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ABSTRACT: Processes controlling CO2 contents of some South Sumatra natural gas reservoirs

Christenson, B. W.1, R.H. Funnell2, R.L. Braithwaite2, A. B. Christie2, and G. Lyon2
(1) Institute of Geological and Nuclear Sciences, Taupo, New Zealand
(2) Institute of Geological & Nuclear Sciences, Lower Hutt, New Zealand

A study of 22 gas fields from the South Sumatra Basin reveals that the natural gases in this region are predominantly of thermogenic origin, and that the CnH2n+2/CO2 ratios of the accumulations are primarily controlled by temperature-dependent fluid-rock equilibrium processes in the reservoirs. The hydrocarbon accumulations show a broad range of thermal maturity, and d13C signatures which are consistent with their generation from a type II kerogen source. CO2 contents range from ca. 5 to 98 mole %, and isotopic evidence indicates that this gas also derives predominantly from the kerogen source materials. There is evidence of only limited and localized mobilization of 13C from sedimentary (ie. marine limestone) sources, and only one of the fields shows a significant magmatic carbon contribution. Fluid-rock equilibrium in the evolving reservoirs is maintained by a combination of FeO-FeO1.5 redox buffers and silicate-carbonate hydrolysis reactions. Departures from chemical equilibrium within the accumulations are readily accounted for by vapor gain/loss (ie. migration) processes, as indicated by 13C mass balance considerations between CH4 and CO2. Apparent non-equilibrium fractionation of 13C between CH4 and CO2 is explained by temperature controlled kinetic effects.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia