Page 123 - Geology of Carbonate Reservoirs

104 STRATIGRAPHIC PRINCIPLES

important steps in exploration for potential reservoir rocks involve (1) determining
platform geometry, whether ramp, shelf, or other; (2) identifying the systems tracts
in which potential reservoir facies should exist; (3) tracing the spatial arrangement
(locations) of those systems tracts through cycles of change in relative sea level; and
(4) determining where in the systems tracts the greatest volume of potential reser-

voir rock exists. Platform architecture can be identified by seismology, surface
geology, or subsurface geology. Until it is determined to be otherwise, exploration
wells are drilled on prospects in which porosity is assumed to be depositional; there-
fore beaches, dunes, and shelf - edge sand bodies may be primary targets. Lowstand,

transgressive, and highstand systems tracts can be identified using sequence -
stratigraphic analyses, and detailed study of the systems tracts can determine if, for
example, the greatest depositional reservoir potential exists in the transgressive,
highstand, or lowstand systems tracts. The sequence stratigrapher will determine if
deposits in lowstand systems tracts accumulated in relatively low - energy settings
causing them to have high mud content; or if the potential reservoir facies in the
high - energy sectors of the transgressive systems tract are less voluminous than those
in the highstand systems tract owing to the short time of sedimentation in any given
space during the transgression. Armed with these analyses and interpretations, the
geological concept for exploration can be completed.
Chronostratigraphic correlations of individual HFS in field development studies,

for example, will identify the location and spatial arrangement of discrete deposi-
tional bodies as previously illustrated in Figure 4.9 . Subsurface seismic and geologi-
cal structure maps can be compared with the spatial distribution of the individual
reservoir bodies to determine the volume of reservoir facies that is enclosed in a
structural trap, for example. If the trap is not structural, other kinds of studies are
done, but for purposes of this example, only structural traps are considered. When
trap structure and reservoir architecture are examined simultaneously, optimized

field development can proceed. As more information is acquired, refi nements can

be made to identify and rank individual flow units within the field.

SUGGESTIONS FOR FURTHER READING

Additional information, illustrations, and discussions on generalized sequence -
stratigraphic models for the different kinds of carbonate platforms can be found in
Sequence Stratigraphy by Emery and Meyers (1996) and Carbonate Sequence

Stratigraphy by Loucks and Sarg (1993) . More general discussions of depositional
environments, facies, and stratigraphy can be found in Sedimentary Environments:
Processes, Facies, and Stratigraphy, 3rd edition, by Reading (1996) and Principles of
Sedimentology and Stratigraphy , 3rd edition, by Boggs (2001) . Carbonate platforms
evolve with time. Some of these changes are discussed in Schlager ’ s, 1981 paper on
the paradox of drowned reefs and carbonate platforms in Schlager and Ginsburg ’ s
1981 discussion of the Bahama carbonate platforms, and in Manfrino and Ginsburg ’ s
(2001) description of the Plio - Pleistocene depositional history of the upper Bahama
Bank. Precisely documented links between sea - level change, carbonate sequence
stratigraphy, and patterns of diagenesis are described by Pomar (1993) and in Pomar
and Ward ’ s (1999) review of reservoir - scale heterogeneity in Miocene depositional
and diagenetic facies on the Island of Mallorca.