Diagenesis at subaerial unconformities within the meteoric zone affect porosity formation by rearranging pore networks without increasing porosity or permeability. Shorter periods of exposure may be associated with greater porosity and may form pore systems resistant to compaction. Once buried below depth of freshwater and marine, the phreatic influence on the subsurface diagenetic environment is significantly different. Each basin has a unique brine history and prediction is difficult, although cementation and pore occlusion predominate. Porosity tends to decrease with depth due to cementation and compaction.
Fractures are extremely important in the delivery of hydrocarbons in carbonates.
Generally, fracture porosity is not related to original rock fabric. Earlyformed fractures tend not to be as effective as latestage fractures for fluid flow. Early-formed fractures in carbonates may be distinguished by their highly variable length. They are filled with sediment and early cement, and are formed by volume change of sediment or by gravity. Meteoric fractures fill with cement or residual soils, cave filling, or caliche.