Geological controls on coal seam gas distribution in the Hunter coalfield, Sydney basin, NSW

Abstract

Evaluation of the origin, distribution, migration and accumulation of coal seam gas (CSG), contributes to a better understanding of the CO2 storage potential of coals and the production of CH4 from coal seams. This study aimed to analyse the origin and distribution of the dominant gas components in coal seams of the Hunter Coalfield of the Sydney Basin, and to discuss the implications of these findings. The geological, coal and CSG reservoir properties were analysed using statistical methods, and burial history models were used to further explain the origin and temporal evolution of the gas. The Hunter Coalfield has been divided into five CSG compartments on the basis of geology, and gas content and composition trends. Gas distribution in the coalfield is compartmentalised, with coal and CSG reservoir properties influencing gas origin and distribution to varying degrees within these compartments. Coals in the first compartment, located south of the Hunter River Cross Fault, are characterized on average by the highest gas contents (~9m3/t), adsorption capacities (~23m3/t), permeabilities (between ~100.0 and ~1.0mD) and vitrinite reflectances (0.56 to 1.15%) in the coalfield. Present day gas contents may partially reflect the ranks and adsorption capacities, with late stage biogenic gas generation replenishing CH4 volumes. Compartments 2, 3 and 4 are located in the central region of the coalfield, with the Hunter-Mooki Thrust Fault and Muswellbrook Anticlines in the east, and the Mount Ogilvie Fault in the west forming the main boundaries. Compartment 2 is characterised as the ‘CH4 rich’ compartment, and has burial history profiles consistent with a shallower depth of burial compared to other compartments. Gas contents in Compartment 3 are particularly low (average ~4m3/t) given that these coals have a similar burial history to those in Compartment 1. It appears that low permeabilities have restricted meteoric water recharge, inhibiting the generation of significant volumes of biogenic CH4. Compartment 4 is considered to be the ‘intermediate gas content’ compartment. The coals were exposed to a shallower maximum depth of burial and greater uplift compared to Compartment 1, but have reached sufficient maturity levels to generate significant volumes of thermogenic gas. It is likely that substantial volumes of heavy hydrocarbon gases have been lost, with the Mount Ogilvie Fault probably acting as the main migration pathway for gas escape. It also seems unlikely that significant biogenic gas generation has taken place in Compartment 4 due to limited cross-formational water flow to recharge deep aquifers, and low permeabilities. Compartment 5 possibly had a similar burial history to that of Compartment 3, and gas contents are comparable to those in Compartment 1, but the gases consist almost entirely of CO2. Despite the lack of data, the most reasonable explanation for the CSG distribution in this compartment is injection of CO2 from the numerous dykes and deep seated igneous intrusions which may have enhanced the adsorptive properties of the coals, and thus the injected CO2 was preferentially stored for millions of years. Generally, gas contents in the coalfield are depth related whereas no relationships have been observed between coal rank and adsorption capacity for the Hunter coals. Detailed studies on the coals of two local study areas occurring within two of the compartments have shown that the main coal and CSG reservoir properties controlling gas distribution in the coalfield are the coal maceral composition, rank, adsorption capacity and permeability. Results show that some relationship exists between gas content and liptinite content in the Glennies Creek area, and that the generation of biogenic CH4 might have taken place using hydrocarbons generated from the liptinite-rich coals. Thus the degree of biogenic gas generation is not only related to the capacity for meteoric water access. This trend was not observed for the Warkworth area, which shows gas distribution patterns similar to Compartment 1. The burial history of the Sydney Basin has been shown to have an overarching control on the temporal evolution of CSG in the Hunter Coalfield. Based on theoretical storage estimates and other factors affecting CO2 storage in coals, the southern and western compartments of the coalfield are considered most prospective, with the theoretical CO2 storage capacity for the coalfield estimated to be ~9512Mt. The southern region also shows the greatest potential for CH4 production given its high gas contents and enhanced permeability.