COMPARISON OF POROSITY‐DEPTH RELATIONSHIPS OF SHALE AND SANDSTONE*

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Abstract
The rate of shale compaction or porosity reduction decreases with increase in burial or compaction. This may be caused by decreasing shale permeability and increasing water viscosity, thus decreasing rate of fluid expulsion with increasing compiction. For a given increase in overburden pressure, the pressure or stress applied to a given grain‐to‐grain contact area decreases with burial becaused the contact area increases with compaction and concomitant grain deformation. This may also cause the reduced rate of compaction with increase in burial. The above abservations may suggest that the shale compaction is primarily controlled by physical forces of sedimentary rocks.On the contrary, the rate of sandstone porosity reduction is known to be relatively constant throughout the burial diagenetic history in many regions provided ther was no significant secondary porosity developed by leaching and fracturing. For a given increase in overburden pressure, the grain‐to‐grain contact area. Yet, rate of porosity reduction for the sandstones is realtively constant, whether they are at a shallow or deep burial depth. This suggests that, for the sandstones, the physical forces have been of secondary importance in reducing the porosity. Chemical and mineralogical agents are believed to have been more significant in sandstones. Numerous regional petrographic studies confiirm the importance of diagenetic cements.

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