Page 129 - Geology of Carbonate Reservoirs

110 DEPOSITIONAL CARBONATE RESERVOIRS

distinctive, fundamental rock properties. Oceanographic research has shown that
carbonate platforms are either ramps or shelves and that each platform type has a

characteristic array of lithofacies defined on fundamental rock properties that char-
acterize each depositional environment from the shoreline to the depths of the
basin. The distinguishing environmental processes for each environmental zone are
assumed to vary only within narrow limits, although the boundaries between zones
are gradational at best. Some examples of the environmental zones and accompany-
ing lithofacies are described in oceanographic studies of modern carbonate plat-

forms, where workers have identified both the prevailing environmental conditions
and their accompanying sedimentary facies; they include the South Florida shelf
(Ginsburg, 1956 ), the Bahama Banks around Andros Island (Purdy, 1963a,b ), the
Yucat á n Platform (Logan et al., 1969 ), Shark Bay, Western Australia (Logan et al.,
1970 ), and the Balearic Platform (Fornos and Ahr, 1997, 2006 ).
Ancient depositional environments are reconstructed by studying the texture,
constituent composition, sedimentary structures, and fossil assemblages that occur in
each subenvironment on ancient carbonate platforms and comparing the interpreted
successions with modern examples. Rock properties produced by sedimentary pro-
cesses in each depositional setting over time comprise unique depositional succes-
sions that can then be used to identify ancient depositional environments. Depositional
successions are 3D bodies of rock illustrated as idealized borehole cores or outcrops
that represent vertical stacks of 2D depositional microfacies. Think of these microfa-

cies as time slices of rocks with specific textures, grain types, sedimentary structures,
and fossils. The slices are stacked one on top of another to make up the 3D succession
that accumulated over time in each specifi c subenvironment until relative sea level
shifted and the subenvironment migrated updip or downdip, became emergent, or
was drowned. The 3D successions and their characteristic rock properties determine
the amount and type of depositional reservoir porosity, the size and shape of the res-
ervoir body, and, ultimately, the economic value of the reservoir.
Carbonate ramps and shelves can be divided into seven subenvironments or
sectors: (1) the attached or detached beach (or barrier island) environment with or
without dunes; (2) the tidal - flat environment and its associated lagoons or adjacent

subtidal waters; (3) the lagoonal environment behind detached barrier islands
(which includes beaches and dunes); (4) the shallow subtidal or neritic environment;
(5) the slope - break environment; (6) the slope, or toe of slope, environment; and
(7) the basinal environment. The location of each environment with respect to the
others — their juxtaposition on the platform — is predictable for ramps and shelves;
therefore when the location of any one environment is known and the platform type
is known, the locations of the corresponding laterally equivalent successions are
predictable. The environmental characteristics of each of these zones are discussed
in the following paragraphs. Those discussions are followed — zone - by - zone — with
descriptions of the fundamental rock properties that correspond to each environ-
ment. Finally, each ideal depositional succession on ramps and shelves is illustrated
to resemble what one would see in borehole cores or outcrops.

5.2.1 The Beach –Dune Environment

Beaches mark the boundary between the mainland and the sea or they may be
separated from the mainland by lagoons. Those that extend from the mainland