meanwhile, the government of Tonga is trying to promote the remaining acreage to attract further exploration in the kingdom.
STRATIGRAPHY
Based on the experience on 'Eua, detailed seismic data and well information, a stratigraphic sequence has been worked out and tabulated for the forearc (Table 1) and also represented on seismic (Fig. 5).
OIL, GAS PROSPECTS
Tonga, though presenting risks, is an excessively high reward area, and any small discovery can make the country immediately an oil exporter.
The Japanese and East European markets are accessible. This is an area that has all the factors important for oil exploration potential.
Source potential: Buchbinder and Halley13 studied Eocene to Pleistocene limestone and volcaniclastic outcrop samples from 'Eua and selected cuttings from Eocene and Miocene shale and siltstone samples from Tongatapu.
The outcrop and borehole samples indicate low total organic carbon (TOC), which may be partly due to weathering in the outcrop samples; the oil seeps on Tongatapu, however, require a source. Analysis of these oil seeps indicate biodegraded oil originated from a marine source.
The geochemical characteristics of the oil seeps indicate an advanced stage of thermal maturity for the biodegraded crude oil. It is quite likely that hydrocarbons have been generated in Tonga in the more deeply buried and thermally mature western section of the platform.
Reprocessing of the regional seismic lines on the South Tonga ridge show three potential flat spots (Figs. 3, 5), possibly indicating hydrocarbons trapped in the subsurface within Miocene to Oligocene volcaniclastic sediments. This also possibly indicates the presence of widely scattered source in Tonga.
The lowest seismic sequence equivalent to Eocene limestones exposed on 'Eua though less continuous, and deposited in active fault graben may have higher source potential.
The wells in Tongatapu did not reach the Eocene sedimentary sequence. While the lithology and stratigraphy of these rocks remain largely unknown, it is believed that they have high source potential.
It is interesting to note that, during the Eocene, when this sequence was being deposited, plate reconstructions indicate that Tonga was close to the New Caledonia and Norfolk ridges, where oil seeps are also reported and Eocene "flysch" sequence is expected to be the source.
Maturity: Not much is known about the geothermal history of the forearc, but the present geothermal gradient is estimated as 30 C./km from the well data. The presence of Miocene intrusions in the wells indicates that the gradient may have been higher in the past.
Gatliff19 indicated existence of the oil window between 3-4 km depth over much of the ridge. It is therefore presumed that any Lower Tertiary source rocks will be within the oil window at present and any hydrocarbons generated could be actively migrating updip and along fault planes, into suitable basin margin traps.
Reservoirs: Deep water re-sedimented volcaniclastics dominate the sequence, some of which consist of submarine fan deposits (Fig. 8) including breccias and conglomerates.
Secondary alteration appears to have reduced the porosity and permeability, but development of better facies in the subsurface cannot be ruled out. The shallow water, high energy tropical limestones within the Eocene and Miocene sequences appear more promising.
Tropical location might have been responsible for the development of carbonate reefs (Figs. 9, 10) and the possibilities of detrital carbonates and fractured massive carbonates also exist.
The Eocene limestones on 'Eua contain numerous caves and pre-Miocene karst topography also would have been responsible for development of massive porosities. Therefore, Eocene limestones appear to be attractive targets.
Seals: The thick sequence of volcaniclastics encountered in the drilling on Tongatapu have very little porosity and permeability, due to cementation by zeolites. These offer potential seals.
Some doubt exists due to the presence of a large number of faults, but the relative lack of competency of volcaniclastics together with their great thickness is not expected to pose any problem.
While the seeps do indicate leakage, their existence is definitely a strong positive evidence for hydrocarbon potential in the area.
Plays: The carbonate reef buildups, particularly in Eocene times and the marine fan deposits are the main attractive plays.
Other secondary targets could be the conglomerates, channel deposits as seen in the 'Eua channel between the islands of Tongatapu and 'Eua. Examples of such features can be seen on seismic (Figs. 8, 9, 10).
TONGA'S DRY HOLES
There are scientific and logical explanations available for all the five wells (Figs. 6, 7) drilled in Tonga being dry.
