CO2CRC Otway project–Soil gas baseline and assurance monitoring 2007–2010

Abstract

A four-stage soil gas monitoring study was conducted from March 2007 to March 2010 in the western Port Campbell Embayment of the onshore Otway Basin, SE Australia. This study evaluated existing vadose zone soil gases over the CO2CRC Otway Project’s site for CO2 storage, and nearby deep natural CO2 accumulations. Carbon dioxide, methane and helium were the main gases analysed and much of the time CH4 concentrations were too low for accurate analysis. Carbon dioxide concentrations ranged from atmospheric background levels of 0.033% up to about 10% of the total soil gas. The δ13CCO2 values are typical for organic matter decomposition as the main source of the CO2 in the soil gas. Modern radiocarbon ages, determined from 14C analysis, and low helium concentrations support the conclusion that CO2 in the soil gas is predominantly from modern organic sources. There was a relative increase in CO2 concentration measured in locations where the Port Campbell Limestone is near the ground surface, which implies additional CO2 has been produced from the weathering of limestone.