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The Pertamina-Cities Service, Ashland, Monsanto, Robina contract area covers Madura Strait, parts of the Java and Bali Seas, and the islands therein--about 57,700 sq mi. The Java Sea sector is part of a geologic platform, the remainder covers the Madura basin.
The Java Sea platform is the southeastern part of the "Sunda shelf" and occupies the stable region between Borneo and East Java. It consists of a basement complex of pre-Tertiary sedimentary, metamorphic, and igneous rocks overlain by varied thicknesses of Tertiary strata. Recurring Tertiary stresses created several prominent downwarps, shatter zones, and broad uplifts. Local folds tend to be associated with fault zones. Transgressive early Tertiary sediments filled the bottoms of the troughs, and were involved in strong tectonic activity. Subsequent sedimentation was more widespread and limestone deposits became important, especially in the southern and eastern parts of the platform. Extensive accumulations of younger Tertiary regressive strata suffered diminishing deformation. "Bioh rmlike" anomalies of several ages are abundant on the platform in the central part of the area and less common in the eastern half.
The Madura basin is the deeper, open-ended, offshore extension of the East Java basin which bounds the Java Sea platform along its southern margin. Several lines of compressional folds are present on the northern flank and localized areas of older folding adjoin the north coast of Java. Madura Island, an anticlinorium, and several islands on trend with it are upfaulted or upwarped parts of the northern flank of the Madura basin.
Geophysical surveys include 25,000 mi of seismic profile. Twelve exploratory wells were drilled on the platform part of this contract area and 8 wells were drilled in the contract area on the north. Numerous early-day tests were drilled on Madura Island; a recent well in the offshore part of the Madura basin failed to reach its primary objectives. Oil and gas shows have
been common throughout the region and exploration continues although tests have resulted only in noncommercial flows.
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