Porosity, permeability and compaction trends for Scandinavian regoliths

Abstract Weathered crystalline and metamorphic basement are proven as hydrocarbon reservoir rock in many areas of the world, including the Norwegian Continental Shelf. The reservoir properties of these rocks vary laterally and vertically as a function of the burial history, initial basement lithologies, sub-aerial exposure to weathering processes, chemical weathering, fault patterns, and these properties are relatively unknown. In this work, laboratory measurements are performed on different regolith types applying varying confining pressures representative for different degrees of basement weathering and burial depth. Porosity and permeability are measured on coherent samples (altered basement) and incoherent samples (disintegrated basement rock and saprolites) from the Utsira High (offshore Norway), and outcrop samples from Bomlo (southwestern Norway) and Ivo Klack (southern Sweden). Coherent samples from Utsira High (well 16/1-15 and 16/1-12) give porosities between 5.10% and 7.95% and permeabilities between 1.08 and 3.30 mD at 5.1 MPa, and indicate a relation between observed micro-fractures and decreasing permeabilities. In general, permeability increases with increasing amount of micro-fractures. The permeability is varying from 0.15 mD at 30 MPa to 3.30 mD at 5.1 MPa. This work presents the first published compaction curves for weathered basement (saprock and saprolites), and to our knowledge first permeability-porosity relationships for saprock and saprolites from three locations (Bomlo, Ivo Klack and Utsira High). The new porosity and permeability measurements of weathered basement rocks can be used as input parameters for reservoir modelling, basin modelling, field planning and hydrocarbon exploration in areas with regoliths. It can also serve as important input into hydrogeology, geothermal modelling, planning of tunnels, and as help to understand landslides and hillslope gullies in weathered basement.