Effects of composition, texture and diagenesis on porosity, permeability and oil recovery in a sandstone reservoir

Abstract

Relationship among depositional texture, composition, diagenesis, porosity, permeability, initial water saturations and residual oil saturations have been investigated in a set of sandstone samples from a North American oil reservor. Pointcount, compositional and textural data obtained from thin sections were used. Grain size, sorting and mineralogy are closely interrelated. As grain size increases, sorting decreases and the proportion of readily soluble detrial grains increases. The type and extent of diagenetic alteration is related to grain size. Silica cement as overgrowths is more common in finer-grained sandstones with abundant monocrystalline quartz, whereas carbonate cement is more common in coarsergrained sandstones. Carbonate dissolution is extensive in some intervals in the reservoir. Sandstone samples were subdivided into tow subgroups in which relationshiops among grain size, porosity and permeability are significantly different: Sandstones with less than 13% porosity are strongly influenced by carbonate cement which controls porosity and permeability, independently of variations of grain size and sorting. Sandstones having porosity greater than 13% have undergone extensive carbonate dissolution, and coarse-grained sandstones have more secondary porosity than the finer-grained due to dissolution of unstable constituents. Both porosity and permeability are positively correlated with grain size in these samples. Initial (connate) water saturation increases with grain size because of the strong correlation between increasing grain size and polycrystalline grains which have intra-grain microporosity. Increasing heterogeneity with increasing grain size is thought to contribute to higher residual oil saturations in coarser-grained, more poorly sorted sandstones. A decrease in oil recovery efficiency with increasing grain size and permeability may be predicted for other sandstone reservoir rocks deposited in similar types of environments.