Page 114 - Geology of Carbonate Reservoirs

ANATOMY OF DEPOSITIONAL UNITS 95

Ideal depositional successions for siliciclastic sandstone environments were
described and standardized by LeBlanc (1972) . Before publishing his ideal deposi-
tional successions, he used them for many years as illustrations in his classes at Shell
Development Company. He illustrated and described successions for eolian, fl uvial,
deltaic, coastal interdeltaic, and marine environments in such a way that practicing
reservoir geologists could use them as aids to identify and map reservoir sandstones
in three dimensions with a minimum of subsurface information. To an extent, the
same method of grouping common lithological attributes in vertical successions and
assigning them to depositional environments can be done with carbonates. The dif-
ferences between carbonates and siliciclastics that were discussed at the beginning
of this book must be kept in mind.
A general review of carbonate depositional facies can be found in the book Facies
Models , a compendium of papers edited by Walker and James (1992) . They discuss
facies that represent a variety of depositional environments, and in most examples,
vertical successions are also described. Those authors do not emphasize the distinc-
tion between two - dimensional facies and three - dimensional successions and the
model successions are not always linked to depositional settings on more than one
kind of platform. Subsurface geology in exploration and production makes exten-
sive use of geological concepts to help predict occurrences of reservoir rocks with
limited information. The concept of standard depositional successions is particularly
helpful in this respect because (1) standard depositional successions are depicted in
the way they would appear in cores or detailed lithological logs from cuttings, and
(2) standard successions are associated with specific environmental sectors on ramps

and shelves. A greatly simplified but somewhat incomplete representation of car-

bonate depositional successions is presented in Ahr (1985) .
Just as standard microfacies depict lithological characteristics in the nine envi-
ronmental cells across Wilson ’ s (1975) idealized platform, the greatly simplifi ed
depositional successions in Ahr (1985) portray vertical assemblages of rock proper-
ties formed over time at specific cells on any carbonate platform, although that

paper did not include discussions of slope environments, the variety of basinal
deposits that are commonly found, or the distinction between beaches attached to
the mainland and detached barrier islands with lagoons and tidal flats behind them.

Standard depositional successions — think of them as depositional models —
presented here represent the typical variety of fundamental rock properties that
can develop in each environmental sector on platforms over time, but the succes-
sions do not have precise chronostratigraphic values. Because of the variations in
the kinds of rock properties that occur in each succession, consideration has to be
given to the amount of unexpected variability that can exist in the successions. Some
license has to be taken to generalize these successions in order to make them more

applicable to platform configurations of essentially any geological age and any place
on the globe. In order to formulate such generalized models, ramps and shelves must
be divided into environmental subdivisions, or sectors, beginning at the shoreline
and ending in the deepest part of the basin. In this book, all platform types — ramps,
shelves, and isolated platforms — can be divided into seven generalized environmen-
tal zones or sectors, each with standardized depositional successions, or models, that
represent typical stratigraphic sequences that might be found in borehole cores.
Sketches of these idealized successions and the various platform types appear at the
end of Chapter 5 .