The formation of grus from granites, Sila Mountains, Calabria, Italy

The storage of hydrocarbons in weathered and fractured crystalline rocks is a relatively new play concept on the Norwegian Continental Shelf. The recent discoveries by Lundin Norway and partners at the Utsira High in the Rolvsnes, Goddo and Jorvik prospects have established the commercial potential of basement reservoirs. When crystalline rocks are exposed at the surface of the Earth, fractures and joints will allow the infiltration of rainwater and plant roots into the rock. Biochemical reactions between the percolating fluids, the rock, fauna and flora may dissolve the primary minerals, replace them with new mineral phases (e.g. clays and opaque oxides), and transport mobile elements out of the system. The result is a weathered material (regolith) that differs in composition and physical properties from the original and unweathered parent rock. Recent studies from the Utsira High have shown that the reservoir potential is impaired in clay rich regoliths, as clays clogs pores and fractures. The best reservoir properties have been observed in clay-poor regoliths, so-called grus. The properties of the regolith is to a large degree controlled by the prevailing climate under which the regolith formed. As an analog to the Utsira High weathering sections, we have studied weathering of granitoids under a Mediterranean type of climate in the Sila Massif, Calabria region of southern Italy. The metamorphic and plutonic terranes of the Sila Massif consist of rocks of Paleozoic intrusive and metamorphic rocks, locally with an unmetamorphized Mesozoic sedimentary cover. The studied profile represented a ~35 m thick profile demonstrating the process of complete disintegration of a coarse-grained, two-mica, granite that displayed an upwards increase in degree of alteration, culminating in an interval of about 2 m grus and 0.6 m soil cover. Compared to the unweathered granite, the plagioclase/(plagioclase+quartz) ratio in the grus has been reduced by 50%, resulting in a clay content of about 5% kaolinite of the total rock. Furthermore, the vermiculization of biotite resulted in a volumetric expansion of biotite that probably was contributing to the disintegration of the rock during weathering. The chemical index of alteration (CIA) increased from around 55 to 60, suggesting intermediate degrees of chemical weathering. Density estimations indicated a reduction from around 2.66 g/cm3 to 1.61 g/cm3 and thin section analysis demonstrated an increase in well-connected porosity from negligible in the fresh rock to around 30-40% in the grus. Together, the analyses showed that weathering in the Sila Mountains produced a highly porous and permeable regolith with excellent capacity to store fluids.