FUTURE BRIGHT FOR TERTIARY CARBONATE RESERVOIRS IN SOUTHEAST ASIA

Mark W. Longman
Consulting Geologist
Lakewood, Colo.

Worldwide, carbonate buildups of all ages serve as reservoirs for approximately 40 billion bbl of recoverable hydrocarbons.1 About 40% of this total, 16 billion bbl of oil equivalent, occurs in Tertiary buildups in Southeast Asia.

This wealth of hydrocarbon reserves makes these, and particularly the Miocene buildups, the single most important set of carbonate buildup reservoirs in the world.

Furthermore, estimated reserves in these Miocene buildups continue to grow. Recent large discoveries by Occidental Petroleum Corp. in the Sarawak basin of eastern Malaysia and by Vaalco and Shell in the Palawan basin of the Philippines are not included in the above calculations.

Also enhancing the significance of the Southeast Asia region as a prime area for hydrocarbons produced from Tertiary buildups are the major development efforts now being planned at previously discovered carbonate reservoirs including the Natuna platform (L structure) in Indonesian waters of the southern South China Sea, 3DA field offshore south of Myanmar, and Liuhua field in the Pearl River Mouth basin off China's south coast (Fig. 1).

Add to these the high level of exploration activity in the area and the abundant modern reefs growing today throughout the region and it should be clear that study of both reefal limestones and carbonate reservoirs can play an important role in many exploration programs in Southeast Asia.

Diverse factors contribute to the common occurrence of Tertiary carbonate reservoirs in Southeast Asia. These include the warm tropical setting of the region throughout the Tertiary, the complex tectonic activity in the area that created numerous and varied types of sedimentary basins, widespread deposition of carbonates in many of these basins, and the common Tertiary source rocks, particularly Paleogene lacustrine deposits and Neogene fluvio-deltaic sediments, that are present.

In addition, locally high heat flow and/or sufficient burial depths have moved many of these source rocks and overlying carbonate buildups into the thermal window for oil and gas generation.

EXPLORATION SUCCESSES

Production from carbonate rocks in Southeast Asia began in the late 1800s with the discovery of Bula field on Seram island in eastern Indonesia.2 3

Although not strictly a carbonate buildup, reservoir rocks in this field include coral-rich calcarenite sands of Pleistocene age at a depth of only a few hundred feet.

Other discoveries producing partly from carbonates were made at about the same time in the area of East Java, but the reservoirs there were relatively deepwater marls of Upper Miocene to Pliocene age rather than buildups.

It was not until the 1930s and 1940s when the Royal/Dutch Shell companies conducted extensive exploration efforts in areas ranging from southern Sumatra to what is now Irian Java on the west end of Papua New Guinea island that carbonate buildups became a focus for exploration.

Among the carbonate buildup reservoirs discovered during this time were Musi, Raja, Kuang, Ogan, and Benakat Timur, all of which produced from the Lower Miocene Baturaja formation in South Sumatra, and Klamono, Mogoi, and Wasian producing from the Upper Miocene Kais limestone in Irian Jaya.

World War II ended most exploration efforts in Southeast Asia. Any emphasis on carbonate buildups that had existed faded as sandstone targets gained dominance. Shell resumed exploration in several areas of Indonesia after the war but had virtually no success in finding carbonate reservoirs. Thus, from the viewpoint of Tertiary buildups, the years from 1943 to about 1968 are notable mainly for the lack of discoveries.

Beginning in the early 1970s, however, carbonates became an increasing focus in exploration, largely because a number of major discoveries were made. Among the first of these were the numerous gas-bearing reefs in the Central Luconia province of the Sarawak basin in eastern Malaysia discovered by Shell beginning about 1969.

These early successes coincided in 1971 with Mobil's discovery of giant Arun gas field in northern Sumatra. From the time it went on production in 1977 through October 1992, Arun field produced 11.3 tcf of gas and 500 million bbl of condensate from a large, deeply buried (10,000 ft) reef complex in the early to early middle Miocene Arun (Peutu) limestone.

Beginning in 1972, Mobil also drilled a number of smaller Miocene reef anomalies in its NSB Block offshore to the northeast of Sumatra.

A number of other smaller but significant discoveries of hydrocarbons in carbonate buildup reservoirs were also made in the early 1970s. Kasim and Walio oil fields in Irian Java were found by Trend in 1972 and 1973, respectively, and have now produced about 50 million bbl and 168 million bbl of oil from buildups in the Upper Miocene Kais limestone.4

At this same time, ARCO was actively exploring for and finding numerous carbonate buildup reservoirs of Miocene age in the Northwest Java basin.

