Page 119 - Geology of Carbonate Reservoirs

100 STRATIGRAPHIC PRINCIPLES

HFS 5
HFS 4
HFS 3
1 1 1
1
HFS 2
2 2
2
2
3
HFS 1

HFS 5
HFS 4
HFS 3

Lithostrat Layer 1
HFS 2
Lithostrat Layer 2

30 M 1 KM HFS 1 L‘ Layer 3

Figure 4.9 A sketch showing the consequences of erroneously correlating by rock type only

(lithostratigraphic correlation), neglecting or not recognizing chronostratigraphic correla-
tions. Four wells are shown penetrating high - frequency sequences (HFS) of grainstones (dot
pattern) and mudstones (darker tones without the dot pattern). The chronostratigraphic

(sequence - stratigraphic) correlations shown on the top part of the figure correctly identify
the HFS chronostratigraphic units. Lithostratigraphic correlations identify only the grain-
stones and because they have similar lithology in each of the four wells, they can be errone-
ously correlated as continuous strata in a “ layer - cake ” form. Grainstones identifi ed with the
number 1 would be correlated as a single layer - cake unit, as would those marked by numbers
2, 3, and 4. The resulting layer - cake lithostratigraphic correlation is shown on the lower half
of the figure. The layer - cake correlations will lead to confusion when it is discovered that

the grainstones are not in flow communication. (Adapted from an illustration in Tinker

(1996) .)

cake correlation would lead to the erroneous expectation that each of the grain-
stones would be in fl ow communication across HFS boundaries. The consequences
of such errors in correlation have obvious and significant impact on determination

of well spacing, volumetric calculations, fi eld development, and enhanced recovery
operations.
Posamentier and James (1993) point out that sequence stratigraphy is a tool for
the study of geological data rather than an end in itself. They note that a variety of
stratigraphic methods exist. Some of the methods include allostratigraphy (subdi-
vides the stratigraphic succession on disconformities, or breaks in the rock record
without regard to their origin or time value), genetic stratigraphy (subdivides stratal