Loss of porosity in the Sunniland Limestone is the result of processes associated with burial of the formation to its present depth of about 3,400 m. Early diagenetic alteration was a relatively minor factor in porosity loss. Significant processes which reduced porosity are mechanical compaction (dewatering, grain fracture, and deformation), chemical compaction (stylolitization, both on a macro and micro scale, and interpenetration of grains along contacts), and burial cementation. Burial cements, the product of chemical compaction and calcite solution transfer, are most evident in grainstones, including Sunniland reservoir rocks.

Three petrographically distinct burial cements are recognized as: (1) scalenohedral or dogtooth calcite, of grain size generally less than 100 µm, filling primary porosity and fractured by burial pressure; (2) equant or blocky calcite, of grain size generally larger than 100 µm, filling primary, secondary, and fracture porosity, and commonly poikilotopic; and (3) equant or baroque dolomite, of grain size generally larger than 100 µm, filling primary, secondary, and fracture porosity, occasionally poikilotopic, with larger grains exhibiting undulatory extinction and curved crystal faces.

These cements account for as much as 30 to 40% porosity loss in some grainstones and are present to a lesser extent in most reservoir rocks.

Isotopic analyses of cements (^dgr¹⁸O, ^dgr¹³C) and water (^dgr¹⁸O) are consistent with an interpretation that these cements formed in the subsurface over a range of temperatures (about 40 to 100°C) in pore water that was continually modified by dissolving Suniland calcite. Quantification of these processes requires accurate predictions of the rate of calcite solution transfer. Estimates of this parameter suggest that very little cement is precipitating now and that most cementation was complete before the Oligocene (burial depth about 3,000 m).

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