The Role of Grain-Coating Microquartz on Preservation of Reservoir Porosity

Nils Einar Aase, Per Arne Bjorkum, and Paul H. Nadeau

Clay coatings have been widley accepted as an important mechanism for preservation of high pororsity in deeply buried sandstones, but few people have realized that similar effects can be produced by microcrystalline quartz coatings. This penomenon can only be expected under very special circumstances, but in such cases it can have profound consequences for exploration. In the Central Graben provenance of the southern North Sea unusually high porosity (20-27%) and permeability (100-1000mD) are found in certain zones in Upper Jurassic sandstones at depths of 3.4-4.4 km. The porosity in these is zones are 5-15% higher than expected based on average porosity-depth trends from Brent and Haltenbanken sandstones. We propose that the high porosity is due to continuous grain coati gs a few microns thick of euhedral microcrystalline quartz crystals (0.1-2µm ). The distribution of microrystalline quartz coatings is controlled by the presence of silicious sponge spicules (Rhaxella), which implies a sedimentological control on the reservoir quality. We present a thermodynamic model showing how continuous microcrystalline quartz coatings inhibit development of normal macrocrystalline quartz overgrowths (which we believe to be sourced mainly from stylolites). High porosities in various Upper Jurassic oil fields (Ula, Gyda and Magnus) have previously been explained by inhibition of quartz cementation by oil. However, we suggest that the microcrystalline quartz coatings provide a more plausible explanation.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California