Publications

Cities Service initiated exploration of the offshore East Java Basin in the late nineteen-sixties, holding the vast (157,000 sq km) North East Java PSC from 1969 until 1979. Cities shot 46,295 km of seismic data and drilled 48 wells, including three on the North Madura Platform. This feature is a stable high flanked by known hydrocarbon kitchens to the northwest (Central Deep) and south (Madura Basin), and it attracted more drilling by later operators. Like the Cities wells, these wildcats found teasing shows of oil and gas but no commercial accumulations. Exploration continued to focus on shelf edge buildups close to hydrocarbon source areas in adjoining depositional troughs. The wells encountered problems with reservoir quality, and sharp velocity gradients confounded attempts to image buried structures with seismic data.Yet another round of exploration on the North Madura High began when the Ketapang PSC was awarded to Gulf in 1998. Geological and geophysical studies highlighted the prominent, east-west trending Ngimbang (Middle Eocene to Early Oligocene) and Kujung (Late Oligocene to Early Miocene) shelf edge carbonates, but hydrocarbon indicators (HCIs) were also identified in more centrally located areas remote from hydrocarbon kitchens. Laborious correction of velocity artifacts caused by shallow channels revealed subtle, longstanding structural closures associated with these HCIs.The new (2001) Bukit Tua and Jenggolo oil and gas discoveries targeted reservoirs in layered Kujung carbonates near the center of the North Madura Platform, 10 to 20 km from the fringing reefs.Porosity development in this area may result from repeated exposure on the crest of the old Madura Platform. Migration pathways connect these reservoirs to distant source kitchens via permeable Kujung I carbonates and near-basement carrier beds such as basal clastics and Ngimbang and Kujung II/III carbonates. Confirmation of porosity and hydrocarbon occurrence in these discoveries opens the stable platform area to exploration. Fringing reefs remain a viable play, as indicated by other recent discoveries in the Ketapang PSC (Bukit Panjang, 2000, Payang, 2001) and nearby West Madura blocks (KE-23B, KE-13, and KE-24, 2000, KE-30, 2001) and Pangkah (Ujung Pangkah, 1998, Sidayu, 2000).