Porosity preservation in reservoir sandstones due to grain-coating illite: a study of the Jurassic Garn Formation from the Kristin and Lavrans fields, offshore Mid-Norway

The Garn Formation is an important reservoir-sandstone in the Hattenbanken area. The Garn Formation from the Kristin field is deeply buried (4.6-4.9 km) but displays higher porosity values compared to the general porosity trend in the region. In the Lavrans field the same formation is extensively quartz cemented with low porosity values despite its shallower depth (4.4-4.5 km). Porosity is controlled by grain coatings preventing quartz overgrowth. It is well known that chlorite coatings inhibit quartz cementation, but our observations show that illite and illite/chlorite coatings can also be effective in preventing quartz precipitation. The grain-coating illite and chlorite in the studied wells may have developed from a smectitic precursor that formed from volcano-clastic material at lower temperatures. Bitumen covers the grains in many of the samples, but the bitumen formed too late to significantly affect quartz overgrowths and has not contributed to preservation of porosity. (C) 2002 Elsevier Science Ltd. All rights reserved.