Silicate mineral authigenesis in the Hutton and NW Hutton fields: implications for sub-surface porosity development

Abstract

The Hutton and NW Hutton fields occur in highly faulted, rotated block structures in Block 211/27 of the East Shetland Basin, Northern North Sea. Fourteen wells that core Brent Group reservoir from these fields have been studied in order to characterize the relationship between clay mineral authigenesis and porosity modification in the sub-surface. Reservoir sands are present in the depth interval 9000–13 000 ft; the shallowest sandstones are subarkoses, contrasting with the deepest sandstones which are mostly feldspar-deficient quartz arenites. The change in composition is a result of diagenetic feldspar dissolution that takes place over the depth interval of study. Authigenic kaolin group minerals and illite are present over the depth range studied, and both exhibit depth-related increases. Three habits of authigenic kaolin group minerals are observed: ‘expanded-mica’, vermiform kaolinite and blocky dickite. The occurrence of vermiform kaolinite is enhanced in Hutton and the adjacent water zone, and does not occur in abundance below 10 500 ft. This habit formed as a result of syn-depositional meteoric water ingress. Structural models suggest that neither field underwent subsequent meteoric water flushing. At the deeper levels of its occurrence range, vermiform kaolinite shows signs of alteration to blocky dickite. Dickite occurs in both intergranular and grain dissolution pores. A depth-related trend of increasing dickite cementation is strongly linked to increasing K-feldspar dissolution. Enhanced dickite cementation is found adjacent to bed contacts in sandstones within mudstone sequences and in regressive sandbodies close to thick mudstone units. Illite shows a more pronounced depth-related increase across the studied interval. Illite precipitation is also genetically linked with detrital feldspar dissolution and is favoured by diminished pore water flow during deep burial. Authigenic clay distribution is partly controlled by local features such as detrital clay coats, in addition to larger-scale parameters such as primary lithology, facies associations, present burial depth and inferred palaeofluid flow. Intergranular porosity declines with increasing burial depth, while grain dissolution porosity increases with depth from Hutton to the shallow and intermediate depth wells of NW Hutton. Generation of enhanced porosity is not related to the unconformities overlying the Brent Group, but to sub-surface fluid migration pathways. Compaction is not significantly operative over the depth interval 10 000–13 000 ft. By 10 000 ft burial depth, up to 50% of the intergranular volume has been lost by mechanical compaction.