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 C_nH_2n+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-FeO_1.5 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