The giant gas field housed in the offshore Natuna platform, described in detail by Rudolph and Lehmann,5 was first drilled by AGIP in 1973. It is now estimated to contain as much as 45 tcf of recoverable hydrocarbon gas.6 Development of this giant field has been delayed so long in part because the reservoir lies far offshore and in part because it contains more than 150 tcf of carbon dioxide mixed in with the methane.

Just a year later in 1974, liapco discovered Rama field during development of its Cinta field sandstone reservoir in the Sunda basin offshore between Sumatra and Java. Rama field subsequently proved to be the largest of more than a dozen carbonate buildup reservoirs in the Sunda basin.

Krisna field, located on a volcanic basement paleohigh and discovered in 1976,7 ranks second. Rama and Krisna fields are estimated to produce about 118 million and 80 million bbl of oil, respectively.

The rate of discovery of carbonate buildup reservoirs slowed a bit in the latter part of the 1970s, but this is when Cities Service and Amoco established the first commercial oil production in the Philippines at Nido and Cadlao fields, respectively. A number of reefal reservoirs, most of which contain gas, were also discovered by Phillips Petroleum on Salawati Island in eastern Indonesia during this time.

The decade of the 1980s brought a new burst of drilling activity in Southeast Asia and with it a number of new carbonate discoveries. Ramba field, estimated to contain as much as 100 million bbl of reserves in the Lower Miocene Baturaja formation, was discovered in the South Sumatra basin by Asamera in 1982.

Just a few years later Stanvac found the somewhat analogous Jene field, which contains reserves of as much as 70 million bbl of oil. Along with these major discoveries, a number of smaller carbonate fields have been found in southern Sumatra. These include Rawa, Air Serdang, and, most recently in 1991, Guruh.

Significant oil discoveries in carbonates elsewhere in Indonesia during the 1980s included Wiriagar in the Bintuni basin of Irian Jaya, BD and MDA offshore northeast Java (although the MDA reservoir consists of planktonic foram-rich limestones and is not a carbonate buildup), and Bima on the structural arch between the Sunda and Northwest Java basins.

Gas discoveries still being evaluated for development include the KE-2 and KE-5 Miocene buildups offshore northeast Java, the Oligocene buildup at Kerendan in the onshore part of the Kutei basin, and the Eocene fractured Ngimbang play in the West Kangean wells.

Many buildups containing hydrocarbons were also found in other parts of Southeast Asia. These include 3DA gas field in the Martaban basin south of Myanmar discovered in 1982, Liuhua field in the Pearl River Mouth basin south of China discovered in 1987, North Matinloc field in the Philippines discovered in 1988, and the Pandora reef complex in the Gulf of Papua in southern Papua New Guinea also in 1988.

Delineation drilling in late 1992 has confirmed the presence of at least one additional gas filled Miocene reef in the Pandora play.

Major new discoveries have also been made in the 1990s. Perhaps the largest of these is the Shell/Oxy discovery of gas and oil in the Malampaya and Camago reef buildups off Northwest Palawan. 8

These fields may contain more than 200 million bbl of oil and 3 tcf of gas. Other carbonate reservoirs discovered off Northwest Palawan 9 include Vaalco's West Linapacan field and Petrocorp's Calauit and South Calauit wells. West Linapacan field, which produces from downdip reef talus, is on production and estimated to contain reserves of 109 million bbl of oil.

Oxy has also been successful in its exploration efforts in the SK-8 Block of the Central Luconia province. Since early 1992, the company has discovered an estimated 5 tcf of gas in Jintan, Selasi, and Serai fields. 10 Development of an LNG plant for these gas reservoirs in combination with the planned expansion of the existing LNG facilities at Bintulu to serve many of Shell's previously discovered reefal reservoirs will greatly increase Malaysia's LNG export capacity.

Another oil bearing carbonate buildup discovered in late 1992 was Lufeng field in the Pearl River Mouth basin north of Liuhua field. Ampolex, operator of this discovery, is still evaluating the production potential of the area.

With these numerous discoveries of hydrocarbon reservoirs in Tertiary carbonate buildups, it is almost certain that the remainder of the 1990s will bring many more similar discoveries. Also adding to the potential for new discoveries is the fact that many deeper water areas once Considered impractical for drilling are now being explored.

