Editor's Note: This is Part II of a two-part series on Southeast Asia's oil exploration history and the expanded use of regional seismic surveys.
Seismic reflection section over the Anambas Graben.
State-of-the-art seismic data acquisition systems technology has been con-sistently utilized in the recent speculative surveys offshore Southeast Asia. Long streamers up to six km length and long variable TWT (two-way time) recording of 10-12 seconds. In Southeast Asia, NEPS (non-exclusive proprietary surveys) have been run by Schlum-berger Geco-Prakla since 1993.
In Indonesia, site of most surveys, they are conducted under the auspices of MIGAS - Myniak dan Gas Bumi of the Ministry of Energy and Mines. The data acquisition and processing parameters utilized in the modern NEPSs have consistently yielded superior seismic resolution. They provide excellent imaging of the main Tertiary markers and unconformities in West Indonesia, the pre-Tertiary unconformities, and the essential pre-Tertiary horizons of East Indonesia.
The seismic interpretation of the 2D and 3D NEPS projects are performed mostly with computer workstations and interpretation software. Seismic interpretation mapping of regional surveys ties the regional seismic markers and produces the regional and the more detailed prospect maps. In deepwater, over the slope and rise, the bathymetry and its correction fundamentally differentiates the time and depth seismic sections.
The interpretation of the NEPS data allows for an assessment of the regional petroleum geology and comparison of structural and stratigraphic components with oil fields tied by these surveys. The following geological and geophysical components are derived from the regional data and compared with analogs:
- Hydrocarbon trap type
- Tectonics and basin inversion
- Reservoir seismic character
- Seal capacity
- Nature of hydrocarbon pathways and timing of migration.
Based on these data, comparisons (analogs) help improve the understanding of new plays in the basins peripheral to the Sunda Shelf and of the frontier plays in Eastern Indonesia. Deepwater forefront exploration plays along the entire archipelago are also derived from NEPS data. Many of the basins of Southeast Asia have undergone tectonic inversion during Late Miocene. Cons-equently, it is important to interpret these events in the seismic sections for the proper understanding of the trap, either as a paleo-high or as a new young anticline post-inversion.
In instances, hydrocarbon-filled reservoirs, either carbonate or sandstone, display typical signatures, either directly as a flat-spot or as a bright-spot, or through AVO (amplitude vs offset) techniques.
Regional seals may, in many instances, be observed directly on records due to the lack of reflectivity, or seismic transparency of shales in seismic sections. The migration pathways and the timing of migration are also increasingly understood through reconstructions based on better imaging of the deeper portions of seismic records.
West Indonesia - Sunda Shelf
Seismic reflection section over East Natuna.
In 1994, MIGAS, the Indonesian Petroleum Ministry, approved the design of a regional speculative survey covering the entire Natuna Sea. In the West Natuna Sea survey, an excellent seismic resolution of all the main Tertiary unconformities was achieved and provided for the accurate mapping of horizons within the zones of tectonic inversion, the deeper syn-rift sequences, and the basement.
In West Natuna, the lacustrine source rocks shows as bright-spots. The main reservoirs are in the Oligocene Gabus Fm. Regional seal is provided by the Arang Fm. The hiatus in the Late Miocene stratigraphy is due to the tectonic inversion at that time. The Pliocene unconformity marks the beginning of the recent deposition.
In East Natuna, the data enabled the interpretation of several episodes of growth within the hydrocarbon bearing carbonate reservoirs and also for understanding of East Natuna basin development through mapping of deeper horizons beneath the carbonates and the thick overburden.
There is no inversion at East Natuna. The main difference between the lithologies of East and West Natuna is the presence of the thick Terumbu carbonates in the northern platform of East Natuna. These are the reservoirs of the main discoveries of East Natuna.
The East Java Sea Basin and the areas onshore northeast Java Island have long been established as hydrocarbon provinces. In the region surrounding Madura Island, exploratory drilling has been successful in developing several oil and gas bearing carbonate reservoirs. The petroleum geology of the East Java Basins is complex with several sub-basins having developed as by-products of major rift zones, which trend NE-SW west of Madura Island and E-W along Madura Island and east of it.
Java PSCs account for 15% of the total acreage under license in Indonesia. Although most of this activity is still centered in the producing provinces offshore West Java, substantial exploration drilling activity is anticipated to grow in the East Java Region, both, onshore and offshore.
The East Java Sea has undergone a renewed exploratory surge, which takes into account the excellent hydrocarbon potential of the region and its relatively unexplored status as compared with the mature West Java Basins.
East Indonesia - Eastern Sunda
Tectonic provinces of East Java, also need a seismic record.
Recent exploratory drilling successes in pre-Tertiary reservoirs of the Timor Sea have established this region as a hydrocarbon province in which potential exists for the discovery of further sizable reserves. This new exploratory focus in the Timor Sea began in 1994 with the Elang oil discovery in the Cooperation Zone "A" - ZOCA between Indonesia and Australia. Upon testing of the lower Montara sand reservoirs of Callovian mid-Jurassic age, this well flowed light oil and opened a new exploration era in the Timor Sea.
Prospect map of the Jurassic unconformity in the Timor Sea.
The Elang field is now in production, and in the five years since its discovery, several other significant oil and gas discoveries were located in the Timor Sea. This included the exploration and delineation drilling of the large Laminaria oil field and Undan/Bayu gas-condensate field.
The prospect mapping of pre-Tertiary horizons conclusively defined deepwater seismic structural closures similar in size to structures that led to the significant discoveries of the adjacent ZOCA acreage. With the results of prospect mapping, Pertamina, the Indonesian State Oil Company, divided the entire region into two blocks and handled a highly successful competitive bidding of the SABO Block, to the east adjoining the ZOCA. SABO Block embraces approximately 4,571 sq km (1.13 million acres).
