Reservoir characterisation of the Jurassic Springbok Sandstone, Surat Basin, Queensland
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2012
Authors
Gallagher, Victoria
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Abstract
The Late Jurassic Springbok Sandstone unconformably overlies the mid-Jurassic Walloon
Subgroup in the northeastern region of the Surat Basin, Queensland. The Walloon Subgroup
contains significant economic coal seam gas (CSG) reserves which are currently under
significant development. Development of CSG for production requires the extraction of
significant volumes of water from the coal seams to enable the gas to flow. The dewatering
process has the potential to affect the Springbok Sandstone as it may be in hydraulic
connection with the coal seams. The aim of this study is to characterise the reservoir quality,
stratigraphic architecture and primary geologic controls on the Springbok Sandstone in order to
gain an understanding of the potential for hydraulic interaction with the underlying coal
measures. The depositional characteristics of the Springbok Sandstone were evaluated by
analysing wireline line logs and the cored intervals of wells, and the petrology and reservoir
quality of the lower section of the Springbok Sandstone was conducted through statistical
analysis of thin sections, XRD data and RCA data.
The Springbok Sandstone has been previously interpreted as a thick sequence of channel
sands interbedded with relatively minor quantities of heterolithic sandstone, siltstone, mudstone
and coal lithologies deposited in a vast intracratonic alluvial plain in warm temperate conditions.
Core and wireline log analyses indicate that the Springbok Sandstone typically consists of a
thin, low-permeability blocky basal sandstone and two major fining-upwards sequences, each
consisting of channel sands at the base that fine-up into overbank and mire environments. An
erosional unconformity exists at the base of the Springbok Sandstone and between each
intraformational unit. The basal sandstone has been heavily eroded in the west of the study
area by the later succession of channel sands. This lower section of the Springbok Sandstone
appears to have been deposited in a high-energy braided stream environment, while the upper
section of the Springbok Sandstone was deposited in a lower energy meandering stream
environment. lsopach maps suggest that paleoflow was towards the centre of the basin and that
there may have been syndepositional deformation of the basin during the Late Jurassic.
The geologic controls on the detrital composition and diagenesis of the Springbok Sandstone
are complex. Analysis of thin sections indicates that the sediment in the lower Springbok
Sandstone section is generally medium grained, well sorted and mineralogically immature
volcanogenic feldspathic litharenite to litharenite. The formation contains abundant well
preserved lithic clasts, feldspars and biotite. The small sample size and mixed sediment
composition of volcanic, plutonic and metamorphic fragments and post-depositional alteration
precluded conclusive petrographic provenance analysis. High clay contents are present in the
volcanic-rich sediments, predominantly including kaolin and highly smectitic mixed-layer illite
interlayers, indicating that the formation may be freshwater sensitive.
The reservoir quality of the lower section of the Springbok Sandstone is principally controlled by
depositional environments, detrital mineralogy, clay alteration and cementation, and ranges
from poor to excellent. These controls are highlighted by the relationship between lithofacies,
grain size, grain composition and diagenetic alteration. The highly heterogeneous cyclic nature
of the Springbok Sandstone has made correlation and prediction of reservoir properties difficult.
Reseruoir quality tends to increase as grain size increases and is best developed in medium to
coarse sandstones while lower energy depositional facies have substantially lower
permeabilities. The basal calcite cemented sandstones potentially represent a sealing lithology
however the unit is not laterally extensive across the study area. The areas with the highest risk
of reseruoir connectivity between the Springbok Sandstone and Walloons Subgroup were
identified where porous sands directly overlie the Walloon Subgroup, particulary in the west of
the study region.
Autocyclic processes were the primary driver for much of the local scale composition variation,
which was largely a function of hydrodynamic softing. Tectonic and climatic influence is also
evident in the stacking pattern of the Springbok Sandstone succession. Mechanical compaction
reduced the porosity and permeabilty of sandstones containing high labile grain content.
Cementation involved early stage glaucony, pyrite, chlorite and siderite mineralisation, followed
by later stage dissolution and kaolinisation of labile grains, illitisation of clays, fracture formation
and poikilotopic calcite mineralisation. Diagenetic processes have diminished the reseruoir
quality of the deeper sandstone samples. Reservoir quality tends to reduce with increased
burial, although moderate to high porosity and permeability are still observed at depth.
The results of this study highlight the heterogeneous nature of the Springbok Sandstone. The
formation comprises a series of heterogeneous sandstone and mudstone units with variable
reservoir quality. The stratigraphic and depositional trends identified in the stratigraphy,
sandstone composition and reservoir quality of the Springbok Sandstone can be used in future
predictive reservoir modelling.
School/Discipline
Australian School of Petroleum
Dissertation Note
Thesis (B.Sc.(Hons.)) -- University of Adelaide, Australian School of Petroleum, 2012.
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Front matter only available electronically. The complete thesis in print form is available from the University of Adelaide Library.