It is this move to ever deeper areas that allowed the Shell/Oxy group to test the Camago and Malampaya structures located in more than 1,000 ft of water off Northwest Palawan.

TECTONICS, DEVELOPMENT

As is widely recognized, the Southeast Asia area is among the most tectonically complex (and tectonically active) regions in the world.

Major collisions between the Australia, Southeast Asia, Caroline, and Philippine tectonic plates (and numerous smaller crustal fragments and plates) occurred during the Tertiary and continue today. Accretion of new terranes on plate margins is also taking place.

This complex tectonic activity in combination with the relative remoteness of parts of the region has hampered exploration efforts in many frontier areas. It also makes clear that understanding the origin and development of the basins is important to understanding the nature and distribution of the carbonate rocks and reservoirs throughout the region.

A previous study of the tectonic evolution of the Southeast Asia region" suggested that 10 major types of basins are present. These range from small, tectonically complex intra-arc basins to broad, shallow, gently deformed, back-arc basins.

Listed approximately in order of their relative importance in terms of carbonate buildups that produce or contain hydrocarbons, these 10 basin types are:

Rift margin
Back-arc
Foreland
Fore-arc
Suture
Intra-arc
Intracratonic
Accretionary wedge
Wrench and rift
Oceanic.

A color coded map of the region shows the distribution of each of these basin types (Fig. 2).

CARBONATE BUILDUPS

Analysis and comparison of the carbonate buildups forming reservoirs in Southeast Asia reveals that they range trom typical "walled reef" coral-rich complexes with up to hundreds of meters of vertical relief to low relief carbonate mudbanks with only a few meters of relief during deposition.

The geometry, facies distribution, and reservoir potential of these different types of buildups is significantly affected by tectonic setting and basin evolution. Particularly important to exploration efforts is understanding that a simple classification system for reef types based simply on shape (e.g., barrier reef, fringing reef, pinnacle reef, etc.) is much less important than understanding the three dimensional geometry of the buildup.

Whereas the modem coral reefs presently found in many parts of Southeast Asia (e.g., in the Kepulauan Seribu of the western Java Sea) are true "walled-reef complexes" that have a marginal rigid reef framework, backreef skeletal sands, and steep fore-reef slopes, this is a relatively uncommon type of reef in certain types of Southeast Asian basins including the oil rich back-arc basins of Sumatra and Java.

Walled-reef complexes formed most readily in rift-margin basins such as those around the South China Sea. Thus it is not surprising that many of the Tertiary carbonate reservoirs in the Pearl River Mouth, Northwest Palawan, Sarawak, and East Natuna basins are true walled-reef complexes.

In such reef complexes, lateral correlation of facies can be difficult because different facies may accumulate nearly vertically through time. Nido field in the Philippines provides a nice example of this (Fig. 3).

In this pinnacle reef reservoir, a centrally drilled vertical well penetrated mainly back-reef carbonate sands, a marginal well penetrated fairly tightly cemented reef framework, and a more distal well penetrated highly fractured fore-reef talus. Recognizing this lateral variation of facies and porosity development can play an important role in reservoir development in such reef complexes.

Another common type of carbonate buildup, particularly in tectonically stable back-arc basins, is the low-relief carbonate mudbank. The sheltered depositional setting in mann, back-arc basins, particularly when the influx of siliciclastic sediments slowed during times of gradually rising sea level (especially in the early Miocene), favored formation of these mudbanks over walled-reef complexes. Examples of fields producing from this type of buildup include Ramba and Air Serdang fields in the South Sumatra basin.

There is good lithologic evidence that Walio and Kasim fields in the Salawati foreland basin of Irian Jaya also accumulated as low-relief carbonate mudbanks. In the case of these fields, however, a relatively long period of vertical stacking of micrite-rich clean and argillaceous carbonates resulted in the low-relief buildups accumulating to relatively great thickness of many hundreds of feet.

Characteristics of carbonate mudbank reservoirs include:

An abundance of skeletal packstones and wackestones containing branching coral fragments and larger benthonic foraminifers;
Abundant depositional micrite matrix;
An absence of rigid reef framework and marine cements,
A generally low-relief, lenticular shape; and
Development on relatively flat carbonate shelves with associated argillaceous carbonates being deposited contemporaneously in slightly deeper water off the buildup.

Reservoir "flow units" in these buildups tend to have formed in response to sea level fluctuations and be laterally correlative across the field. This is particularly well shown in a schematic cross section across Air Serdang field (Fig. 4).

