Miocene Reefs, Offshore North Sumatra
Authors: Mundt Phil A. 
Published: December 2007

During the last two years Mobil has conducted in aggressive program of seismic work, wildcat and appraisal offshore North Sumatra. At times up to 3 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. Eleven new wildcats were drilled resulting in 7 oil and gas discoveries.
Most reefs am pinnacle-like with up to 1100 feet of vertical and up to 3000 acres of areal closure. At NSB-A gas field a "build-out" reef complex developed, with some 10,000 acres of areal closure. Reefs grew preferentially on pre-existing basement high trends. Dominant organisms preferentially on pre-existing basement high trends. Dominant organisms include massive and platy corals, calcareous algae, bivalves, echinoids and liner foraminifera. Secondary processes of solution, recrystalization, 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 it effect on porosity.
Commercial gas reserves in the order of 2 TCF have been established in four separate fields. The gas contains up to 1 – 15 percent H2S and CO2 content is 28 – 31%. For production purposes offshore dehydration will 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 pour-point 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.
During the last two years Mobil Exploration Indonesia his conducted an active exploration/appraisal program in the Malacca Straits, offshore North Sumatra. At times up to 2 drillships and one semi-submersible were active. Fig. 1 shows the location of the NSB are situated about 60 km offshore in approximately 350 feet of water. This block of acreage, some 2.5 million acres in size, lies on the north side of the North Sumatra basin and on the west flank of the Sunda Shelf.

Stratigraphy
Fig. 2 is a north-south diagrammatic section across the NSB block and demonstrates generalized stratigraphic units. Economic basement in this area is slightly altered dolomites and argillites of Mesozoic or early Tertiary age. Altered granite was encountered in the Dl well. The basement ranges from a depth of 16,000 feet at the southwest comer of the NSB block to less than 4,000 feet near the Indonesia-Malaysia international boundary.
Basement topography is generally smooth to gently undulating with north-south high trends formed by fault scamps. Sediments directly overlying basement consist predominantly of calcarenites and calcareous sandstones of the lower-upper Miocene Belumai Formation. These sediments show very little deformation except for regional dip to the southwest, and moderate drape over basement highs. Numerous pinnacle and biohermal reefs are present within the Belumai Formation. These reefs, developed present within the Belumai Formation. These reefs, developed on basement or Belumai highs, are given the local name Malacca Member. As seen in the stratigraphic section, the Belumai Formation with its Malacca reef developments is time-transgressive onto the Sunda Shelf.
Ratios of Sr87 to Sr86 show the age of Malacca/Belumai carbonates to range from 7 to 23 million years.
The Baong Formation overlies the Belumai/Malacca units in most of the area. It is a dark-colored shale and forms an effective seal for the underlying carbonate reservoirs. Downdip, at depths greater than about 8,000 feet, the Baong shale is considered to be a hydrocarbon source for both oil and gas. A thin bed of sandstone is present within the Baong Formation over much of the area and contains oil and gas in local closures.
Keutapang, Seurula and Julu Rayeu sands and shales comprise the rest of the Stratigraphic column. These units display only monotonous regional dip to the southwest, being too far north to be affected by tectonic activity related to the Barisan Orogeny. These three formations together range in total thickness from 4,000 feet to 10,000 feet. Updip, where the Baong Formation is missing, lower shales of the Keutapang Formation serve as a seal for the Malacca reefs. A thin sand in the middle Keutapang contains gas and condensate at the 'A' field.

Exploration History
The NSO block, originally some 7.9 million acres in size, was awarded to Mobil in 1968. Subsequent relinquishments reduced the acreage to its present size of 3.2 million acres. A boundary dispute with Malaysia also served to rearrange the acreage; the NSO Extension block was assigned to compensate for acreage lost in the boundary settlement. The NSO block in its final form comprises 3 disconnected segments, of which the NSB area is one.
Prior to 1979 seismic control consisted of 3,970 km of fine on a Prior to 1979 seismic control consisted of 3,970 km of fine on a grid spacing averaging about 2 × 4 km in the NSB area of interest. Up to that time, five wells were drilled including NSB-Al, a Malacca gas discovery made in 1972.
In late 1979 the results of 2 wildcat wells, and some new seismic, revealed that literally dozens of pinnacle reefs were present.