The geology, stratigraphy and coal seam gas characteristics of the Walloon subgroup - northeastern Surat Basin

Scott, Steven Geoffrey (2008) The geology, stratigraphy and coal seam gas characteristics of the Walloon subgroup - northeastern Surat Basin. PhD thesis, James Cook University.

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Abstract

The Mesozoic Surat Basin constitutes part of the Great Australian Basin, which is composed of the Clarence, Moreton, Surat, Eromanga and Carpentaria basins and is located in southern Queensland and northern New South Wales. Within the northeastern basin, the Middle Jurassic Walloon Subgroup is the dominant coal-bearing unit. The Walloon Subgroup is best developed within the northeastern region of the Surat Basin between Wandoan in the north, Moonie in the south, as far east as the Kumbarilla Ridge and to the Mimosa Syncline in the west. This region offered the best likelihood for thick and continuous coal intersections and was chosen for this study.

The objects of this study were to revise the stratigraphy and sedimentology and to review and understand the coal and coal seam gas characteristics of the Walloon Subgroup. Coals of the Walloon Subgroup have become the source of one of Queensland's fastest growing energy industries, producing over 100 TJ of gas daily into southern Queensland's gas pipeline network. Growth has occurred quickly over the past five years. Prior to 2000 only one well had been drilled that deliberately targeted the Walloon Subgroup coals to assess their coal seam gas potential. Since 2000, five companies have drilled over 260 core, exploration, appraisal and development wells to assess and develop this gas resource.

Review and revision of Walloon Subgroup stratigraphy utilised the work of many previous workers and additionally over 300 well intersections, 85 of which were specifically drilled for coal seam gas exploration. While the upgrading of the Walloon Subgroup had been proposed as early as 1981, this proposal was based on only a small number of wells that had mostly only intersected incomplete sections of the subgroup in a limited area of the northeastern basin. The present study is based on over 300 well intersections, with over 20 fully cored intersections spread across the entire northeastern basin. The unit has been raised to subgroup status and divided into four distinct formations: the Durabilla Formation, Taroom Coal Measures, Tangalooma Sandstone and Juandah Coal Measures. Coal seams are concentrated within the Juandah and Taroom Coal Measures with the seams mostly being thinly interlaminated with carbonaceous mudstone, mudstone, siltstone and sandstone but can be identified and correlated across the northeastern basin. While the coal plies are lenticular, the coal packages are continuous across the study area. These coal packages have been named. Within the Juandah Coal Measures they are, from shallowest to deepest, Kogan, Macalister Upper, Macalister Lower, Nangram, Wambo, Iona and Argyle. Within the Taroom Coal Measures, they are Auburn, Bulwer and Condamine.

Sedimentology of the Walloon Subgroup in the Surat Basin was reviewed and categorised using lithofacies associations developed for the Walloon Coal Measures in the Moreton Basin. Over 2,000m of core was logged from seven fully cored wells located across the northeastern Surat Basin to understand the nature and change (if any) of the sediments. Lithofacies associations developed in the Moreton Basin are shown to be relevant for the subgroup in the northeastern Surat Basin. Seven lithofacies were developed which could be grouped into three associations related to their environment of deposition: Major Alluvial Channel Facies (A1); Quiet Water – Lake/Abandoned Channel Facies (B1); Quiet Water – Lake/Abandoned Channel Facies (B2); Crevasse Splay/Channel (B3); Major Channel Levee (B4); Swamp (C1) and Ash Fall Pyroclastics (C2). These seven lithofacies are present in each of the subgroup's component formations and while the proportions of these lithofacies was not consistent across the northeastern basin, each of the formations was composed of a number of fining upwards sequences, beginning with either the A1 or B4 facies and culminating in the C1 facies. These fining upwards sequences are consistent enough to have each of the end facies, the coal swamp (C1), correlatable across the northeastern basin.

Detailed graphic core logs and a cross section using the seven core wells were drawn to show the relationships of the lithofacies associations to each other as well as how they changed across the basin. This work confirmed that the Walloon Subgroup was deposited on a broad alluvial plain, with the sedimentary architecture being very similar across the subgroup, from the Durabilla Formation up to the Juandah Coal Measures.

While the Walloon Subgroup is the fine-grained end member of one of the fining upwards sequences in the Surat Basin, it still contains a significant volume of sandstone. During the work to delineate lithofacies associations across the northeastern basin, sand/mud ratios were also determined from the cores in the seven selected wells. Sandstone makes up between 50 and 70% of the sequence while coal comprises only between 15 and 30% of the sequence. This result contrasts with earlier work that showed the Walloon Subgroup comprising less than 20% sandstone in the Surat Basin. This earlier work was based on electrical and gamma ray logs from 166 conventional petroleum wells and because of the high percentage of volcano-lithic components of many of the sandstones found within the Walloon Subgroup, many of these sandstones do not display low gamma ray counts. This then explains the apparent discrepancy.