CONCLUSIONS

From this brief overview of Tertiary carbonate reservoirs in Southeast Asia, it is clear that the productive buildups vary widely in terms of their distribution, overall size, facies geometry, and hydrocarbon reserves

Understanding the relationship between tectonic setting, basin type, and relative fluctuations in sea level can provide information useful to defining optimum exploration targets as well as reservoir characteristics, porosity development, and potential for charging with hydrocarbons from underlying or associated source rocks.

Some of Southeast Asia's major oil producing areas such as the South Sumatra and Northeast Java basins were explored for many decades before large carbonate buildup reservoirs were discovered. This was in part due to the widespread misconception that sandstones offered much more attractive exploration targets.

Now with the discoveries of dozens of major fields including Ramba and Jene in the South Sumatra basin, and KE-5 in Northeast Java, the importance of carbonate exploration targets is rarely overlooked (Fig. 5).

Many of the rift margin basins around parts of the South China Sea remain underexplored, particularly along the long coastline of Viet Nam. The potential for large Miocene walled-reef complexes in this area is high.

Other more stable basin types such as the back-arc and foreland basins of eastern Indonesia also contain good potential as sites for finding low-relief carbonate buildup reservoirs. . Certainly, the numerous major carbonate reservoirs discovered during the 1980s and 1990s should serve to fuel future exploration in the tectonically complex but exciting frontier provinces of the Southeast Asia region.

ACKNOWLEDGMENTS

Ultimate recovery, and reserve estimates in this article are Petroconsultants calculations, not official government or company figures.

REFERENCES

Greenlee, S.M., and Lehmann, P.J., Stratigraphic framework of productive carbonate buildups, submitted for inclusion in the AAPG Memoir on carbonate sequence stratigraphy, 1993, in press.
Zillman, N.J., and Paten, R.I., Petroleum prospects, Bula basin, Seram, Indonesia, APEA Journal, Vol. 15, 1975, pp. 73-80.
Price, P.L., O'Sullivan, T., and Alexander, R., The nature and occurrence of oil in Seram, Indonesia, IPA 16th annual convention, Vol. 1, 1987, pp. 141-173.
Livingstone, H.J., Sincock, B.W., Syarief, A.M., Sriwidadi, and Wilson. I.N., Comparison of Walio and Kasim reefs, Salawati basin, Western Irian Jaya, Indonesia, in Siemers, C.T., Longman, M.W., Park, R.K., and Kaldi, J.G., eds., Carbonate rocks and reservoirs of Indonesia: a core workshop, IPA Core Workshop Notes No. 1, 1992, pp. 4-1 to 4 14.
Rudolph, K.W., and Lehmann, P.J., Platform evolution and sequence stratigraphy of the Natuna platform, South China Sea, in Crevello, P.D., Wilson, J.L., Sarg, J.F., and Read, J.F., eds., Controls on carbonate platform and basin development, SEPM Special Publication 44, 1989, pp. 353-361.
Sumarno, A., Djabbar, S., and Fuller, H.H., "The time is right" for Natuna gas development, Asian Oil & Gas, June 1992, pp. 14-18.
Ardila, L.E., The Krisna High: Its geologic setting and related hydrocarbon accumulations, Offshore Southeast Asia 1982 Conference, Seapex proceedings, Vol. VI, 1983, pp. 10-23.
Scherer, M., Lambers, E.J.T., Steffens, G.S., and Phillips, J.R., Shell appraising deepwater discovery off Philippines, OGJ, May 10, 1993, p. 48.
Hefty tests buoy Philippine oil sector, OGJ, Apr. 13, 1992, p. 27.
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Packham, G., and Shaw, R., Southeast Asian Tectonics: Multiclient Study, Petroconsultants Australasia Pty. Ltd., six chapters plus enclosures, 1991.
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AUTHOR

Mark Longman joined Cities Service Co. as a research geologist in the mid-1970s. Much of his work with Cities Service involved the study of Tertiary carbonate depositional systems and diagenesis in reef systems around the world. Special emphasis was placed on understanding the lithologies, depositional environments, and distribution of reservoir-quality porosity in the Nido and Matinloc discoveries in the Philippines. In the 1980s he worked as an exploration geologist for Coastal Oil & Gas and Buttercup Energy before becoming a consultant in 1984. He is the author of a number of published papers on various aspects of carbonate rocks and reservoirs and served as an adjunct professor on carbonates at the Colorado School of Mines. He has a PhD in geology from the University of Texas.