SUMMARY
There are several simplistic models that can be used to summarize the observed variation in carbonate play types.

Carbonate reservoirs that form stratigraphic traps may have lens-like, sheet-like or ribbon geometry. Carbonate bodies with lens-like geometries include biohermal or reef buildups that form during and after rapid relative sea level rises as well as down-slope debris fans. In contrast the sheet-like geometries may include muddy or grain-dominated progradational carbonate sheets that are terminated by exposure or flooding or prolonged stillstands. Trapping of hydrocarbons in reservoirs with sheet-like geometry requires some structuring before migration. Finally, plays with ribbon geometry are formed by carbonates that accumulate at the platform margin. These are commonly reefs or shoals but may be sand-filled tidal channels or beaches.

Lens Geometry

Bioherms may occur updip behind the major shelf-edge trend (left figure) or at the base of the break in slope or deeper portions of a ramp (figure below). The best reservoir facies commonly occurs along the margin of the buildup where original interparticle porosity occurs in mud-free carbonate sands or organically-bound rubble. The interiors of the buildups are usually muddy deposits which have little or no reservoir potential unless fenestral porosity remains or dolomitization occurs. The common seal is shale which covers and fills in around the buildups when they fail to keep pace with relative rise insea level and drown.

The deeper-water buildups usually begin as a mud mound with a pioneer faunal community. With establishment of the community, the rate of sedimentation accelerates over the mounds as the organisms trap and precipitate more carbonate. The development of the buildups is interrupted if the basin is isolated and becomes evaporitic. Silurian reefs in the Michigan Basin show evidence of several interruptions in development caused by basin isolation. The best reservoir quality in these very thick buildups is developed near the top of the reef where mud-free carbonate sands, fresh-water leaching and dolomitization are common. The source beds for these buildups are the laterally adjacent basinal carbonates and shales and in some cases the underlying limestones.

Sheet Geometry

Progradational carbonate sand sheets form as shoreline deposits, marine bars or tidal deltas on the seaward portions of carbonate platforms and mark the upper portions of shoaling upward sequences. The most porous fades form where bottom agitation is at maximum and interstitial lime mud is winnowed. Sedimentation may be terminated by exposure, which in turn causes the development of secondary leached porosity (figure above).

If deposition of carbonate sands is terminated by burial beneath deeper water sediments, their primary porosity is likely to be preserved (left figure). Similarly primary porosity will likely be preserved in carbonate sands terminated by the deposition of evaporites (figure below). The evaporites form a seal that prevents the movement of fresh water through the overlying strata and shields these sands from diagenesis. A reservoir example of this model is the Jurassic Arab "D" of Saudi Arabia. On the other hand, if a regional fresh groundwater system is confined beneath the seal, significant diagenesis including dolomitization may occur. Reservoir examples of this type occur in the Jurassic Smackover Formation of the U. S. Gulf Coast. Possible source rocks in each case are the underlying carbonate muds or shales. The seal could be updip progradational shales, evaporites or offshore marine shales deposited during a subsequent sea level rise.

Progradational sheets of muddy subtidal carbonate, usually associated with interior basins, may form plays if the muds are dolomitized (left figure). These fine carbonates are deposited in a shoaling cycle that is terminated by the deposition of supratidal evaporites. Source rocks are most probably the underlying muddy carbonates and shales. Reservoir porosity, which is a result of the dolomitizatlon, is characteristically patchy. Examples of this model include the Ordovician and Silurian of the Williston Basin.

Ribbon Geometry

The final play type, ribbon geometry reefs or shoals, which mark the margin of relatively steep-sided platforms, usually generates the most interest from explorationists (right figure). The potential reservoirs form as part of a package of prograding marginal sediments. Updip muddy carbonates or evaporites as well as overlying shales provide seals, and basinal deeper-water limestones or shales may serve as sources. If localized structuring does not occur before migration, the porous platform margin deposits will not become reservoirs. Instead, they will act as a pathway for the migrating oil that becomes trapped updip within the platform sequence. Downdip debris that forms an apron to a relatively dense carbonate margin is productive in the Lower Cretaceous of southeastern Mexico.