David P. Johnson

Coal microstructure and micropermeability and their effects on natural gas recovery

Abstract Natural gas (methane) production from coal seams rather than from porous sandstone reservoirs is now recognized as a valuable and recoverable energy source in the USA, Australia and the UK. The Permo-Triassic Bowen Basin, Queensland, Australia possesses well defined coal seams which contain major methane resources. However, commercial gas production to date has been hampered by the low permeabilities of coal seams. Recovery of this valuable resource will be assisted by a fundamental understanding of the microstructures in coals and their influence on transmissibility of methane. This study examines coal microstructure (e.g. micrometre-sized fractures and cavities) which vary in width from 0.05 to 20 μm. These microstructures fall within the limits of meso- and macroporosity. Scanning electron microscope examination of the microstructure shows three porosity types: fracture porosity, phyteral porosity and matrix porosity. Fracture porosity is generally associated with bright coals although microfractures are present in maceral fragments from the dull coal layers. Characteristically the macro- and microfractures form a continuous structural fabric through the bright coal layers. In contrast, phyteral and matrix porosity is associated with the dull coal layers that are composed of plant fragments or a heterogeneous mixture of macerals. The continuity of the observed micrometre-sized fractures and cavities suggests that they make a significant contribution to overall permeability, and therefore play a major role in the transmissibility of methane at a level between diffusion at the micropore level and laminar flow at the cleat level. The effectiveness of gas drainage through the observed microstructures, however is likely to vary according to: 1. (1) the type of microstructure present in each coal type; 2. (2) microstructure density, orientation and continuity; 3. (3) the amount of infilling in the voids; 4. (4) the degree of coalification; and 5. (5) the presence or absence of clay layers in the coal seam. This is important, as a knowledge of the microstructure system and its relationship to coal type and coal rank plays an important consideration in gas drainage modelling.

New evidence for episodic post-glacial sea-level rise, central Great Barrier Reef, Australia

Abstract We present an extensive database of 364 radiocarbon dates from coastal and marine sediments of the central Great Barrier Reef (GBR) shelf, of which 110 are previously unpublished. The elevation data have been reduced to a common datum (Australian Height Datum, AHD) and the various sources of error have been assessed. Using modern lithological and biological relationships with sea level, the elements of the radiocarbon database have been converted into sea-level indicators. The upper bound of the assembled dataset corresponds to a best-estimate sea-level curve, and the full dataset provides a narrow envelope for sea-level rise on the GBR shelf for the last 11–12 kyr (not including hydro-isostatic crustal flexing). The envelope is consistent with episodic rise in post-glacial sea levels. The rising post-glacial sea level probably included Stillstands (or minor falls), at ca. −45 m AHD (at ca. 10.5 kyr B.P.), −5 m (7.8 kyr B.P.), −2 m (ca. 6 kyr B.P.) and +1.7 m (5.5 kyr B.P.). There is evidence for a significant fall in sea level between stillstands at −11 m (8.5 kyr B.P.) and −17 m (8.2 kyr B.P.). Stillstand durations apparently ranged between The vertical spread in the derived sea-level data is very wide. The use of shell material for dating seems unreliable and prone to large and unpredictable errors. Data from bulk mangrove muds appear reliable for determination of ancient sea level, but may at times result in sea level being placed up to 4 m below the true level. In-situ biogenic carbonates such as preserved oyster beds and coral micro-atolls are the most reliable indicators of sea-level position, while deposits of mangrove mud give a useful first-order approximation of ancient sea levels. Caution should be used in drawing ‘sea-level curves’ from few data points. We conclude that the post-11–12 kyr B.P. relative rise in sea level was episodic on the central GBR continental margin. More data are required to define clearly sea-level change up to ca. −20 m at 9 kyr B.P.

Terrestrial vegetation change inferred from n-alkane σ13C analysis in the marine environment

Abstract We report gas chromatography-isotope ratio monitoring-mass spectrometry (GC-IRM-MS) measurements of the δ13C values of individual biomarker compounds (n-alkanes) extracted from a 3 m marine sediment core taken near the mouth of the Johnstone River, North Queensland, Australia. The technique allows a purely terrestrial isotope signal to be discerned despite mixing of terrestrial and marine-derived carbon. The results indicate that there has been a 2% increase in the δ13C values of terrestrially derived n-alkanes (C29−C33) since clearing of the forested Johnstone River drainage basin for sugarcane and pasture began in the late 19th century. A much slower ∼1% increase in δ13C values after 6,000 years BP and prior to European settlement may be related to a decrease in rainfall in the basin, or to an increase in the abundance of C4 plants as a result of increased aboriginal burning. The results from the sediment core are consistent with data obtained for modern river sediments from forested and cleared subcatchments within the basin, and demonstrate that the δ13C values of terrestrially derived n-alkanes in the marine environment can be used to assess basin-wide vegetation changes in adjacent river catchments on geological timescales.

