The presence of natural methane in Great Artesian Basin aquifers of the Surat Basin

Abstract

The Surat Basin portion of the Great Artesian Basin (GAB) in Queensland has long been known to contain natural gas from both conventional and CSG sources. Commercial gas extraction from conventional sources target the Evergreen and Precipice Formations, which are among the lowermost of the Surat Basin stratigraphic units; however, evidence exists of methane occurrences in waterbores, which in most cases, access aquifers much shallower than recognised conventional gas or CSG targets. Large-scale development of CSG in the Surat and southern Bowen basins has highlighted the presence of gas in aquifers overlying and underlying the coal measures. Potential issues associated with gas in waterbores include health and safety risks, and the difficulty of establishing baseline groundwater bore conditions against which potential CSG impacts can be compared. Australia Pacific LNG has been investigating the presence of gas in the aquifers across the basin. The program has involved the routine measurement of wellhead gas concentrations and analysis of dissolved gas in waterbores. Stable isotope analysis of the dissolved methane (δ13C-methane and δD-methane) has been undertaken to ‘fingerprint’ aquifer gasses to ascertain their provenance. More recently, δ13C-CO2 has been added to the suite of isotopes. Initial results confirm the presence of natural methane across the study area and in all of the GAB aquifers sampled. Isotopic analysis indicates a distinct difference in isotopic signatures between the methane from the coal measures and that of the overlying aquifers from which most groundwater is extracted.