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Imaging Horizons and Fracture Patterns in Coal Using Magnetic Data for Coal Seam Gas Exploration in Eastern Australia

Abstract

In the Red Hill area, part of the northern Bowen Basin, eastern Australia, where there is active exploration for Coal Seam Gas, a study was undertaken to map the top and base of coal seam sequences and delineate fault and fracture patterns using high resolution magnetic data. A helicopter survey had been acquired over an area adjacent to an open-cut coal mine. One seismic line extended a short distance into the survey area and information from 9 wells that penetrated the coal-bearing strata was used. The target coal units for the study were the Fort Cooper and Moranbah coal measures from the Blackwater Group of Late Permian age. The mapped formations were the Top of the Fair Hill Formation from within the Fort Cooper coal measures and Goonyella Middle Seam (Base) which is a regional primary target within the Moranbah coal measures. These coals were deposited under dominantly fluvial flood plain environments. Both of the coal measures targeted have tuff bands from volcanism that occurred at the time of deposition, making them more likely to be detected using magnetic data. Using a unique technology based on spectral analysis (Multi-Window Test, MWT), a series of multiple depth estimates were made at stations on a regular mesh of 400×400m over the study area as well as at the location of wells. Having established which formations could be detected at the wells, the depth estimates were correlated to produce coarse ‘skeleton’ maps of the two formations. Parameters determined from the initial interpretation stage were used to produce a detailed map of each horizon (100×100m). A second technique called Automatic Curve Matching (ACM) was applied to the magnetic data to interpret single magnetic anomalies derived from sources within the sediments. The results from this analysis were imported into petroleum interpretation packages and magnetic lineaments in various depth bands were delineated to produce a structural model of the area. Stress orientation was interpreted by analysing the magnetic susceptibility of the ACM results. From the completed skeleton of the top of coal, a band of susceptibility results, of around 10m above the coal to 100m below the coal surface, was created. From this, lineaments were interpreted by examining and assessing the alignment of solutions. The outcome was a series of lineaments showing fracture patterns at the surface of the coal. The azimuths of the interpreted lineaments are then grouped by dominant trends for easier analysis.