EXPLORATION Sequence stratigraphy buoys W. Indonesia basins

Western Indonesia-1

Steven Courteney
Wairarapa Geological Services
Wairarapa, New Zealand

In an environment of relatively low oil prices it is essential to minimize risk in new exploration. This necessity is particularly true for mature hydrocarbon provinces where the "traditional" plays have been fully appraised and several "nonconventional plays" have been tried and have failed.

New exploration in these mature hydrocarbon pro- vinces also has a tendency to reapply the older, traditional geological techniques, such as lithostratigraphy, which have become established in the technical literature and consequently new exploration opportunities can be missed.

Western Indonesia is one such mature hydrocarbon province where more than 300 fields are presently producing in 12 geological basins. A further 100 fields have either been abandoned or shut-in.

Since oil exploration began in Indonesia in 1870 more than 3,000 exploratory wells have been drilled in Western Indonesia, which means a significant data base exists; however, many of the published technical papers describing Indonesian petroleum geology are based on lithostratigraphy.

This article presents a correlative framework based on the application of sequence stratigraphy, which has been established for the hydrocarbon productive basins.

Part 1 illustrates this correlative framework with an example each from areas in Sumatra, Java, and Kalimantan. These examples review the "hydrocarbon system" in each area from the perspective of source, reservoir, seal, and timing of structure.

In Part 2 the ideas developed are expanded and integrated into an examination of the future hydrocarbon potential for western Indonesia, which is refreshingly different and perhaps more encouraging than those made using the "traditional tools" of lithostratigraphy and biostratigraphy.

Introduction

Fig. 1 [87468 bytes] shows the major Tertiary basins, excluding the present day fore-arc basins, of western Indonesia together with the outlines of the three areas chosen for this review.

The areas are: the Tiga Puluh arch (which separates the Central and South Sumatra basins), the Tanjung Raya area of the Barito basin in Southeast Kalimantan, and the East Java basin.

A large amount of published data is available for each area, and this reinterpretation with an emphasis on sequence stratigraphy results in a significantly different view of the "hydrocarbon system" of each area as compared with the "traditional" view.

Discussion
Tiga Puluh arch

The Tiga Puluh arch is a major northeast-southwest trending Pre-Tertiary high that separates the Central and South Sumatra basinsFig. 1 [87468 bytes]. Recent movements have resulted in the uplift of part of the arch to form the Tiga Puluh mountains, which trend northwest-southeast.

Several oil fields have been discovered in the area of the Tiga Puluh arch. To the north, most of the oil fields are associated with a north-northwest to south-southeast trending structural element known as the Lirik trend. To the south, the oil and gas fields are closely associated with the major northeast-southwest trending Jambi depression, though most of the fields on the south flank of the Tiga Puluh arch are also associated with recent movements along northwest-southeast trending fault systems.

In the literature the clastic reservoirs of the oil fields on the northern flank of the Tiga Puluh arch, locally called the Tualang and Lakat formations, are generally correlated with the Talang Akar formation sands on the southern flank of the arch. These sands are productive in small Sengeti field. The Tualang formation is assigned an Early Miocene age (N.4) and the Lakat formation a Late Oligocene age (P.22), whereas the Talang Akar formation is assigned a Late Oligocene to Early Miocene age (P.22-N.4).

Stanvac1 has shown that the Lakat and Tualang formations are diachronous on the northern flank of the Tiga Puluh arch. This interpretation, modified to reflect the ages used by Haq et al.2 and an older age of the continental Kelesa formation (which is based on regional sequence stratigraphy), is shown on the seismic line through the Japura structure and Parum field (Fig. 2 [62282 bytes]).

Fig. 3 [34942 bytes] shows a well correlation between Parum-1 and Sengeti-2 which clearly shows the difference in ages of the formations that had previously been correlated. In Parum field the Tualang formation sands and the Lakat formation sands are Early Miocene in age (Burdigalian and Aquitanian, respectively). The productive Talang Akar sands in Sengeti field are Late Oligocene (Chattian).

Fig. 4 [78509 bytes] shows a sequence interpretation of a seismic line through Kenali Asam field, which is the largest field on the southern flank of the Tiga Puluh arch and produces from Middle to Late Miocene sands.

