Porosity evolution of sandstones related to vitrinite reflectance

Abstract

Porosity of potential reservoir sandstones undergoes a complex, multistage evolution or diagenesis during burial. Organic matter deposited in shales also changes during burial, an irreversible process called thermal maturation. Measures of thermal maturation, such as vitrinite reflectance ( R o), are routinely available for subsurface samples. So, the question is: can vitrinite reflectance be used to predict the evolutionary stage and quality of sandstone porosity prior to drilling? No universal quantitative correlation between R o and sandstone porosity can exist, for there are too many variables. However, R o can indicate the stage of sandstone-porosity evolution. The first stage is marked by progressive destruction of primary porosity, and coincides with low levels of thermal maturation of kerogen, up to an R o of 0.4–0.5. The second stage involves dissolution and flushing away of cements and detrital minerals by acid solutions to create secondary porosity. The acids are produced by decarboxylation of kerogen in shales in the R o range of about 0.4–0.7. The third stage involves destruction of secondary porosity by cementation and compaction, or, ideally, filling of secondary porosity by oil. Because the oil window begins at an R o of about 0.6, that is about where the third stage begins. It continues until porosity and permeability are reduced to low, noneconomic levels, at which point the diagenetic evolution has reached porosity basement, somewhere between R o = 1.4 and 2.5. Above R o = 2.5 are unmistakable indications of incipient metamorphism in the sandstones. Because R o can be modeled and projected into undrilled parts of a basin, it should be possible to predict the stage of porosity evolution of sandstones in advance of drilling, a useful capability.