Page 105 - Geology of Carbonate Reservoirs

86 STRATIGRAPHIC PRINCIPLES

surfaces. Correlative conformities or bedding surfaces in successions where the

entire record is present must also be identified by their ages as determined from
fossils or geochemical dates. Without knowing the geometry of the time surfaces in
successions of layered rocks, it is not possible to reconstruct the stratigraphic history

of basin filling, or to relate facies arrays to platform configurations. Without geo-
chronology, reservoir anatomy may not be mappable.
4.2.3 Time–Rock Units

Rock layers deposited during a specific amount of geological time are time – rock
units according to the North American Code of Stratigraphic Nomenclature.
Geological time is divided into eons (Cryptozoic and Phanerozoic), eras (Paleozoic,
Mesozoic, and Cenozoic), periods (Cambrian through Quaternary), epochs, and
ages. Rocks deposited during eons, eras, periods, epochs, and ages are designated as
eonothems, erathems, systems, series , and stages , respectively (Table 4.1 ). For example,
all rocks deposited during the Quaternary Period comprise the time – rock unit
known as the Quaternary System, including time – rock units of shorter span belong-
ing to the Pleistocene Series and the stages that correspond to the ages of the

Pleistocene Epoch. Time – rock boundaries are defined by relative or absolute age
dates, not by rock characteristics. It is important to remember that time surfaces are
not always parallel to rock unit boundaries.

4.3 CORRELATION

Stratigraphic correlation is the process of linking age - equivalent or lithologically
similar strata in two or more different places. These links are formed in several ways,
including (1) by establishing physical continuity, (2) by establishing identical age,
(3) by identifying beds that occur in the same order within a sequence, or (4) by
matching wireline log or geophysical characteristics and assuming that those signa-
tures are reliable substitutes for direct observations of lithology and chronology.
Correlations based on log and geophysical similarities are the least reliable and
generally represent the kind of work done in the subsurface when outcrops, bore-
hole samples, or cores are not available. Physical continuity of beds can be deter-
mined by “ walking - out ” or physically tracing the beds from one location to another.
The same geological age of beds in different locations can be established by fi nding
identical marker fossils or assemblages of fossils at each location or by determining

TABLE 4.1 Rock, Time, and Time – Rock Units
Lithostratigraphic (Rock) Geochronological Chronostratigraphic
Units (Time) Units (Time – Rock) Units
Group Eon Eonothem
Formation Era Erathem
Member Period System
Bed Epoch Series
Age Stage