Sea-level controls on the post-glacial development of the Great Barrier Reef, Queensland

Abstract The central Great Barrier Reef is situated on a 125 km wide continental shelf with a shallow shelf-break at ca. 70 m. In consequence, the earlier post-glacial reef tract was confined to a narrow fringing belt along the present upper slope and outer shelf. The reef-province only started to assume its modern dimensions after ca. 10 ky B.P., when sea-level stabilised at depths around 28 m. Reef growth since 18 ky B.P. has been controlled by the episodic nature of the post-glacial transgression, being related to shorelines S8 to S1 developed at estimated eustatic depths and ages of 114 m 18 ky B.P. , 88 m 17 ky B.P. , 75 m 15 ky B.P. , 56 m 12 ky B.P. , 45 m 11 ky B.P. , 39 m 10 ky B.P. , 28 m 9.5 ky B.P. , 23 m 9 ky B.P. , 9 m 7.5 ky B.P. and 0 m 6.5 ky B.P. Rates of sea-level rise as high as 12 m ky−1 occurred between stillstands, outpacing coral growth response and resulting in many mid-outer shelf reefs being drowned in situ.

Coal microstructure and secondary mineralization: their effect on methane recovery

Abstract Methane production from coal seams rather than porous sandstone reservoirs is now recognized as a valuable and recoverable energy source in Australia. The Bowen Basin of Australia possesses well defined coal seams that contain major methane resources. However, commercial gas production to date has been hampered by the low permeabilities of the coal seams. Recovery of this valuable resource will be assisted by a fundamental understanding of coal microstructures and presence of mineralization, and their influence on the gas flow behaviour through coal. This paper examines the relationships between coal type, microstructure, secondary mineralization and gas flow behaviour. The study demonstrates that Bowen Basin coals should not be viewed as simply a dual porosity system of micropores which are surrounded by cleats. Instead, studies using scanning electron microscopy show the Bowen Basin coals have a third porosity system comprising a hierarchy of micron-sized fractures and micronsized cavities at a level between the micropores and the cleat/macropore system, which vary according to coal type. To determine the influence such microstructures have on the flow of gas through the coal matrix, sorption experiments were carried out on small solid blocks of coal, using a new gravimetric technique. The results demonstrate that a clear distinction exists between diffusivity of dull and bright coals in response to coal microstructure. The gas sorption data suggests that both dull and bright coals can be divided into two categories: coal which have a rapid sorption behaviour and coals which have a slow sorption behaviour. The results of the sorption experiments indicate that size, continuity, connectivity of the microstructures, and the extent of minerals infilling the fractures and cavities, play a significant contribution to overall permeability, and are likely to play a major rate-limiting factor in the flow of methane through coal at a level between diffusion at the micropore level and laminar flow at the cleat level. The studies indicate that the flow behaviour of gas through coal seams in the Bowen Basin is unlikely to be solely dependent on the cleat system but rather a combination of the cleat, microstructure and secondary mineralization in coal.

The Cyclone Winifred storm bed, central Great Barrier Reef shelf, Australia

ABSTRACT Shelf sediments were collected immediately before (8-20 January 1986), immediatly after (9-10 February 1986), and one year after (27 February 1987) Cyclone Winifred crossed the central Great Barrier Reef shelf on 1 February 1986. The storm produced a normally graded, mixed terrigenous-carbonate storm bed more than 11 cm thick, covering an area at least 1,200 km2, and extending more than 30 km offshore to water depths greater than 40 m. The storm layer encompasses two lithologically distinct, shelf-parallel facies belts. The nearshore (0-25 m depth) storm bed is 311 cm thick, thins offshore, and grades upward from medium sand to silty clay. Offshore (25-43 m depth) the storm bed thickens to more than 11 cm and consists of normally graded, well sorted, relict quartz and skeletal, gravelly sands capped by a thin mud veneer. The cross-shelf distributions of grain size, mineralogy, carbonate content, and carbon isotope ratios were very similar before and after Winifred, suggesting the storm bed did not result from offshore-directed storm currents but, rather, in situ resuspension and settling of the shelf sand with shoreward transport of mud. In contrast to storm sedimentation models which predict thinning and fining offshore, Winifred created a layer that becomes thicker and coarser grained in deeper water in response to cross-shelf substrate changes. After one year the Winifred storm layer was well preserved nearshore ( 30 m water depth) below fairweather wave base. Preservation may not be a matter of insulating the layer from physical reworking below fairweather wave base but of burying it where rates of sediment accumulation outpace bioturbation. The effects of storms similar in size to Winifred may be well concealed in ancient shelf sequences subject to rapid biological remixing of storm layers.

