Page 188 - Geology of Carbonate Reservoirs

POROSITY REDUCTION BY DIAGENESIS 169

As burial proceeds and interstitial water composition changes, cement mineral-
ogy and crystal form change accordingly. Water composition changes during burial
as upward - migrating expulsion waters mingle with in situ interstitial water, which
may have already been through a variety of rock – water reactions. Calcite, dolomite,
and other mineral cements may form, depending on water composition and chemi-
cal equilibrium for each mineral species. If burial calcite forms, it commonly takes

the form of large, clear crystals that fill remaining pores and may encompass pore
spaces around several constituent grains. These multipore, large crystals are called
poikilotopic crystals , a name borrowed from igneous petrology. Because Mg is
depleted in subsurface waters as compared to the marine phreatic environment,
burial calcites are low - Mg calcites, like freshwater varieties. Burial calcites may
include relatively higher iron content than marine or freshwater calcites, depending
on availability of iron and the oxidation – reduction state of the cementing environ-
ment. Higher iron content in the calcite lattice is characteristic of reducing burial
environments and is easily recognized by applying a potassium ferricyanide stain to
thin sections. Ferroan calcites (high iron content) are stained with a blue tint in this
solution. In summary, cementation can occur in marine, fresh, or subsurface (burial)
waters. Different episodes of cementation can be recognized with CL and ordinary
petrography enabling the petrographer to establish a cement stratigraphy for a
given rock specimen. This method has been refined and automated with computer -

assisted image capturing techniques to enable petrographers to reconstruct the
cementation history of carbonate reservoir flow units and barriers (Mowers and

Budd, 1996 ; Witkowski et al., 2000 ).
Noncarbonate cements are less abundant in carbonate reservoirs, but when they
are present they can greatly reduce effective porosity. One of the most common
cements found in carbonate reservoirs is anhydrite. It is usually associated with
evaporite deposits that commonly cap shallowing - upward sequences, but it can be
introduced as late - stage burial cements as well. Dolomite, including saddle dolomite,
may occur as cement as well as a replacement mineral. Saddle dolomite is common
as a vug and fracture lining cement in many reservoirs. Chalcedony, a variety of
silica cement, is moderately common as pore - reducing cement, especially in associa-
tion with late - stage burial cements such as saddle dolomite, sphalerite, and fl uorite.
These minerals are commonly associated with residual heavy hydrocarbons, sug-
gesting that the exotic cementation took place in association with hydrocarbon
migration to form the MVT mineral association discussed previously in this
chapter.

Porosity reduction by cementation can usually be identified by cross - cutting
relationships in cement stratigraphy. Isopachous cements that rim the framework
grains may be the first cementation event that occurred in the marine phreatic zone,

for example. Coarser, centripetal cements that further reduce porosity may follow
these early cements. Finally, remaining effective porosity might be plugged with
poikilotopic cements formed in the deeper - burial environment. All of the cements

might be cross - cut by mineral - filled fractures or by exotic deep - burial cements.
Exotic mineral cements and mineralized fractures are good indications that new
permeability avenues were created, allowing upward migration of mineralizing
fluids. Such exotic fluids are commonly associated with migrating hydrocarbons and

the new permeability avenues are usually fractures that postdate other diagenetic
events.