GEOPHYSICAL, GEOCHEMICAL AND STABLE ISOTOPE PORE WATER ANALYSIS ON SEDIMENTARY STRATA ABOVE A LONGWALL MINE, SYDNEY BASIN

The extraction of longwall coal panels in thick sedimentary basins results in fracturing in the overburden which then propagates to some level above the longwall. The level of fracturing is dependent on the longwall geometry, thickness of the overburden and geology of the strata above. Thick claystone stratum in the overburden is considered a limiting zone for groundwater flow. The aim of this paper is to understand the changes in the vertical depth profiles of δ¹⁸O and δ²H combined with geochemical and mineralogical analysis and geophysical methods. To understand recharge and flux, we analysed the stable isotopes from pore water on core samples from thick heterogeneous sandstone and claystone units from a temperate climate and compared those to rainfall signature, surface water and water from the underlying goaf area. The isotope analysis was supplemented by chloride analysis on 1:5 crushed rock suspension which indicated depositional environment and hydrogeological properties. Porosity and bulk density calculated from measured water content on core samples were compared with density obtained from downhole geophysical logging. This was undertaken to investigate if there was correlation between porosity and groundwater flux. The results indicate good correlation between δ¹⁸O and δ²H and chloride measured from 1:5 crushed rock suspension. A transition from sandstone to claystone indicates that diffuse flow is the main transport mechanism in claystone. There is evidence for advective transport in sandstone however this remains subject to further research. The core pore water isotope results reflect the heterogeneity of the Hawkesbury and Narrabeen sandstones and preferential horizontal flow that causes distinctive variations in the isotope profile with depth. Detailed fine scale mineralogy analysis using XRF and XRD confirms significant heterogeneity of the strata at centimetre scale thickness. Off axis equilibration laser spectrometry provides a useful method for understanding of flow and recharge processes in highly stratified sedimentary basins when supported with other methods such as geophysical and mineralogical analysis.