Page 150 - The Petroleum System From Source to Trap

144 Jordan and Wilson

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LIME M U D CARBONATE CEMENT
CARBONATE G R AINS FRAME B U ILDERS

Figure 7.2. The Dunham (1962) classification of carbonate rocks, with two minor modifications: (1) mud is defined at the sedi­
mentologic silt-sand boundary of 62.5 J.lm and (2) up to 5% lime mud is allowed in the category of grainstones.

various amounts of lime mud matrix with various sizes 7.3, which shows the most common symbols used to
of bioclasts and peloids (with local concentrations of describe carbonate reservoirs. (A complete listing of
intraclasts or ooids). The size and shape of particles in nearly 500 lithofacies symbols used in describing silici­
this type of sediment are dependent on the breakdown clastics, carbonates, and other rocks is available from the
fabric of constituent parts of calcified plants and animals author upon request.)
and the degree of burrowing, ingesting, and reworking Carbonate lithofacies fall into several distinct litho­
of the sediment. Moderate water movement and logic associations, ranging from various types of lime
mechanical processes of sediment winnowing, so signifi­ mudstones to wackestones, packstones, grainstones,
cant to siliciclastic deposition, are much less important in boundstones, and dolomites. These six main textural
carbonates. families are grouped in Figure 7.4 according to the most
Two minor modifications of Dunham's (1962) classifi­ common types of constituent grains. This grouping
cation scheme are recommended. First, the size of what is shows that there is a limited number of combinations of
considered lime mud is increased from Dunham's limit carbonate rock textures and compositions to be dealt
of 35 )lm to 62.5 )lm, thus standardizing the use of the with in nature. By using the generalized bioclastic
term mud in the sedimentologic sense. Second, the symbol, ),, which represents an assortment of normal
amount of lime mud allowable in a grainstone is marine fossils (usually fragmented) from any given
increased from % 1 to 5%, thus widening the scope of the geologic time period, the number of common carbonate
term grainstone and incorporating many porous mud­ lithofacies observed reduces to about 26 (Figure 7.4) and
lean packstones (Figure 7.2). provides bounda.ry conditions for the variability of
By using symbols to represent words (or word carbonate lithofacies throughout geologic time. Wilson
strings) for lithic descriptors and sedimentary structures, (1975) and Flugel (1982) used a typical shelf to basin
grain composition, remarks, and/ or porosity and by profile as a framework for compiling a list of standard
adding these to the textural terms of Dunham (Jordan, microfacies of commonly recurring carbonate rock types
1985), the compilation and comparison of large amounts through geologic time (Table 7.2). Based on this, it is
of carbonate lithofacies information becomes more evident that the natural variation in carbonate rock
manageable. This "lithofacies shorthand" is a rigorous textures and compositions occurring on shelves and
semi-quantitative approach that normalizes facies platforms through time is limited and that, to some
descriptions for comparison and mapping purposes. extent, carbonate reservoir facies are predictable in their
This system is based on the lithofacies symbols in Figure distribution.