Page 182 - Geology of Carbonate Reservoirs

DIAGENETICALLY ENHANCED POROSITY 163

dominates, and permeability is provided largely by interconnected solution chan-

nels. ” In both the paleocave reservoirs described by Loucks (1999) and the systems
described by Purdy and Waltham (1999) , it is matrix porosity that ultimately deter-
mines the volume of producible hydrocarbons. Knowing the array of pore charac-
teristics that may appear with increasing depth of burial in collapsed caves, it is
probably equally important to distinguish between caves in uplifted karst terranes,
those developed during passive sea - level lowering, and those developed as coastal,
anchialine (mixing - zone) caves.

6.4.4 Porosity Enhancement by Replacement
The most common replacement mineral in carbonate reservoirs is dolomite. Replace-
ment of limestone by dolomite is discussed at the beginning of this chapter. In
general, replacement of limestone by dolomite can actually reduce porosity rather

than enhance it (Lucia, 2000 ) . Permeability is increased only in cases where lime-
stone has been replaced totally by dolomite and intercrystalline porosity is pre-
served without “ excess ” dolomite cement. Porosity enhancement in dolostone
replacements after limestones may have been accomplished largely by dissolution
of limestone simultaneously with dolomite replacement leaving a residual, porous
network of dolomite crystals. Other common replacement minerals include anhy-

drite, silica as chert and flint, and sulfide minerals such as MVT ore minerals. MVT
ore emplacement is not generally associated with porosity enhancement or with live
hydrocarbons, although MVT ores are commonly associated with bitumen.
6.4.5 Recognizing Enhanced Porosity

Enhanced porosity is easy to recognize. It is not as easy to determine the cause or
causes of diagenetic enhancement, how many times the pores were altered, and to
what geographic extent the alterations modified reservoir zones. Porosity enhance-

ment by dissolution is arguably the easiest to identify and sort out. Caverns, vugs,
enlarged inter - and intragranular pores, and molds obviously represent enlargement
by dissolution. Other types of diagenetically enhanced porosity may not be as easy
to recognize but they still represent pore enlargement, improved storage capacity,
and perhaps improved fl ow capacity. Recognition of reservoirs with purely deposi-
tional porosity is, as we have seen previously, a matter of identifying pore charac-
teristics that correspond closely with depositional successions and facies. Purely
diagenetic porosity and extensively altered hybrids (diagenetic attributes dominant)
have few characteristics in common with depositional facies. Methods used to deter-
mine reservoir boundaries in those cases depend on the kinds of diagenesis that
have influenced porosity, in which diagenetic environment the changes occurred,

and the relative timing of the alterations. Dissolution is, as previously discussed,
commonly associated with exposure surfaces, unconformities, soil zones, and karst
surfaces.
Replacement by dolomite is commonly associated with lagoonal or tidal - fl at
evaporites such that stratigraphic models of evaporite facies may predict the occur-
rence of seepage - reflux dolomite replacements. Replacement by evaporites, sili-

cates, sulfides, or other minerals usually reduces depositional porosity instead of