Controls on methane sorption capacity of Mesoproterozoic gas shales from the Beetaloo Sub-basin, Australia and global shales

Abstract

Exploration in the Beetaloo Sub-basin in Australia's Northern Territory has indicated gas-saturated, quartz-rich source rocks that are gas mature and laterally continuous over large areas. In the Sub-basin shale targets have been identified within the Velkerri Formation and the Kyalla Formation. These shales are of Mesoproterozoic age and thus significantly older than any of the North American shale plays and most other shales currently under investigation for their gas production potential.In this work, we characterise two sets of drill cutting samples from the Beetaloo Sub-basin for their composition, pore structure, and CH4 adsorption capacity. The objective of this study is to assess the adsorption potential of these shales and investigate the properties controlling this. Measurements from other researchers are included in the analysis to determine to what degree commonly characterised properties such as bulk clay and total organic carbon (TOC) content control gas adsorption capacity on a larger scale and thus provide an indication of the range of adsorption behaviours that may be expected.The adsorption measurements are carried out at reservoir conditions - that is high pressures (up to 30 MPa) and high temperatures (up to 110 °C). The results show that the organic-rich (TOC: 3.7–6.2%) middle Velkerri shale samples have a significantly higher adsorption capacity (expressed by the Langmuir volume) than the clay-rich, organic-lean (TOC: 0.85–1.8%) lower Kyalla shale samples: 2.89–3.38 m3/t compared to 1.88–2.81 m3/t. This is in agreement with the higher average micropore volume of the middle Velkerri shale samples.Results of our analysis demonstrate that, depending on a shale's composition, different properties control gas adsorption in shale, though in the organic-rich middle Velkerri B shale samples the controlling paramters cannot be clearly determined. A positive correlation between TOC and CH4 adsorption capacity is observed, but only up to a TOC of 4.5%. Above this point the adsorption capacity appears to decrease again. The lack of a strictly positive correlation between TOC and adsorption capacity is likely caused by other parameters affecting sorption behaviour, in particular variations in organic matter type and thermal maturity. However, analysis of a global adsorption data set of marine shales of non-differentiated maturity nevertheless indicates that, on a larger scale, the TOC content can provide a reasonable first estimate of a shale's CH4 adsorption capacity.In the organic-lean lower Kyalla shale samples clay minerals control microporosity and CH4 adsorption. It is the high illite/muscovite content (30–40%) in particular that is indicated to be the main contributor to microporosity and gas adsorption capacity. Results from the analysis of the global shale adsorption data set are in agreement with these findings, showing that for organic-lean shales with a TOC < 2% clay may be the primary control on CH4 adsorption.