Diagenetic characteristics of low permeability, non-reservoir chalks from the Central North Sea

The characteristics and diagenetic history of low permeability, non-reservoir chalk of the Central North Sea have been investigated to establish how chalk lithologies evolve into rocks capable of acting as a pressure seal. The permeability of 25 chalk samples have been measured using the Transient Pulse Decay method and shows that most non-reservoir chalks have low permeability (typically of tens of nanoDarcys) irrespective of depth (2000-3900 m), clay content, facies or stratigraphic formation. The samples were further characterised using a number of techniques including petrological analysis, insoluble residue measurements and porosimetry. Petrological studies have revealed that mechanical compaction is a relatively minor process in clean (non-argillaceous) chalks due to a framework of grains that resist compaction. Chemical compaction is the dominant porosity occluding mechanism in clean chalks, principally consisting of overgrowth cementation and grain-to-grain pressure solution. In contrast, the diagenetic history of argillaceous chalks is more complex with extensive mechanical compaction occurring, in addition to irregular cementation and widespread dissolution. The interpretation that the style of diagenesis is controlled by clay content is supported by analysis of pore size distributions as clean chalks show a gradual reduction in mean pore size with depth due to gradual occlusion by cements while argillaceous chalks show constantly small mean pore radii (5-20 nm) as a result of early mechanical compaction coupled with dissolution. The only sample within the set which did not exhibit low permeability possesses inclined stylolites, suggesting that locally stylolites can act as fluid pathways. The results show that both clean and argillaceous chalk diagenetic pathways result in low permeability rocks and the diversity of rock types that exhibit low permeability suggests that seals are pervasive throughout the Chalk Group. Non-reservoir chalks can therefore act as significant barriers to fluid flow, trapping hydrocarbons, as well as significant pressure seals within the Central North Sea, trapping high pressures beneath the Chalk Group. (C) 2008 Elsevier Ltd. All rights reserved.