Siderite cement Tirrawarra Sandstone, isotope

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. 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.

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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