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A suite of selected oolitic limestones ranging in age from Devonian to Pleistocene was studied petrographically with light microscopy and SEM observation of resin pore casts, and petrophysically with measurements of porosity, gas and liquid permeability, and with mercury injection capillary pressure curves. A new genetic classification of porosity types and related processes in oolitic limestones is presented which is based on the chronological order of their occurrence in the natural history of the rock, following the terminology and concepts of Choquette and Pray.
The 11 subdivisions of the proposed classification are: Primary Porosity: pre-deposition, deposition; Secondary Porosity: eogenetic dissolution, eogenetic drusy cementation, eogenetic compaction, mesogenetic cementation, mesogenetic dissolution, mesogenetic compaction, late mesogenetic cementation, telogenetic recrystallization and telogenetic dissolution. Furthermore, it is possible to subdivide the investigated samples into five evolutionary stages showing the gradual reduction of primary porosity mainly through cementation, and into six stages showing the modification of secondary porosity mainly through dissolution.
Concurrently, 11 laboratory experiments were performed in which samples of a tightly cemented Mississippian oolitic calcarenite were submitted to simulated burial conditions, and undersaturated carbonic acid solution was forced to pass through them. The result was selective dissolution of the ooid cortical layers, with the sparite cement preserved undissolved. It is concluded that oomoldic porosity can result from textural variation between components and does not necessarily imply that the ooids had an unstable mineralogy.
The understanding of the complex time and space relationships between the different types of porosity in oolitic limestones is critical for reconstructing their depositional-diagenetic history and for evaluating their economic importance as potential hydrocarbon reservoirs.
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