Mechanical Compaction of Sandstones during Deep Burial

Quentin Fisher, Christine Souque, and Anren Li
Rock Deformation Research Limited, Leeds, United Kingdom

Mechanical compaction of sand-rich sediments is usually thought to be restricted to shallow burial depths. Recent studies of sandstone from petroleum reservoirs and outcrops indicate, however, that pervasive mechanical compaction involving porosity collapse and grain-fracturing may dramatically reduce the permeability of sandstones during deep burial. Sediments particularly prone to mechanical compaction are those that have high porosity and medium to coarse grain-size, which become exposed to high mean effective stresses. Diagenetic reactions such as quartz cementation ensure that the strength of sandstone usually increases at a rate that is sufficient, compared to the rate of increase in effective stress, to prevent porosity collapse during deep burial. Low geothermal gradients, rapid rates of burial and the presence of grain-coating clays may, however, reduce the rate of porosity reduction making some sediments, particularly within some of the deep water frontier areas, prone to mechanical compaction during deep burial. Diagenetic modeling provides a good indication as to which sandstone reservoirs would be susceptible to mechanical compaction during deep burial. However, the extent of grain-fracturing varies as a function of structural position. Here we describe outcrop examples from Azerbaijan, Portugal and France, which have experienced pervasive porosity collapse. It is shown that grain-fracturing is sometimes enhanced in tightly folded horizons at the hinges of periclines and in the vicinity of salt domes. The dependence of grain-fracturing on structural position may means that sophisticated geomechanical reconstructions are required to predict reservoir quality in some deeply buried reservoirs.