[1] We present the interpretation of newly acquired high-quality industry-standard deep seismic reflection and swath bathymetry data to provide insight into the structural style and evolution of the Mentawai Fault Zone (MFZ). The MFZ lies along the boundary between the accretionary wedge and the proposed continental backstop. This zone exhibits arcuate ridges on the seafloor, convex toward the east. Beneath these ridges the structures developed as landward-vergent imbricated backthrusts in the inner part of the accretionary wedge and higher-angle backthrusts that deformed the forearc basin sediments. In the forearc high, anticlines were developed due to the seaward-vergent forearc high thrusts originating in the accretionary wedge. The imbricated backthrusts may have initiated during the Early-Middle Miocene contemporaneously with the slide and back-rotation of forearc high thrusts. In the Late Miocene, the higher-angle backthrusts were initiated. Continuous contraction induced the frontal higher-angle backthrusts and formed a fold-thrust belt toward the east during the Pliocene. The folds and thrusts were disturbed by diapirs and mud volcanoes. Backthrusting and fold-thrust belts developed in the MFZ may explain the compressional features observed at the boundary between the accretionary wedge and continental backstop along the southern Sumatra margin. The backthrusts along the MFZ are waning in activity and hence the risk of a large earthquake and associated tsunami at the present time should be small.

The aim of the National Geoscience Mapping Accord Cooper-Eromanga Basins Project was to develop a quantitative petroleum generation model for the Cooper and Eromanga Basins by delineating basin fill, thermal history and generation potential of key stratigraphic intervals. Bio- and lithostratigraphic frameworks were developed that were uniform across state boundaries. Similarly cross-border seismic horizon maps were prepared for the C horizon (top Cadna-owie Formation), P horizon (top Patchawarra Formation) and Z horizon (base Eromanga/Cooper Basins). Derivative maps, such as isopach maps, were prepared from the seismic horizon maps.Burial geohistory plots were constructed using standard decompaction techniques, a fluctuating sea level and palaeo-waterdepths. Using terrestrial compaction and a palaeo-elevation for the Winton Formation, tectonic subsidence during the Winton Formation deposition and erosion is the same as the background Eromanga Basin trend—this differs significantly from previous studies which attributed apparently rapid deposition of the Winton Formation to basement subsidence. A dynamic topography model explains many of the features of basin history during the Cretaceous. Palaeo-temperature modelling showed a high heatflow peak from 90–85 Ma. The origin of this peak is unknown. There is also a peak over the last two–five million years.Expulsion maps were prepared for the source rock units studied. In preparing these maps the following assumptions were made:expulsion is proportional to maturity and source rock richness;maturity is proportional to peak temperature; andpeak temperature is proportional to palaeo-heatflow and palaeo-burial.The geohistory modelling involved 111 control points. The major expulsion is in the mid-Cretaceous with minor amounts in the late Tertiary. Maturity maps were prepared by draping seismic structure over maturity values at control points. Draping of maturity maps over expulsion values at the control points was used to produce expulsion maps. Hydrocarbon generation was calculated using a composite kerogen kinetic model. Volumes generated are theoretically large, up to 120 BBL m2 of kitchen area at Tirrawarra North. Maps were prepared for the Patchawarra and Toolachee Formations in the Cooper Basin and the Birkhead and Poolowanna Formations in the Eromanga Basins. In addition, maps were prepared for Tertiary expulsion. The Permian units represent the dominant source as Jurassic source rocks have only generated in the deepest parts of the Eromanga Basin.

Thermal history data from wells in the Vulcan Sub- basin and adjacent platforms show clear evidence that many reservoir sections have experienced relatively shortlived, high- temperature flushes during the Late Tertiary. These transient heat pulses are related to slow migration of hot fluids and hydrocarbons from adjacent depocentres, or rapid flow of deep-seated brines during Late Miocene- Early Pliocene tectonic reactivation. The hot fluids have been focussed into structured reservoir sections via high- permeability pathways and reactivated faults. As a consequence, most exploration wells are not truly representative of the thermal regime of nearby source kitchens.In order to constrain the regional thermal and expulsion history of the region, and to address the issue of thermal pulses, burial history analysis of 44 wells and 18 depocentre sites was carried out. This analysis utilises a simplified transient heat pulse model developed as part of the WinBury™ burial and thermal geohistory modelling software. The transient and steady-state thermal history models are constrained by reflectance and fluorescence maturity data, together with apatite fission track analysis and fluid inclusion palaeo-temperature data.

Geohistory analysis is a technique for plotting the cumulative subsidence of various chronostratigraphic horizons encountered in an exploration well or section. The intervals between horizons are corrected for sediment compaction and are plotted relative to an estimated ancient seabed (or land surface) as a function of geological time. Speed and precision have been added to this procedure by automatic computer processing and plotting. A variable eustatic sealevel datum has also been introduced. Accurate quantitative assessment of depth sub-seabed (or subsurface) of any horizon may be combined with a model of heatflow history for the well location to compute a maturation integral and theoretical vitrinite reflectance for that horizon as a function of geological time. Comparison of theory with present observations may be used to refine the model heatflow history. Palaeomaturation may be readily assessed.An offshore Perth Basin example is used to illustrate the effects of palaeosealevel, palaeoheatflow and unconformities and hiatuses. Analysis of a cross-section through three wells is used to demonstrate the potential for accurate calculation of a palinspastic section, palaeomaturation and migration direction. Geohistory analysis is demonstrated to be an essential tool in rigorous basin evaluation.

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