Porosity/depth trends in reservoir sandstones: assessing the quantitative effects of varying pore-pressure, temperature history and mineralogy, Norwegian Shelf data

Abstract

AbstractEmpirical porosity/depth trends from the Norwegian Continental Shelf are discussed in the light of models on compactional processes during burial. Linear best-fit lines are useful as a first approximation, illustrating the general decline in porosity with depth. Porosity increases non-linearly with pore-pressure, and notably higher than normal porosity is found in very high overpressure reservoirs. Hydrostatic pressure sandstones from the Haltenbanken area are on average more porous than equivalents from the northern Viking Graben. This may be related to the burial history, as generally more subsidence occurred in Late Pliocene and Pleistocene in the Haltenbanken basins. The porosity/depth data show considerable scatter, and this is to a large degree related to the initial mineralogical composition. Correlated with mineralogical analyses, the reservoir sandstones show a trend towards lower porosity with increasing clay content at shallow burial, probably due to more extensive mechanical compaction. At depths >3.0–3.5 km, the porosity loss is highest in sandstones with low clay contents. This may be explained by a lower rate of quartz cementation in sandstones with abundant intergranular clays or clay coatings around quartz grains. These data show that pre-burial mineralogy is a very important factor in porosity prediction of sandstones, in many cases more important than the degree of overpressure.