Grain Coatings in Deep Reservoirs – Sandstones Last Defense against Chemical Compaction

Jens S. Jahren, Knut Bjørlykke, and Per Aagaard
University of Oslo, Oslo, Norway

Grain coatings may stop or delay cementation and thereby preserve porosity during deep burial and high temperatures. The preservation potential is a function of factors like type of coating, coating thickness and grain coverage. Coating can preserve good reservoir condition 1-2 km deeper than in adjacent uncoated reservoirs. The most common mineral coatings are chlorite and microcrystalline quartz but other coatings like oxides might also be important. The rate limiting step in the quartz redistribution process is precipitation. Precipitation is known to be a direct function of the available area for cement growth. The rate of precipitation is therefore insensitive to stress and chemical compaction driven by quartz cementation will continue and reduced porosity and rock volume (shrinkage) as a function of temperature. At low strain rates this chemical compaction will result in a relaxation of differential stresses due to the rock shrinkage and the horizontal stress may become close to the vertical stress. In well coated rocks the coating will prevent cementation as long as the grain coating is chemically stable and pervasive. The stress will then build up to levels to either provide high enough supersaturation to overgrow the coating directly or to crush the quartz grains. Crushing of grains due to high effective stresses at grain contacts and low bulk rock strength from lack of cementation will produce new fresh intra grain quartz surfaces perfect for cementation. Crushing of grains will also break pervasive coatings at the grain-pore boundary and provide avenues for quartz growth out of the fractures and into the pore space. Coating formation will be illustrated both by experimental formation of coatings and with examples from reservoir sandstones from the North Sea and Haltenbanken areas Offshore Norway.