Page 160 - The Petroleum System From Source to Trap

7. Carbonate Reservoir Rocks 153

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Figure 7.13. Map views of carbonate lithofacies patterns across an idealized carbonate shelf during dominantly aggrada­
tional stages of sedimentation associated with transgressive systems tracts (TSTs). (A) Inner shelf, (B) middle shelf, (C)
middle outer shelf, and (D) basin. (See Figure 7.9 for lithofacies legend.)
Highstand systems tracts in clastic-dominated not common in shallow intracratonic basins, but are
systems tracts are notably progradational. In contrast, logically found downslope of thick, older, well ­
wide carbonate lagoons (middle shelf deposits) may fill developed carbonate platforms. Sources for lowstand
in with a progradational pattern building out from the sediments are linear and not from point sources, such as
shoreline or with a regular layering of cyclic sediments, deltaic systems associated with clastic-dominated
as the outer shelf aggrades or possibly progrades. shelves.
Clastic- and carbonate-dominated shelves respond In summary, sequence stratigraphy as applied to
differently to sea level lowstands. Clastic material on carbonate lithofacies describes the detailed patterns of
stranded shelves is uncemented and is easily reworked shelf building and basin filling in carbonate systems
and transported across the shelf into the adjoining basin. tracts. Original depositional environments favorable for
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In contrast, carb n ate sediments, upon subaerial primary and diagenetic (secondary) porosity can be posi­
exposure, are subject to cementation and/ or dissolution tioned on a sequence stratigraphic model. In addition,
but not to reworking as a second-cycle sand. disconformable surfaces can be easily recognized, indi­
Throughout the development of depositional cating areas where more intense diagenetic processes
sequences, two prime fairways of opportunity for the such as meteoric dissolution, surface calichification, and
development of carbonate reservoir facies exist: an inner dolomitization can be expected. Benefits of this approach
and an outer shelf fairway. Other sites of porosity devel­ are ( 1 ) a framework for integrating all appropriate
opment include shoals and patch reefs of the middle seismic and well data, which make porosity and perm e­
shelf, porous sand-rich slope and lowstand fan deposits, ability more predictable; (2) a natural division of the sedi­
and pinnacle reefs in slope settings. Carbonate LSWs are mentary column into lithogenetic units where correla-