EXPLORATION Middle Eocene, older sequences in rifts key to potential in western Indonesia

Steven Courteney
Wairarapa Geological Services
Wairarapa, New Zealand

In Part 1 of this article three areas, the Tiga Puluh arch (Sumatra), the Tanjung Raya area (Kalimantan), and the East Java basin in western Indonesia were discussed in terms of sequence stratigraphy.

Such data have been interpreted for each basin in western Indonesia and two example correlations are discussed below (see Fig. 9 for location map).

Well correlations

A correlation of Late Oligocene to Middle Miocene TB1 & TB2 Supercycles (Fig. 10) indicates the reservoir intervals of some of the largest fields in Southeast Asia. The key points of this correlation are summarized below:

In the Central Sumatra basin Minas oil field, with reserves of over 4 billion bbl, produces from nonmarine Chattian (Late Oligocene) sandstones and fluvio-deltaic sands of Aquitanian age (Early Miocene). The overlying Burdigalian shales (Early Miocene) form a regional seal. To the northwest giant Duri oil field produces from sandstones of Chattian (nonmarine) and Aquitanian age (fluvio-deltaic) as well as the overlying fluvio-deltaic sands of Burdigalian age.
In the North Sumatra basin the giant gas bearing Arun reef of Burdigalian age (Early Miocene) unconformably overlies marine shales of Chattian age (Late Oligocene). A smaller reefal development forms the upper reservoir in Krisna field of the Sunda basin. In Krisna field the majority of the production is from the carbonates of Aquitanian age. The underlying Chattian sands form a stratigraphic trap against the Krisna high.
In the South Sumatra basin marginal Sengeti oil field and relatively small Tanjung Laban oil field both produce from fluvio-deltaic sands of Chattian age (Late Oligocene). In the literature these reservoirs are often dated as Early Miocene. In Tanjung Laban field the overlying Burdigalian carbonates are also productive.
In the West Natuna basin Chattian to Burdigalian age (Late Oligocene to Early Miocene) fluvio-deltaic sands deposited in a lacustrine environment are the productive reservoirs of Belida field.

No effective source rocks exist in the TB1 and TB2 Supercycles. Effective source rocks are defined as rocks containing organic carbon (which is capable of generating hydrocarbons) that have reached maturity and have expelled hydrocarbons. These hydrocarbons have migrated to and charged traps, the resulting hydrocarbon pools being large enough to be considered commercial.

A correlation of the Middle Eocene to Early Oligocene TA3 & TA4 Supercycles (Fig. 11) illustrates the widespread occurrence of source rocks in these sequences. The main points of this correlation are summarized below:

The lacustrine Pema- tang formation is the source rock for all the hydrocarbons in the Central Sumatra basin.7
To the southeast the lacustrine Banuwati formation has been identified as the main source rock in the Sunda basin by Prayitno et al.8
Farther east in the Lombok basin Phillips et al.9 published data on Eocene source rocks. They interpreted the depositional environment of the source intervals to be fluvio-deltaic. However, a later reinterpretation of the limited data3 suggested that the hydrocarbons reservoired in the fields were sourced from rocks deposited in a shallow lacustrine environment.
To the northeast the source rocks for the Barito basin are Middle Eocene lacustrine shales.
Illustrative logs are also shown of wells in the Northwest Java and Northeast Kalimantan basin which penetrated Middle Eocene age sediments. The shales, or their lateral equivalents, in these wells are considered to be potential source rocks.
The illustrative logs of Pematang (Central Sumatra), BZZ (Northwest Java), Jatibarang (Northwest Java), Pagerungan (Lombok), and Kambitin (Barito) fields, together with the gas zone that was apparently tested in the Northeast Kalimantan well all show the reservoir potential of the TA3 and TA4 supercyles.

Conclusion

A correlation chart of the major basins of Western Indonesia is shown on Fig. 12. This chart was prepared on the basis of the regional correlations using sequence stratigraphy discussed above.

Reservoir intervals shown on Fig. 12 are those of producing, shut-in, and abandoned fields together with marginally sub-commercial fields and new discoveries. Minor reservoir intervals that have yielded noncommercial flows and/or shows are not included.

Around 0.08% of the estimated proven in-place resources are reservoired in Pre-Tertiary rocks which are not shown on the correlation chart.

In every basin of western Indonesia significant thickness of sediments exist within rifts. These sediments are interpreted as Middle Eocene, or older, to Early Oligocene and contain the effective source rocks of western Indonesia together with reservoirs and seals. Where wells have not penetrated the whole Tertiary section (e.g. East Java basin) or where the older sediments cannot be dated using biostratigraphy (e.g. South Sumatra basin), sequence stratigraphy applied to seismic and well logs enables the identification of these older sediments.

This is the key to the future exploration potential of western Indonesia. The Middle Eocene and older sequences are restricted to the rifts, and potential traps will tend to be inversion structures (e.g. Jatibarang field, Northwest Java), or stratigraphic/structural traps on the hanging walls of the rifts. With few exceptions, in the deeper parts of the rifts, these older sequences have been eroded from the foot walls.

The major risk with this play is the quality of the Paleogene reservoirs. More detailed work is required to better understand the reservoir distribution in these syn-rift and immediate post-rift sequences.

The Paleogene of western Indonesia has been reviewed and estimated potential in-place resources of greater than 6.61 billion bbl of oil equivalent (unrisked) have been calculated. The distribution of these resources by basin is indicated on Table 2 [25964 bytes].

Neogene reservoirs should only be considered as viable exploration targets where possible communication to Paleogene source sequences can be demonstrated. Unfortunately, because of the limited extent of the Paleogene rifts and the extensive regional seals in the early Neogene, few areas remain in western Indonesia where additional reserves will be discovered in Neogene reservoirs.

Acknowledgments

The following publications were used in the preparation of the poster which formed the basis of this article:

Proceedings of the Indonesia Petroleum Association.

Pertamina-IPA Oil & Gas Fields Atlas.

Pertamina-IPA Seismic Atlas of Indonesian Oil & Gas Fields.

Proceedings of the Ikatan Ahli Geologi Indonesia.

Proceedings of the Southeast Asian Petroleum Exploration Society.

ASEAN Council on Petroleum Technical Papers.

AAPG Bulletin.

Masera-AAPG Data Systems International Developments in Exploratory Drilling.

References

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