Characteristics of Fluid of Jabung Sub-Basin, South Sumatra, Indonesia

I. N. Suta¹, Nugrahani Nugrahani², and Jonih Rachmat^2
1Exploration, PetroChina, Jakarta, Indonesia
²Exploration, BPMIGAS, Jakarta, Indonesia

Post-relinquishment stage Jabung Block has a restricted working area down to 1600 sqkms, lying in between two series of prolific grabens. Each graben has limited size of no more than 12 x 19 sq miles (20 x 30 sq kms) with accumulated sediment loads over 15,000 ft (4500 m) thick, denoted as the main kitchen produced mainly gas with additionally oil from the Kerogen Type-II and III and matured from depth of 6,000 ft (1830 m) to 20,000 ft (6,100 m).

Jabung sub-basin lies in the northern nose of the prominent South Sumatra Basin that has produced total cumulative of 2.1 BBO and 2.5 TCF of oil and gas, is Paleogene back arc basin. Its evolution experienced synrift, regional subsidence, and up-lift provided source rocks, reservoirs, seals, as well as preservation of hydrocarbon. The productive Tertiary reservoir consist of alluvial-fan to fluvial facies, delta facies, lacustrine, and marine facies, within evolution of fault controlled depocenter provided sediment development, subsidence and shrinkage.

Oil tends to exist as thinely rim ranging from 10 ft (3 m) to more than 50 ft (15 m) thick underlying the majority of thick gas ranging from 100 ft (30 m) to 600 ft (180 m). To the west of the sub-basin, the synrift alluvial - fluvial Oligocene-Miocene LTAF sands become the main target for both oil and gas production. The post-rift of carbonate, deltaic and marine reservoirs of UTAF to GUF and ABF are inconsistently become the secondary targets in several fields for gas and condensate production. The fluid column in this upper reservoirs vary from field to field, some fields have no indication of hydrocarbon appearance. To the east of the sub-basin, the upper reservoirs of deltaic-marine GUF to ABF sands become the main productive reservoir for oil and gas. The lower reservoir of LTAF - UTAF sands has so far been a risky target of having extremely high CO₂ content sourced from the underlying pre-Tertiary carbonates. Nevertheless, the western area has a minor influence of the CO₂ presence in the lower target, and negligible in the upper reservoirs.

Variety fluid contacts within and among the fields are common, reflect highly reservoir and field compartmentalization.

The current total cumulative production of Jabung Block is over 56 MMBO and nearly 500 BCFG since its commerciality in 1997.

The Exploration success ratio increased to 60% by recent discovery of three gas fields, in addition of existing 5 major oil and gas fields.

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