--> Regional Gas Geochemistry of Indonesia: Genetic Characterization and Habitat of Natural Gases, Satyana, Awang H.; Parulian, Lambok; Purwaningsih, Margaretha; Utama, Kusuma, #90100 (2009)

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Regional Gas Geochemistry of Indonesia: Genetic Characterization and Habitat of Natural Gases

Satyana, Awang H.1
 Parulian, Lambok2
 Purwaningsih, Margaretha3
 Utama, Kusuma1

1Exploration, BPMIGAS, Jakarta, Indonesia.
2
Exploration, Chevron,
Pakanbaru, Indonesia.
3
Exploration, ConocoPhillips,
Jakarta, Indonesia.

Exploration in
Indonesia during the last ten years has resulted in discoveries of large gas fields. Understanding on the origin, distribution, and habitat of natural gases is important for continued gas exploration. This paper presents the first regional geochemistry study of natural gases in Indonesian basins and has used data from approximately 350 gas occurrences to derive regional conclusions.

Based on molecular composition and isotope data, both thermogenic and biogenic (bacterial) gas types can be recognized in
Indonesia. Mixing between the two types is also commonly observed. The thermogenic gases are characterized by a normalized methane concentration of less than 95 % and carbon isotope ratios (δ13CCH4) heavier (more positive) than -45 ‰. The pure biogenic gases have a methane component of 98 % or more and δ13CCH4 values lighter (more negative) than -60 ‰. Mixed thermogenic and biogenic gases have methane components of 95-98 % and δ13CCH4 values in the range -45 to -60 ‰.

Thermogenic gases predominate in most gas provinces in
Indonesia and can be found in the basins of Sumatra, Natuna, Java, Kalimantan, Makassar Strait, Sulawesi, Papua, and Timor-Arafura. The gases were resulted from both primary gas generation of gas-prone kerogen and from secondary gas generation of oil cracking. The biogenic gases are found mainly in the fore-arc basins to the west of Sumatra, in the East Java Basin, and in the foredeep areas of the Sorong Fault Zone in northern Papua.

High concentrations of CO2 mainly occur in North and
South Sumatra, East Natuna, and onshore Java. Based on the “heavy” values for δ13CCO2, most CO2 occurrences have an inorganic origin by either thermal destruction of carbonates or volcanic degassing. H2S concentrations are moderate to high (500 to >10,000 ppm) in some gas fields in North Sumatra, South Sumatra, East Java, East Sulawesi and Salawati Basins. All occurrences of high H2S relate to thermo-chemical sulfate reduction of deep, hot carbonate sequences.

Indonesia is well known for its early- to mid-Tertiary petroleum systems (Eocene to Miocene) and the bulk of the gas resources have been generated from and are found in rocks of 40-5 My in age. However, there are also significant gas volumes found in Mesozoic and Plio-Pleistocene systems. Paleozoic systems are not important yet but they may contribute to future gas discoveries in some frontier basins in Eastern Indonesia.

AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil