REGIONAL GAS GEOCHEMISTRY OF INDONESIA: GENETIC CHARACTERIZATION AND HABITAT OF NATURAL GASES

摘要

Strong growth of gas reserves in Indonesia as mature oil fields are depleted will cause a continued shift from oil to gas production. Exploration during the last ten years has resulted in discoveries of large gas fields widely distributed across the Indonesian archipelago. Understanding 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 regionally applicable conclusions.Based on molecular composition and isotope data, both fhermogenic 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 (δ~(13)C_(CH4)) heavier (more positive) than -45 %o. The pure biogenic gases have a methane component of 98% or more and δ~(13)C_(CH4) values lighter (more negative) than -60 %o. Mixed thermogenic and biogenic gases have methane components of 95-98 % and δ~(13)C_(CH4) values in the range -45 to -60 %o.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 result from both primary gas generation from gas-prone kerogen and from secondary gas generation from oil cracking. The biogenic gases are found mainly in the fore-arc basins west of Sumatra, in the East Java Basin, and in the foredeep area of the Sorong Fault Zone in northern Papua.High concentrations of CO_2 mainly occur in North and South Sumatra, East Natuna, and onshore Java. Based on the "heavy" values for δ~(13)C_(co2), most CO_2 occurrences have an inorganic origin by either thermal destruction of carbonates or volcanic degassing. H_2S 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 H_2S relate to thermo-chemical sulfate reduction of deep, hot carbonate sequences.Indonesia is well known for its mid-Tertiary petroleum systems (Eocene to Miocene) and the bulk of the nation's gas resources have been generated from and are found in rocks 40-5 My. However, there are also significant volumes of natural gas 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.