Thermal history and geological controls on the distribution of coal seam gases in the southern Sydney Basin, Australia

Publication Year:
1993
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Repository URL:
http://ro.uow.edu.au/theses/346
Author(s):
Faiz, Mohinudeen Mohamed
Publisher(s):
Department of Geology - Faculty of Science
thesis / dissertation description
Coal seams of the southern Sydney Basin contain large volumes of gas, mainly methane (CH4) and carbon dioxide (C02) with subordinate volumes of longer chain hydrocarbons (C2+) and nitrogen (Nz). Data from exploration boreholes, underground mines and laboratory sorption-desorption tests are used to investigate the composition and distribution of gases in the coal seams. The influences of thermal history, coal composition, rank, geological structure, stratigraphy and igneous activity are evaluated.The coals of the Late Permian lllawarra Coal Measures attained ranks of up to medium- and low volatile bituminous coals as a result of high palaeo-heat flows (up to 2.5 HFU) and deep burial (up to 2500 m) in the Cretaceous. During the interval between Early Triassic and Middle Jurassic, large volumes of COz and H20 and subsidiary amounts of hydrocarbons were generated by coalification. The major phase of CH4 and other hydrocarbon generation occurred between the Middle Jurassic and Late Cretaceous at burial depths between 1.5 krn and 2.5 km.The gas retention and emission characteristics of coals are primarily dependent on pore-size distribution. The Permian coals studied composed of mostly vitrinite and inertinite and the porosity of these coals is dominated by micro- and meso-pores « 50 nm). Pores associated with the mineral matter are mainly macro-pores (> 50 nm) hence the presence of mineral matter in coal relatively decreases its micro-porosity.The gas sorption in coals mainly occurs in the micro-pores and increases with fixed carbon content, vitrinite reflectance and content but decreases with increasing mineral matter, volatile matter and moisture content. The CO2 sorption capacity of coal is between two to three times higher than that for C~. Coal also desorbs CO2 at a significantly higher rate than CH4• For a given gas, the desorption rates are related to the abundance of macro-pores and fractures and accordingly increases with mineral matter and vitrinite content. Hence the residual gas content decreases with the increasing COz, mineral matter and vitrinite content.CH4 and longer chain hydrocarbons presently occurring within the Illawarra Coal Measures were generated during coalification, whereas most of the COz was introduced into the sequence in association with the periodic igneous activity since the Middle Jurassic. Most of the CO2 generated during the early stages of coalification have been expelled from the coal measures. The variations in the gas composition are mainly related to structural features and depth; the volume of CO2 increases towards structural highs with highest concentration occurring in anticlines and near some faults. Structural lows contain dominantly CH4 but local pockets of CO2 sometimes occur adjacent to faults and dykes. Furthermore, in majority of the boreholes, the volume of CO2 increases with decreasing depth. These variations are mainly related to the migration and solubility properties of CO2.The quantity of ethane and longer chain hydrocarbons occurring at depths less than 500m is very small « 0.1%) but increases with depth. It is postulated that this trend is related to theready expulsion of most of the early-formed longer chain hydrocarbons from shallow depths and the subsequent invasion by later-formed CH4.The total in-situ gas content of coal, measured from core desorption test, varies from < 1 m'It to 20 m3/t. The desorbable gas content at depths shallower than 200 m is negligible but, on average, increases by approximately 4 m'It per 100m in increase depth up to 600 m, and thereafter the rate of increases is significantly low. However, at a given depth, individual values can show up to 80% variability depending on gas composition, geological structure, coal composition and rank. Elevated gas contents occur where the gas is dominantly CO2 whereas anomalously low gas contents occur near highly faulted zones.