Page 103 - Geology of Carbonate Reservoirs

84 STRATIGRAPHIC PRINCIPLES

themselves may not conform to time boundaries. A group is more than one forma-
tion, formations are subdivided into members, and members are divisible into lentils

and tongues, or beds. Formations and members are generally identifiable at reservoir

scale, sometimes so easily identifiable that they can be correlated for great distances.
Traditional subsurface correlations based on wireline log signatures or sample
“ tops ” are lithostratigraphic rather than chronostratigraphic. Of course, correlations
based only on rock units can lead to errors in identifying flow units. One of the main

reasons for the current popularity of sequence stratigraphy is that it focuses on
chronostratigraphic rather than lithostratigraphic correlations. Lithostratigraphy
groups rocks of common character but not necessarily of the same age, a potential

flaw that can cause one to overlook internal flow barriers or baffles that conform
to time surfaces. Geologists commonly try to establish correspondence between
wireline log characteristics and lithology in order to draw subsurface correlation
sections and to generate a variety of subsurface maps. This traditional method has
a built - in potential for error, however, because precise determination of individual
flow units within a reservoir body usually requires some time reference, especially

if the reservoir incorporates time - transgressive facies. At a larger scale, over hun-
dreds of kilometers, for example, correlation of individual rock units without time
reference is hardly possible because of the major differences in depositional
characteristics that exist across large areas over time.

4.2.2 Time Units
Relative geological time is determined by fossils. For example, the trilobite Olenellus
is useful worldwide as an indicator of Early Cambrian age. Such fossils, commonly
called marker fossils or index fossils, ideally have short ranges and widespread dis-
tribution in the rock record, enabling the geologist to use them to identify specifi c
time intervals within geological periods. In some situations, a single fossil species or
genus does not provide enough information to make a precise age determination,
but in association with other fossil species or genera in the rocks, the age of the
assemblage can be determined. Accurate stratigraphic correlation depends on geo-
chronology — establishing the age of the strata. Sometimes fossils may not be present,
but accurate correlations can still be made with isotopes, fission tracks, or other

geochemical methods. For example, a layer of volcanic ash represents an event — an
instant in geological time — that precisely separates in time the beds below and
above the ash layer. Unconformities represent gaps in the rock record produced by
erosion or nondeposition and they can be used as indicators to mark the age of fi rst
deposition above the unconformity. Beds below the unconformity may be widely
different in geological age because some parts of the unconformity surface may have
been eroded more deeply than others. Consider a regional unconformity that covers
hundreds of square kilometers, slopes seaward, and then becomes gradually sub-
merged by a relative rise in sea level. The advancing sea will submerge the uncon-

formity bit by bit, at first depositing a body of shallow - water sediments. Then, with
further sea - level rise, the shallow - water sediments at the first depositional site will

be covered with a younger layer of deeper - water deposits. As that deeper - water
deposit was being laid down, a coeval shallow - water facies was deposited landward
from the first one. The upslope migration of these progressively younger deposits

illustrates the concept of time - transgressive facies (Figure 4.4 ). That is, facies may be