Abstract

During the last three years Mobil conducted an aggressive program of seismic work, wildcat and appraisal drilling offshore North Sumatra. At times up to three drillships or semi-submersibles were active.

Activity focussed on the NSB area offshore NW Sumatra where a significant gas discovery (NSB-A1) in 357 feet of water was known from earlier drilling. Seismic work in 1979-80, hindered by positioning problems, revealed a province of Miocene pinnacle reefs. Up to 70 reefs were mapped in an area of 1800 square kilometers on the Malacca Shelf. Twelve new wildcats were drilled resulting in eight oil and gas discoveries.

Most reefs are pinnacle-like with up to 1100 feet of vertical and up to 3000 acres of areal closure. At NSB-A gas field, a “buildout” reef complex developed, with some 10,000 acres of areal closure. Reefs grew preferentially on pre-existing basement high trends. Dominant organisms include massive and platy corals, calcareous algae, bivalves, echinoids and larger foraminifera. Secondary processes of solution, recrystallization, chalkification and dolomitization over-rode the original reef facies and altered reservoir properties. Limestone with good secondary porosity appears to be restricted to regionally high areas which were subaerially exposed. Dolomitization seems to be erratic both in geographic distribution and in its effects on porosity.

Commercial gas reserves in the order of 2 TCF have been established in four separate fields. The gas contains up to 1-1.5 percent H₂S and CO₂ content is 28-31%. For production purposes offshore dehydration and compression facilities will be required plus a 100 km pipeline to the LNG plant at Lho Seumawe.

Some of the southeasterly wells tested high gravity, low pourpoint crude at rates of 4000-5600 b/d. At present plans are being developed for a floating oil production system utilizing subsea completions, a test barge, SPM loading and tanker storage.