Abstract

Exploration activity over the last three years has outlined a hydrocarbon bearing province of carbonate buildups of Miocene age in Mobil's North Sumatra offshore area (NSO). Major activity concentrated on some 2,000 square kilometers of the Sunda/Malacca Shelf on the eastern flank of the North Sumatra Basin (NSB).

In 1972, a significant gas discovery (NSB–A1) in 357 feet of water established the presence of reefs. NSB–H1 drilled in 1979 tested the first black oil in the Malacca Straits and prompted increased activity. Detailed seismic mapping in 1979–81 revealed more than 70 carbonate buildups. Within this reef area, four oil and four gas discoveries resulted from the drilling of 12 wildcats in the last three years.

Most carbonate buildups are pinnacle-like reefs, with up to 1,100 feet of vertical and 3,000 acres of areal closure, located on basement highs and commonly with an underlying and/or interbedded clastic sequence of variable thickness. Mounded form and distinct lateral variation in seismic character together with termination of events on the flanks and drape in shallower beds define the structures.

Reservoir properties are due to original sedimentary environments overprinted by diagenetic facies. Current carbonate porosities range up to 30 percent. Dissolution, recrystallization, chalkification and dolomitization have significant effects. Dolomitization is commonly selective but can be pervasive, both increasing and decreasing porosity. Fracturing is indicated by lost circulation while drilling, in cores, on logs, and by test data.

Commercial gas and oil reserves have been established and various production systems are currently being studied. Flow rates up to 11 MMCFD were tested from the gas reservoirs which are normally pressured. The gas contains up to 1.5 percent H₂S and 31 percent CO₂. Development of the sour gas will require dehydration and compression facilities plus a 100 km pipeline to the LNG plant at Lho Seumawe.

High gravity, low pour point crude oil flowed at rates up to 5,600 BOPD in the NSB–L1 well. Current plans for the oil discoveries envisage a floating oil production system utilizing subsea completions, a test barge, single point mooring (SPM) loading and tanker storage.