Geochemistry of gases in the Malay Basin


Author : Douglas Waples and Mahadir RamlyPublication : Bulletin of the Geological Society of MalaysiaVolume : 39Page : 241-258Year : 1996


Description

Bulletin of the Geological Society of Malaysia, Volume 39, July 1996, pp. 241 - 258

 

Geochemistry of gases in the Malay Basin

DOUGLAS WAPLES AND MAHADIR RAMLY

Petronas Carigali Sdn. Bhd., P.O. Box 12407, 50776 Kuala Lumpur, Malaysia

 

Abstract: Based on carbon-isotope ratios for CO2, methane, ethane, and propane, and on CO2 contents and the relative proportions of methane, ethane, and propane, we have identified three end-member gas types in the Malay Basin: biogenic gas, thermal gas, and basement gas. The thermal gas has been divided into two subgroups: "normal" thermal gas originating at relatively shallow depths, and "deep" thermal gas from more-mature source rocks.

Most gas samples studied in the Malay Basin are composed of mixtures of two or all three of the end members. Gases with a significant biogenic component are limited to the northeast corner of the basin, and are not associated with large accumulations. The biogenic gas was probably generated locally since the end of the Middle Miocene, and does not appear to offer an important exploration target in the Malay Basin.

Gases dominated by CO2 are predominantly sourced from the basement. They are found along a discontinuous trend from Dulang to Ular, and along another from Bunga Raya to Bunga Pakma. Because these gases have migrated vertically from the basement, they dominate only where extensive fault systems extend all the way to the basement. Although some accumulations along this trend are very large, targets are at risk of being dominated by CO2, with risk increasing with increasing proximity to basement.

The area in the north central part of the basin contains gas that appears to be mainly of "normal" thermal origin. Accumulations are of moderate size. Lack of contamination by basement gas and "deep" thermal gas in this area suggests a lack of deep faults. Lack of fault-related vertical migration pathways will limit the volume of hydrocarbon gas in this area, and thus downgrades its exploration potential, except where there is local evidence for deep vertical faults.

Much of the basin contains gas that is a mixture of "normal" thermal hydrocarbons, "deep" thermal hydrocarbons, and CO2 from the basement in varying proportions. "Deep" thermal gas seems to dominate over "normal" thermal gas in the large accumulations, suggesting that the key to exploring for large gas reserves is to find areas where vertical faults are adequate to drain the deep strata responsible for generating the large volumes of "deep" thermal gas, but where there is also evidence that these faults do not extend all the way to the basement. The region between Damar and Tujoh, where large reserves are present with only moderate amounts of CO2, may serve as a model for this type of migration. Integration of these data with analysis of structural styles should provide important guidelines for future gas exploration in the Malay Basin.