Page 145 - Geology of Carbonate Reservoirs

126 DEPOSITIONAL CARBONATE RESERVOIRS

Figure 5.5 Air photo of shelf - edge grainstone buildups on the Great Bahama Bank. These
buildups, or sand – wave complexes, form the rim on an isolated, rimmed shelf. Spillover lobes
indicate direction of transport from open ocean to bank interior (right to left) during tidal
exchange. The sand – wave complex borders a precipitous slope break that is swept by power-

ful tidal currents. From right to left, each spillover lobe is about 1 km wide.

landward from the shelf - edge reef and about 40 km along depositional strike. A
diagrammatic cross - section and facies map illustrating the White Bank sand – wave
complex are included in the memoir by Enos and Perkins (1977) . Similar grainstone
buildups occur behind the Guadalupian (Permian) reef in West Texas and New
Mexico, and Wilson (1975) describes reef - related grainstone buildups on lower
Cretaceous slope breaks in the Middle East (Thamama Group) and the ancestral
Gulf of Mexico (Stuart City reef trend). In essence, typical slope - break facies consist
of reefs and associated sand sheets or sand – wave complexes. On the Great Bahama
Banks oolite sand waves occur on slope breaks without reefs (Figure 5.5 ). Sediments
inboard from these slope - break grainstones are shallow subtidal wackestones to
mudstones, or alternatively, platform interior wackestones to mudstones. Slope -
break grainstones and reefs present three main kinds of depositional porosity:

intergranular porosity in grainstones (including the detrital infill around reef masses),
intraframe porosity within the entire skeletal or cementstone structure of the reef,
and intraparticle porosity within individual skeletons in the reef mass. Of these pore
types, intergranular porosity has the greatest potential to provide reservoir continu-
ity and connectivity but may also have the highest potential for cementation by
early marine diagenesis. Intraframe porosity and intraparticle porosity in fossil
skeletons have a tendency to behave as separate vugs, thereby reducing their con-
nectivity potential. Early diagenesis may drastically alter this pattern, especially if
aragonitic skeletal components are abundant. Aragonite is metastable and quickly
dissolves in undersaturated water to form moldic pores and may just as quickly be

reprecipitated as pore - filling cement.
5.2.9 The Slope Environment

Bathymetric characteristics of slope environments vary depending on the mechani-
cal slope stability, the slope angle, and the rigor of the physical oceanographic