Tirrawarra Sandstone

The Gidgealpa Field currently produces gas from the Early Permian Patchawarra Formation and Tirrawarra Sandstone, the Late Permian Toolachee Formation, and oil from the Jurassic Namur, Birk-head, Hutton and Poolowanna Formations (Fig. 2). The abundant dispersed organic matter in the intraformational shales and siltstones, and possibly... 

Fig. 2. Stratigraphic column for the southern Cooper basin and main tectonic events. The fluvio-deltaic Tirrawarra Sandstone was deposited during the early subsidence history of the Cooper basin when Australia was at high southern latitudes. Modified from Apak et al. (1993), Apak (1994) and Seggie et al. (1994).

A total of 130 core samples from the Tirrawarra Sandstone were collected from 14 wells of the Moorari and Fly Lake Fields. Sedimentological descriptions of the cores were made and samples from rocks deposited in a variety of depositional environments were collected. 

Table 1. Derivation of the Tirrawarra Sandstone core samples. Mineralogical compositions were determined by semiquantitative bulk-rock XRD analysis. Siderite is the only carbonate cement present, but occurs in varying proportions...

Table 3. Carbon and oxygen isotope data of the Tirrawarra Sandstone siderite cements. A good match is observed between measured (6 0 5as) and calculated oxygen isotope values determined from image analysis results.

Computerized colour conversion of the grey-scale BSE image using the manually defined pixel threshold values. In the Tirrawarra Sandstone each major... 

Fig. 3. Example of the different depositional environments and progradational and retrogradational cycles of the Tirrawarra Sandstone in the Moorari and Fly Lake Fields. The gamma ray log trace is derived from the Moorari 9 well.

Fig. 4. Petrographic, BSE and colour image characteristics of Tirrawarra Sandstone siderites. (A) Plane-polarized view of the main siderite cement generations that can be distinguished under the optical microscope in this case, which is the exception rather than the rule. SI has a brownish colour, whereas S2 and S3 are clear and colourless. SI is typically engulfed by S2. Note the concentration of fluid inclusions in S2, and the irregular serrated boundary between S2 and S3 (arrow), implying some dissolution of S2 prior to precipitation of S3. Sample Ml-9598, Moorari 1, 2925.5 m.

Tirrawarra Sandstones in the Moorari and Fly Lake Fields consist mainly of medium-grained, moderately sorted sublitharenites (mostly mica schist and phyllite, shale and siltstone clasts) (classification of Folk, 1974). A variety of authigenic minerals are recognized, including syntaxial quartz overgrowths, minor illite, patchy kaolinite and siderite. Attention is focused on the siderite and only a short description of the other diagenetic minerals is provided here. 

Fig. 5. BSE image of the view shown in Fig. 4(A). SI displays a light colour, whereas the surrounding S2 (medium grey) is characterized by a variable internal composition and complex zoning. S3 is a relatively homogeneous, late-generation pore-filling cement. Note the irregular dissolution boundary between S1 and S2, and between S2 and S3 (arrows). These cement relationships are typical of Tirrawarra Sandstones. Sample Ml-9598, Moorari 1, 2925.5 m.

Fig. 10. Ternary diagrams showing the compositional ranges of different generations of siderite cement in the Tirrawarra Sandstone. The early generation of the siderite cement (SI) is very rich in Fe, whereas the middle (S2) and late generations (S3) have much higher substitution of Mg and fall within the realm of sideroplesite and pistomsite. S2 and S3 have almost identical compositions except that S2 has a slightly higher Ca content.

Fig. 13. Fluid inclusion homogenization temperatures for the middle (S2) and late generations (S3) of the Tirrawarra Sandstone siderites. The fluid inclusions are considered to be of primary origin, and did not experience stretching. S2 formed at much lower temperatures than S3.

Fig. 15. Cross-plot of carbon and oxygen isotope values for the Tirrawarra Sandstone siderites. The bulk-rock isotope signatures are the cumulative product of the varying proportions of the different siderite cement generations (SI, S2, S3). The pure end-member compositions of the different siderite cement generations can be estimated from samples which are dominated by a single cement generation (circled areas). Only the end-member isotope compositions of the different cement generations should be taken into consideration when evaluating multigeneration siderite cements.

Tirrawarra Sandstone (Fig. 15). The conclusion is supported by the presence of a dissolution boundary between each major cement generation (Figs 4A and 5). 

In the Tirrawarra Sandstone, carbon isotope values show that major changes in conditions occurred between the precipitation of S1 and the later sider-ite cement generations, S2 and S3. Whereas the sample where SI is the only siderite cement present has a 6 C composition of about +1.45%o, those samples dominated by S2 or S3 are much more depleted in C (Fig. 15). 

Fig. 17. Generalized paragenetic sequence for the Tirrawarra Sandstone in the Fly Lake-Moorari area. Cooper basin. The interpretation is based on the integration of petrographic, isotope and fluid inclusions results. The estimated timing of oil generation and migration is indicated (shaded zone). S1, early generation of siderite cement S2, middle generation of siderite cement S3, late generation of siderite cement D1 and D2, first and second phases of siderite dissolution, respectively.

Tirrawarra Sandstone, either meteoric invasion or source-rock maturation probably accounts for the association of authigenic kaolinite patches with siderite spar displaying dissolution (Fig. 4B). 

Petrography and core study, Tirrawarra Sandstone, Moorari and Tirrawarra Fields, 2 vols. Unpublished report, SAGASCO Ltd, 138 pp. 

Core facies, petrology, and permeability of Tirrawarra Sandstone, Moorari Field, Cooper Basin, South Australia (abstract). Bull. Am. /lij. Petrol. GeoL, 72, 162. 

Martin, K.R. (1981) Petrology and Diagenesis of the Tirrawarra Sandstone in Tirrawarra Field. Unpublished report, Santos Ltd, 17 pp. 

Wild, E.K. l9 l)The sedimeniology and reservoir quality of the Kinnerton Sandstone Formation, UK. and the Tirrawarra Sandstone, S. Australia. PhD thesis. Department of Geology, University of Bristol. 

Williams, B.P.J. Wild, E.K. (1984) The Tirrawarra Sandstone and Merrimelia Formation of the Southern Cooper Basin, South Australia—the sedimentation and evolution of a glaciofluvial system. Aust. Petrol. Explor. Asi. J., 24, 377-392. 

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