Page 170 - Geology of Carbonate Reservoirs

DIAGENETIC POROSITY 151

with karst processes (dissolution) may correspond with paleotopography and posi-
tions of ancient water tables instead of depositional fabric or facies, or it may cor-
respond to the distribution of metastable precursor minerals such as aragonite or

Mg-calcite . In such cases, selective dissolution of metastable constituents can produce
patchworks of moldic or vuggy pores that mark the previous position of aragonitic
or magnesium - calcite components, or simply the position of ancient water tables.
Neomorphism of limestone resulting in a net porosity increase is uncommon because

limestone neomorphism usually produces close - fitting crystal mosaics in which com-

promise crystal boundaries (close fit between crystal faces at angles of 120 ° ) are
abundant. Intercrystalline porosity is low in this case and the resulting sheet - like
pore throats are too small (micrometer - sized) to allow easy liquid fl ow, although
microporous gas reservoirs are common.

Porosity created by diagenesis, not simply modified by it, can be grouped into
four end - member categories: (1) intercrystalline, (2) moldic, (3) vuggy, and (4) cav-
ernous pores. A variety of combinations can exist. Some examples include solution -
enhanced versions of inter - and intragranular pores, intercrystalline pores,
solution - enlarged fractures or joints, and complex porosity in solution - collapse or
karst - related breccias. Stylolites may act as pores, but they are usually fl ow barriers
because they are typically plugged with insoluble residue. Intercrystalline porosity
is common in dolomite replacements of limestone. It is rare in neomorphosed lime-
stones with one noteworthy exception: microporous, microrhombic low - Mg calcite
that forms from metastable precursors, probably Mg - calcite, by a stabilization
process that is not well understood (Ahr, 1989 ; Dravis, 1989 ; Moshier, 1989 ). This
form of “ chalky ” porosity is the primary reservoir rock in many natural gas fi elds,

including many Cotton Valley Limestone (Jurassic) fields in the East Texas basin.
Intercrystalline porosity, literally pore spaces between crystals, requires neomor-
phism, replacement, and perhaps enlarging grain boundaries by dissolution. Replace-
ment of limestone by dolomite has been cited as a mechanism by which, in theory,
a solid volume decrease of up to 13% can be accomplished (Weyl, 1960 ). Porosity
measurements on synthetic dolomite in laboratory studies by Bubb and Perry (1968)
were within the limits of the theoretical predictions, but in practice, the theory has
not been shown to work. Weyl ’ s paper gained wide acceptance because many pro-
ductive reservoirs with intercrystalline porosity are in dolostones. However, for such
2+
a volume change to occur requires substitution of Mg into the CaCO 3 lattice along
2+ 2+ 2−
with removal of Ca . If Mg and CO 3 are supplied simultaneously to the dolomi-
tizing reaction, there is a volume increase of 75 – 88% (Morrow, 1990 ). Replacement
of limestone by dolomite does not automatically produce a 13% increase in total
reservoir porosity (Lucia, 2000 ). Why then do most dolomite reservoirs with a given
porosity have greater permeability than limestone reservoirs with the same porosity
(Wardlaw, 1979 )? Intercrystalline porosity decreases with initial increase in the
dolomite/calcite ratio in ancient carbonates up to about 50% replacement by dolo-
mite (Murray, 1960 ). But Murray found that as dolomite content increased beyond
50%, intercrystalline porosity increased proportionately, leading him to conclude
that high percentages of intercrystalline porosity are the result of dissolution of the
last remaining calcite host rock. His observations are purely empirical and have not
been duplicated in the laboratory. His interpretation that dolomite was concentrated
by subtle dissolution that removed the last remaining calcite from between the
replacement dolomite crystals is compelling. This process would produce high