Abstract Detailed geologic studies of hydrocarbon reservoirs in platform carbonates commonly show that reservoir zones occur within 1–15-m-thick, upward-coarsening successions of lithofacies, i.e., within upward-shallowing cycles. Our understanding of the depositional history and reservoir characteristics of such cycles and their component facies is enhanced by observations of ancient outcrop examples and models derived from modern analogs. Outcrops of the Permian San Andres Formation along the Algerita Escarpment of the Guadalupe Mountains contain cycles 3–12 m thick, with thin mudstone/wackestone bases, overlain by burrowed wackestones and packstones, and capped by thick massive to planar or cross-bedded packstones and grainstones. These facies formed during relative rise and/or stabilization of sea level during which carbonate sand shoals developed. The outcrops also display lateral facies relationships within the cycles on the scale of hundreds of meters that are representative of those commonly observed in analogous hydrocarbon reservoirs of the Permian basin of Texas and New Mexico. Core and surface sediment mapping in the Holocene Joulters Cays ooidshoal complex of Great Bahama Bank reveals the three-dimensional complexity of an upward-coarsening and shallowing cycle. This facies mosaic is like that observed in two dimensions at Algerita Escarpment or in one dimension in a core from a reservoir. This modern example points out difficulties in interpretation and correlation of grainstone cycles in subsurface studies of platform carbonate reservoirs. The modern shoal complex, which extends over 400 km 2 , varies greatly in thickness but averages 4 m thick. Shoal growth, largely in a response to a relative rise of sea level, records rapid expansion of ooid sands, island formation and associated meteoric diagenesis, local shoal stabilization and reworking by burrowing, and generation of hardground layers. Sand generation and topography varied greatly in the Joulters Cays area during flooding of the platform and development of the shoal. Such variation should be expected in ancient examples, as was observed at Algerita Escarpment. Within the upper grain-dominated part of the cycle at Joulters, depositional facies geometries and early diagenetic alteration contribute to fine-scale heterogeneities. This is at a scale equivalent to documented hydrocarbon reservoir heterogeneities.

Abstract Coring of Holocene sediments in the Joulters Cays area of Great Bahama Bank has revealed five facies. Three facies — ooid grainstone, ooid packstone and fine-peloid packstone — are responsible for the buildup of the shoal. Ooid grainstone occurs in a narrow band along the windward-facing borders of the shoal, an occurrence that coincides with the formation of clean ooid sands today. Bankward of these grainstones, ooid packstone overlies fine-peloid packstone with both forming a wedge of sediment that thins in all directions away from the grainstones. Interpretation of the packstone sequence suggests the shoal was more active in the past over a larger portion of its area than it is today and the packstones represent reworking of grainstones through burrowing during development of a vast sand flat. In addition to providing modern counterparts for facies, grain types, structures and diagenetic overprint that are common in ancient ooid sands, the Joulters example graphically shows the rapidity with which complex facies relations can form. The shoal, covering over 400 km 2 and averaging 4 m in thickness, formed during the last 5000 years. Nevertheless, the facies record of shoal growth, largely a response to changes in relative sea level, indicates significant changes such as rapid expansion of ooid sands, island formation and major reworking of sands by burrowing. In some facies, the depositional texture, sedimentary structures and grain types have all been modified since deposition. These changes point out the difficulty of interpretation of cores from ancient examples.

Abstract Porous dolomites are present below a distinctive stratigraphic marker within the lower Quintuco Formation (Lower Cretaceous, Berriasian-lower Valanginian) in the eastern Neuquén basin, Argentina. Dolomitized pack-stones and wackestones with moldic and sucrosic porosity provide the main reservoir facies in Rio Neuquén field and perhaps other oil fields in the area. Lower Quintuco carbonates are comprised of: (1) oolitic grainstones, (2) burrowed, dolomitized oolite-skeletal-peloid packstones/wackestones, (3) dolomudstones and bedded anhydrites, and (4) very fine-grained, superficially coated oolite grainstones. These sediments are commonly packaged into shoaling- and coarsening-upward parasequences. Reservoir-quality porosity and permeability exist almost exclusively in burrowed, dolomitized packs tones and wackestones. These strata are interpreted as off-bar facies deposited on the landward side of bar complexes, similar to modern facies analogs known in the Joulters Cay area of the Bahamas. In the lower Quintuco Formation, dolomite preferentially replaced carbonate mud. Below an inferred widespread paleo-exposure surface, ooid-skeletal-peloid grains were then dissolved to leave an open pore network with abundant moldic and intercrystalline porosity.

Abstract A Quaternary example of an ooid shoal complex comprised of northern Andros Island (Pleistocene) and Joulters Cays (Holocene) provides insights into ancient ooid shoal deposition and preservation. Criteria that are commonly used in the interpretation of fossil ooid shoals are preserved in the Quaternary example. These include grain composition, lateral variability of shoal subenvironments and adjacent carbonate environments, topography, and to some extent, sedimentary structures. Two subenvironments in particular are easily recognized: (1) the mobile fringe is characterized by high ooid concentrations and well-preserved sedimentary lamination and is topographically elevated, and (2) the stabilized sand flat contains fewer ooids and occurs at a lower elevation. Lagoons, reefs, and offshore skeletal sands are located laterally adjacent to ooid shoals. Muddy tidal flats, however, are not. Evidence of extensive exposure is also well preserved in the Quaternary analog. Sea-level fluctuations as little as 5 m have a profound effect on the site of ooid shoal accumulation and the creation of exposure surfaces. A comparison of the Quaternary analog to Mississippian oolitic rocks reveals similar grain compositions, sedimentary structures, and to some extent, adjacent carbonate environments. The mobile fringe and stabilized sand flats can also be recognized. In contrast, original topography, exposure surfaces, and eolian features are seldom reported for Mississippian oolitic rocks.