Previously only the seams of the upper Juandah Coal Measures (Macalister Upper and Lower) and Taroom Coal Measures (Condamine) had been extentsively studied for coal quality due to their suitability for open-cut coal mining. During this study over 570 coal samples from 23 fully and partially cored wells, located across the northeastern Surat Basin, were analysed to determine the characteristics of all seams present in the Walloon Subgroup. All samples were analysed to determine ash yield, moisture content and relative density. Samples from wells drilled during the latter stages of this study were also analysed for volatile matter and fixed carbon content. Due to the variable and high mineral matter content of the coals, most data needed to be recalculated to a dry, ash-free (d.a.f.) basis or a mineral matter free (m.m.f.) basis for comparison purposes.

Coals from the Walloon Subgroup are high-volatile, bituminous coal with a moisture content ranging between 1.5 and 10.5% (weighted average 4.6%), volatile matter content ranging between 12.6 and 49.1% (weighted average 36.0%), an ash yield ranging from 5.1 to 78.7% (weighted average 27.7%) and a fixed carbon content between 4.3 and 46.7% (weighted average 31.5%). On an individual seam basis, moisture content decreases through the Juandah Coal Measures but seems to increase marginally at the top of the Taroom Coal Measures before again decreasing through this formation. Volatile matter content of the individual seams has an inverse relationship to that of the moisture content. Volatile matter content increases steadily through the Juandah Coal Measures, peaking in the Tangalooma Sandstone before dipping at the top of the Taroom Coal Measures, increasing towards the middle of the unit before dipping again at the base of the formation. Ash yield is relatively constant through the Juandah Coal Measures with the exception of the Wambo Seam, which has a much higher ash yield than the other Juandah Coal Measures seams. There is then a quantum shift with the thin seams of the Tangalooma Sandstone having significantly greater ash yields than those of the overlying Juandah Coal Measures. The seams of the Taroom Coal Measures have a lower ash yield than those of the Tangalooma Sandstone but greater than those within the Juandah Coal Measures. Relative density across the seams mirrors the changes seen in the seam's ash yield. Relative densities for seams in the Juandah Coal Measures are relatively similar with the exception being the Wambo Seam, which is higher. There is then an increase for the Tangalooma Sandstone seams and then a decrease through the Taroom Coal Measures.

Coal samples were also analysed for their maceral composition and vitrinite reflectance with 215 samples analysed. Initially composite samples taken for adsorption analysis were also petrologically analysed for maceral composition and vitrinite reflectance. Only in the most recent sampling have individual plies analysed for gas content and coal quality also been petrologically analysed. While 125 samples from three wells have been analysed to this extent so far, a number of characteristics are apparent.

Petrographically the coals of the Walloon Subgroup are dominated by vitrinite and liptinite with only minor to trace amounts of inertinite. Vitrinite content ranges from 62.4 to 90.6% (weighted average 76.8%), liptinite content ranges from 7.6 to 33.4% (weighted average 19.5%) and inertinite content ranges from 0 to 18.8% (weighted average 3.7%). On an individual seam basis the vitrinite content is very similar for the seams of the Juandah Coal Measures and which in turn are also similar to the seams of the Tangalooma Sandstone. The seams of the Taroom Coal Measures are also similar in vitrinite content but the lowermost seam (Condamine Seam) has the highest vitrinite content of all seams in the Walloon Subgroup. Liptinite content for the individual seams increases through the Juandah Coal Measures and peaks in the seams in the Tangalooma Sandstone before decreasing through the Taroom Coal Measures. Inertinite content for the Walloon Subgroup coal seams shows the most variation. The seams in the upper Juandah Coal Measures (Kogan, Macalister Upper, Macalister Lower and Nangram) have the highest inertinite content with the seams in the lower Juandah Coal Measures, Tangalooma Sandstone and Taroom Coal Measures having only a trace amount. Vitrinite reflectance across the Walloon Subgroup seams shows a steady increase through the unit.

