Depth Versus Age

Abstract Introduction Geologic Setting Stratigraphic Setting Porosity Determination Data Set Porosity vs. Depth Depth vs. Age Effect of Pore- Water Composition Limestone & Dolomite Porosity Summary References

Although Figure 5 shows a strong relation between porosity and depth, it is possible that there is also a correlation between porosity and time, because older rocks tend to be buried more deeply. This hypothesis can be tested by comparing porosity measurements from rocks of equal depths but different ages. If porosity is time dependent, older rocks should be consistently less porous than younger rocks at equal depths.

In the data set of this study, there are depth overlaps between carbonate strata of different ages. As Stephenson (1977) pointed out, however, one must be careful not to ascribe impossibly large effects to a single variable. There is some question whether the age differences at given depths in the south Florida data are great enough to conclusively prove or disprove a porosity dependence on time.

With this caution in mind, porosity-depth curves representing each of the time-stratigraphic units used in this report are compared in Figure 7. In zones of depth overlap, the older unit is more porous than the younger unit in three cases, the older unit is less porous in two cases, and in one case there is essentially no porosity difference. Furthermore, porosity differences in zones of depth overlap are within the expected range of scatter indicated by the standard error of estimate. Thus, there is no systematic porosity difference between older and younger carbonate rocks at equal depths, which suggests that the porosity decrease shown in Figure 5 is primarily a function of depth rather than of time.

Fig. 7 - Exponential porosity-depth curves for the time divisions of Figure 2. There is no systematic porosity difference between older and younger rocks where they overlap in depth. [larger image]

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