Peter Baillie, TGS-NOPEC Geophysical Co.Sumatra forearc is unprospective; there is not sufficient sedimentary section in Bone Bay to have generated hydrocarbons; Cendarawasih Bay is underlain by oceanic crust and is not prospective. If you answered “true” to all of these, many experts would agree that you are correct. Until relatively recently, all of these statements were believed to be true.
In fact, there are many widely held ideas about the geological structure underlying Indonesia’s deepwater basins that are now being turned on their heads in the face of new geophysical and geological evidence. In a recent mega-study of Indonesia’s deepwater basins, evidence was found indicating that the Sumatra forearc may not be a forearc at all, Bone Bay has a potentially prolific (and previously unidentified) synrift section, and Cenderawasih Bay is underlain by something quite different than oceanic crust.
These are just a few examples demonstrating that the geology and petroleum potential of Indonesia’s deepwater basins are very poorly understood and that many preconceptions about prospectivity are based on lack of data rather than rational analysis. TGSNOPEC Geophysical Co. (TGS) and the Indonesian Ministry of Energy and Mineral Resources (MIGAS) are bringing new insights into the realities of these areas with the IndoDeep exploration program, a megastudy over Indonesia’s deepwater basins covering nearly 610,000 sq miles (1 million sq km) of the region. The IndoDeep program integrates 2-D seismic data and gravity and magnetic data with core data and multibeam sonar technology that can help detect oil seeps and other active geological processes on the ocean floor.
TGS began acquisition of data for the IndoDeep program in late 2006 and completed the project in early 2008. The dataset includes 21,924 line miles (36,000 line km) of high-resolution, long-offset 2-D seismic data; 154,440 sq miles (400,000 sq km) of multibeam sonar (bathymetry and backscatter) data; 85,260 miles (140,000 km) of gravity and magnetic data; 50 heat flow probes; and the collection and analysis of 1,150 core samples. Now that the data have been acquired and the study is underway, theories such as the previously held notions about Sumatra, Bone Bay, and Cenderawasih are being challenged as the data reveals new clues about the geological evolution and prospectivity of the archipelago.
Theories thwarted
The IndoDeep program has already challenged some of the widely held beliefs about several of the deepwater regions.
One example is the Sumatra “forearc” area west of the island of Sumatra. The study indicates that the very notion that this represents a forearc area may not be accurate. The results of the seismic, multibeam, and coring surveys indicate that the oblique collision of the Australo-India and Eurasian plates offshore of Sumatra has resulted in the formation of a series of wrench basins. In addition, a synrift section overlain by Miocene and Pliocene carbonates has been identified on the seismic data. This is significant because a similar synrift section has generated billions of barrels of oil in nearby onshore areas and throughout much of southeast Asia. Furthermore, core analysis indicates the presence of thermogenic hydrocarbons in the study area. An additional 5,664 miles (9,300 km) of 2-D seismic data is currently being acquired in the Sumatra region.
Another region that has been shown to be geologically interesting is Cendera-wasih Bay. An emergent and previously unknown fold-and-thrust belt has been found on the eastern side of the bay. In addition, a significant sedimentary section was found underlying a substrate of sedimentary and metamorphic rocks. These rocks include reefs that are apparently similar to producing reefs in western parts of the island of New Guinea.
Potential proven
A similar synrift trend was found in Bone Bay, which was once believed to have neither significant sedimentary section nor hydrocarbons.
In addition to the spectacular results from the seismic data, the coring geochemistry was an unprecedented success. All areas showed the presence of thermogenic hydrocarbons, with 46% of cores returning thermogenic gas hits and 13% showing oil seeps. These results are double the world average for this type of survey, and that success is attributed to a combination of factors. First of all, the targets were found using backscatter, or the intensity of the returning sonar “ping” which gives an indication of the hardness or lithology of the seabed. Other key components of the project’s success include the direct navigation of the piston corer onto the target and the active tectonics of the Indonesian archipelago.
Another interesting result of the project is the repeated evidence of gas hydrates in the region. The data acquired over the Sunda area between the islands of Java and Sumatra has revealed the existence of significant amounts of gas hydrates, which may well be an energy source for the distant future. Other instances of gas hydrates were found in the Makassar Straits between the islands of Borneo and Sulawesi, in eastern Indonesia in the Seram Trough near the island of Waiego and the westernmost end of New Guinea.
IndoDeep insight
Indonesia is still a significant hydrocarbon producer with more than 950,000 boe/d from eight basins. Output, however, has been declining at an average annual rate of 5%, and potentially large stores of hydrocarbons remain to be discovered. Up until now, the 30 frontier sedimentary basins were lacking modern geophysical data to indicate their potential.
By combining modern geophysical data with geochemistry from carefully mapped oil and gas seeps, the IndoDeep project could effect a change in the speed and efficiency with which exploration companies find hydrocarbons in frontier basins and assess their commercial potential.
The bathymetric data generate spectacularly beautiful images of the seafloor and illustrate active geological processes. The combination of multibeam bathymetry and 2-D seismic data provides a virtual 3-D image to help unravel the geological history of these very complicated areas while costing significantly less than 3-D. A deepwater basin can now be fast-tracked from an area of little or no exploration interest to a hydrocarbon exploration hot spot.
As anticipated, the IndoDeep program is generating a huge amount of new data and fresh ideas about Indonesia’s geology. Early indications are that the complete dataset will challenge previously held concepts of the hydrocarbon potential of Indonesia’s frontier basins, which will prove valuable to explorers and the people of Indonesia for many years to come.
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