Porosity reduction during contact metamorphism in sandstone reservoirs

Mechanical and chemical compaction are two fundamental processes that reduce the porosity of sedimentary rocks and affect reservoir quality. These processes can occur during burial and subsequent diagenesis, as well as during contact metamorphism. Although mechanical and chemical compaction have been studied extensively in diagenetic sedimentary systems, only limited studies have been carried out in the context of metamorphic rocks. Here, we show that as quartzite forms from sandstone during contact metamorphism, both mechanical and chemical compaction cause significant loss of porosity. Rocks sampled from the Inmar formation in Maktesh Ramon (Israel) - where quartzite formed near contacts with dykes - were analyzed with high resolution scanning electron microscopy and cathodoluminescence. Using image segmentation to differentiate between quartz cement and detrital grains, we observed both grain rearrangement and the brittle fracturing of grains near intrusive contacts. In the sample exhibiting the highest level of compaction, mechanical processes reduced porosity by 18 - 26%, while chemical compaction caused a reduction of 11 %. This result indicates that both mechanical and chemical processes can contribute to porosity reduction and the formation of barriers to fluid flow in sandstone reservoirs containing intrusive bodies.