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Recognition and Measurement of Basin Inversion |
1 Geotrack International Pty Ltd, PO Box 4120, Melbourne University, Victoria 3052, Australia
2 Geotrack International UK Office, 30 Upper High Street, Thame, Oxfordshire OX9 3EX, UK
Studies of inverted basins based solely on the preserved section provide only partial insight into the thermal and tectonic history of basin development. Equally important is that part of the history represented by section removed by erosion during inversion. Thermal History Reconstruction, involving application of Apatite Fission Track Analysis (AFTATM) and Vitrinite Reflectance (VR) to define the timing and magnitude of key erosional and/or thermal episodes, and integration of this information with data from the preserved section, provides a more complete description. Case histories from the UK, New Zealand and Australia are presented to demonstrate this approach. Much of northern and eastern England, the Irish Sea and the Southern North Sea was affected by heating apparently due largely to additional burial, followed by early Tertiary cooling due to regional uplift and erosion, synchronous with recognized basin inversion events. Heating associated with this episode caused generation of hydrocarbons throughout the region. Areas conventionally regarded as not inverted appear to have been subjected to km-scale uplift and erosion, which was not restricted to classic ‘inversion axes’. Failure to allow for these effects can lead to a significant underestimation of regional maturity trends. Thermal History Reconstruction is also essential in areas that have undergone more than one episode of inversion. For example in Inner Moray Firth well 12/16—1 maximum palaeotemperatures in the Carboniferous section were reached prior to Hercynian inversion and the chances of preserving any hydrocarbons generated from Carboniferous or older source rocks appear poor. In southeastern Australia, early Cretaceous rift basins underwent mid-Cretaceous inversion at a time of high palaeogeothermal gradients (c. 60°C km–1), and basement margins were also inverted at that time. Again, early generation implies that the chances of preservation of hydrocarbons are small. By integrating stratigraphic and structural relationships within the preserved section with data on the palaeo-thermal history of a section derived from direct measurements of palaeotemperature profiles, the complete history of hydrocarbon generation may be reconstructed with confidence.
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