The following account gives, in brief, the possible explanations for not finding any hydrocarbons in these wells.
- 1 Kumifonua was drilled by Shell in 1971 (Figs. 6, 7) in the vicinity of the Nuku 'alofa oil seeps without any seismic control. The well was drilled to a depth of 1,685 m in Lower Miocene, but could not reach the targeted Eocene limestones because of limited rig capacity.
- 2 Kumifonua was also drilled by Shell in 1971 to find an updip location of KF-1 (Figs. 6, 7) and test Eocene rocks but without any seismic control, and the well ended in Lower Miocene at a depth of 1,886 m without reaching the targeted Eocene limestones.
- 1 Kumimonu was drilled by Webb Resources in 1978 (Fig. 6) on the basis of Vibroseis seismic on a mapped high. The well was drilled to 2,553 m and abandoned in a middle Miocene intrusive.
Re-interpretation of seismic indicates absence of closure, and additionally the targeted Eocene limestone could not be reached.
- 2 Kumimonu was drilled by Webb Resources for testing a seismic anomaly believed to be a 'reef.' The well was drilled to 2,128 m. After drilling through early Oligocene and possibly "early Eocene volcaniclastics," igneous rocks subsequently determined to be early Miocene intrusive (sill) were drilled.
More than 130 m of intrusive was drilled before the well was abandoned. Later reprocessing and reinterpretation indicated that the so called 'reef' appeared to be in fact a lens (Fig. 8) of reworked volcaniclastic material (seismic line V-8).
Pflueger and Havard19 postulated that during the middle Oligocene depositional hiatus, this material was dumped in a pre-existing channel to form a local fan deposit. Barney9 proposed that the location of the fan deposit may be related to reef development in the underlying sequence.
A few limestone fragments recovered near the base of this well support this proposition.
The absence of any hydrocarbon accumulations in this well could be due to absence of suitable structure. Cap could be a further problem.
- 3 Kumimonu was also drilled by Webb, west of Nukualofa in between KF1 and KF2. The well was intended to test deeper targets, which could not be penetrated by Shell in its drilling (KF1 KF2) in 1971.
The well was located in an oil seep area and was to test a bright spot. It was drilled to 2,633 m and still remained in Oligocene and therefore could not test the Eocene target.
From the foregoing, it is clear that the Eocene targets still remain untested in Tonga area and deeper wells are necessary to arrive to any meaningful conclusion for the prospects.
'Eua channel between the islands of Tongatapu and 'Eua shows many seismic anomalies (Fig. 10) in the lowest stratigraphic sequence correlated with the Eocene limestones on 'Eua. Their seismic signatures appear to suggest existence of reefs with the largest features covering an area of about 6 sq km.
The overall prospectivity, of course, is dependent on the quality of lateral as well as vertical seals. These prospects lie on the margin of a basin to the south of Tongatapu, where the sedimentary column appears to be more than 4,000 m thick, with potential for mature source and a simple migration path updip to the basin margin.
The large area of the Tonga ridge south of Tongatapu is another area relatively unexplored, primarily because of greater water depths, where over 400 m depths are common and shallow water areas are limited.
There is a regional grid of seismic data with grid spacing of 25 km (Fig. 6). The sediments appear thick with mature source potential. The area needs further attention where reef structures are also expected.
Shell shot seismic in 1971, in the north of Tongatapu area and some reefs have been mapped. Additional seismic is required for further assessment. Further north, the basement is shallow with not much sediments and mature hydrocarbons could be expected only if the geothermal gradient is high.
DATA BASE
The existing data (Table 2) can be procured from the government of Tonga (Saimone P. Helu, Geologist, Ministry of Lands, Survey and Natural Resources, P.O. Box 5, Nuku 'alofa, Tonga, Telex 66269 PRIMO TS and fax 676-23888).
All the data in open file are available for inspection and procurement. A detailed list of well records, seismic data and relevant references are available on request.
LEGISLATION, OBLIGATIONS
The petroleum legislation in Tonga was revised in 1985 and now exists in five parts:
- Petroleum Mining Act 1969.
