Abstract

More than twenty carbonate build-up features developed on the top of a Late Eocene, shallow water, carbonate shelfal sequence. These build-up features grew over basement highs, spreading east-west across the Sepanjang island–JS25 area in the central, eastern part of the Kangean block offshore northern Bali.

One well, the JS25-1, was drilled in this trend by Cities Service, and penetrated more than 1000 feet of highly recrystallized Late Eocene limestone. Primary porosity is largely destroyed in this well because of recrystallization, however, secondary solution porosity and extensive fracturing of the limestone has produced good reservoir zones.

The Kangean production sharing contract block was awarded to ARBNI in 1980. Since then, three seismic surveys have been conducted in the study area, in 1981, 1982 and 1983/1984 respectively.

Selected seismic lines clearly demonstrate drape of younger beds over the build-ups as well as onlap onto the reefs. Discontinuous and flat reflectors noted in the carbonate build-ups may be due to velocity anomalies caused by fractured limestone and/or fluid contacts.

Interval velocity and frequency displays of seismic line K-1231 across the JS25-1 well show zones of possible good porosity.