Gas Geochemistry of Betara Complex, Jabung Area, South Sumatra Basin – Genetic Characterization & Habitat of Natural Gases (2008)



Jabung area located in northern part of South Sumatra Basin has been proven as prolific hydrocarbon in Jambi Province. It is proved by oil and gas production since 1997 after the discovering of North Geragai and Makmur Fields in 1995 and North East Betara Field in 1997. There has been boosted of growth of gas reserves in Jabung area after subsequent discoveries of North Betara, Gemah, West Betara, South Betara, Suko and Panen Fields. This will impact a shift from oil to gas production as oil depleted in Jabung area. A better understanding of the gas geochemistry, origin, distribution, and habitat is important for focusing gas exploration that result in more efficient future gas exploration strategy in Jabung area. Based on concentration, molecular and isotope data from 25 samples, thermogenic gas type is commonly observed in Betara complex. True thermogenic gases of Betara complex are simply characterized by : wet gas methane (CH4) component of less than 80 % or wet gas components (C2+) are more than 20 % after normalization of hydrocarbon gases (by excluding nonhydrocarbon gases) and ratios of 13-carbon isotope to 12-carbon (d13CCH4) values are heavier (more positive) than -48 ‰. In Betara Complex, high trends (>48 vol %) of nonhydrocarbon CO2 gas mainly occur in Lower Talang Akar Formation of the eastern compartment of the North East Betara Field. Based on heavy values of d13CCO2, CO2 gas occurrences are related to inorganic origin by carbonate thermal destruction in North East Betara Fields. Low trend (<5% vol %) and medium values of d13CCO2 related to organic matter maturity. Most of the reservoirs are in Gumai, Upper Talang Akar and Lower Talang Akar Formations of South West Betara, West Betara and South Betara fields. Concentration of nonhydrocarbon gas of H2S is nil in Betara Complex. Sources and reservoirs of natural gases in Betara complex encompass almost whole of Oligocene to Miocene sediments. Gas habitats of the Jambi sub-basin are strongly controlled by stratigrapic, structural, and geologic evolution of the basin.


Author : Lambok P. Marpaung, I Nyoman Suta, Awang H. Satyana, Johnson A. Paju