- Petroleum Mining (Amendment) Act 1985.
- Petroleum (General Provisions) Regulations 1985.
- Model Petroleum Agreement.
- Model Exploration License.
The 1969 act forms the basis of the legislation, and covers both onshore and offshore areas. Two types of licenses are defined: a nonexclusive exploration license and the exclusive petroleum agreement. In 1985, the act was amended and detailed regulations and model exploration and petroleum agreements were prepared.
The petroleum regulations set out definitions, financial provisions, the keeping of records and accounts, the confidentiality of data, rights reserved by government, and miscellaneous duties of the company.
The basis of all agreements is to negotiate an agreed work program in the licensed area, within the Exclusive Economic Zone (EEZ). All the areas available for license are within the 320 km.
The main prospective area of the Tonga ridge has been divided into 119 blocks, each of which is 10 min of latitude by 10 min of longitude (about 340 sq km).
Blocks were defined as far as 23 30 south, which forms the Tonga territorial boundary as defined by Royal Proclamation in 1988, but areas further south and within the 320 km EEZ are also available for license.
Normally the maximum area covered by an agreement is preferred to be 5,100 sq km, but more than one license may be held by the same company. All data are held in confidence by the government until the license is terminated or the area covered by the data is relinquished with special provisions.
The non-exclusive exploration license is valid for 2 years, with a possible extension for a further 12 months. The licensee has the right to carry out geological and geophysical surveys, but is not entitled to drill any wells, except with the consent of the Minister of Lands.
The license is designed to allow exploration without entering into a petroleum agreement.
In the Model Petroleum Agreement, royalty is set at 15% with an additional Petroleum Profits Tax (APT). The PPT is flexible and is based on the profitability of fields. It is charged only after an investor has earned a prescribed rate of return and as profits increase, the rate of APT increases.
This progressive regime is included to encourage exploration and the profitable development of small discoveries, but at the same time allowing greater taxation on large and very profitable discoveries.
It is expected that future licenses are to be issued based on the model agreement, but financial terms and conditions are open to negotiations. The government of Tonga recognizes that with the current state of the oil market, and being in a "frontier area," it is necessary to encourage exploration with favourable terms and conditions.
ACKNOWLEDGMENT
The authors are thankful to the government of Tonga for permission to publish this paper. Commonwealth Fund for Technical Cooperation gave financial support for the efforts of senior author. Free use of published and unpublished reports of Tonga has been made.
REFERENCES
- Kroenke, L.W., Cenozoic Tectonic Development of the South West Pacific, UNESCAP, CCOP/SOPAC Tech. Bull. 6, 1984.
- Hoffmeister, J.E., Geology of 'Eua, Tonga, Bernice P. Bishop Museum, Bull. 96, 1932, 93 p.
- Ladd, H.S., Eocene Molluscs from 'Eua, Tonga: U.S. Geological Survey Professional Paper 640C, 1970, 12 p.
- Sterns, H.T., Geological Setting of an Eocene Fossil Deposit on 'Eua Island, Tonga, Geological Society of America Bull., Vol. 82, 1971, pp. 2,541-51.
- Mulder, C.J., and Nieuwenhuizen, C.V,D., Appraisal of the Tonga Concession and Recommendations for Future Work, The Hague, BIPM, Exploration and Production Report EP42347, 68 p. with appendices (on file, Ministry of Lands, Nuku'alofa, Tonga, 1971.
- Tongilava, S.L., and Kroenke, L., Oil Prospecting in Tonga, 1968-1974, South Pacific Marine Geological Notes, Vol. 1, No. 1, 1975, pp. 1-8.
- Robertson Research (North America) Limited, Paleobathymetric Interpretation of Nine Stratigraphical Sections and Kuminfonua 1 and 2 Well Sections, Kingdom of Tonga, Project No. RRNA/767/346, Memo. No. 62 (on file, Ministry of Lands, Nuku'alofa, Tonga), 1975.
