Evaluation of seals and associated reservoir rocks: Triassic Nappamerri group and Cuddapan Formation, Cooper Basin, South Australia

Abstract

This study evaluates the seal potential (capacity, geometry and integrity) of four sealing lithofacies and the reservoir quality of five reservoir lithofacies from the Early-Middle Triassic Nappamerri Group and late Triassic Cuddapan Formation, Cooper Basin in South Australia. The Nappamerri Group and Cuddapan Formation sediments are heterolithic fluvial units and comprise both sealing and reservoir lithologies. Five reservoir lithofacies: gravels from the base of fluvial channels, fluvial channel sands, chute sands, crevasse splay sands and sands from the top of point bars and four sealing lithofacies: diagenetically modified fluvial sandstones, lacustrine mudstones, palaeosols and floodplain mudstones were identified. Reservoir quality, pore and pore throat size distributions are controlled by petrographic composition, texture and the subsequent diagenetic overprint. These are influenced by depositional environment. Seal capacity was evaluated using mercury injection capillary pressure analyses and was calculated for both oil and gas accumulations. The highest column of hydrocarbon is associated with lacustrine mudstones (1353 m oil and 727 m gas). The next highest column of hydrocarbon can be supported by palaeosols and floodplain mudstones (1132 m oil and 609 m gas). The lowest hydrocarbon column (37 m oil and 20 m gas) is supported by diagenetically modified fluvial sandstones. The sensitivity of column height to interfacial tension, the wettability and the subsurface extrapolation of fluid densities were tested using empirical hydrocarbon fluid property data from the Cooper Basin' Recognition of the sealing lithologies on wire-line logs was completed by calibrating the cores to petrophysical cross-plots. Seals were evaluated in terms of their geometry (thickness and areal extent). Sealing intervals were correlated and mapped for thickness and quality across the Merrimelia Field. Seal integrity was qualitatively determined on the basis of petrographic criteria, core description and literature analogs. The sealing potential of the four lithofacies identified in the study area was ranked. The best sealing potential exists within the palaeosols and floodplain mudstones. These lithofacies occur as two sealing intervals; they are thick, extend over the entire Merrimelia Field and have high measured seal capacities. Although of relatively higher seal capacity and homogeneity, tacustrine mudstones have a lower seal potential because they are thin and occur only locally. The poorest seal potential occurs within the diagenetically modified sandstones. This lithofacies is thin, with unpredictable distribution, poor rock mechanical properties and low seal capacity. This study demonstrates that the Nappamerri Group contains a variety of sealing lithofacies with different seal potential; it can no longer be regarded as a simple regional seal but must be treated as a series of intraformational seals. Detailed knowledge of the seal potential will improve the assessment of the seal risk and thus help guide future exploration in the Cooper Basin.