The role of hydrogeology, in situ stress and temperature logs in the distribution and delineation of coal seam gas regimes

Abstract

This thesis explores mechanisms that determine coal seam gas (CSG) distribution and methods for its delineation. Understanding the distribution of gas content and composition underpins exploration and forecasting, as well as estimation of fugitive emissions from coal mines. Coal seam gas origins are variable, and thermogenic hydrocarbon accumulations are often supplemented by inorganic carbon dioxide and microbial methane in many reservoirs. The generation of these gases is dependent on geological and hydrogeological parameters relating to reservoir geometry and permeability. Specifically, this thesis examined: • Hydro-geochemical controls on gas distributions and the apparent vertical zonation of gas reservoirs in the Sydney Basin, Australia; • The role of in situ stress in regulating water and gas migration (and/or accumulation); and • Utilisation of wireline temperature logging to enhance existing gas and geological exploration methods. The Sydney Basin is a coal-bearing sedimentary basin in eastern Australia. It is bounded by a series of highlands in the north, west and south and drains towards the centre and then to the east of the basin. Coal seam gas occurrence is laterally extensive and comprises layers of biogenic and thermogenic hydrocarbons and carbon dioxide. The zonation of these gases is regular and cross-cuts regional bedding dip; however, the sequence of gases varies with geographical position within the basin. Inland areas host a CO2-rich zone between the shallow biogenic and deep thermogenic hydrocarbon layers, whereas coastal locations are devoid of CO2, even in the vicinity of igneous intrusives. Gas contents typically increase with depth and peak at around 600-800m, below which volumes decrease to the base of the coal-bearing sequences. Carbon isotope data mirror this trend; both δ13C-CH4 and δ13C-CO2 increase with depth down to 800m, and then stabilise. These results confirm the respective biogenic and thermogenic hydrocarbon origins; however, carbon dioxide results are more complex. Conventional interpretation of CO2 origin is limited to deep-seated magmatic sources; however, many of the δ13C-CO2 values in the basin are outside of the traditionally assigned range. Investigations reveal that meteoric water enriched with positive cations (such as fresh rainwater in highland recharge areas) routinely dissolve carbonate mineralisation and transport bicarbonate down-gradient. Groundwater chemistry evolves along flow paths from fresh to saline composition and this causes re-precipitation of minerals. In some areas, the bicarbonate saturated waters can get trapped and, due to partial-pressure and groundwater salinity changes, liberate CO2 gas which then adsorbs to the coal matrix. Saline groundwaters in coastal regions preclude the development of CO2-rich gas accumulations, instead hosting extensive hydrocarbon reservoirs. Groundwater infiltration and gas migration are dependent on permeability that primarily occurs via fractures and coal cleats. Horizontal stress is critical in determining whether the cleat or fracture sets stay open and form conduits. Reinterpretation of existing data showed that differential horizontal stress magnitude varies with depth in zones regardless of the host formation lithology or stratigraphy; similar to that exhibited by the gas compositional layering. This means that the horizontal stress isotropy varies between zones; displaying higher values in the shallower and deeper parts of the strata (associated with biogenic and thermogenic gas reservoirs, respectively), and lower values in the middle section around 600-850m that hosts the mixed gas zone. This results in more fracturing in the middle zone and is intensified by the pore pressure overcoming the effective vertical stress. The upwelling, deep formation waters interact with the meteoric influx and result in the development of a peak gas horizon in this zone. The observations show that the vertically zonal nature of the stress environment controls the hydrogeological setting, which in turn facilitates the gas distribution. Therefore, groundwater monitoring methods, such as wireline temperature logging, could be utilised for mapping gas distribution. This tool is used to discern downward and upward flow and identify along-bedding flow and for approximation of permeability. The changes in temperature gradients identify flow type boundaries that coincide with changes in gas characteristics. This is particularly pertinent where gas compositional changes occur; in a case study presented as part of this investigation, surface meteoric influx discernible from the temperature logs coincide with the shallow biogenic methane zone, which is underlain by a highly compartmentalised and isolated strata interval of some 100-300m thickness hosting high concentrations of CO2 gas. In an adjacent and less compartmentalised region, the biogenic methane zone persists to deeper horizons but with increasingly less infiltration evident with depth concomitant with increasing CO2 compositions. This case study provides proof of concept for the utilisation of temperature logs towards enhancing coal seam gas exploration and optimising production and estimation processes.