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Reservoir Characterisation Of The Tirrawarra Sandstone In The Moorari And Fly Lake Fields, Southern Cooper Basin, South Australia

Rezaee, Mohammad R.

Doctor of Philosophy, 1996

University of Adelaide

Abstract

This study examines the depositional and diagenetic controls on reservoir quality of the oil-bearing Permian Tirrawarra Sandstone in the Cooper Basin, South Australia. A new depositional classification of the unit is presented, together with prediction of reservoir quality. A detailed diagenetic study of the unit has been made, including models for the formation of quartz and siderite cements. The depositional and diagenetic data have been integrated with log data to produce new algorithms for calculating effective porosity and effective water saturation in kaolin-bearing sandstones.

Seven facies associations are recognised in the Tirrawarra Sandstone in the Moorari and Fly Lake Fields. These facies associations are interpreted to have been deposited in seven depositional environments including braid-delta (medial & distal), back barrier marsh, beach barrier, lacustrine, meandering fluvial and aeolian environments. Each sedimentary environment has a characteristic detrital composition controlled by its distance from the sedimentary source and the energy of the depositional environment. The relative proportions of quartz grains and rock fragments vary considerably between each sedimentary environment. The sandstone composition strongly influences the diagenetic history and subsequent reservoir quality.

The most important diagenetic processes which have modified porosity in the Tirrawarra Sandstone are mechanical compaction and quartz cementation. Mechanical compaction, evaluated by a compaction index equation, is greater in sandstones rich in rock fragments, whereas quartz cement is more developed in the quartz-rich sandstones. In this study a quantitative estimation of sandstone composition control on diagenetic events has made it possible to predict the reservoir quality of the Tirrawarra Sandstone for different depositional environments.

Based on fluid inclusion and electron microprobe analyses and CL observations, three zones of quartz cement were recognised. The first zone (Zl), formed at about 65oC, has a positive effect on reservoir quality and is derived from early dissolution of feldspar grains by acidic meteoric water flushing during late Triassic tectonic activity. The second zone of quartz cement (Z2), formed at about 100oC, originated from late dissolution of feldspar grains by organic acids generated during kerogen maturation. The third zone of quartz cement (Z3), precipitated after oil migration at about 130oC, originated mostly from stylolitization and pressure dissolution of quartz grains. The first zone of quartz cement prevented subsequent compaction and had a positive effect on the Tirrawarra Sandstone reservoir quality, whereas the later zones occluded remaining porosity.

The only carbonate cement present in the Tirrawarra Sandstone is siderite. Application of the back-scattered electron image analysis technique, together with bulk-rock isotope and fluid inclusion studies, has led to the identification of three main generations of siderite cement. The first and second siderite cement generations were each followed by at least one dissolution event. The first generation of siderite (S1) is an homogeneous Fe-rich siderite with a d13C signature of +1.45%o, which probably formed during low-temperature methanogenesis (=/< 30oC). The second generation of siderite cement (S2) is an Mg-rich, inhomogeneous siderite characterised by complex zoning, with a d13C signature of -8.5%o. This siderite cement is thought to have formed mainly in association with the decarboxylation of organic matter at temperatures between 64 and 76oC. The third and final siderite precipitation event (S3) produced an Mg-rich, evenly banded, pore-filling siderite with average d13C of -11%o. This siderite is also interpreted to have formed during kerogen maturation, albeit at more elevated temperatures (98-110oC). The results of this study show that organic processes controlled siderite cementation over a range of different burial conditions in the study area. Integrating video-imaging and image analysis software provides an efficient means of quantifying the different siderite cement generations seen under the BSE. The results of this investigation show that caution must be exercised in the interpretation of bulk- rock isotope signatures. This study has produced a new method which enables the determination of end-member d18O and d13C compositions of individual cement generations in cases where pure, or nearly pure samples of end-member carbonate cement generations are not available for isotope analysis. Since the method can be semi-automated, the technique provides a potentially powerful tool for improved bulk-rock isotope interpretations in clastics containing multi-generation carbonate cements.

It appears that siderite cements in the Tirrawarra Sandstone record tectonic activity in the form of irregular growth and dissolution highlighted by compositional zoning with stages of strong dissolution recording particularly active times when pore waters changed composition dramatically. Some zoning could be related in part to tectonic pulses. The heterogenous, pitted and zoned S2 is thought to have formed during a time of active tectonism in the Cooper Basin whereas the evenly banded nature of S3 suggests that it precipitated during a quiet tectonic period when pore waters remained relatively constant. The temperature recorded by each of the siderite stages allows their precipitation to be tied to a burial history curve and, by making some simple assumptions about that history, the timing of cementation can be estimated. This can be a additional tool for calibrating the thermal history of an area.

The main clay minerals present in the Tirrawarra Sandstone are kaolinite and illite. Illite largely formed from the alteration of labile components of rock fragments. Kaolinite formed during two stages. The first stage formed relatively early, at temperatures around 60oC, when acidic meteoric water invaded the reservoir and led to dissolution of feldspar grains. The second generation of kaolinite formed during late dissolution of feldspar grains by organic acids during kerogen maturation, at temperatures around 80oC. Back-scattered electron image analysis indicated an average of 20% microporosity is associated with kaolinite in the Tirrawarra Sandstone.

Tirrawarra Sandstone samples can be grouped into eight classes. The classes are separated according to the relative proportions of different kinds of porosity (microporosity, and primary and secondary macroporosity). The reservoir quality increases from class eight to class one and the classes have a predictable relationship with the sedimentary environment. Class one samples, associated with meander, aeolian or beach-barrier environments, have primary macroporosity with partial quartz cement. Class eight, on the other hand, associated with a braid-delta environment, is dominated by microporosity.

Integration of petrographical and wireline log data led to the introduction of a new empirical equation for porosity determination from sonic porosity, derived from the new equation, versus core porosity shows an r2 value of 0.8, remembering that total porosity is being calculated using this equation. Knowing the amount of microporosity, however, it is possible to estimate the amount of macroporosity. By knowing the volume of clay and the percentage of associated microporosity, it is possible to determine the amount of macroporosity in the Tirrawarra Sandstone. This investigation also indicates that rock parameters such as composition, cementation and clay content have a clear and measurable effect on acoustic transit time. The rock parameters which affect transit time, also influence porosity, but for sandstones with a given composition, the most important parameter controlling acoustic transit time is porosity.

In this study a new equation is introduced to estimate effective water saturation in the Tirrawarra Sandstone. As the equation is based on integration of resistivity and sonic log from wells in the Moorari and Fly Lake fields where obtaining the volume of clay is difficult and unreliable, it may be useful for estimation of water saturation. This equation, which reduces calculated water saturation by about 10% for Tirrawarra Sandstone, is likely to be applicable for other kaolinite-bearing sandstones.


Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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