The study deals with the diagenesis and reservoir quality prediction of deeply buried Rotliegend sandstone from Northern Permian Basin (NPB) in the Central Graben. Cored intervals from two wells (1/3-5 and 2/10-2) have been investigated by petrographical and petrophysical analysis.
Quartz cementation is the main process destroying porosity in deeply buried quartz rich sandstone reservoirs of the North Sea. Quartz cement precipitation takes place as syntaxial overgrowths, which starts at temperatures above 70oC-80oC. The Rotliegend sandstone is buried at the depth below 4km. As quartz cementation follows a time/temperature integral, all the pore spaces are expected to be filled with quartz cement in Rotliegend sandstone. Porosity preserving mechanisms such as grain coatings and hydrocarbon emplacement may preserve porosity at greater depths. In deeply buried sandstone reservoirs of the North Sea, grain coatings are most important porosity preserving mechanism covering the detrital grain surfaces and giving no surface area available for quartz cementation.
All above mentioned porosity preserving mechanisms have been investigated, but detailed petrographic analysis confirms that only illite coating has been found most frequently covering the detrital grain surfaces and effectively inhibiting quartz cementation. Although illite coating has preserved porosity but on the other hand, it has severely damaged the permeability of Rotliegend sandstone. Quartz cementation has negligible effect on porosity loss. The pore-filling authigenic kaolinite and carbonate cements have mainly caused the porosity reduction. The presence of unaltered kaolinite together with K-feldspar at this depth suggests that the process of illitization of kaolinite is still going on.
Samples from well 2/10-2 contain very high intergranular volume (IGV) values and less affected from mechanical compaction, indicating that early carbonate cement have helped preserving high IGV, whereas, samples from well 1/3-5 are more mechanically compacted representing relatively low IGV values.