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Porosity and permeability measurements on 926 core plugs were taken from 18 exploration, wildcat and producing wells from the Lower Cretaceous Flacourt Formation, Barrow Sub-basin and analysed for their spatial and temporal variability using both sedimentological and geostatistical methods.The porosity and permeability distribution shows a strong relationship to depositional facies and diagenetic modification. The samples are variably cemented with porosity ranging from 1.3 to 39.4 per cent and permeability from less than 0.01 to 24,400 md. Both porosity and permeability exhibit strong heterogeneity as a result of change of the depositional facies within the same well and the amount of detrital matrix within each facies.The degree of spatial and temporal variability observed in both porosity and permeability data is well reflected by the Levy-stable statistical analysis and variogram modelling. The heterogeneity is shown to be strongly controlled by texture, composition and sedimentary structures of the specific sedimentary facies. The heterogeneity of both permeability and porosity for each individual facies was characterised by the correlation length in the vertical direction from both the variogram and Levy C-gram analyses, and Levy index parameter.The sands of the Flacourt Formation, which form the principal reservoirs in the Barrow Sub-basin, were deposited in a paralic environment. Five major facies were recognised including a tidal channel, an upper shoreface, a middle shoreface, a lower shoreface and a background facies.Sedimentological and statistical analysis of the porosity and permeability of the Flacourt Formation shoreface sandstones enabled us to better understand the heterogenity of the petrophysical properties. The finding can be used to understand similar heterogeneous reservoirs elsewhere, and will provide a basis for reservoir simulations. We have shown that the approach of using genetically related sedimentary facies, can significantly improve the predicability of the porosity and permeability distribution.

Siderite cement is one of the most volumetrically important diagenetic minerals in the Late Triassic to Early Cretaceous sandstones of the Barrow Sub-basin. It constitutes up to 60 per cent of the rock volume, and where abundant, occludes the primary intergranular porosity. Petrogriiphic, chemical and isotopic studies indicate the early precipitation of much of this siderite prior to significant compaction. Siderite samples and concretions were taken from a variety of depositional environments ranging from fluvial to deep marine from Late Triassic to Early Cretaceous sequences.Of the early phases, three distinct siderite types were recognised and vary according to depositional environment. The first type, mostly collected from fluvial deposits, is Fe-rich with a mean composition of (Fe96.3 Mg1.8 Ca0.9 Mn1.0) C03. The second type of siderite cement is relatively Mg-rich, Ca-poor and has a higher Mn content, with a mean composition of (Fe87.1 Mg9.6 Ca1.2 Mn2.1) C03. The third type of siderite cement is typically Mg−, Ca-rich, with a low Mn content and an average composition of (Fe78.7 Mg12.4 Ca8.4 Mn0.5) C03. The second and third siderite cements occur in marine facies. The δ13C and δ180 values for siderite cements range from −2.8 to −14.3 %. PDB and 17.4 to 28.2 %. SMOW, respectively.Petrographic and chemical isotopic studies and other sedimentological data from siderite can be used to distinguish between different depositional environments. Chemical and isotopic compositions of the early authigenic siderites indicate precipitation from fluids with significant meteoric input. Siderite cements formed during sulphate reduction and early methanogenesis from mixed marine and meteoric pore-waters at temperatures below 30°C. While an influx of meteoric water to the fluvial and deltaic sediments of the Triassic Mungaroo Formation is easily envisaged, the siderites show that some mixing of sea water is also required. The concept of introduction of meteoric water to the marine sediments of the Birdrong Formation requires an appreciation of the sea level fluctuations at the time. In these situations, the recognition of meteoric or marine input to an early siderite cement can assist in the determination of sea level fluctuations.