- Tappin, D.R., Herzer, R.H., and Stevenson, A.J., Structure and Stratigraphy of the Southern Tonga Ridge 22-26 S. Lat., in Ballance, P., Herzer, R.H., and Stevenson, A.J. (eds.), Geology and Offshore Resources of Pacific Island Arcs-Tonga II, Circum-Pacific Council for Energy & Mineral Resources, Earth Science Series, CPCEMR, Houston, Tex.
- Barney, W.M., An evaluation of the hydrocarbon potential of the Tongatapu-'Eua through, Kingdom of Tonga, confidential report to Gary Williams Oil Producer, Kingdom of Tonga, open file petroleum report No. 37, 1983.
- Maung, T.V., Anscombe, K.J., and Tongilava, S.L., Assessment of Petroleum Potential of the Southern and Northern parts of the Tonga Platform, CCOP/SOPAC Technical Report 18, 1981, 57 p.
- Cunningham, J.K. and Anscombe K.J., Geology of 'Eua and other Islands, Kingdom of Tonga, Circum-Pacific Council for Energy & Mineral Resources Earth Science Series, Vol. 2, 1985, pp. 221-257.
- Alexander, C., 2-D Gravity and Magnetic Modeling of Subsurface Domical Structure 11/14, Circum-Pacific Council for Energy & Mineral Resources Earth Science Series, Vol. 2, 1985, pp. 197-202.
- Buchbinder, B.; Halley, R.B., Source Rock Evaluation of Outcrop and Borehole Samples from Tongatapu and 'Eua Islands, Tonga, and from Viti Levu and Vanua Levu Islands, Fiji, in Scholl, D.W. and Vallier T.L. (Eds), Geology and offshore Resources of Pacific Island Arcs Tonga Region, Circum-Pacific Council for Energy & Mineral Resources Earth Science Series, Vol. 2, 1985, pp. 335-350.
- Kroenke, L.W. and Yan, C.Y., Palinspastic reconstruction of Cenozoic plate boundaries in the southwest Pacific, joint CCOP/SOPAC-IOC Working Group on South Pacific Tectonics & Resources (STAR), Fourth Session, Suva, Fiji, October 1985, UNESCO-IOC Reports of meetings kit experts and equivalent bodies (maps not published).
- Scholl, P.W., Vallier, T.L. (eds.), Geology and Offshore Resources of Pacific Island Arcs - Tonga Region. Circum Pacific Council for Energy & Mineral Resources Earth Science Series, Vol. 2, 1985, 488 p.
- Gatliff, R.W.; Pfluegger, J.C.; Havard, K.R. and Helu S.P., Structure, Seismic Stratigraphy and Petroleum Potential of the Tongatapu-Eua area of the Kingdom of Tonga, in Ballance, P.; Herzer, R.H,, and Stevenson, A.J. (Eds), Geology and Offshore Resources of Pacific Island Arcs Tonga II, Circum-Pacific Council for Energy & Mineral Resources, Earth Science Series, CPEMR Houston, Tex., 1989.
- Herzer, R.H.; Exon, N.F., Structure and Basin Analysis of the Southern Tonga Forearc, Circcum-Pacific Council for Energy & Mineral Resources Earth Science Series, Vol. 2, CPCEMR, Houston, Tex., pp. 55-73.
- Sandstrom, M.W., Organic Geo-chemistry of Dredge Samples and Oil Seeps from the Southern Tonga Platform, Circum Pacific Council for Energy & Mineral Resources Earth Science Series, Vol. 2, 1985, pp. 121-129.
- Pflueger, J.C. and Havard, K.R., Hydrocarbon Prospectivity of the Kingdom of Tonga, CCOP/SOPAC Technical Secretariat Report CO/1989/04/TG, Kingdom of Tonga open file petroleum report No. 43, 1989.
- Gatliff, R.W., The Petroleum Prospects of the Kingdom of Tonga, promotion brochure, published by SOPAC on behalf of Australian International Development Assistance Bureau, ISBN: 0 642 155208, 1990.
- Warters, H.R., Oil & Gas Potential in the Kingdom of Tonga, final report by Warpet Exploration Ltd. for Webb Tonga Inc. (unpublished), 1976.
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