Page 143 - Geology of Carbonate Reservoirs

124 DEPOSITIONAL CARBONATE RESERVOIRS

environment, whether it is a ramp, rimmed shelf, or open shelf, can be monotonous.
Flat or gently sloping surfaces do not offer much resistance to the passage of waves,
tides, and currents in low - to moderate - energy settings. This means that there is little
winnowing to concentrate grainy sediments. Grainy facies are characteristic of the
strandline on ramps and some open shelves, and the slope break on rimmed shelves.
For grainy facies to exist in the neritic zone requires some topographic feature to
focus waves, tides, and currents. If suitable habitat exists and environmental quality
is good, patch reefs may develop to create local bathymetric highs that interact with
incoming waves and currents. Bioclastic sands are commonly derived from the
breakdown of reef organisms and from opportunistic dwellers in the reef habitat.
These sands accumulate on and around bathymetric highs to form local grainstone
and packstone accumulations, which along with the patch - reef buildups stand in
stark contrast to the muddy facies that characterize most of the neritic seabed away
from the highs. Most shallow neritic depositional successions consist of monotonous
wackestones and mudstones with two possible variants: (1) patch reefs and their

flanking grainstones and packstones, or (2) grainstone shoals formed as winnowed
lag deposits on paleo - highs. If commercial reservoirs with depositional porosity exist
in the ordinary shallow neritic environment, they will almost certainly be either
patch reefs or grainy facies on paleo - highs. Examples of such reservoirs on isolated
highs surrounded by subtidal mudstones and wackestones are the Mississippian
skeletal grainstones at Conley Field, Hardeman Basin, Texas (Ahr and Walters,
1985 ), the James Limestone (Cretaceous) reefs on a “ turtle ” structure in the East
Texas salt basin, and many of the Smackover (Jurassic) oolite buildups on salt struc-
tures in the ancestral Gulf of Mexico salt basins from Texas to Alabama. Porosity
enhancement by diagenesis, and especially by fracturing, can transform the monoto-
nous neritic mudstones and wackestones into excellent reservoirs, however. At the
other end of the environmental spectrum, extreme climatic conditions such as those

in the “ roaring 40s ” latitudes in the southern hemisphere, fierce winds, waves, cur-
rents, and storms sweep detrital sediments out of the shallow subtidal zone. A
modern example of this situation exists off the coast of southern Australia. There is
no deposition in the shallow subtidal zone on this open shelf because the hydrologic
regime is so powerful. Coarse sediments are winnowed and redeposited by strong
waves and currents at depths of up to 140 m (Nelson et al., 1982 ; James et al., 1992 ;
Boreen et al., 1993 ). Clearly, it is the low - to moderate - energy platforms on which

significant deposits of shallow subtidal facies will accumulate.
Chalks Most chalks were deposited in the subtidal shelf or inner and middle ramp

environments in epicratonic basins, or in shallow seas that flooded continents, espe-
cially northwestern Europe, western North America, and the Middle East during
Middle to Late Cretaceous times. Although most chalks are shallow marine deposits,
some were deposited in basinal settings. To avoid duplication, the discussion on
chalk deposits is included in a later section on basinal facies.

5.2.7 The Slope -Break Environment
Modern bathymetric charts show that rimmed carbonate shelves like those in South
Florida and the Bahama Banks have slope breaks at 10 m or less, well within the
highly oxygenated, nutrient - rich, euphotic zone. Slope breaks on tropical rimmed