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Petrology And Reservoir Quality Of Sandstones From The Late Triassic Mungaroo And Brigadier Formations, Rankin Trend, Northern Carnarvon Basin, Australia

McNamara, Christopher P.

Geoscience Honours Degree, 2011

University of Adelaide


The Late Triassic Mungaroo and Brigadier Formations contain significant hydrocarbon reservoirs in the Rankin Trend of the Northern Carnarvon Basin. This study aimed to assess the controls on the composition and reservoir quality of sandstones from the Late Triassic succession of the Rankin Trend, in order to improve reservoir understanding and to further optimise development opportunities.

The sandstone composition and reservoir quality were assessed by analysing the cored intervals of four wells; North Rankin-5, Goodwyn-5, Goodwyn-6 and Yodel-2. The compositional data were compiled from 169 existing thin section modal point-counts and 38 existing x-ray diffraction analyses. Reservoir quality data were compiled from existing petrographic descriptions and routine core analysis data for all sandstone sample points. A lithofacies analysis was performed on the sampled intervals to provide sedimentological context to the petrology data and to evaluate the depositional facies previously identified in the core.

Mungaroo Formation sandstones are mostly subarkose and quartzarenite with minor sublitharenite and rare arkose, lithic arkose, feldspathic litharenite and litharenite. Sands are typically fine- to coarse-grained and moderately- to well-sorted. Framework grains are dominated by monocrystalline quartz, with minor polycrystalline quartz, feldspar and lithic fragments (mean Q90.7F7.4L1.9). Mica, heavy minerals, organic matter and detrital clay are rare. Authigenic phases are dominated by kaolin clays (and less common illite) and quartz overgrowths. Other locally important minerals include pyrite, siderite, calcite and ankerite. Feldspar and labile lithic grains are extensively altered and dissolved. Most grains are compacted with elongate and point grain contacts. Floating grain textures and physico-chemical compaction are rare.

Brigadier Formation sandstones exhibit a consistent, highly mature composition of quartzarenites and subarkoses. These are typically fine- to medium-grained and well- to very-well sorted. Detrital grains are dominated by monocrystalline quartz, with minor polycrystalline quartz, k-feldspar and detrital clay (mean Q94.8F5.0L0.2). Lithic grains, micas, heavy minerals and organic matter are rare. Plagioclase and metamorphic lithic grains are absent. Most sandstones exhibit significant mechanical compaction and cementation, along with minor grain dissolution. Cements are dominated by kaolin clays (and less common illite) and quartz overgrowths, along with locally abundant siderite and pyrite. Calcite and iron oxide cements are rare. Kaolin dominantly forms in patches where feldspar and labile grains have altered or dissolved.

The system controlling sandstone composition is complex and reflect the parent lithology and the entire history of its modifications by weathering, recycling, transport, mixing, deposition and diagenesis. Craton interior and recycled orogen provenance setting are inferred from the relative portions of quartz, feldspar and lithic grains. Extraformational sedimentary rock fragments indicate at least a component of the sediment is polycyclic. There is a broad trend of increasing compositional maturity with time. Compositions become consistent and highly mature from the TR28 interval to the end of the Brigadier Formation. This trend may be related to a provenance change towards the end of the Norian, or a gradual shift in paleoclimate to more warm and humid conditions.

Autogenic processes are responsible for much of the local scale composition variation and are largely a function of hydrodynamic sorting processes. The dominantly fluvio-deltaic sediments of the Mungaroo Formation preserve a greater deal of heterogeneity associated with these variations. The highly consistent and mature composition of the Brigadier Formation may be a function of the marginal marine depositional system promoting the homogenisation of sediment compositions and winnowing of unstable grains.

Reservoir quality in the studied sandstones ranges from poor to excellent. Most samples contain modest to high amounts of clean, interconnected primary intergranular porosity, supplemented by scattered secondary pores (He injection porosity: mean 17.7 %, range 0.0-29.1 %; visual porosity: mean 10.4 %, range 0.0-24.3 %). Permeabilities are mostly fair to excellent and are generally higher in the Mungaroo Formation (mean 1324.8 mD, range 0.0-20980.0 mD) compared with the Brigadier Formation (mean 522.7 mD, range 0.0-3300.0 mD). Negligible values are mostly associated with pervasive carbonate cementation.

The reservoir quality of sandstones are principally controlled by primary depositional processes. This control is highlighted in the reservoir characteristics between grain size classes and depositional facies. Reservoir quality tends to increase as grain size increases, and is best developed in massive and cross- stratified sands. Channelized facies consistently exhibit the best reservoir characteristics, while other depositional facies have substantially lower permeabilities.

Dominantly, diagenetic processes have diminished the reservoir characteristics. Compaction is responsible for marginally more porosity reduction than cementation. Cementation has involved early stage pyrite and siderite mineralisations, followed by later stage kaolinisation of labile grains, quartz overgrowth cementation, illlitisation of clays, and finally calcite and ankerite mineralisations. Reservoir quality tends to reduce with increased burial, though moderate to high porosity and permeability are still observed at depth.

This study represents a significant review of the sandstone petrology and reservoir quality of the Mungaroo and Brigadier Formations on the Rankin Trend of the Northern Carnarvon Basin and the identified stratigraphic and depositional trends in sand composition and reservoir quality can be used to assist in predictive reservoir modeling.

Australian School of Petroleum



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