Comparison of deeply buried paleoregolith profiles, Norwegian North Sea, with outcrops from southern Sweden and Georgia, USA — Implications for petroleum exploration

Abstract

For the first time on the Norwegian Continental Shelf, deeply buried paleoregolith profiles have been identified as part of the petroleum reservoirs in recently discovered oil fields on the Utsira High, Norwegian North Sea. Reservoir properties (porosity and permeability) in the granitic basement on the Utsira High are mainly the result of physical and chemical alteration of the rock occurring in the near-surface environment during sub-aerial exposure of the high in the Mesozoic. Evaluating the reservoir potential of altered basement rocks requires a different approach than in conventional petroleum exploration. In this paper, macroscopic, mineralogical and micromorphological alteration features observed in two deeply buried paleoregolith profiles are compared with surface paleoregoliths from Ivö Klack, Sweden and Georgia, USA. The paleoregolith profiles are subdivided into specific weathering facies (altered coherent rock facies, saprock facies and saprolite facies) based on the rock fabric and mechanical strength. The reservoir potential of each weathering facies is controlled by the type and degree of alteration. In the altered coherent rock facies, porosity and permeability is mainly controlled by joints and microfractures that developed prior to subaerial exposure of the granitic pluton. In the saprock facies, intensified chemical dissolution of plagioclase enhanced porosity and the development of mesofractures improved the connectivity between pores. In the saprolite facies, progressive dissolution of plagioclase creates porosity, but the precipitation of clays within voids and mesofractures has a destructive effect on the overall reservoir properties.The deeply buried paleoregolith profiles from the Utsira High display comparable macroscopic, mineralogical and micromorphological alteration features to what was observed in surface paleoregoliths from Ivö Klack and Georgia. Outcrop studies may therefore be an important tool when evaluating the reservoir potential in subsurface paleoregoliths.