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The porosity/permeability relationships of the common carbonate rock types have been studied, with emphasis on the variety of pore types in upward-shoaling grainstone sequences including: Smackover, Lansing, Salem, and San Andres Formations. A result of these studies is an improved conceptual understanding of permeability gained from the cross-plotting of porosity and permeability data from plugs and whole cores accompanied by textural and fabric analyses of rock samples, thin-sections, serial sections, and pore casts. Importantly, many depositional rock types cross-plot as distinctly different populations, commonly yielding linear trends on semi-log paper. These trends indicate a degree of order in the seemingly chaotic pore systems of carbonate rocks which have undergon cementation and/or compaction. For grainstone samples, there appears to be a change in the slope of this trend between the compaction and cementation phases of diagenesis.
Once the depositional texture and fabric of the rock are defined in terms of porosity and permeability, the evaluation of fractures and secondary porosity can be addressed. The secondary porosity is observed to be as high as 14% of the rock volume in the Smackover example and 21% of an oolitic sample from the Lansing Formation. Pore casts and serial sections reveal that the grain-moldic porosity is poorly connected to the intragranular pore system and contributes little to the permeability of the rock. This insight allows quantitative estimates of this type of secondary
porosity using standard porosity and permeability data. The geologic and quantitative analysis of the various pore types and porosity/permeability relationships also aided in the interpretation of the log data from the reservoirs studied.
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