Towards the west the RABE Block covers 6,500 sq km (1.6 million acres). Because the water depths range from 600 to 1,200 meters, frontier incentives were awarded for this acreage, and exploratory drilling is about to get started. - Cretaceous horizon exposed at Misool, surrounded by deep Miocene troughs.
The Indonesian area of the Timor Sea is the outward shelf and slope extension of the northern Bonaparte Basin. The regional structural configuration of this basin results from two rift systems, an older Late Devonian-Early Carboniferous which is oriented NW-SE and a younger one which was initiated during the Triassic and is orientated NE-SW. The rifting associated with this phase culminated in the Late Jurassic, Oxfordian-Kimmeridgian, as a result of the continental break-up and is clearly expressed by the "break-up unconformity." Another significant unconformity, also seen as a major seismic event, is the Aptian Disconformity, which marks the end of the break-up phase.
The final period of structural movement occurred during the Late Miocene when the Australian Plate collided with the continental margin off Timor. Similarly, the Bonaparte Basin was the result of movement within the outer shelf and slope, which consists of an alternating series of normally-tilted fault blocks.
Faulting is widespread and occurred initially during Late Triassic to Early Jurassic. Significant younger fault realignments occurred after Late Miocene. Hydrocarbon plays in the Timor Sea are associated with block faulting, mostly within elongate horst blocks. The Jurassic unconformity immediately overlies the targeted Montara fluvial sandstone reservoirs. Therefore, mapping of this unconformity, and of the structuring beneath it, accurately represents the prospectivity of this sector of the Timor Sea.
Prospect evaluation mapping consisted of detailed maps of the Jurassic Unconformity and maps of structuring immediately below this unconformity. At the unconformity TWT, these are represented by four-way dip closures, tilted horst blocks fault closures, and hourglass structures. Below the Jurassic unconformity there are numerous stratigraphic traps and, in instances, anticlinal features formed within graben boundaries.
The Indonesian West Arafura Sea is essentially unexplored. However, in Australian waters, two older discoveries were effected in the 1970s. Substantial gas flowed upon the testing of Jurassic sands in the Sunrise-1 and Troubadour-1 wells. Drilling resumed in this area only in 1997 with the spudding of the Sunset-1 well, which tested gas and condensate in the Jurassic sands.
Further to the east several wells were drilled in the Australian Arafura Sea near the Calder Graben and in the Goulburn Graben. These wells targeted Jurassic and Paleozoic reservoirs. The most significant well is Arafura-1 well which encountered oil in calcareous Devonian and Lower Ordovician dolomites. These oils originate from marine Cambrian source rocks.
The reservoir of the large Malampaya gas field, a deepwater Miocene carbonate build-up. In the Palawan Basin 2D regional speculative surveying unraveled all of the deepwater carbonate plays.
A new pre-Tertiary petroleum province may exist in the outer shelf and slope of the West Arafura Sea. These may include the proven "Jurassic Play" of the ZOCA Timor Sea, where the newly found reserves established the existence of large Late Jurassic mature source rocks, and the "Paleozoic Play" which is still to be delineated. With the results of this prospect mapping, MIGAS, Ministry of Mines and Energy of the Government of Indonesia, and Pertamina, the State Oil Company of Indonesia, announced and conducted the tender of two blocks, LETI and MASELA. This represents the first exploration efforts in a large, mostly deepwater area of the Arafura Sea.
The areas offshore south Irian Jaya encompass several proven petroliferous basins and distinct tectonic settings. Oil has been produced from Miocene carbonate reservoirs onshore Salawati Basin for more than fifty years. Recently, in the last five years, giant gas reserves were found both onshore and offshore Bintuni Basin. These are trapped on pre-Tertiary Cretaceous, Jurassic and Permian reservoirs.
Offshore south Irian Jaya, there are several prominent features such as the elongated Misool High that trend NW-SE and traverse the Salawati and Bintuni basins. Because of recent tectonism, the geologic realms around the Misool High are quite distinct. In the Salawati Basin offshore oil and gas has been found in the Miocene Kais carbonates, in similar carbonate build-ups to the ones found onshore
The areas offshore south Irian Jaya encompass several proven petroliferous basins and distinct tectonic settings. Oil has been produced from Miocene carbonate reservoirs onshore Salawati Basin for more than fifty years. Recently, in the last five years, giant gas reserves have been found both onshore and offshore Bintuni Basin. These are trapped on pre-Tertiary Cretaceous, Jurassic and Permian reservoirs.
Makassar deepwater exploration has had renewed impetus with the discoveries of the Miocene reservoirs at West Seno. West Seno Field lies in 3,160 ft water depth and its development approval follows the POD (plan of development) for the deepwater Merah Field also offshore East Kalimantan. The 3D surveys thus have the purpose of prospect mapping similar distal sands offshore Mahakam. These, characteristically have high porosity and permeability.
Conclusion
In the offshore areas of Southeast Asia, non-exclusive seismic surveys are creating regional studies in mature basins, relatively unexplored areas, and deepwater frontiers. Modern data are now available over most of the offshore Tertiary basins including the Gulf of Tonkin (South China Sea), Palawan Basin (Philippines), Natuna Sea (Indonesia), Andaman Sea (Myanmar and North Sumatra), East Java Sea (Madura Island and North Bali), and Makassar Strait (Kalimantan).
Seismic prospects interpreted from these data are the basis for new license offerings. Upstream companies can benefit from exploration incentives granted for pre-Tertiary reservoirs, deep-water, and gas exploration while their explorers benefit from state-of-the-art seismic data. ;
References
A complete list of references is available from the author. Contact: Email: [email protected].
