Controls On Reservoir Development And Quality In A Glacial Sequence; A Study Of The Late Palaeozoic, Cooper Basin, South Australia And Queensland, Australia
Doctor of Philosophy, 2000
University of Adelaide
The Merrimelia Formation is a complex mosaic of glacial facies in which the Tirrawarra Sandstone and Merrimelia Formation exhibit an interfingering relationship. This study, has defined this relationship further, indicating that the Tirrawarra Sandstone should be included in the Merrimelia Formation as a "facies type" as both Merrimelia and Tirrawarra sediments form an integrated suite of sediments. Within this context, all glacio-fluvial braided outwash sandstones are called "Tirrawarra type" sandstones.
The kaleidoscope of facies which forms the Merrimelia-Tirrawarra glacial complex (MTGC) have been deposited directly from the action of glacial meltwater and sediment output; a consequence of the Permo-Carboniferous Gondwanan glaciation which covered the early Cooper Basin. Merrimelia (and Tirrawarra) sediments were deposited in both terminoglacial or proglacial depositional realms depending on the relative position of Gondwanan glaciers and ice sheet.
Merrimelia Formation sediments contain up to 76 different rock fragment species. Rock fragments in Merrimelia (and Tirrawarra) sediments are either intrabasinal or extrabasinal. Intrabasinal rock fragments are mainly derived from the underlying Warburton Basin units, whereas extrabasinal rock fragments have been sourced from outside the Cooper Basin and transported via the action of glaciers and glacial meltwaters. Three rock fragment domains were defined. The northern rock fragment domain area was possibly sourced from the north (Arunta Block) while the western and southern rock fragment domains were possibly sourced from the west (Officer Basin), southwest (Mt. Painter region), and the south (Benagerie Ridge, Willyama Supergroup) respectively.
The composition of authigenic illite (poly type 1Md) in argillaceous Merrimelia sediments, was found to be consistent with a mixed layered I/S clay. This mixed layered clay exhibited an I/S ratio of 0.95. Preliminary illite dating data combined with geohistory analysis suggests that illite has formed between 75ºC and 95ºC. This formation range matches petrographic observations.
Controls on illite growth in MTGC sediments include, temperature, cation supply, pore water acidity, water/rock ratio and the size of pore spaces. Initially illite formed in a closed system where reactant supply was sourced locally (closed system) and, in the later stages of diagenesis, illite is neoformed by interaction with circulating pore fluids (open system).
A new petrographic technique was developed for the study of illite in the Merrimelia Formation; illite fluorescence. Illite fluorescence microscopy was found to be an effective tool in observing relationships between diagenetic illite and other authigenic phases. Illite fluorescence microscopy in combination with image analysis, was also used to quickly and accurately assess illite proportions from normally prepared, unpolished thin sections. This study has shown that by utilising the fluorescent properties of illite, a more accurate assessment of reservoir quality of illitic glacial sediments is possible.
A pilot illite dating study was undertaken. The results showed good agreement with diagenetic observation indicating that standard illite dating techniques can be successfully applied to the Cooper Basin.
Using diagenetic observations to better defined geohistory parametres, led to a greater understanding of the thermal history of the southern Cooper Basin region, where aquifer effects on the thermal regime were found to be more extensive than previously considered.
Standard and fluorescent thin section petrography, SEM, TEM and microprobe analyses of Merrimelia sediments delineated a paragenetic sequence similar to that previously published for the Tirrawarra Sandstone. A generalised paragenetic sequence for the whole MTGC was subsequently constructed. Variations in the Merrimelia and Tirrawarra paragenetic sequences are localised and attributable to lithology variations.
Provenance of sediment source regions can exert a strong control on reservoir quality, as rock fragment litho-type and proportion are crucial to compactional effects and clay authigenesis. These latter processes are linked to diagenetic adjustments, and the thermal regime operating in the basin. The principle diagenetic events which effect reservoir performance in the MTGC are compaction, quartz precipitation (Tirrawarra sediments) and the formation of authigenic clays (kaolin - Tirrawarra Sandstone and illite - Merrimelia Formation).
Hydrocarbon bearing Merrimelia sandstone packages at Malgoona and Merrimelia fields have permeabilities that vary enormously. This variability has been attributed to facies as matrix-rich facies in terminoglacial environments form poorly permeable intervals and proglacial quartzose facies exhibit better reservoir attributes. Thus differing facies types govern the textural and mineralogical maturity of sediments controlling any reservoir potential a sediment may possess. Deltaic, braidplain and glacio-aeolian sandstones have the greatest reservoir potential of all MTGC facies types.
The presence of illite does not necessarily negate a potential reservoir interval as distribution is as important as mere presence. The longitudinal bar sandstones at Malgoona Field show boxwork illite morphologies isolated as clumps formed from the breakdown of rock fragments and feldspars. This mode of illite occurrence has little effect on reservoir quality as the clay does not block pore throats. Conversely only a small proportion of illite in the porous glacio-aeolian sandstones of the Merrimelia Field drastically diminishes the quality of these reservoirs.
In general, illite was found to dictate reservoir quality in all Merrimelia facies. Consequently, understanding the morphology, formation, timing and distribution of illite is critical for risking the likelihood of favourable reservoir attributes in the Merrimelia Formation.
Development of potential reservoir sediments in the MTGC is controlled by:
- Climatic changes.
- Basin topography.
- Sediment distribution.
- Sediment composition.
Reservoir quality in the MTGC is primarily controlled by:
- Facies type (terminoglacial or proglacial).
- Diagenetic overprint (illite and quartz cement authigenesis).
- Geothermal gradient (acceleration of illite and quartz cement formation).
- Rock fragment composition (competent or incompetent).
- Mechanical compaction.
- Rock fragment proportion.