Page 113 - Geology of Carbonate Reservoirs

94 STRATIGRAPHIC PRINCIPLES

Basin Open Sea Deep Shelf Foreslope Organic Winnowed Shelf Lagoon Restricted Evaporites
Circulation
Shelf Margin Build Up Edge Sands Open Shelf and on Sabkhas-
Circulation Salinas
Tidal Flat
1 2 3 4 5 6 7 8 9
Wide Belts Facies Profile
Very Narrow Belts Wide Belts
Debris flows Giant talus Downslope Islands. Tidal deltas. Tidal flats. Anhydrite domes.
and turbidites blocks, mounds. Dunes. Lagoonal Channels. Tepee structures.
in fine laminate infilled large Reef knolls. Barrier bars. ponds. Natural levees. Laminated 2nd Order
strata. cavities. Boundstone Passes and Typical shelf Ponds. crusts of gypsum
Mounds on Downslope patches. channels. mounds. Algal mat belts. Salinas
toe of slope. mounds. Fringing and Columnar (Evaporative Sedimentary Bodies
barrier frame- algal mats. ponds)
work reef. Channels and Sabkhas
Spur and tidal bars (Evaporative
groove. of lime sand. flats)
Figure 4.8 The Wilson standard carbonate platform with its nine environmental subdivisions

and their accompanying standard microfacies. The concept of standard platform geometry
with characteristic facies representing different environmental subdivisions paved the way
for modern methods of sedimentary facies analysis and sequence stratigraphy. (Adapted from
an illustration in Wilson (1975) .)

of which has distinctive rock and fossil characteristics. These vertical successions vary
as accommodation, sediment input, sediment retention, hydrological regime, and
biotic characteristics change over time at the site of deposition. For example, the
typical shallow subtidal facies on both ramps and shelves consists of mudstones and
wackestones with occasional patches of packstones. If conditions are right, patch
reefs may develop in the shallow subtidal environment, thereby introducing dis-
tinctly different facies characteristics compared to the laterally equivalent mud-
stones and wackestones. These variations must be considered when ideal successions
are constructed to serve as depositional models. Depositional successions are records
of sedimentation during the accumulation of stacked, 2D microfacies within each
platform sector. Variations in sedimentary and biotic characteristics notwithstanding,
standardized depositional successions can be used in subsurface reservoir geology
as long as they record rock and fossil characteristics that occurred in the same plat-
form sector over time and with more or less constant environmental conditions. For
example, tidal flats and lagoons typically occur behind barrier islands on ramps. As

long as the hydrologic regimes, bathymetry, and subsidence allow sedimentation to
continue without dramatic change, the resulting successions will provide unique
records of tidal - flat deposition. The interplay between subsidence and sedimentation

will dictate how the typical tidal - flat microfacies — lagoonal, intertidal, and supratidal

microfacies — are stacked. Remember that in chronostratigraphic context, individual
depositional successions are assumed to represent short time intervals, continuous
sedimentation, and limited spatial migration of environmental sectors on platforms.
During high - amplitude, basin - scale changes in relative sea level, individual microfa-
cies (stacked, individual blankets that make up successions) may migrate consider-
able distances up or down depositional dip to stay in step with their shifting
environments. These migrating facies can be time - transgressive, as we have discussed
earlier; consequently, it is very important to identify time markers when doing cor-
relations between successions at fi eld or platform scale.