Page 107 - Geology of Carbonate Reservoirs

88 STRATIGRAPHIC PRINCIPLES

of rock properties per se, and log correlations have no time value. Chronostrati-
graphic information must be determined independently and superimposed on
lithostratigraphic correlations in order to distinguish between age - equivalent and
time - transgressive rock units. Likewise, geophysical information such as seismo-
graph records is not a direct representation of rock properties. Seismic traces, like

wireline log traces, have no time significance. This fact is vitally important to the
worker who may try to establish direct correspondence between seismic stratigra-
phy and sequence stratigraphy. Without independent time markers superimposed

on the seismograms, no specific age can be assigned a priori to seismic refl ectors. If
ages are not known, sequences cannot be correctly identifi ed or mapped. Indepen-
dent geochronology is usually determined from age dates established in boreholes
along, or projected into, seismic lines.

4.4 ANATOMY OF DEPOSITIONAL UNITS

Depositional sedimentary bodies have characteristic 3D shapes. In most of these
shapes, the dimensions of length, width, and thickness are different. Shapes may be
elongate, fl attened, or otherwise streamlined in response to physical and biological
sedimentary processes. In addition, the long axis of sedimentary bodies may be ori-
ented parallel or at some angle to depositional strike. Depositional strike is the
compass direction perpendicular to depositional dip. Depositional dip is the amount
and direction of slope across the platform from shore to basin. Beaches and shelf - edge
calcarenites, for example, are elongate parallel to depositional strike. Knowledge of
3D anatomy for common depositional models is critically important in both explora-
tion and development geology. If the standard depositional unit is identified, then its

characteristic shape and orientation with regard to depositional strike can be pre-
dicted. This information is necessary to determine drilling locations, for calculating

the volume of hydrocarbons in place, for optimum field development, and for para-
meters to use in reservoir simulation. In other words, knowledge of depositional
anatomy is necessary to predict the volume and spatial orientation of the reservoir.

Typical depositional shapes are usually classified by the ratios of their axial
dimensions. Bodies with one axis much longer than two shorter ones are classifi ed
as stringers, for example. Sheet, equidimensional, elongate, and channel - form shapes
are illustrated in Figure 4.6 . Beaches, dunes, subtidal sand waves, and some channel -
fill deposits are elongate; reefs, mounds, or other types of biological and chemogenic

buildups are usually equidimensional. Reefs and other forms of carbonate buildups
that form in shallow - water environments take on streamlined shapes in response to
the incoming waves, tides, and currents. Massive pavements, boulder beds, or resis-
tant skeletal frameworks develop on the windward sides of the buildup. Finer, loose
debris accumulates in the leeward “ shadow ” of the buildups sometimes forming
streamlined hooks or spits shaped by the refracted waves and currents. Sheet depos-
its typically represent reworked deposits that are relict from previous environments.
Sheet calcarenites such as those along the margin of the West Florida platform
are relict grainstones that have been reworked on the drowned Pleistocene shelf
(Figure 4.7 ).
When depositional bodies represent one or two episodes of sedimentation in a
specific cell on a platform, it is not difficult to predict the size of the bodies, their