Focusing and Timing of Overpressured Diagenetic Fluid Movement Predicts Deep Poroperm: Central Graben, UK

R. Stuart Haszeldine, M. Wilkinson, D. Darby, A. E. Fallick

The Upper Jurassic "Fulmar Sandstone" was deposited diachronously during marine transgression and differential tectonic subsidence. Post-rift thermal subsidence was complicated by salt movement. This sandstone hosts many condensate accumulations which are being actively appraised to become the first HPHT commercial developments in the U.K. In such highly pressured areas, delineation and development wells can cost $30M each, so that good predictions are important.

As part of a regionally coordinated project with EFCal, new palynology and sedimentological work provided a sequence stratigraphic framework across the UK Central Graben. Our core petrography shows similar diagenetic sequences in all wells, dominated by dolomite cementation. However, important petrographic differences exist in cement volume: regionally, with depth and locally within one core. We have made stable and radiogenic isotopic analyses of carbonates and silicates. This enables us to reconstruct the pore-water history, both across the region and with progressive burial. We have also used present-day pressure data and the BasinMod code to understand the geopressuring processes in this Graben, their timing, and quantified their deep potential to drive known fluid volumes.

We find that the timing of illite growth is systematically linked to fluid expulsion, both regionally and within one core; this is not tectonically driven event diagenesis. Porosity-filling cement volumes correlate poorly with sequence stratigraphic boundaries. However, there can be an excellent correlation between present-day core poroperm and sequence stratigraphic boundaries. Effective deep poroperm is secondary, generated by compactional pore fluids. These are focused onto predictable regional structures, and onto predictable surfaces within the sandstone package.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995