CARBON-DIOXIDE GENERATION FROM COALS IN TARANAKI BASIN, NEW-ZEALAND -IMPLICATIONS FOR PETROLEUM MIGRATION IN SOUTHEAST-ASIAN TERTIARY BASINS

Elevated CO2 levels in gas accumulations within the onshore area of th e Taranaki Basin can be quantitatively accounted for by decarboxylatio n of vitrinite-rich Tertiary coals over the lignite to early high-vola tile bituminous coal-rank range, The highest levels are found within s andstone reservoirs vertically adjacent to Eocene coal seams that have realized approximately 50% of their CO2 generative potential. The gas exists as a supercritical fluid under reservoir conditions and appear s to be generated in significant amounts, comparable to liquid hydroca rbons. CO2 may aid oil expulsion from coals directly when intense CO2 generation coincides with major oil generation, or indirectly by creat ing pressure-induced microfractures during the relatively rapid genera tion of large amounts of CO2 prior to the main phase of oil generation , For these effects to be realized, the rate of CO2 production relativ e to removal is considered critical, and favorable conditions are prob ably restricted to basins that have experienced relatively rapid heati ng. In this respect, the Taranaki Basin provides a model for CO2 evolu tion in many southeast Asian Tertiary basins.