Page 189 - Geology of Carbonate Reservoirs

170 DIAGENETIC CARBONATE RESERVOIRS

6.5.5 Recognizing Diagenetically Reduced Porosity

Porosity reduction by cementation, compaction, replacement, or recrystallization is
usually easy to recognize but it may not be as easy to determine how many episodes

of diagenesis have occurred to reach the final levels of porosity and permeability in
the reservoir rocks. Multiple episodes of diagenesis involving several types of altera-
tion over a long time, especially when early episodes are cross - cut by later ones,
make it challenging to interpret the origin and spatial distribution of the fi nal
poroperm characteristics in altered reservoirs. Diagenetically reduced porosity is
analyzed using most of the same methods that are employed to analyze diageneti-

cally enhanced porosity. That is, the first item on a checklist is to determine if poros-
ity is purely diagenetic or a hybrid of diagenetic and depositional or fracture porosity.

Hybrids of diagenetic and depositional pore types can be identified by fi nding cor-
respondence between types of diagenesis (cementation, compaction, replacement,
or recrystallization) and depositional facies. It should be relatively easy to recognize
porosity loss due to diagenetic changes. It is not always easy to determine which of
the diagenetic events caused the greatest reduction in porosity and when that event,
or events, happened during burial. In hybrid pore types where depositional attri-
butes dominate, petrophysical characteristics refl ect more of the depositional char-
acter than of the diagenetic alteration; therefore it is not as critical to trace zones
that have been cemented or compacted, or where recrystallization or replacement
have reduced depositional porosity. On the other hand, in hybrid porosity domi-
nated by diagenetic attributes, it is critically important to identify the type of dia-
genesis, the diagenetic environment in which it occurred, and the time sequence of

diagenetic events as each event modified the original depositional pore geometry.
When diagenetic attributes dominate, porosity does not follow facies boundaries.
Instead, it may follow certain phases in stratigraphic cycles, it may follow the shape
of ancient water tables, it may follow unconformities, or it may follow paleotopog-
raphy. Ruppel (1992) described a compartmentalized reservoir of Leonardian
(Permian) age in West Texas, where stratigraphic cyclicity (mainly shallowing -
upward cycles) and paleotopography played dominant roles in shaping the fi nal
pore geometry. Grainy facies tended to be associated with upper parts of shallow-
ing - upward cycles and with paleotopographic highs. The highs were also associated
with dissolution diagenesis that enhanced intergranular pores in grain - supported
rocks. Low porosity tended to be associated with tidal - flat facies with poorly con-

nected fenestral porosity, pore - filling anhydrite, and fine siliciclastics. Pore - reducing

diagenesis may correspond to paleotopographic or paleohydrological boundaries.
Kopaska - Merkel and Mann (1993) described stratigraphic cyclicity in Jurassic
Smackover facies from Alabama, where shallowing - upward cycles ended with low -
porosity horizons including paleosols, evaporites, and sebkha deposits. It is relatively
common in Permian carbonate reservoirs of the southwestern United States to fi nd
pores plugged by anhydrite that capped shallowing - upward stratigraphic cycles.
Calcium sulfate from the tops of cycles was dissolved, percolated downward as a

dense brine, and reprecipitated as pore - filling cement in rocks immediately below

cycle capping grainstones (Amare, 1996 ). Pore - filling carbonate cements may occur
in certain stratigraphic horizons below unconformities or simply in progressively
deeper positions along a paleoaquifer. In either case, cementation below a zone of
exposure and dissolution commonly coincides with the outline of a paleotopo-