Microbial Activity in Australian Cbm Reservoirs

Mohinudeen Faiz and Phil Hendry
Petroleum, CSIRO, North Ryde, NSW, Australia

Coalbed Methane (CBM) in Australian basins comprise methane, carbon dioxide, ethane and higher hydrocarbons. Gas and coal geochemistry data indicate extensive microbial activity, especially in coal seams shallower than about 600 m. Microbial activity causing secondary biogenic gas generation possibly occurred subsequent to uplift of the eastern Australian basins during the Late Cretaceous and Tertiary. Stable isotope data indicate that CO₂ reduction is the main pathway of secondary biogenic methane generation in the eastern Australian coals.

The gas saturation levels of coal seams are highly variable depending on the thermal maturity, burial history and groundwater flow. In coal seams that have been uplifted and where meteoric water recharge has occured, secondary microbial activity has enhanced the methane saturation levels. Without such secondary gas replenishment however, many of these coals remains significantly undersaturated. In the Sydney Basin, for example, the CBM production rates are up to an order of magnitude higher in areas where coal seams have been re-saturated with secondary biogenic gas compared to areas containing only thermogenic gas.

The types of microflora that are generating gas in Australia CBM reservoirs are unknown. The CSIRO is currently investigating the processes of biogenic gas generation in Australian coals including the ability of the indigenous microflora in a range of Australian coals to generate methane under laboratory conditions. Experiments will also be conducted to assess the viability of injecting suitable micro-organisms and/or nutrients into depleting and under-saturated coal seams to enhance methane production.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas