Relationship between coal composition, fracture abundance and initial reservoir permeability: A case study in the Walloon Coal Measures, Surat Basin, Australia

Abstract

The Surat Basin, which is a major eastern Australian Coal Seam Gas (CSG) reservoir, produces gas from its Middle to Late Jurassic sub-bituminous Walloon Coal Measures, and shows variability in initial reservoir permeability across the basin. For a given lithostatic stress, the presence of vitrinite group or telovitrinite macerals within the coal can influence the fracture abundance within the coal reservoirs of similar rank, which in turn potentially impacts the initial reservoir permeability of the CSG reservoirs. The predictive relationship between megascopic banding character or lithotype, microscopic maceral composition and fracture intensity within CSG reservoirs is well explored in bituminous coals, but not so much in sub-bituminous coals. Here, we analyse wellbore image log data along with the conventional geophysical logs, maceral analysis data, core descriptions validated by core photos from the well completion reports and interpreted permeability data from well tests to systematically investigate the relationship between lithotype, maceral composition with a focus on vitrinite/telovitrinite, and fracture intensity in the sub-bituminous rank Walloon coals, to test their influence on permeability trends across the basin.Permeability-depth trends in the Walloon coals vary regionally across the basin, and show different inclinations depending on coal measures stratigraphy, coal lithotype composition, fracture intensity and stress orientation between local anticlines, monoclines and synclines. Depth normalised permeability and thickness-weighted telovitrinite content data from 55 wells were analysed. Furthermore, analysis of 3.8 km of borehole image logs, along with 156 maceral composition data points and thickness-weighted average lithotypes derived from the core description across 10 vertical wells, revealed a moderately positive relationship between the coal type and fracture abundance. This indicates some coal compositional control on the fracture abundance at this lower rank. The derived average total fracture intensity (cleat and sinusoidal coal fracture) from borehole image logs and thickness-weighted average telovitrinite content from the core analysis also exhibited an exponential increase, with the increasing depth normalised permeability derived from the well test data within the 10 analysed wells. This relationship suggests the influence of the variation in coal composition and fracture intensity on the rheological behaviour of the CSG reservoir within the Walloon Coal Measures. Spatial distribution of depth normalised permeability from the 55 wells across different domains and their relationship with the maceral composition within the study area also suggests the influence of geological structures on the depth normalised permeability. Therefore, though coal composition controls the fracture intensity, under favourable stress and fracture conditions the relationship can be substantially modified in parts of the Surat Basin, and altogether these factors control the rheological behaviour of the Walloon Coal Measures.