Diagenesis and Reservoir Quality of the Hugin Formation Sandstones in the North Sea: A Petrographical Approach
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The Middle Jurassic Hugin Formation in the Sleipner area has been studied in five wells in the southern Viking Graben in the Norwegian sector of the North Sea. The main objective is to understand how the diagenetic development has influenced the reservoir quality by the use of petrographical methods, in combination with core plug measurements of porosities and permeabilities. The Hugin Formation sandstones are characterized by textural and compositional maturity, and are classified as quartz arenites, subarkoses and sublitharenites. Mechanical compaction and diagenetic minerals (such as calcite cement and quartz overgrowths) contributed the most to the reduction of reservoir quality. Porosity measurements from the core plugs vary from 6 to 16%, which is somewhat below the average regional trend at similar depths. Calcite cement does not follow any specific trend related to depth, but the amount of quartz cement clearly increases with increasing depth.The main source of quartz cement is dissolution of detrital quartz grains at stylolites. The stylolites are formed where thin laminae of clay and insoluble organic material occur between detrital quartz grains, and the dissolved silica precipitated close to the stylolites. Authigenic clay minerals are also involved in the reduction of the reservoir quality. Kaolinite and illite are present in varying amounts throughout the wells, and their abundance is closely related to the presence of alkali feldspar. It is believed that the variation of alkali feldspar is responsible for the varying degree of illitization seen in the Hugin Formation. The illitization of feldspar is an additional source of the quartz cement. Secondary dissolution of alkali feldspar has occasionally contributed to preserve the reservoir quality, and this process is most commonly seen in the subarkoses.