A successful approach to basin analysis requires the broad-scale reconstruction of the three dimensional depositional systems in relation to concurrent structural development of the basin. The Gidgealpa-Merrimelia-lnnamincka (GMI) Trend is a prominent, asymmetric, mildly compressional anticlinal trend located in the Late Carboniferous to Triassic Cooper Basin. Its northwest flank is controlled by high angle thrust faults which were reactivated repeatedly throughout geological time. The present study addresses both the structural style and depositional character of the GMI Trend, focusing on selected areas. It is an integrated approach utilising wire-line logs, seismic interpretation, isopach and structural maps and detailed palynology. This approach has produced a detailed chronostratigraphic subdivision of the Permo-Triassic sequence, particularly the Patchawarra Formation, which points to evidence of synsedimentary tectonics. Evidence from crestal unconformities suggests that the GMI Trend was uplifted during at least four distinct structural episodes. These phases of uplift result from the rejuvenation of pre-Permian faults. Regional investigation of chronostratigraphic units incorporating palynological information, clearly demonstrates the palaeogeography and the presence of internal unconformities within the Patchawarra Formation. Subsurface distribution of hydrocarbon pools and improved definition of areas of prospectivity relate to the episodic uplifts. Although known hydrocarbon reserves have largely accumulated in structural traps, additional potential exploration targets in the Permian sequence exist in stratigraphic, combination, pinchout and downflank fault traps as well as onlap plays along the mid flank areas of the GMI Trend.
The northeast-trending Nappamerri Syncline and its flanking high to the southeast, the Murteree-Nappacoongee (NM) Trend, show structural development throughout the deposition of the Permo-Triassic Cooper Basin sequences. Pre-existing topography, such as around Moomba, influenced early depositional patterns within the area. Rates of sedimentation were influenced by the rate of uplift of source areas around the basin, largely as a consequence of buckling. Periods of active tectonism resulted in non-deposition and stripping of sediments from uplifted blocks while deposition continued in deeper areas. Hangingwall blocks, pushed up along reverse faults, strongly influenced the areas of sedimentation and the facies being deposited. This is particularly evident along the NM Trend. Phases of uplift were immediately followed by increased sedimentation where fluvial deposition dominated. The effect of displacement along the northwest-trending basement lineaments was to subdivide the northeast trends into compartments which contain similar facies within each time slice. Adjacent compartments may display different facies and different tectonic histories. An inversion episode, particularly evident in the Big Lake/Moomba area, resulted in a reversal of the depocentre and was related to phases of Sakmarian compression.Facies distribution and sandstone percentage maps of the chronostratigraphic units of the Patchawarra Formation suggest that a northeast-trending major channel system entered this part of basin along the NW edge of the Murteree area.
The genetic differentiation of Permian Lower Triassic quartz sandstones and other rock-types has elucidated more fully our understanding of sedimentary processes during the course of time in the southern Cooper Basin. Core and ditch samples analyses in conjunction with log signatures provide a strong stratigraphic tool for constructing various types of maps which help in the exploration and development of hydrocarbons in this basin. Various percentage maps using gross genetic members have delineated areas of stream sediment transport, deltaic centres and shoreline-offshore sediments. Although channel-lag to point-bar sandstone reservoirs still offer gas and liquid potential, stratigraphic delta front-shoreline sandstones exhibiting excellent reservoir characteristics are also considered to be worthwhile targets in the basin.Various depositional models are usually more widespread than sites of local anticlines which appear to have undergone varying rates of sinking, relative to one another and also to adjacent low areas during intervals of Permian-Lower Triassic sediment accumulation. Although unconformities are probably widespread in marginal areas of the basin, deposition is considered to have been essentially continuous in parts of the Nappamerri 'synclinoria- like' low - the main depositional and structural axis of the basin. Facies analysis tends to support the belief that anticlinal complexes have never shown abrupt relief during the sediment accumulation of the Gidgealpa Group even though in some areas crestal unconformities are common. It is suggested that local 'structural growth' is genetically related to the differential downwarping of the basin. Episodes of greater mobility of the basin rims are indicated by both unconformities and facies relationships. It is apparent, however, that facies patterns indicate only local entry of sediment transport into the basin particularly in its southern portions. During some phases of sediment accumulation climate may have blurred the larger-scale earth movements. It is suggested by this local study that similar genetic models and mapping techniques could be applied to other Permian basins in the region.
The term structural growth is applied to subtle earth movements contemporaneous with sediment accumulation. To assess this factor, one must consider the roles of lateral sandstone accretion, river avulsion and sediment dispersal in the context of basin setting, climate and geological time.Interpretation of stratigraphic relationships and isopach and structural maps of areas comprising structural highs in the subsurface of the Permian Cooper Basin is based on the correlation of thin chronostratigraphic units in the Patchawarra Formation (Tirrawarra Field) and the Toolachee Formation (Big Lake-Moomba fields). Unit C of the Toolachee Formation in the Moomba area comprises several fining upwards sequences that appear cyclic and sheet-like. This facies relationship is similar to those in the immediate downflank areas of Big Lake. However, upflank in crestal areas, fluviatile sequences are more typical of multistorey sandstones. These crestal sandstones are the result of structural growth contemporaneous with deposition from a fluviatile meandering system during a significant interval of geological time. Meandering river systems are considered to have crossed the Big Lake-Moomba area locally from south-west to north-east. Isopach and percentage sandstone maps indicate that streams preferred an avenue of transport towards the Nappamerri Trough.Stratigraphic analysis of Patchawarra Formation Unit 2B in the Tirrawarra Field suggests a slightly greater rate of local subsidence there than in Big Lake and Moomba fields, resulting in a decreasing sand body interconnection. Although occasional fining upward sequences correlate across the field area, multistorey sandstone development occurs in an arcuate belt on its western side. This is the result of local structural growth and faulting contemporaneous with fluviatile deposition. Potential petroleum reservoirs of fluviatile origin occur in numerous basins worldwide. In basins exhibiting low rates of differential subsidence, potential reservoirs and seals occur as sheet-like bodies in structural traps near the margins of major depocentres. Knowledge of local and basinal differential subsidence and fluviatile architecture provides confidence for field exploitation and exploration for new plays.
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