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ABSTRACT: The Origin of Overpressure in the Nappamerri Trough, Cooper Basin, South Australia

van Ruth, Peter J.1, Richard R. Hillis1, and Richard E. Swarbrick2
(1) Adelaide University, Adelaide, Australia
(2) University of Durham, Durham, England

Overpressure occurs within the Nappamerri Trough, Cooper Basin, South Australia. Typically the overpressured compartments are below 2500 m in depth. The origin of overpressure in the Cooper Basin is not related to rapid Tertiary burial as the Cooper Basin has not undergone significant sedimentation since the late Cretaceous. The maximum thickness of the Tertiary is 300 m. Indeed the Tertiary has been dominated by uplift. Modelling of overpressure suggests that the two main effective overpressure generating mechanisms are compaction disequilibrium and fluid expansion at depth (in particular the production of gas). This study aimed to establish the relative contributions of each mechanism in the Cooper Basin.

Changes in porosity were compared with changes in vertical effective stress acting on the rock matrix in order to distinguish between the two types of overpressure. Porosity was modelled from wireline log data, especially the sonic log and calibrated against the density and neutron porosity logs where available. Vertical effective stress was calculated from the density log and pore pressures using the Terzaghi vertical effective stress relationship. The porosity- vertical effective stress data was then compared to the normal compaction relationship established in the Cooper Basin. In addition, the burial/temperature history and TOC % data from the Cooper Basin were considered. If overpressure in the Cooper Basin was generated due to compaction disequilibrium during the late Cretaceous the seals to overpressure appear to be very effective. Alternatively, overpressure may have been maintained or generated by the generation of gas in the deep, hot Nappamerri Trough.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia