Page 178 - Geology of Carbonate Reservoirs

DIAGENETICALLY ENHANCED POROSITY 159

earlier in this book, the term neomorphism originated with Folk (1965) , who defi ned
it to include both true recrystallization and mineralogical inversion. True recrystal-
lization occurs when very small crystals of calcite dissolve and reprecipitate as
larger, neomorphic spar crystals. Inversion is the process by which metastable miner-
als change to stable minerals, generally undergoing some compositional as well as
crystallographic changes such as the inversion of strontium - rich aragonite or Mg -
calcite to low - Mg calcite. Recrystallization is also used to describe the processes of
crystal reformation such as those that take place during metamorphism of limestone
to marble or as new crystals form during application of great stresses. This type of
recrystallization can be accompanied by elimination of crystal imperfections and

fluid inclusions to produce nearly perfect lattice structures.
Examples of porosity enhancement associated with stabilizing neomorphism are
described from Cotton Valley Formation (Jurassic) reservoirs in the East Texas
Basin (Ahr, 1989 ; Dravis, 1989 ; Moshier, 1989 ). One of the most striking examples
of neomorphic microporosity is found at Overton Field, Texas. There, individual ooid
grainstone deposits (Figure 6.7 a) exhibit micrometer - sized, microrhombic micropo-
rosity (Figure 6.7 b) interpreted to have resulted from neomorphic stabilization of
metastable ooid mineralogy to present - day low - Mg calcite. The mechanism of the

A B

Figure 6.7 (a) Thin section photomicrograph of an altered oolite grainstone in the Jurassic

Cotton Valley Formation of East Texas. The photomicrograph shows the degree of degrada-
tional diagenesis that has almost obliterated the original microstructure of the ooid. Only
traces of concentric structure of the original ooid cortices are visible in the now - altered,
“ chalky ” ooid that makes up a large part of the natural gas reservoir rock in Exxon - Mobil ’ s

Overton Field in Texas. The width of the photo is 2.5 mm. (b) An SEM micrograph of the
microstructure in the large ooid in the center of the photo in (a). This microporous, microrhom-
bic microcalcite fabric resulted from a type of recrystallization that is not well understood
but is widespread and makes up many gas reservoir rocks around the world. The bar scale

faintly visible in the lower right - hand corner of the image is 5 μ m long. (From Ahr (1989) .)