Tirrawarra Sandstone cement

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.

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.

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. 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.

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