Abstract: Porosity Preservation in Shallow-Marine Carbonate Rocks
J. C. Wendte, H. W. Mueller, III, C. G. Kendall
A major problem in carbonate geology is the lack of understanding of porosity preservation in some ancient carbonate rocks. Normally, carbonate sediments with porosities between 35 and 70% are converted through diagenesis to rocks with only a few percent porosity. Studies concerned with postdepositional changes in carbonate deposits have focused on cementation and/or compaction accompanying the massive porosity reduction in most ancient carbonate rocks. Only minor attention has been given to understanding preservation of porosity in the select few carbonate rocks which have reservoir potential.
Potential limestone reservoirs are lime grainstones, low-mud packstones, and boundstones where porosity-destroying diagenetic processes have been inhibited or arrested. In such sequences porosity is preserved during passage through the near-surface freshwater phreatic zone. As many carbonate sequences consist of repeated upward-shoaling cycles terminating in supratidal or coastal-plain sediments, multiple periods of freshwater sparry calcite cementation can affect any one cycle. Factors which control freshwater cementation include rate of burial, development of impermeable seals, paleoclimate, and distance from continental land masses. Optimum combinations of these factors will inhibit or arrest freshwater cementation and thus preserve porosity. Such porosity in grain carbonate rocks s reduced gradually by closer grain packing, physical interpenetration of grains, pressure solution, and attendant calcite cementation during burial.
Early dolomitization preserves porosity by creating a rigid framework inhibiting compaction in pelletal mudstones and by increasing rock strength impeding compaction during deep burial in uncemented grainstones. Solution of relict calcite and aragonite grains in dolomites also creates porosity.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC