Modern
sequence stratigraphy is a direct outgrowth of the concept of
unconformity bound stratigraphic units as proposed by Sloss (1964).
In simplest terms, Sloss's (1964) sequence stratigraphy was a
tool to identify major unconformity-bound packages (e.g., Sauk,
Kaskaskia) for the purpose of correlation across large areas of
the craton. We now call these 2nd order sequences.
The second major evolutionary step in the conceptual evolution
to modern sequence stratigraphy was seismic stratigraphy. AAPG
Memoir 26 (Payton et al. 1977) captures, in a series of manuscripts
by Exxon geologists, a methodology for identifying Slossian type
unconformity-bound sequences using reflection seismic data. The
emphasis of this methodology was the use of seismic geometry and
termination styles to delineate unconformity-bound seismic sequences,
derivation of eustatic curves using coastal onlap (Vail et al.
1977a), and global sequence correlation based on biostratigraphic
control (Vail et al. 1977b). Another key aspect of this research
(Vail et al. 1977c), which was also echoed by Brown and Fisher
(1977) among others, was the chronostratigraphic significance
of seismic reflectors. The observation that a single shelf-to-basin
reflector mimicked a depositional timeline opened the door for
sedimentologic analysis of the internal make-up of seismic sequences.
This seismic chronostratigraphy led to the third key development,
which was concept of sequence stratigraphy.
The transition from seismic stratigraphy to sequence stratigraphy
represents a major conceptual jump. Focus shifted from global
mapping of unconformity-bound packages to the interpretation of
the internal reflector patterns of stratigraphic sequences, which
came to be viewed as genetically-significant depositional sequences.
Eighties-style sequence stratigraphy (Vail 1987; Posementier and
Vail 1988; Van Wagoner et al. 1988) focused on viewing sequences
as unconformity-bound units that are also genetic depositional
cycles associated with global sea-level change. In sequence stratigraphic
terms, it was proposed that depositional sequences contain a predictable
suite of systems tracts and lithofacies tracts tied to a sinusoidal
eustatic curve, and can be correlated worldwide. This global correlation
model can also be used locally to predict reservoir, source, and
seal lithofacies within the idealized genetic packages by developing
a standard suite of lithofacies tracts associated with lowstand,
transgressive, and highstand systems tracts.
For geologists interested in constructing accurate stratigraphic
frameworks for reservoir characterization, the analysis of the
internal anatomy of sequences is more important than the global
correlation methodologies. One significant evolutionary step occurred
after the introduction of sequence stratigraphy, which came about
through the application of sequence concepts to reservoir characterization
and to outcrop investigations. Using wireline logs and cores from
the subsurface combined with outcrop cross sections, it became
apparent that an additional level of stratigraphic sequences could
be resolved within seismically-definable depositional sequences.
The development of this outcrop-core-log based sequence stratigraphy
has been referred to as high-resolution, or high-frequency sequence
stratigraphy, and represents the critical scale at which reservoir-scale
sequence descriptions should be carried out. High-frequency sequence
stratigraphy recognizes that when the more detailed subsurface
data from wireline logs are examined within a seismically-defined
depositional sequence, a number of unconformity bound sequences
can be recognized. These high-frequency sequences and their component
cycles represent the scale of observation critical to reservoir
model construction. |