Page 173 - Geology of Carbonate Reservoirs

154 DIAGENETIC CARBONATE RESERVOIRS

Vadose Zone
Water Table
Vadose Caves
Marine
Phreatic
Meteoric Phreatic Zone
Phreatic Caves

Mixing Zone

Subsurface Zone

Figure 6.4 A sketch showing the principal diagenetic environments from the surface vadose
environment through freshwater and marine phreatic, to the mixing zone, to the shallow -
and deep - burial environments. (Adapted from an illustration in McIlreath and Morrow
(1990) .)
expulsion during compaction, among other possible sources that can create hybrid
water composition.

Diagenetic environments are classified mainly on the basis of water chemistry
and location with respect to the earth ’ s surface. Fresh water, brackish water, seawa-
ter, and expulsion brines from the subsurface are the main kinds of diagenetic fl uids
that come in contact with carbonate rocks during their diagenetic histories. True
connate water (original water of deposition trapped in buried rock) probably does
not exist because extensive studies on subsurface brine compositions show that
those brines have dramatically different compositions than do surface waters in
marine and terrestrial settings. Instead, subsurface waters are mixtures of trapped
surface waters, expulsion fluids from mechanical compaction during burial, and

diagenetic fluids from rock – water interactions during burial. Shallow - and deep -

burial environments are distinguished not by prescribed depth zones but by pres-
ence or absence of clues to elevated temperature and pressure along with evidence
of exotic water chemistry typical of basinal brines. Other clues include diagenetic
fabric, mineralogy and cement characteristics, and geochemical indicators of envi-
ronmental parameters. Some fabric characteristics that indicate pressure include
penetrative grain contacts ( “ overcompaction ” ) and stylolites. Mineralogical indica-
tions of deeper burial include the presence of high - temperature minerals such as
saddle dolomite, high levels of kerogen metamorphism, and the amount, crystallo-

graphic character, and isotopic composition of pore - filling cements. Carbon and

oxygen isotopic compositions in combination with fluid inclusion geochemistry are
commonly used to identify the type or source of carbon and the temperature of
crystallization, respectively. There are many more clues and techniques that can be
used to identify the kinds of diagenetic alteration and the environments in which