Subsurface-derived secondary oomoldic porosity is an important factor in reservoir development in the south Texas Smackover Formation. Much of the section penetrated is impermeable; however, reservoirs as thick as 33 ft (10 m), with porosity ranging from 4 to 26% and permeabilities ranging from 0.1 to 6.5 md, have been cored at depths below 18,000 ft (9,486 m).
In the grainstone facies, four general stages of diagenesis affected porosity: Stage 1 (marine-phreatic environment), precipitation of an isopachous carbonate cement and extensive grain micritization; Stage 2 (shallow-meteoric environment), precipitation of very coarse-crystalline syntaxial calcite and fine-crystalline equant calcite, dissolution of aragonitic skeletal grains, and incipient solution-compaction; Stage 3 (regional fluid-mixing environment), intrapore precipitation of and grain/matrix replacement by fine to medium-crystalline rhombic dolomite; and Stage 4 (subsurface environment associated with basinal fluid expulsion), dissolution of ooids and dolomite resulting from decarboxylation of kerogen, microstylolitization by solution compaction, and precipitation of coarse-crystalline calcite and baroque dolomite. The magnitude of each general diagenetic stage varies regionally.
Oomoldic porosity is present only in the updip, highly dolomitized grainstone facies. The dolomite formed a chemically rigid matrix that allowed the calcite ooids to be dissolved without solution compaction between grains. In the downdip, poorly dolomitized facies there was no chemically rigid framework, and dissolution proceeded by solution compaction resulting in loss of porosity.
Exploration in the deep south Texas Smackover Formation cannot depend upon finding reservoirs consisting of preserved, early types of porosity, but must depend upon defining areas where late subsurface-derived oomoldic porosity formed reservoirs.