Examples of Porosity Distribution
Plots of sandstone porosity versus depth are easily constructed using core analyses and/or log interpretations from wells. Petrographic analysis of the sandstones and application of the criteria and concepts developed in this paper allow a much more complete interpretation of such porosity/depth plots than has hitherto been possible. Of minerals such as feldspars, micas, amphiboles, and pyroxenes precipitate as mesogenetic cements mainly consisting of clay minerals and zeolites (Hayes, 1979). Chemical compaction and associated intrastratal precipitation of cements may collectively be termed framework diagenesis, analogous to quartz diagenesis. Mesogenetic carbonatization in these sandstones involves more replacement and less cementation when compared with quartz arenites. Carbonatization tends to culminate during the early phase of mature stage “A” at about the same level of thermomaturation as in quartz arenites.
Figure 72 represents a porosity/depth plot illustrating the rate of decline of primary and secondary sandstone porosity of a well in the Mackenzie Delta which penetrated Tertiary sandstones of intermediate mineralogical maturity and Lower Cretaceous quartz arenites. The porosity of the Tertiary sandstones above a depth of 2,100 meters is predominantly primary and declines rapidly with depth. Primary porosity has been reduced by both framework diagenesis and by carbonate cementation. Both processes are apparently active at the present time. Below 2100 meters the sandstone porosity is essentially secondary and declines with depth at a much slower rate compared with that of primary porosity. Active maximum decarbonatization occurs between 2,400 and 2,500 meters. The relatively high porosities below 4,000 meters do not necessarily indicate