Geology of the Arun Gas Field
Authors: Soeparjadi R.A. 
Published: December 2007

Abstract
The Arun gas field, discovered in 1971 by the Pertamina-Mobil Arun-A1 well, is located in the Pertamina-Mobil Arun-A1 well, is located in the Province of Aceh in North Sumatra. The field straddles the Province of Aceh in North Sumatra. The field straddles the coastal plain between the Barisan Mts. and the Strait of Malacca, 140 miles (225 km) northwest of Medan. Condensate-rich gas is found in reef and associated carbonate facies of Lower and Middle Miocene Age that in places exceed 1,000 ft (305 m) in thickness. These carbonates occur near the base of a Tertiary Age sedimentary section having a thickness of more than 10,000 ft (3048 m). The carbonate rocks are underlain by a clastic sequence of variable thickness that rests on economic basement of pre-Tertiary Age. The reef carbonates occur on a large paleo topographic high trending in a general north-south direction. The gas accumulation is mainly stratigraphic, having been trapped in a porous reefoid facies that is overlapped by Lower Baong (Middle and Upper Miocene) shales. Geochemical studies indicate that the Arun gas is derived directly from the Baong shale. Subsurface studies and drilling results indicate a field size of more than 21,400 acres (86 × 106 m2). The structure is approximately 11. 5 miles (18. 5 km) long and 3. 1 miles (5. 0 km) wide. Arun field is characterized by an abnormally high pressure of 7,100 psig (49 MPa) and a temperature of 352 deg. F (178 deg. C) at 10,000 ft (3048 m) subsea. Well productivities are high because of the good permeability of the limestone. The pay thickness permeability of the limestone. The pay thickness averages 500 ft (152 m). Initially the reservoir contained 16. 2 × 10 12 scf (459 × 109 std m3) of dry gas in place.

Introduction
Mobil's exploration efforts in the North Sumatra basin area since acquisition of the Bee Block in 1968 have led to the discovery of a number of important gas and gas-condensate accumulations. The Arun gas-condensate field, discovered in 1971 by the Pertamina-Mobil Arun-A1 well, proved to be of giant proportions, with a field size of more than 21,400 acres (86 × 106 m2) and containing reserves of 16. 2 × 1012 scf (464 × 109 std m3) of dry gas initially in place, with approximately 50 bbl condensate/MMscf gas (281 dm3 condensate/m3 gas). Additional seismic, drilling, and production data acquired during the past decade have enhanced knowledge about the Arun reef and allowed a better understanding of its complexities. This paper complements and updates previously published papers by Graves and Weegar and by Houpt published papers by Graves and Weegar and by Houpt and Kersting.

Regional Setting Location
The Arun gas-condensate field is located in the Province of Aceh, North Sumatra, Indonesia, and occupies a central position within Mobil's Bee Block (Fig. 1). The field lies immediately west of the village of Lho Sukon, or 140 miles (225 km) northwest of Medan, the capital of the region. The field is overlain for the most part by the relatively flat coastal plain extending from the foothills of the Barisan Mts. to the coast; rice paddies form the main cultivation in this flat area. Important means of access to the Arun field are by air, with Mobil-operated aircraft maintaining a regular service between Point A, a central control and power site in the field, and Medan's Polonia Airport, and thence to Singapore. Heavy equipment and material are transported by barges up the Peutu River to the Lho Sukon landing.

Geology
JPT
P. 1163