In several calcrete profiles from South Australia, needle-fibre calcite is concentrated in the channels and voids of nodules, hardpans, carbonate silt and platy calcrete. The morphology, spatial arrangements and crystallography of the needle-fibre calcite were determined using scanning and transmission electron microscopy. Needle-fibre calcite is composed of single crystals of calcite containing less than 0 5 wt % Mg. Two distinct morphological classes are evident: (1) small single rods or micro-rods, 1 �m in length and 0.1 �m or less in diameter and (2) larger needle-fibres 2-120 �m in length and 0.5-6 �m in diameter which display a variety of habits including multiple rods and serrated forms. The basic unit for the second group is that of a double rod which may be modified by cementation to other rods, and by epitaxial growth or dissolution. Spatial arrangements of needle-fibre calcite are determined by their association with organic matter. The large fibres form within mycelial strands. Lysis of the strand by rod-shaped bacteria releases the needle-fibres for redistribution in the profile. The micro-rods are calcified rod-shaped bacteria.
Scanning electron microscope (SEM) studies of calcareous soils and calcretes from South Australia reveal a fossilized community of soil micro-organisms dominated by filamentous structures preserved in fine detail by calcite. In the various calcrete lithological facies, the filaments form dense mats within channels and voids, and also occur within the matrix where they are intimately associated with micrite. The calcite forming the filaments has a variety of crystal habits: the nature of the microcrystals is specific to each filament but varies significantly between adjacent filaments. In the calcareous soils there are various stages between the primary filaments and the calcite encrusted structures characteristic of the calcretes, suggesting that in vivo biochemical processes dominate the mechanisms of calcification. This hypothesis is supported by the specificity of the habit of calcite microcrystals on each filament. It is suggested that the organisms deposit calcite microcrystals within the mucilaginous sheath or in the cell wall (or both) as a detoxification mechanism in response to their highly calcareous environment. Based on the identification of structures resembling fruiting bodies, at least some of the filaments appear to have been fungal hyphae, which are known to be responsible for stabilizing macroaggregates in soils. Calcified filaments may produce permanently stabilized macroaggregates which provide the locus for further carbonate precipitation, leading to eventual induration of the soil.
The Pretty Hill Formation in the Penola Trough is a productive gas reservoir in the Katnook, Ladbroke Grove and Haselgrove fields. Thin sections, X-ray diffraction, scanning electron microscopy and electron microprobe analyses have been used to characterise the mineralogy of core samples from eight wells. The reservoir sandstones are typically fine to medium grained, moderately sorted feldspathic litharenites. Framework grains comprise detrital quartz, feldspars (albite, microcline and anorthite), lithics (dominantly volcanic), mica and accessory minerals. Authigenic minerals of chlorite, laumontite, carbonate, quartz, feldspar, sphene, anatase, glaucony and illite are present in all wells. Kaolinite is restricted to Ladbroke Grove-1. Chlorite, laumontite and carbonate are volumetrically the most important authigenic minerals.There is a wide range in core plug porosity (one to 23 per cent) and permeability (10"3to 103 md) in the reservoir sandstones. In samples with high percentages of authigenic clays microporosity is important. Regional trends indicate reservoir quality decreases with increasing depth but superimposed on this trend is the influence of the detrital and authigenic mineralogy. Cleaner, coarser sublitharenites and subarkoses have good reservoir characteristics but where lithics concentrate in the finer feldspathic litharenites and litharenites deformation of these ductile grains has limited porosity and permeability. Authigenic minerals have both reduced and enhanced reservoir quality. Chlorite rims with associated microporosity have decreased the impact of mechanical compaction and inhibited silicification. Pore filling cements of laumontite and carbonate have occluded intergranular pores and replaced grains. Secondary porosity produced by the dissolution of these cements in the gas zones has significantly improved reservoir quality.Other information gained from the mineralogical study could influence future exploration and production. Lack of contrast on resistivity logs between gas and water zones is not due to the mineralogy of the Pretty Hill Formation. However, the restriction of early diagenetic laumontite to the water zones of gas producing wells does indicate the location of the gas-water contact. Laumontite was dissolved from the gas zone by an increase in C02 prior to hydrocarbon migration. Use of acids to enhance permeability in the Pretty Hill Formation should take into account the probable formation damage caused by reactions with the clays. Kaolin- ite could dissolve to produce a silica gel and the high Fe3+ content of the chlorite will result in a gel unless iron chelators are used in the mud acid. The depositional environment of the Pretty Hill Formation has historically been interpreted as braided fluvial stream deposits interfingering with finer grained lacustrine shales and siltstone. However, this model can not explain the presence of glaucony grains, unless the glaucony has been reworked, but there is no unequivocal evidence to support this hypothesis.
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