Page 183 - Geology of Carbonate Reservoirs

164 DIAGENETIC CARBONATE RESERVOIRS

enhancing it. Recrystallization is commonly associated with neomorphic microspar,
or a tightly interlocked crystal mosaic that results from the dissolution and repre-
cipitation of metastable carbonates such as aragonite or Mg - calcite with the end
result being porosity reduction instead of enhancement. One example of recrystal-
lization that does enhance porosity is the microcrystalline microporosity inter-
preted by Ahr (1989) to be the result of neomorphic stabilization of aragonite or
Mg - calcite.
Diagenetically enhanced porosity may correspond to the outline of a paleoaqui-

fer, some topographic feature that influenced paleohydrology, or to fractures and
joints. This may mean that the enhanced porosity conforms to the outline of a
present or paleostructural high, to the slope of a paleoaquifer, to the shape of an
evaporite lagoon, or to the boundaries of some other kind of system in which chemi-
cally reactive waters came in contact with carbonate rocks — good examples of which
are the extensive coastal - zone caves of Mexico.
The job is to trace the “ diagenetic pathway, ” pinpointing the processes that
created reservoir porosity and determining the relative timing of diagenetic events
during burial history. This is not always simple because several events that occurred

at different times during burial may have shaped the final pore system much like

multiple exposures influence the final image on a photograph. Fortunately, some
simplifying generalizations can help solve the problem. Virtually all diagenetic pro-

cesses require reactive fluids to pass through connected pores in the host rock,

reacting with the rock as the fluids migrate. Large - scale removal of rock as in the
formation of caves, caverns, towers, and other karst features requires enormous
amounts of undersaturated water. Such great volumes of water are virtually impos-
sible to obtain except at the earth ’ s surface or in shallow aquifers. It follows that
large - scale dissolution diagenesis is a priori an early event in burial history. The same
is generally true for extensive networks of vugs, molds, and channels. Dolomitization
reactions are commonly, but not exclusively, associated with evaporites and shallow-
ing - upward cycles in carbonate strata. Pore enhancement by burial diagenesis must
follow some pattern of preserved porosity and permeability, usually a depositional
fabric or metastable original mineralogical composition inherited from the original
rock. The conduit through which burial fluids migrated may not match exactly the

size and shape of the original depositional facies; rather, burial alteration may cor-
respond to paleotopography or present structure where ancient or present struc-
tural contours intersect facies surfaces, creating an interesting coincidence of
three - dimensional geometry and geology.

6.5 POROSITY REDUCTION BY DIAGENESIS

Diagenetic processes that reduce porosity include some of the same ones that
enhance porosity — recrystallization and replacement — but cementation and com-
paction stand alone as mechanisms that only reduce porosity. Cementation happens

when crystalline precipitates fill pore spaces. This can happen just after deposition
and at any time during burial. Because cementation can occur immediately after
deposition, it is tempting to infer that most porosity reduction is accomplished
that way, but cemented rocks may be compacted to cause even further porosity
reduction.