Page 174 - Geology of Carbonate Reservoirs

DIAGENETIC ENVIRONMENTS AND FACIES 155

they formed. Some of them will be mentioned in this chapter, but no attempt is
made to present a comprehensive review of carbonate diagenesis.

6.3.1 Diagenetic Facies

Depositional facies were defined earlier as laterally continuous, mappable horizons
with well - defined associations of fundamental rock properties and, in some cases,

fossils. Depositional facies are usually described as sedimentary blankets of rela-
tively local extent and limited thickness. Diagenetic facies are also laterally continu-
ous zones that display mappable patterns of diagenetic alteration. For example,
rocks exposed to freshwater dissolution at an unconformity or at an ancient water
table could be grouped into diagenetic facies on the basis of the amount and kind
of diagenetic changes they exhibit. These diagenetic patterns can be treated as
“ facies ” to make the task of mapping somewhat similar to the mapping of deposi-
tional facies. In this case, the diagenetic characteristics are mapped instead of depo-
sitional rock properties. Mapping diagenetic facies requires information about the
vertical and lateral extent of the stratigraphic horizon that is associated with the
diagenesis. Care must be taken to ensure that the diagenetic facies include only
those rocks that were associated with that specifi c stratigraphic horizon during one
diagenetic episode because it is possible that more than one episode may have
occurred at different times during burial. Stratigraphic markers are usually neces-
sary to establish datums for mapping diagenetic facies. Such markers include
hardgrounds, unconformities, paleosols, or other features typically associated with
subareal exposure or ancient water tables.
Diagenetic facies are different from depositional facies and the methods used to
predict their spatial distribution are correspondingly different. Diagenetic altera-
tions may or may not have followed depositional facies boundaries and they may
not be blanket - like in form. Sinkholes in karst topography are good examples of
one diagenetic terrane that cuts across both lateral and vertical depositional bound-
aries. Instead of following traditional depositional boundaries, diagenetic facies may
reflect the outlines of ancient water tables, or zones where evaporative concentra-

tion formed enriched brines that percolated through the shallow subsurface and
selectively replaced metastable carbonates. Diagenetic facies may have followed
ancient topography, where highs were exposed to weathering and erosion while
nearby submerged low areas remained unaffected. They may mark subsurface
regions where rock – water interactions were part of circulation systems driven by
temperature or concentration gradients as in Kohout circulation, a system of plat-
form - wide circulation cells associated with regional diagenetic alteration (Kohout,
1967 ). Zones of extensive compaction and pressure solution may occur in halos
around tectonic features where stresses were concentrated. Away from stress con-
centrations, beds of the same lithology and the same age will not show the same
degree of mechanical alteration. Because nonselective diagenesis cuts across depo-
sitional boundaries, it is important to identify the distinguishing characteristics of
each type of alteration.
Diagenetic facies maps can be constructed using these distinguishing character-

istics even though it may be difficult at first to identify the size and shape of the
altered zones in the subsurface. Of course, reservoir rocks have almost always been
affected by more than one episode of diagenesis during their burial history; therefore