Page 156 - Geology of Carbonate Reservoirs

DIAGNOSIS AND MAPPING OF DEPOSITIONAL RESERVOIRS 137

Figure 5.12 Slope - break or shelf - edge successions are characteristic of rimmed shelf margins.
Slope - break successions may include biogenic reefs and associated grainstones, fl oatstones,

and rudstones. The proportions of skeletal reef, nonbedded detrital infill, and crossbedded
grainstones that typify slope - break successions vary depending on relative sea - level history
and location with respect to windward and leeward sides of the slope break. Similar succes-
sions may occur around local or regional slope breaks on islands or antecedent topographic
highs, but they are easily recognized by their association with local topography rather than
with continuous, laterally extensive shelf margins. Slope - break successions typically occur in
tropical environments; they are rare or absent in temperate settings because skeletal reefs
and oolitic grainstones are rare to absent in temperate environments.

take - home lesson is that biogenic and chemical carbonates may accumulate as local
thicks on the crests of preexisting highs. This never occurs with siliciclastic
sedimentation.

5.4 DIAGNOSIS AND MAPPING OF DEPOSITIONAL RESERVOIRS

To qualify as a depositional carbonate reservoir, reservoir porosity must be domi-
nated by depositional rock properties such as texture, grain type (constituent com-
position), fabric, or sedimentary structures identified by direct observation of rock

samples. Wireline logs, seismic profiles, and even borehole images do not provide

enough information to identify and classify porosity. Although it is possible to iden-
tify and classify porosity by examining samples from one well, it is not possible to

predict the size and shape of the reservoir at field scale without additional informa-
tion on stratigraphy and structure. This is usually obtained as additional wells are

drilled in a field. Predicting the size and shape of the reservoir body is the most