Page 156 - The Petroleum System From Source to Trap

7. Carbonate Reservoir Rocks 149

Figure 7.9. Block diagram of
carbonate lithofacies patterns
on a drop-off profile across an
idealized carbonate shelf during
dominantly progradational
stages of sedimentation during
the formation of highstand
systems tracts (HSTs), shelf
margin wedges (SMWs), and
lowstand wedges (L.SWs).

- ClB 'I'B li\B or VB )..G CSS mB0 or M
INNER SHELF
REEFS
AG •G CG � or 0G AW/P )..eP/G lAW or �SH
1:<=>1 SHOALS or NEAR-REEF MIDDLE and MIDDLE-OUTER SHELF
l::;::::::::::::::q AP >,•P a.W l),W SH or lM
BASIN
OFF-REEF, INTER-REEF, BACK­
REEF, and SLOPE DEPOSITS
LAND
members: the ramp and the drop-off profiles (Read, 1982; The inner shelf zone extends from shallow subtidal to
Wilson and Jordan, 1983). The relative position of sea high (storm) tide levels. It includes nearshore subtidal
level on either of these profiles is important in that the environments, coastal lagoons, tidal flats or sabkhas, and
width of subtidal shelf facies can vary significantly beach environments. Consequently, lithofacies variation
(Irwin, 1965; Shaw, 1964). may be considerable along strike. For example, a 10-km
The relative position of sea level is a major point of length of coastline may be rocky (e.g., an eroding shore
emphasis in the development of sequence stratigraphy that hosts fringing reefs), sandy (a typical beach), or
(van Wagoner et al., 1990; Schlager, 1992). Narrow belts muddy (an algal flat where stromatolites bind and trap
of subtidal shelf facies (at most, a few tens of kilometers lime mud). The term facies mosaic has been used to
wide) correspond to lowstand wedges of carbonate sedi­ describe this high degree of variability associated with
mentation deposited in front of older stranded shelves, inner shelf facies patterns.
whereas wide belts (up to several hundred kilometers At the time of deposition, this facies belt ranges from a
wide) correspond to highstand systems tracts, deposited few kilometers wide to a maxium of about 15 km wide
on broad, flooded shelves (Shaw, 1964; van Wagoner et where the gradient of the sea floor is low. The shiftg of
al., 1990). At any given stage of sea level, a shelf can be inner shelf environments through time produces a wide
divided into inner, middle, and outer zones. (See Figures fairway for exploration, best exemplified by three
7.9, 7.10, and 7.12 for block diagrams of carbonate lithofa­ Permian San Andres zones of production from tidal flat
cies patterns across an idealized shelf and atoll.) deposits in a belt about 30 km wide on the northern shelf
In general, the inner shelf setting is characterized by of the Delaware basin of West Texas (Meissner, 1974).
lithofacies containing euryhaline faunas (lacking Generally, if arid climates prevail, inner shelf lithofacies
organisms associated with normal marine salinity), by in the profile of Figure 7.9 would likely be modified to
sedimentary structures, or by lithic sequences indicating include dolomites (mainly dolomitized mudstones, M0)
the proximity of a shoreline. Examples include ostracod and possibly evaporites. In addition, the zone of
wackestones (="W) and algal stromatolite boundstones nearshore faunal restriction (the inner shelf zone) would
(rnB) deposited in nearshore lagoonal and intertidal envi­ extend farther out into subtidal environments.
ronments, respectively. Low-angle accretion cross strati­ Inner shelf deposits can also potentially form around
fication and evaporite beds are also associated with inner the shoreline of any island. However, most islands, espe­
shelf environments. cially those in moderate- to high-energy middle shelf