Late Secondary Porosity Associated with Overpressure Leak-Off: Evidence from the Fulmar Formation, Central North Sea
David Darby, Mark Wilkinson, and Stuart Haszeldine
The Upper Jurassic Fulmar sandstone of the Central North Sea is characterised by deep burial (greater than 4000m burial depth) and extremely high levels of overpressure (up to 40MPa overpressure at 4500m depth). High overpressure may support high porosity at great depths, and so the relationship between overpressure and porosity is of considerable commercial importance in deep prospects. Simulating the porosity evolution and overpressure of the sandstones via a two-dimensional basin model reveals that for the majority of Fulmar Fm. sandstones, the porosity evolution is a simple pattern of reduction during burial to present-day values of 15-20%. However, in wells sited close to structurally-elevated overpressure leak-off points, the porosity is anomalously high (up to 35% at 4500m depth).
We suggest that pressure-induced vertical fluid flow at overpressure leak points allows removal of the products of feldspar dissolution. This process allows porosity to increase from an end-Miocene low of 15-20% to the present-day values of 30-35%. Developing voids require support by overpressure, and so this increase in porosity must have occurred as the sandstones became highly over-pressured during rapid Plio-Pleistocene subsidence. We infer that overpressured sandstones may behave as open geochemical systems, and that 10-15% porosity has formed in regions of the Central Graben during deep burial in the last 5 Ma.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California