The Mahakam Delta Province: An Ever-Changing Picture and a Bright Future
Authors: Duval B.C. , de Janvry G. Choppin , Loiret Bernard 
Published: October 2008

The Mahakam delta area is an exceptionally prolific hydrocarbon province.

An extensive reinterpretation of geological and geophysical data has recently been completed allowing the petroleum systems of the entire area to be clearly understood.

The handicaps related to the huge volume of data and the scarcity of accurate stratigraphic information were overcome by using specially designed data sheets and by applying sequence stratigraphy and a number of specific and specially designed geological tools which proved very effective.

The Mahakam delta is seen to still have substantial remaining potential and this kind of study appears applicable to similar deltaic settings.

INTRODUCTION

The Mahakam delta is a Tertiary delta, located (fig. I) on the eastern coast of Kalimantan, the Indonesian part of the Borneo island. The sedimentary cover extends from Upper Eocene to Quaternary with a maximum thickness of about 12000 m developed directly beneath the present delta (MAGNIER, 1975). Large amounts of oil and gas from a number of fields (fig. 2) have been found in a Middle to Upper Miocene deltaic sequence (GERARD, 1973).

Exploration started as early as 1888 with field geological investigations in the vicinity of oil and gas seepages. It resulted in the oil discoveries of Sanga Sanga (1897) and Samboja (1909) in uplifted Middle Miocene sands, at 393 relatively shallow depths. In the 1970's, several giant gas and oil fields were first discovered offshore, later onshore and in the delta area, principally because of major improvements in seismic methods through mangrove swamps.

By 1985, proven and probable initial reserves amounted to about 2.6 billions barrels of oil and between 20 to 30 TCF of gas.

The acreage still operated by Total Indonesie in 1990 extended over approximately 10000 km2 with two contract areas: Mahakam PSC (1) and Tengah JOA (2) mainly located offshore (water-depth up to 200 m). Exploration of this area started in 1967 when Japex was awarded a vast area offshore the Mahakam delta. After a first well, Total farmed in and became operator in 1970. Five other dry wells were drilled in 1971 before making several major discoveries: Bekapai (1972), Handil (1974), Tambora (1974), Tunu (1977). More recently, two significant gas discoveries have been made in Sisi (1986) in the outermost part of the delta and in NW Peciko (1991) in a more internal position.

This Total Indonesie's acreage position extends over three distinct geological units (fig. 2), located within the outer part of the Kutei basin, with well defined characteristics;

The North Mahakam area, centered on the modern delta, is limited outwards by a belt of listric faults (fig. 3) at the front of the Mahakam deltaic system. It contains the bulk of hydrocarbon reserves.

The South Mahakam area corresponds to a narrow NWSE strip related to the downwarping of the Pre-Tertiary basement, below the Mahakam deltaic systems. It is intersected by several major NW-SE faults with a high dip-slip component. Sedimentation is mainly marine with minor deltaic tongues related to the ancestral Balikpapan and Adang rivers. Only a few minor oil accumulations have been found in this subsidiary Sepinggan delta area. The carbonate Paternoster platform is related to a basement high which acts as a southern boundary to the Kutei basin. All the wells that have been drilled in this area are dry.