anhydrite cement

What controls the distribution of these anhydrite cements is not properly understood and therefore their presence is difficult to predict.

During deeper burial, the intergranular and intragranular pores have been partially or completely occluded by anhydrite cement at some localities (Fig. 16b).

In the second well, however, the sonic log indicates that the upper unit is much tighter and the core descriptions suggest that the pores have been filled by an anhydrite cement.

Here, the intergranular pores have been filled by an anhydrite cement leaving only leached porosity, and the sonic log displays a very clear downward-decreasing porosity profile in the uppermost unit of oolitic grainstone (Fig. 18).