In both the Central and South Sumatra basins regional work3 has indicated that the effective source rocks on the flanks of the Tiga Puluh arch are Middle Eocene in age. This interval is present locally (between the purple, or 39.5 Ma, sequence boundary and Pre-Tertiary reflector) on the southeastern flank of the Japura structure on Fig. 2 [62282 bytes]. On Fig. 4 [78509 bytes] this interval is visible below Kenali Asam field and farther to the northwest in the foot wall of the Jambi fault.

Modeling indicates migration occurred on both flanks of the Tiga Puluh arch during the Late Miocene and in the case of inversion structures such as Kenali Asam field re-migration in the Pleistocene is strongly inferred.

The seals and timing of structure on both the northern and southern flanks of the Tiga Puluh arch are indicated on Table 1 [51543 bytes].

Southeast Kalimantan

The Tanjung Raya area, Southeast Kalimantan, covers the producing fields of the Barito basin (Fig. 1 [87468 bytes]). Tanjung and Kambitin fields are among the few cited in the literature as producing from Eocene sediments.

Ideas differ, however, on the precise age of the reservoir units within Eocene since the earliest datable larger foraminifera (Late Eocene to Early Oligocene) are found in the marine shales overlying the reservoir units. The older non-marine shales contain no age diagnostic fossils. By reviewing all the available data and integrating this with the data from the Kutei basin the following conclusions can be made:

• The main producing zone (Z-860) in Tanjung field, the productive intervals in Kambitin field, and the sands that yielded significant flows in Bagok-1 are all Late Eocene in age (Figs. 5 [78509 bytes] and 6 [26917 bytes]). This interpretation differs from the lithostratigraphic interpretation of Fig. 5 [25813 bytes] published by Kusuma and Darin,4 which put the main producing zone of Tanjung field in Middle Eocene.

• The effective source rocks of the Barito basin are interpreted as being shallow lacustrine shales.3 Lacustrine environments only existed in the Middle Eocene in the Barito basin.

It should be noted that Tanjung field also produces from fractured Pre-Tertiary granites and from Middle Eocene sands. Warukin field produces from Middle Mio- cene (Serravallian) sandstones.

The seal, timing of structure, and migration in the Tanjung Raya area are shown on Table 1 [51543 bytes].

East Java basin

The most important biostratigraphic type well, Bodjonegoro-1, cited in the literature for Indonesia is located in the East Java basin (Fig. 1 [87468 bytes]). Recent well results and the observations of van Gorsel5 strongly indicate that the whole section penetrated by Bodjonegoro-1 is reworked (see Courteney,6 in press, for a detailed discussion).

As a consequence of the unreliability of this well, sequence stratigraphy has been applied to the East Java basin by correlating from the offshore gas field to the east and the wells in the Java Sea to the north. In these areas the effects of possible reworking are minimal.

The correlations, while necessitating jumps at the coastline, are reliable for the 10.5 Ma, 15.5 Ma, and 21.0 Ma sequence boundaries. The deeper 30.0 Ma and 39.5 Ma sequence boundaries are interpreted from the significant angular unconformities in the rift sequences with limited control from deep wells such as Ngimbang-1. Such angular unconformities are recognized in the rift sections of all the basins in Western Indonesia.

Fig. 7 [79812 bytes] shows a composite seismic line across Kawengan field, which is the largest field in the East Java basin and is also located on the same structural trend as the Bodjonegoro-1 well. The difference between the sequence stratigraphic interpretation and the older, established interpretation is best illustrated by the 10.5 Ma event (light blue), which is just below the TDs of the development wells in the field. According to the older interpretations these sands are supposed to be Middle Miocene whereas the se- quence stratigraphy indicates a Late Miocene age.

Fig. 8 [51688 bytes] shows a seismic line farther to the west which crosses abandoned Tungkul and Loesi (or Lusi) fields. Here the age of the producing horizons is also interpreted as being around 5 million years older than previously reported.

Both seismic sections also show the syn-rift sequence (Pre-Tertiary, or red, to 39.5 Ma, or purple) which contains the effective source rocks of the East Java basin. The resulting reappraisal of the hydrocarbon system of the East Java basin is summarized on Table 1 [51543 bytes].

Part 2 of this article will build on these three examples to present a regional correlation of western Indonesia.

The Author