Abstract

Clay coatings have been widely accepted by many workers as an explanation for preserving high porosity in deeply buried sandstones, but few workers have realized that similar effects can be produced by microcrystalline quartz coatings. This phenomenon can be expected only under special circumstances, but in such cases it can have profound consequences for exploration. In the Central Graben area of the southern North Sea, unusually high porosity (20-27%) and permeability (100-1000 md) are found in certain zones in Upper Jurassic sandstones at depths of 3.4-4.4 km. The porosity in these zones is 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 coatings of euhedral microcrystalline quartz crystals that are 0.1-2 micrometers thick. The distribution of microcrystalline quartz coatings is controlled by the presence of siliceous 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 sourced mainly from stylolites. High porosities in parts of various Upper Jurassic oil fields (Ula and Gyda) have previously been explained by inhibition of quartz cementation by early hydrocarbon charge. We suggest that the microcrystalline quartz coatings provide a more plausible explanation.

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