Page 96 - Geology of Carbonate Reservoirs

CARBONATE DEPOSITIONAL PLATFORMS 77

imposed on the depositional template. The timing during burial and the mechanisms

by which diagenesis has modified the depositional template will be discussed in
Chapter 6 . Knowing where to look and how to recognize clues that can lead to
fi nding porous and permeable rocks is the key to successful exploration and devel-
opment. It is beyond the scope of this book to provide detailed discussions of basin
origin and evolution. Fortunately, an extensive literature already exists on those
subjects. The focus of this chapter is on the fundamentals of stratigraphy, particularly
those that relate to carbonate reservoir rocks.
Stratigraphy is the study of layered rocks. It is commonly referred to as the
mother science of geology. The study of reservoir geology depends entirely on the
application of stratigraphic principles to interpret the origin of a reservoir, predict
its spatial dimensions, and map individual fl ow units. We learned in Chapter 1 that
carbonates are intrabasinal in origin and consist mainly of biogenic or chemical
constituents. The anatomy — the size and shape — of all depositional and many hybrid
carbonate reservoirs conforms directly or indirectly to the geometry of depositional
bodies. Those bodies accumulated or grew (in the case of reefs) on a variety of

seafloor bathymetric confi gurations.
All depositional surfaces, regardless of their physiography, or bathymetry, are
called platforms , and each platform type has its own distinguishing stratigraphic
characteristics. Reservoir and nonreservoir strata accumulate in predictable pat-
terns and locations on platforms depending on how platform physiography inter-
acted with environmental dynamics — generally meaning wind or water movement.
Platform sediment accumulations are organized in depositional facies that occur in
predictable, lateral arrays from shoreline to basin fl oor. These arrays may be cycli-
cally repeated — vertically stacked — over time. Depositional facies are also called
lithogenetic units because each represents sediment deposited under one set of
environmental conditions. Facies boundaries may not correspond with time bound-
aries, however. Facies boundaries may coincide with boundaries of reservoir poros-
ity and permeability if the origin of the porosity is depositional or facies - selective
diagenetic. Some kinds of fractures may also correspond in part or in total to facies
boundaries because brittle behavior is commonly related to stratal thickness, grain
size, and stratiform mineral occurrences. To establish a sense of scale and strati-
graphic perspective, we begin with the types of carbonate depositional platforms
and their characteristic facies arrays.

4.1 CARBONATE DEPOSITIONAL PLATFORMS

Few classifications for carbonate platforms existed before the 1970s. Some of the

early schemes include those of Wilson (1970, 1974) , Ahr (1973) , and Ginsburg and
James (1974) . But it was not until an integrated scheme to classify a variety of plat-
form types was presented by Read (1982, 1985) that the stage was set for the devel-
opment of present - day terminology. In current usage, carbonate platform is the
informal term for all depositional surfaces upon which shallow - water carbonate
facies are deposited. Depositional surfaces include submerged continental margins,
submerged perimeters around islands, or the shallow margins of intracratonic seas.
Platform therefore includes any depositional surface that cannot immediately be
assigned to a more specific category (Tucker and Wright, 1990 ; Burchette and