Page 169 - Geology of Carbonate Reservoirs

150 DIAGENETIC CARBONATE RESERVOIRS

Neomorphism (Recrystrallization)

Porphyroid

Coalescive

Micrite Microspar
Figure 6.3 Diagram illustrating neomorphism — the diagenetic change from micrometer -
sized, crystallites in lime mud to a mosaic of subsequant calcite crystals called neomorphic
microspar by Folk (1965) . Aggradational neomorphism is systematic and uniform enlarge-
ment of crystallites, porphyritic neomorphism is enlargement in patches. (Adapted from an
illustration in Folk (1965) .)

incorrectly called “ chalkification. ” In this case, rather than true chalk, a microrhom-

bic microcrystalline calcitic fabric is formed by diagenetic alteration of aragonite or
Mg - calcite ooids (Ahr, 1989 ; Dravis, 1989 ; Moshier, 1989 ).
Replacement is the process of complete change from one mineral to another.
Dolomite commonly replaces calcite and aragonite, although replacement by miner-
als such as anhydrite and SiO 2 are almost as common and both calcite and silica
commonly replace evaporite precursors (Ulmer - Scholle and Scholle, 1994 ).

6.2 DIAGENETIC POROSITY

Diagenetic porosity can be formed by dissolution — the enlargement of existing
pores (e.g., solution - enlarged intergranular pores) or the creation of new ones (e.g.,
karst features). It can be formed by replacement with successor minerals that ulti-
mately occur in some fashion that is associated with greater porosity than the parent
rock (dolomitization), and it can be formed by dissolution or stabilization followed
by reprecipitation that yields a porous, neomorphic product (e.g., microporosity).
When porosity is created or enhanced by these processes, the previous texture,
fabric, and sometimes the mineralogy of the host rock may be radically changed.
Purely diagenetic porosity is not as common as diagenetically altered depositional
porosity. However, some purely diagenetic alterations exist that may not correspond
with depositional characteristics. Intercrystalline porosity in dolomite (replacement)
may be unrelated to depositional facies boundaries or fabrics, as we learned in car-
bonate porosity classifications. Vuggy or cavernous porosity and porosity associated