Over 570 coal samples from the Springbok Sandstone, Juandah Coal Measures, Tangalooma Sandstone and Taroom Coal Measures from 23 fully and partially cored wells were desorbed to determine their gas content and composition. The gas content was determined using the slow desorption method according to Australian Standards. All seams within the Walloon Subgroup contain coal seam gas. On a dried, ash free basis, gas content ranges from 1.15 to 14.48 m³/t (weighted average 5.33 m³/t). Gas content increases through the seams of the Juandah Coal Measures, peaks in the seams of the Tangalooma Sandstone and then decreases through the seams of the Taroom Coal Measures. This trend is very similar to the one displayed for the seams' liptinite content and indicates that within the coals of the Walloon Subgroup, liptinite content controls gas content. Gas saturation of the seams within the Walloon Subgroup again show an increase through the Juandah Coal Measures, peaking in seams of the Tangalooma Sandstone and then decreasing through the Taroom Coal Measures with the exception of the Kogan Seam which has a very high saturation. This trend also mirrors the gas content and liptinite content trend.

To further characterise the coal seam gas potential of the coals of the Walloon Subgroup the methane adsorption properties of the coals was also determined. One hundred and seventeen samples were analysed to determine Langmuir volume and Langmuir pressure. These characteristics display a similar trend to that shown by the gas content. Using the Langmuir constants, the gas saturation of each seam could be determined. Overall the coals of the Walloon Subgroup are undersaturated and again a similar trend to that displayed for gas content and liptinite content is evident.

Overall a number of physical characteristics of the Walloon Subgroup coals influence the quantity and quality of the coal seam gas present in the subgroup. These traits include depth, seam continuity, formation contacts and coal petrology.

To further understand the coal seam gas characteristics of the Walloon Subgroup coals, sampling points were divided into groups dependent on their geographic location. The study area is divided into three structural regions; the Mimosa Syncline, Undulla Nose and the Chinchilla-Goondiwindi Slope and when coal characteristics, gas contents and gas saturations of each region are compared some differences and trends become evident. Broadly speaking the coals of the Juandah and Taroom Coal Measures are very similar and any differences are due to changes in depth and so the coals' maturity. What is evident is that there is a difference in gas content and saturation and a marginal difference between liptinite contents across the three regions.

Modelling of the results of the study showed there are a number of stratigraphic and geologic features controlling the coal seam gas characteristics of the coals of the Walloon Subgroup. Regional geologic controls which increase gas content include the generation of secondary biogenic methane by the action of bacterially charged meteoric water entering the coal seams through the coals' outcrop and/or subgroup zones, the introduction of thermogenically generated gas from much deeper seams, and an increase in gas content due to an increase in coal maturity by an increase in the geothermal gradient. Regional geologic controls can also decrease gas content. These controls include the loss of gas due to a loss of seal between the Walloon Subgroup and the overlying permeable "Proud Sandstone" member of the Springbok Sandstone, a decrease in hydrostatic pressure due to the shallowing of the coal seams close to and at the seams' subcrops/outcrops and a loss of seal due to large regional faults which appear to act as conduits for gas.

Stratigraphic controls on gas properties include coal characteristics and composition. Gas contents and saturations are expected to increase steadily with depth as a function of increasing hydrostatic pressure but there are anomalous jumps through the Juandah Coal Measures and a decrease through the seams of the Taroom Coal Measures. This trend in gas content and saturation is mirrored in the trend displayed by ash content and liptinite content and so are strongly suggestive as being linked.

It has been demonstrated that the Walloon Subgroup can be divided into four separate formations which can be recognised across the northeastern Surat Basin. Two of these formations, the Juandah and Taroom Coal Measures, contain significant coal and coal seam gas resources. The Walloon Subgroup was deposited on an alluvial plain, which commenced with a high sedimentary accommodation rate but towards the end of depositional time had slowed considerably. Coal quality within the subgroup varies only marginally across the study area and the coal seam gas content within the coals varies with depth but does not uniformly increase with depth. This non-uniformity of gas content can only be partially explained by differing coal maceral composition of the different seams and is also due to a number of controlling factors including structural setting, depositional setting and coal petrology.

Item ID: 32089
Item Type: Thesis (PhD)
Keywords: Mesozoic Surat Basin; Middle Jurassic Walloon Subgroup; stratigraphy; stratigraphy; coal; coal seam gas characteristics
Additional Information:

Publications arising from this thesis are available from the Related URLs field. The publications are:

Scott, S.G., and Crosdale, P. (2000) The Walloon Coal Measures - the next coal seam gas target? Australian Petroleum Production and Exploration Association Journal, 40. pp. 86-93.

Related URLs:
Date Deposited: 29 Apr 2014 06:14
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040309 Petroleum and Coal Geology @ 34%
04 EARTH SCIENCES > 0403 Geology > 040310 Sedimentology @ 33%
04 EARTH SCIENCES > 0403 Geology > 040311 Stratigraphy (incl Biostratigraphy and Sequence Stratigraphy) @ 33%
SEO Codes: 85 ENERGY > 8501 Energy Exploration > 850199 Energy Exploration not elsewhere classified @ 50%
97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 50%
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