Page 118 - Geology of Carbonate Reservoirs

SEQUENCE STRATIGRAPHY 99

4.5 SEQUENCE STRATIGRAPHY

Sequence stratigraphy puts depositional rock bodies in a chronostratigraphic frame-
work. It is a way to identify sedimentary bodies by their position in the stacks

of bedded rocks deposited in fluctuating environments between one major
stratigraphic boundary, such as a major erosional unconformity (Type 1 Sequence
Boundary), and the next.

4.5.1 Definitions and Scales of Observation
We have described standard microfacies as the sediments that blanket the surfaces
of environmental sectors across platforms. Ideal successions consist of vertically
stacked arrays of microfacies that characterize each environmental zone on ramps
and shelves. Both standard microfacies and depositional successions represent
deposits in limited geographical areas, or sectors, on platforms. These model succes-
sions are sketched to represent idealized versions of cored intervals or outcrop
sections. Interpreting the depositional history of large areas requires correlation of
many such depositional successions over large distances. It also requires knowledge
of the relationship between rock units and time units. In short, there is a hierarchy
of scale in time and space in stratigraphy. There are methods to describe this hier-

archy, to define boundaries between the units within it, and to define the genetic

significance and geological history of the entire package. Currently, the method of
choice is sequence stratigraphy.
Modern sequence stratigraphy evolved largely from petroleum industry research
on the geometry of depositional bodies interpreted from seismic records, or seismic
stratigraphy . The historical roots and fundamental concepts of seismic and sequence
stratigraphy can be found in Vail et al. (1977a,b) and Brown and Fisher (1977) .

Sequence stratigraphy can be defined as “ the subdivision of basin fills into genetic

packages bounded by unconformities and their correlative conformities ” (Emery and
Meyers, 1996 ) . In ordinary language, that means that sequence stratigraphy is a

method of correlating depositional bodies that share a common origin and that are
bounded by surfaces that have some time value. In today ’ s literature there are

several variations on that definition, but this one was chosen because it is concise
and relatively easy to understand. Sequence stratigraphy differs fundamentally from
lithostratigraphy because it is based on time – rock units instead of rock units. An
illustration of this fundamental difference is shown in Figure 4.4 , where facies with
identical lithological characteristics are deposited as time - transgressive lithofacies.
The consequences of these differences for the reservoir geologist or engineer are
illustrated in Figure 4.9 , which shows the difference between lithostratigraphic and
sequence - stratigraphic correlation of basinward - prograding grainstone successions.
Each grainstone body and its lateral equivalents represent deposition during a series
of prograding, high - frequency depositional sequences, or HFS. The reservoir is
compartmentalized along boundaries within the chronostratigraphic HFS boundar-
ies. When sequence - stratigraphic interpretations are employed, the compartmental-
ization is recognized. On the other hand, if chronostratigraphic relationships are
ignored and correlations are based only on lithostratigraphic similarity, as would
commonly be done with wireline log traces, the different grainstone units would be
correlated as if they were three laterally continuous reservoir zones. Such a layer -