Page 157 - The Petroleum System From Source to Trap

150 Jordan and Wilson

Figure 7.10. Block diagram of
carbonate lithofacies patterns
across an idealized oceanic
atoll during dominantly aggra·
dational stages of sedimenta­
tion associated with trans­
gressive systems tracts
(TSTs). (See Figure 7.9 for
lithofacies legend.)

I OUTER OUTER
BASIN I SLOPE SHELF I MIDDLE SHELF SHELF I sLOPE I BASIN

settings or those associated with open ocean atolls, show point toward shore. Beyond the skeletal grainstone­
little evidence in the rock record that they were packstone halo in deeper water off the reef, a back­
emergent. Unless tidal flat deposits formed, island ground sedimentation of skeletal packstone (/,p) occurs,
deposits are difficult to distinguish from subtidal shoal deposited in interreef lagoonal settings. These packstones
deposits (Ebanks, 1975). surround and include all the individual reefs of a typical
The middle shelf setting (Wilson and Jordan, 1983) middle shelf reef complex (Figure 7.11). Farther beyond
consists of a broad band of lithofacies that separates the reef complex, the regional background of middle
inner and outer shelf settings. Middle shelf environments shelf sedimentation is generally muddier, e.g., consisting
generally include vast areas of subtidal sediments of skeletal wackestone-packstone (),w /P), argillaceous
dominated by lime mud (e.g., skeletal wackestones, \W) skeletal wackestone-packstone (l ), W /P), benthonic
and account for the bulk of carbonate deposits on a foram-rich shale (�SH), or benthonic-foram planktonic­
typical shelf. Burrowing in these sediments is common; foram packstone (�c!OP).
by definition, they host normal marine faunas and floras. The outer shelf setting consists of a moderately
Water depths in middle shelf environments vary from narrow facies belt (2-8 km wide) of grainstone shoals or
about 2-3m to 60 m). Shelves commonly lie within boundstone facies, forming either a linear shelf edge
normal wave base (about 10 m deep), unless subsidence shoal or a barrier reef. Outer shelf facies form consistent
exceeds the tendency to build up to sea level. linear to curvilinear trends that provide remarkably
Within the regional backdrop of muddy sedimenta­ reliable exploration fairways along depositional strike.
tion across the middle shelf, sand shoals (grainstone Figure 7.9 show the basic lithofacies pattern associated
deposits), patch reefs (boundstones and related grain­ with progradational sedimentation characteristic of
stones), and patch reef complexes can occur, all of which highstand systems tracts. A similar pattern can form
have high potential as reservoir facies. Patch reefs and farther downslope as lowstand systems tracts. The sea
shoals can reflect considerable environmental variation: floor slopes gently from the shoreline to perhaps several
some are emergent forming islands of storm-tossed tens of meters deep somewhere in the middle shelf envi­
detritus; some are capped with boundstone; others are ronment, then rises toward the shoaling environments of
sand shoals with small heads of reef framework; and still the outer shelf, and finally drops down the steep slope
others are sand shoal deposits devoid of any frame (the upper part of which may be vertical) to form an
builders (Figures 7.9, 7.10, and 7.11). apron out into the basin. There is a general trend from
Generally, a halo of skeletal grainstone-packstone muddy carbonate textures nearshore, to more grain-rich
(),G-P) occurs around individual patch reefs, demon­ textures offshore near the shelf break and slope (or near
strating the radial transport of reef-derived material local middle shelf highs with reefs or grainstone shoal
away from the reef core and out into the middle shelf deposits), and finally to muddy textures out in the basin.
lagoon. Dominant paleowind directions can be inferred The productivity of carbonate grains in the outer shelf
from asymmetric reef halos or from shoals whose spits and slope is high, rather like a well run "carbonate