Page 109 - Geology of Carbonate Reservoirs

90 STRATIGRAPHIC PRINCIPLES

QUARTZ SAND
BIOCLASTIC SAND

ARAGONITE
MUD
PLANKTONIC
SILT

WEST FLORIDA SHELF

100 Km

Figure 4.7 An example of a sheet - form deposit of bioclastic lime sand that extends over
hundreds of square miles where grain - rich deposits of Pleistocene age cover much of the
modern West Florida shelf. They have remained unburied and spread across wide areas
because post - Pleistocene sedimentation rates have been low and reworking by waves and
currents has kept the seabed mobile. (Adapted from an illustration in Purser (1980) .)

barriers), they can compartmentalize the reservoir or act as permeability barriers.
It is especially difficult to dissect deposits where there is little contrast between

sedimentary properties in layers of different ages or episodes of sedimentation.
Reefs and homogeneous oolite sand bodies are good examples of depositional units
that develop episodically. They are not easy to dissect into time slices because depo-
sitional rates vary in space as the bodies form and because there may be few
systematic differences in rock properties from one layer to another. Unconformities
may further complicate the problem by removing different amounts of rock at dif-
ferent parts of a depositional body, leaving a truncated surface that exposes rocks
of different age and possibly different petrophysical characteristics. An example of
a compound depositional body with both reservoir and nonreservoir zones exists in
Jurassic oolite buildups at Overton Field, Texas. Here, reservoir porosity corre-
sponds with diagenetically altered oolites. Nonreservoir zones are represented by
beds of unaltered oolites. There are few, if any, distinctive differences in depositional

rock properties to provide data for field development or for finding another pros-

pect in the trend. The solution to the problem lies in knowing the geological cause
of the diagenetic alteration. Alteration commonly has subtle connections to