Positive relief over buried post-glacial channels, Great Barrier Reef Province, Australia

Abstract Continuous seismic profiling on the continental shelf, central Great Barrier Reef Province, Australia, has delineated a major sub-bottom reflector interpreted as the pre-Holocene surface. Channels eroded in this surface are infilled and overfilled by post-glacial sediments. Filling is ascribed to estuarine backfilling during sea-level rise followed by deposition of delta front deposits. In many cases these fills create positive relief over buried channels. Hence modern shelf morphology may not be an accurate guide to the positions of Pleistocene channels across the shelf.

Sedimentation in a complex convergent margin: the Papua New Guinea collision zone of the western Solomon Sea

Abstract Bathymetric, sidescan and `groundtruthed acoustic facies maps incorporating 50,000 km 2 of HAWAII MR1 swath and 3.5 kHz data have been used to construct a tectono-sedimentary model for sedimentation along the western Solomon Sea region of the Papua New Guinea collision zone. The southern underthrust plate along the collision zone includes the Morobe and Trobriand tectono-sedimentary provinces. This region has an extensive platform to the east, a narrow shelf, and a moderately steep slope descending to the plate margin. Along the edge of the underthrust plate sediment accumulates mainly in numerous isolated slope basins and several canyon-fed slope fans. The former are 5 to 20 km across and filled with fine-grained hemipelagic sediment. The latter are larger (40 to 70 km across), coarser-grained and elongate along the margin. The overriding plate along the Papua New Guinea collision zone includes the Huon, Finsch, Siassi and New Britain tectono-sedimentary provinces. The edge of this plate has no shelf, is very steep, and sediments accumulate in fewer isolated slope basins and more numerous canyon-fed slope fans, which are also elongate along the margin. The basin floor is constrained along two elongate deep sea trenches (separated by oceanic lithosphere). Sediment supply from the northern overriding plate to the basin floor is limited, as most submarine canyons terminate at slope fans, slope basins or terraces. Sediment supply from the southern underthrusting plate is also believed to be low, as approximately half of the tributary submarine canyons debouch onto slope fans before joining the axial Markham Canyon. The sediment supplied to the western end of the trench is delivered axially down the collisional suture, much of it apparently derived from emergent landmasses to the west. This pattern of sediment delivery implies that the fill in ancient deep water collisional basins may be largely unrelated to terranes on either side of the basin, but derived from terranes further afield.

Cold-climate, fluvial to paralic coal-forming environments in the Permian Collinsville Coal Measures, Bowen Basin, Australia

Abstract The Middle Permian Collinsville Coal Measures of the northern Bowen Basin illustrate a range of cold to cold-temperate, coal-forming environments. Cold climate is indicated by Glossopteris flora in the coal measures, and by restricted marine fauna dominated by brachiopods and bryozoa in correlative marine sequences of the Back Creek Group which contains also abundant lonestones (dropstones). Sedimentation was characterised by an overall transgression, interrupted by local fluvial and coastal progradation in a shallow, epicontinental sea during a relatively quiescent tectonic period. Six sedimentary environments are represented: fluvial, fluvio-paralic, barrier-strandplain, back-barrier, tidal flat and open marine. The basal coal formed from peat of swamps of abandoned areas of gravelly braided streams, and is massive, dull, and with high ash (20%), low sulphur (1%) contents. Overlying coals developed from peats formed in fluvio-paralic and paralic environments, and thicker seams are generally brighter, with low to moderate ash (8–17%) and moderate to high total sulphur (1–6%) contents. Seams associated with fluvial influence show splits and high ash yield, while seams associated with coastal deposits show high sulphur levels (up to 21%). In contrast to reported models of coal-forming environments, no clearly defined deltaic or inter-distributary bay-fill sequences were identified in the area studied. Rather, vast freshwater wetlands backed low-gradient, progradational coasts locally having bars and barriers. The barriers were not prerequisites for substantial peat accumulation, although may have locally assisted peatland development by raising the profile of coastal equilibrium.

Convergent margin extension associated with arc‐continent collision: The Finsch Deep, Papua New Guinea

The Finsch Deep is an asymmetric rhomboidal basin, with a maximum depth of 5400 m, situated to the north of the Solomon Sea Triple Junction, Papua New Guinea. Anomalous slip vector azimuths, regional plate velocities, earthquake hypocenters, and bathymetric depth relative to surrounding terranes all indicate that the Finsch Deep is not a passive feature developed behind the subduction front. Our interpretation of all available data, including seismological studies, detailed bathymetry, side-scan character, and seismic sections, suggests that the Finsch Deep has developed due to N-S extension in the transition zone from continental collision west of the Solomon Sea Triple Junction to oceanic subduction to the east. The key mechanisms thought to drive “convergent margin extension” within the New Guinea Collision are slab pull and oblique subduction. Researchers studying other convergent margins have recognized underplating and tectonic erosion as driving mechanisms for extension; though we have no evidence to either support or discount their presence within the New Guinea Collision.