Page 108 - Geology of Carbonate Reservoirs

ANATOMY OF DEPOSITIONAL UNITS 89

Width = Miles
Thickness = A Few Feet

Equidimensional Elongate Channel Form

Length / Width Ratio
< 3/1 > 3/1 > 3/1 Branched

Figure 4.6 Sheets or blankets: typical shapes of depositional bodies deposited in carbonate

environments adapted from an illustration in Potter (1963) . Sheet forms are usually a few
feet thick at most and may cover many square miles in area; they usually represent reworked
material deposited during previous lowstands on outer parts of platforms. Elongate shapes
are typical of strandplain deposits, reefs and mounds are usually more - or - less equidimen-
sional in plan, and channel forms may be found in tidal deltas, turbidites, and density current
deposits.

spatial orientation, and the porosity and permeability distribution within them. A
case history is helpful to illustrate this concept. Pleistocene oolites occur in a wide
belt at present water depths of about 100 m off the western coast of Yucat á n. Oolites
and shallow - water benthic organisms identify these Pleistocene facies as trans-
gressed remnants of shoreline deposits similar to the modern barriers and beaches
on the northeastern coast of Yucat á n at Isla Cancun and Isla Mujeres, among others.
Knowing that beaches parallel depositional strike, it is possible to predict the ori-
entation of the body, and data on the dimensions of the beach at one or two locations
enables predictive maps to be made on the size and shape of the potential reservoir.
These are external, large - scale characteristics that relate beaches to their laterally
equivalent facies across the entire depositional platform. Two or more cores of the
Pleistocene oolites at different locations on the Campeche Bank should provide
enough information to construct a map to predict the size, shape, and location of
the oolite facies trend as long as the bathymetry remained relatively constant. The
oolite body will extend along depositional strike following the paleobathymetry of
the Pleistocene shoreline. Any changes in bathymetry such as local highs would
cause deviations in the shoreline trend, emphasizing that it is always necessary to
take bathymetry into account when making facies maps.
If several episodes of sedimentation are repeated and a compound depositional

body forms within one cell on the platform, it may be difficult to separate strata
that represent each episode. If some of the strata are nonreservoir rocks (baffl es or