The origin of overpressure in the Carnarvon Basin, Western Australia: implications for pore pressure prediction

Abstract

Porosity–effective stress analysis of 37 wells in the Carnarvon Basin, Australia, has identified 12 wells where the overpressure was generated by disequilibrium compaction and four wells where the overpressure was generated by fluid expansion. Disequilibrium compaction was identified as the dominant overpressure-generating mechanism in wells along the Rankin Trend as far south as Gorgon 1 and Spar 1. Fluid expansion was identified as the dominant mechanism of overpressure generation in wells along the Barrow Trend and around the Alpha Arch. Disequilibrium compaction-generated overpressures occur, as would be expected, where the Tertiary sediment thickness is greatest and fluid expansion overpressures where the Tertiary is thinnest. Indeed, where the N-1 (35 Ma) reflector is greater than c. 1500 m below seabed, disequilibrium compaction overpressures are observed and where itis shallower than c. 1500 m, fluid expansion overpressures are observed. Acoustic log-based pore pressure detection using Eaton's (1975) method yielded accurate estimates with an exponent of three in Wilcox 1 where the overpressure was generated by disequilibrium compaction, and an exponent of six in Bambra 1 where the overpressure was generated by fluid expansion. If the Eaton (1975) technique is to be applied to seismic processing velocities for pore pressure prediction, then an exponent of three can be used where the N-1 horizon is greater than c. 1500 m and an exponent of six where the N-1 horizon is less than c. 1500 m.