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The AAPG/Datapages Combined Publications Database

Indonesian Petroleum Association


IPA-AAPG Deepwater and Frontier Symposium, 2004
Pages 11-30

Multibeam Exploration in the Makassar Strait

John Decker, Philip A. Teas, Joseph A. Curiale, Elizabeth A. E. Johnson, Daniel L. Orange


During 2003, Unocal and its partners, Eni, Inpex, Santos, Zodan, and Zudavi, conducted a marine exploration survey in 3 PSCs in the greater Makassar Strait area. The survey had five components: 1) multibeam bathymetry and backscatter to identify fluid expulsion features at the sea bottom, 2) gravity and magnetics to determine regional basement fabric, 3) high-resolution shallow seismic to constrain near surface structure, 4) navigated ≤ 6 m piston cores to obtain sea bed samples for hydrocarbon geochemistry and sedimentology, and 5) heat flow measurements to evaluate geothermal gradients and thermal conductivity. The objectives of this program were to identify locations of possible hydrocarbon leakage at the sea floor, to sample observed anomalies using very accurately positioned navigating piston cores, and to analyze the cores for hydrocarbon content. Our hypothesis was that thermogenic hydrocarbon seeps at the sea floor could provide information regarding favorable subsurface exploration targets. We also conducted a similar program over known subsurface oil and gas accumulations to serve as a reference for comparison. One PSC, Sangkarang, was found to have no indications of thermogenic hydrocarbons in 109 samples from 33 cores. A PSC commitment well, Lombosang 1, confirmed the lack of charge in one portion of this basin.

The Papalang and Popodi PSCs surveys identified numerous anomalous seafloor bathymetry and backscatter features. Most of the features were relatively small, and taken together, constituted only a small percentage of the seafloor. Analyses of sediments acquired with navigated cores showed that a very high number of the anomalous seafloor targets were characterized by biogenic and thermogenic gas seeps, and a few authentic oil seeps. Highest gas counts were obtained in areas of very high backscatter caused by communities of chemosynthetic clams and associated authigenic carbonate at or near the sea bottom (both successfully sampled with the piston core). These clams live off sulfate reducing bacteria that proliferate in the vicinity of active hydrocarbon seepage. The exact seep locations remain confidential, but wells are currently scheduled to test prospects in both PSCs. The multibeam exploration program conducted in the Makassar Strait has proven to be a useful tool in differentiating among play areas and prospects within a trend. It has been used to support additional data collection and to suggest new well locations where results were positive, and it has served to support relinquishment where results were negative.

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