Reservoir characterization of the Stø formation in the Hammerfest Basin, SW Barents Sea

Abstract

A reservoir characterization of the Stø formation has been performed in two wells linked to the Snøhvit field within the Hammerfest Basin in the South-Western Barents Sea. The reservoir quality of the Stø formation has been described as a function of provenance, depositional environment and diagenesis where both a petrographic- and a petrophysical approach has been implemented. The primary focus has been to investigate processes which have had an influence on the reservoir quality in the Stø formation, with special regard to porosity. Provenance and depositional environment have been of great importance for the resultant reservoir quality within the Stø formation as these processes determined initial rock properties such as mineralogy, sorting and grain size. The mature mineralogy of the Stø formation has been linked to leaching and reworking of the sediments prior to final deposition, whereas the detrital clay content has been related to a transgressive development of the shoreline in a shallow marine setting. IGV results with respect to grain size indicate that intervals consisting of very fine sand had the best preserved porosity after mechanical compaction, whereas the same intervals indicate significant porosity reduction during the chemical compaction regime, due to quartz cementation. Highly cemented intervals have been linked to the distance between stylolites and the relative amounts of micro-stylolites, which is thought to be the most prominent silica dissolving mechanism. A relationship between quartz cement and porosity shows that quartz cementation is the major porosity reducing mechanism at depth within the Stø formation. The lower part of the Stø formation have the best preserved porosity and lies in the range of 8-13%, whereas upper part of the Stø formation have porosity values ranging from 4-14%. However, the majority of the porosity data are below 8% within the upper part of the Stø formation. Authigenic kaolinite and illite has been suggested to have a minor influence on the reservoir quality within the Stø formation, due to the relatively small amounts of these clay minerals compared to the bulk volume. Thin clay laminations were sporadically observed within the upper part of the Stø formation that also is intervals with a high quartz cement volume. These intervals may reduce the vertical permeability.