Page 155 - Geology of Carbonate Reservoirs

136 DEPOSITIONAL CARBONATE RESERVOIRS

May/may not
be present

Figure 5.11 The shallow subtidal succession is present on all shelves and ramps. In tropical

environments this succession may or may not include skeletal patch reefs with fl anking skel-
etal grainstones and packstones depending on water clarity, nutrients, oxygenation, and suit-
able substrate to support reef buildups. The default subtidal succession on rimmed shelves
and low - energy ramps is dominated by mudstones and wackestones. High - energy ramps and
open shelves may exhibit more grain - dominated successions. Restricted interiors on rimmed
shelves may have lower taxonomic diversity and muddier substrates than ramps and open
shelves at the same latitudes because ramps and open shelves are subjected to ocean waves
and currents with attendant nutrients, oxygen, salinity, and moderated temperature.

erode it. In some cases, the high may even be emergent, as in the case of
islands.
Carbonates, unlike siliciclastics, are formed by biogenic and chemical processes
as well as by detrital sedimentation. Abundant biological or chemical carbonate
sedimentation may occur on highs with little or no comparable sedimentation in
adjacent deeper water. This is particularly common in the case of reef and certain

grainstone accumulations on tops and flanks of paleo - highs. Oolite grainstones are
commonly found on the crests of salt - generated highs that marked the Jurassic
seabed around the Gulf of Mexico rim during deposition of the Smackover and
Cotton Valley Formations. Paleo - highs are favored locations for reefs, too. Reefs are
commonly located on paleo - highs such as salt domes, horst blocks, or relict, ero-
sional features. Bioclastic grainstones may accumulate preferentially on the crests
of subtidal shoals because the skeletal producers favored the shallow crest of the
high over the deeper, less favorable zones on the adjacent seabed. In these instances,
relief on the bathymetric highs is increased by greater rates of deposition on the
highs and comparatively lower rates of deposition in the adjacent deeper zones. In
such cases, care must be taken to interpret interval isopach maps because they will
exhibit thick deposits on antecedent highs and thins that outline coeval, antecedent
lows — just the reverse of what one might expect for detrital sedimentation. The