Clay coating preserving high porosities in deeply buried intervals of the Stø Formation

Abstract

The Stø Formation is the most important reservoir interval in the Norwegian Barents Sea, however the reservoir quality can be highly affected by the detrimental effects of quartz cement where there have been extensive post depositional burial. Core plug data from well 7219/8-2 in the Southwestern Barents Sea shows abnormally high porosity and permeability values in certain units of the deeply buried and otherwise highly quartz cemented Stø Formation. The amount of quartz cement in the samples is inversely proportional to the porosity. Samples with high and low porosities are similar texturally and mineralogically, but the high porosity samples have a layer of illitic clay coating the majority of the detrital quartz grains. Illitic clay coating present at grain contacts can result in a lowered IGV given they aid in the dissolution of quartz at interfaces, also creating a source of dissolved silica. Clay induced dissolution means that silica saturation is not a limiting factor in quartz cementation in these samples. The results show that the illitic clay coating is capable of limiting the amount of authigenic quartz overgrowth from 20-23% in samples with negligible grain coating to 5-11% in the intervals with high coating coverage. The illitic clay coating inhibits quartz overgrowth by limiting the surface area available for nucleation on detrital grains. The Stø Formation comprises mainly shallow marine deposits of highly reworked clean sandstone. Abnormally high porosities appear to be linked to settings where sediments of a more proximal location are preserved without extensive reworking. The grain coating clay is illitic and most likely originates from clay infiltration processes prior to final deposition. The difference in extent of clay coating in similar facies can mostly be correlated with varying amount of post depositional reworking. This study suggests that there is a potential for considerable porosity and permeability to be preserved in deeply buried sandstones in the Barents Sea. This study could be important in the future exploration activity of deeply buried structures in the area.

Characteristics and Controlling Factors of Tight Gas Sandstones from the Upper Shanxi and Lower Shihezi Formations in the Northern Sulige Area, Ordos Basin, China

“…The presence of a chlorite rim typically impacts the development of high-quality reservoirs in two contrasting ways. On one hand, when it attaches to the surface of quartz particles, the film hinders secondary enlargement of quartz and is more beneficial to pore preservation during the diagenetic evolution process. − On the other hand, the several micrometer thick film acts as a barrier to fluid seepage, thus closing up the pores between particles, which is not conducive to fluid migration and preservation. ,, …”

Characterization of Upper Cretaceous Matulla and Wata clastic reservoirs from October field, Central Gulf of Suez, Egypt

“…6d) which generally forms in the early diagenesis (Dowey et al, 2012;Freiburg et al, 2016) and favoured by the presence of iron-rich clays (Zhu et al, 2017). Such chlorite rims reduces surface area for further quartz cementation and stops overgrowth, thus helps in porosity preservation (Nguyen et al, 2013;Hansen et al, 2017). Clay pseudomatrix is commonly observed in all the samples from Matulla and Wata, which results from the compaction/alteration of argillaceous rock fragments.…”

Tectonic, sedimentary and diagenetic controls on sediment maturity of lower Cambrian quartz arenite from southwestern Baltica

“…Since the highest amount of quartz cement is associated with the highest IGV and low content of detrital phyllosilicates, quartz cement passively filled the pore space and caused the IGV to remain the same as before quartz cementation commenced (cf. Hansen, Løvstad, Müller, & Jahren, 2017). If clay-induced dissolution at grain contacts was a major source for quartz cement, the IGV would be expected to decrease with increasing cementation (Ramm, 1992;Rittenhouse, 1971).…”