Page 38 - Geology of Carbonate Reservoirs

FUNDAMENTAL ROCK PROPERTIES 19

Current

6 in
A

B C

Figure 2.3 (a) Depositional fabric in detrital rocks. Grain orientation and alignment pro-

duced by currents at time of deposition. The larger grains at the top of the figure are imbri-
cated and those at the bottom of the figure are simply oriented with long axes parallel to the

direction of flow. Permeability is highest in the direction of grain alignment. (b) Diagenetic
fabric. Complete replacement of limestone by dolomite creates a diagenetic fabric totally
unrelated to depositional rock properties. In this case the dolomite rhombohedra occur in an
“ open ” fabric with a great amount of intercrystalline porosity. (c) Biogenic (skeletal growth)
fabric. The great variety of internal growth fabrics created by reef - building organisms creates
depositional fabrics dramatically different from those in detrital (particulate) rocks. The
photo illustrates a Pleistocene patch reef from Windley Key, Florida. Porosity and permeabil-

ity in the coral framestone are influenced by biologically constructed, skeletal structures, not
by granular or crystalline fabrics.
with detrital sediments, (2) skeletal elements such as branches or leaves that acted

as “ baffles ” that were subsequently buried in the sediment they helped to trap, and
(3) closely bound fabrics generated by encrusting organisms. The skeletal micro-
structure of many organisms is porous and may provide intraskeletal porosity, even
in nonreef deposits. The pores within sponge, coral, bryozoan, stromatoporoid, or
rudist skeletons, for example, are intraparticle pores, although the individual skele-
tons are part of larger reef structures. All three fabric categories are closely related

to reservoir properties because fabric influences pore to pore throat geometry and
may influence directional permeability. An example of combined biogenic and

detrital fabric is illustrated in a Pleistocene coral framestone reef with detrital inter-
beds (Figure 2.3 c).