Influence of fluid pressure on the diagenesis of clastic sediments

Abstract

As the exploration of hydrocarbon moves into more complex and deeper basinal settings the need to understand the effect of high pressures and high temperatures (HPHT) on reservoir quality and rock properties becomes more important. The complex fluvial sandstones of the Skagerrak Formation are the host for a number of HPHT reservoirs in the Central North Sea and exhibit anomalously high porosities and permeabilities considering their present-day depth of burial (>4500 m). The Skagerrak Formation reservoirs used in this study have encountered overpressures of >40 MPa and temperatures up to ~185°C at present-day maximum burial. This study has combined detailed petrographic analyses, core analysis and pressure history modelling to assess the impact of high pore fluid pressures (up to 80 MPa), differing vertical effective stress (VES) and authigenic clay mineral grain coatings on reservoir quality. It has been recognized that fluvial channel sandstones of the Skagerrak Formation in the UK sector have experienced, due to shallow onset and continuous maintenance of overpressure, significantly less mechanical compaction than their equivalents in the Norwegian sector. This difference in mechanical compaction has had a significant impact upon reservoir quality, even though the presence of chlorite grain coatings and the reduced VES inhibited extensive macroquartz cement overgrowths across all Skagerrak Formation reservoir sandstones. It is the combined effect of shallow overpressure onset, continuous overpressure maintenance and well-developed authigenic chlorite grain coatings which maintained anomalously high reservoir quality in the deeply buried HPHT reservoir sandstones of the Skagerrak Formation in the Central Graben, North Sea