ASIA/PACIFIC: Exploration targets shift in Indonesian seismic evaluation

Part I: Cycles in technical performance

Th 6011osseis1

Editor's Note: This is Part I of a two-part series on Southeast Asia's oil exploration history and the expanded use of regional seismic surveys.

Th 6011osseis1
Water depths offshore Southeast Asia extend to over 5,000 meters.
Click here to enlarge image

The Southeast Asia region, mostly Indonesia, encompasses the world's largest archipelago, with more than 15,000 islands, extending east to west over a distance of more than 3,500 miles and north to south over more than 1,300 miles. The ocean area in the Indonesian Archipelago is about four times greater than all land areas.

In the Sunda Shelf of Western Indonesia, sea depths shallower than 200 meters delimit this rather pervasive shelf. This large shelf embraces the Malacca Straits, Natuna Sea, offshore Southeast Sumatra, offshore North Java, East Java Sea and offshore Kalimantan.

There are more than 50 sedimentary basins in Southeast Asia, most of which are situated in Indonesia, and which may generate and trap hydrocarbons. The main productive basins are North Sumatra, Central Sumatra, South Sumatra, Sunda, Northwest Java, East Java, Barito, Kutei, Tarakan and Salawati. To date, productive oil and gas fields of Indonesia have recovered more than 15 billion bbl of oil and about 20 tcf of gas. At present, Indonesia produces approximately 1.5 million b/d of oil and is the world leader in LNG exports.

Geologic setting

Th 0611osseis02
Southeast Asia has over 50 sedimentary basins (yellow), most of which are offshore.
Click here to enlarge image

Within the Sunda Craton of Western Indonesia, basins are Tertiary back-arc basins. The Tertiary basins have undergone a common pattern of cyclical sedimentation that consisted of a series of transgressions and regressions. These sedimentary cycles are common to the entire Sunda Shelf of Southeast Asia; hence, the stratigraphic columns of the Indonesian Tertiary Basins are similar.

The most important transgressions occurred from Eocene to Early Oligocene, and from Late Oligocene to Early-Middle Miocene. Regressions occurred in Mid-Miocene, the main regressive event occurred from Late Miocene to Early Pliocene.

The regressive cycles mark periods of intense tectonic activity, which caused tectonic inversion of basins with uplift of the previously back-arc rifts and realignment of the main fault systems. Numerous structural and stratigraphic plays are recognized within the Indonesian basins, the main reservoir types are transgressive and regressive clastic sequences within structural closures and pinch-outs, carbonate build-ups, and reefs.

Source rocks consist mostly of lacustrine shales and coals, which are abundant in the Eocene and Oligocene syn-rift sequences. Seals extend regionally, although occasionally interbedded shale sequences may cap smaller structures. Depocenters are located within close distance of producing reservoirs, while the most effective migration paths are through sand carrier beds and leaky faults which are proximal to reservoirs.

Field discoveries

Th 6011os08
Many 2D seismic lines reveal significant structures like this tectonic inversion north of Madura.
Click here to enlarge image

Historically, discovery rates have been high both onshore and offshore, with the success ratio, including exploration and appraisal wells, being close to 50%. By far, the most important onshore fields are the two giant oil fields of Central Sumatra, Duri, and Minas. Offshore, the largest oil fields are the "B", "E", Cinta, and Widuri fields off Northwest Java, and the giant Attaka and Handil oil fields, off East Kalimantan. Recently the deepwater oil plays became prominent in the Makassar Strait with a series of successes in stacked distal Miocene sands.

The giant Arun gas and condensate field in North Sumatra and the cluster of giant gas fields in the Mahakam Delta Area in East Kalimantan are the most prominent gas fields in production. In recent years, reserve replacement was dependent upon the development of the giant gas fields off East Kalimantan and in the Natuna Sea. The addition of major new oil reserves, hinges upon exploration successes in the under-explored pre- Tertiary basins of Eastern Indonesia and in the deepwater plays of Tertiary and pre-Tertiary basins.

The deepwater region of the Makassar Strait is site of the most exciting exploration play of the modern era in Indonesia. Also, a new hydrocarbon play has emerged recently with the significant oil and gas discoveries on the pre-Tertiary sequence reservoirs of Eastern Indonesia, onshore Seram, offshore in the Timor Sea, and south of Irian Jaya.

Bintuni Bay off South Irian Jaya is the site of the most important discovery of the last decade, the giant Wiriagar (Tangguh) gas field. The major gas field of western Indonesia, the super-giant Natuna gas field, remains undeveloped. Future exploration will be geared to deepwater reservoirs peripheral to the Sunda Shelf and towards the delineation of the pre-Tertiary sequences of Eastern Indonesia.

Most oil reserves of Indonesia were found onshore Sumatra before the advent of production sharing contracts (PSC). It is interesting to note that giant fields such as Minas, Duri, Talang Akar, Rantau, and most of the onshore reserves, were found by exploring the geological surface expression of large anticlines and oil seeps.

The advent of PSC's also marks the strategic focusing on offshore exploration and the onset of digital geophysical technology. Still, all of the reserves found onshore and offshore by PSC's account for less than half of the total oil reserves found by the early explorers. However, the main gas reserves of Indonesia were all discovered during the PSC era.

Early exploration

Early exploration drilling was based on surface mapping of large anticlines and on the geographical distribution of oil seeps. Geologists endured daily hardships in their field work, which in many instances had to be postponed due to transportation and logistics difficulties, or worse, malaria.

Petroleum geophysical exploration started around 1930, when the first torsion balance gravity surveys and seismic refraction surveys were undertaken in Central and South Sumatra, these surveys roughly delineated basin basement topography. The first magnetic surveys were conducted shortly thereafter. Also, at this time the first electrical logging of wells in Indonesia were conducted. Seismic reflection surveys were introduced in 1936.

In South Sumatra, the first successful well location chosen with the seismic reflection method was the Musi-4 well in 1940. Regionally, the tectonics of the Indonesian Archipelago was unraveled and documented by Vening Meinesz in a 1954 Geological Society of America publication. His accurate work was based on topographic, bathymetric, and gravity measurements.

Geochemical analysis of oil seeps has been conducted since the last century starting with the oil seep analysis ordered by A.J. Zilker which led to the discovery of the Telaga Said Field near Rantau, North Sumatra. Geochemical surveys were first conducted by Stanvac in Central and South Sumatra during 1954 and 1955.

Geochemical fingerprinting of oils relating source rocks and reservoirs became recognized in Indonesia in the 1980s with studies on the oil generative potential of the Early Miocene, Oligocene, and Eocene coals and shales.

Recent exploration

Modern seismic exploration techniques in Indonesia are coincidental with the renewed exploration efforts of the 1960s and early 1970s and were introduced by the PSC contractors. Onshore seismic surveys utilizing the new CDP technique were conducted in Sumatra, Java, Kalimantan, and Irian Jaya. An outstanding discovery was the Arun Field (gas and condensate), a seismic stratigraphic play.

Offshore exploration of the Northwest Java Sea and East Kalimantan led to the findings of the "B", "E", and Cinta fields off Northwest Java, and to the giant oil and gas discoveries of Attaka, Handil, and Bekapai at the Mahakam Delta, off East Kalimantan. The super giant Natuna Alpha gas field was discovered in the Natuna Sea. These findings were based upon interpretation of stacked CDP (common depth point), unmigrated seismic profiles, and underscored the enormous potential of seismic technology for efficiently resolving deeper seismic structures with distinct structural attitudes respective to the overburden. Deeper oil pools were found prior to the extensive utilization of seismic; however, these discoveries were located mostly through exploration tails beneath shallow oil fields.

Digitally processed, migrated seismic profiles became a standard processing output during the 1980s. In the 1990 decade, seismic acquisition with long cables, pre-stack time migration, and pre-stack depth migration of seismic sections led to precise, focused, seismic interpretation of offshore features.

3D surveys emerge

While exploration-drilling activity has shown a slight decline during the last decade, development drilling and rehabilitation of older fields have emerged in tandem with a growing reliance on 3D seismic reflection surveys. Most of the important fields have been surveyed by 3D seismic. This technique is established as a cost-effective tool in interpretation mapping, in the designing of field development, and in production optimization through infill development drilling.

The much-improved seismic imaging of reservoirs, achieved with 3D methods, has lessened the risk of drilling a dry hole when developing a field. Greater field output has been attained with 3D seismic interpretation, by either correctly placing the development infill vertical holes or by horizontally deviating the well towards the best reservoir trajectories. These are often defined by seismic attributes of horizontal time-slices derived from 3D seismic interpretation. Undeveloped fields benefit the most from 3D technology.

Effective seismic data collection and processing turnaround have been best performed offshore to the point that 3D seismic surveys are also becoming an exploration tool. Most modern 3D vessels can acquire data very fast because they are equipped with multiple air gun sources and multiple receiver streamers.

High-speed links with onshore computers may provide for real-time processing, however, most surveys are processed onboard up to the seismic migration stage. For reduced turnaround time, full survey processed "fast-tracks" and part survey sub-sets, "quick-looks" that are low-fold subsets of the full data set, expedite prospect evaluation. The emergence of 3D surveys in Southeast Asia was made in tandem with another revolution in the seismic industry, the advent of massive speculative seismic surveys.

Speculative surveys

During the last decade, the seismic industry has undergone profound changes worldwide because of hardware and software implementation based on forefront technology. These affected the traditional seismic industry business as a whole and new business development models took the industry to uncharted territory. Non-exclusive surveys (NEPS) rather than proprietary data gathering suddenly began to dominate the market. Presently, the speculative data libraries contain more than 4,000,000 km of 2D data and more than 900,000 sq km of 3D data.

Modern vessels equipped with long streamers, accurate navigation, and state-of-the-art seismic recording features are designated to undertake the regional speculative projects back-to-back. With rapid processing turnaround time, these data are readily available for all of bid rounds and for the evaluation of all partnership and exploration and production blocks. Furthermore, the cost of speculative surveys, which also includes gravity and magnetics is attractive to upstream companies for the study of larger areas.

Upstream companies quickly referenced regional speculative 2D survey data as a reliable tool to investigate the regional geology of the shelf, slope, and ultra-deepwater areas. By accessing these data the companies add to their prospect inventories and geologic study of blocks offered in the bid rounds. ;

References

A complete list of references is available from the author. Contact: Tel. (55)(21) 824-6821, Fax (55)(21) 533-0526, Email: fainstein@rio-de-janeiro.geco-prakla.slb.com

More in Home