Abstract Shallow core boring transects on West Caicos record the late Pleistocene evolution of the leeward margin of Caicos Platform. Throughout the Pleistocene and Holocene, this leeward margin progressively evolved from an open margin to one blocked by emergent topography, resulting in a dramatic shift in depositional facies. A leeward-margin barrier reef formed behind the protection of emergent oolitic grainstone ridges that persisted through the last two Pleistocene and the Holocene interglacial sea-level high stands. Soil crusts separate depositional sequences and define emergent periods. The youngest Pleistocene interglacial produced a classic shallowing upwards barrier reef that built up to a reef flat about 4 meters above present sea level. Pleistocene and Holocene reef sequences are each capped by oolitic grainstones that were spread across the reefs by longshore currents that also promoted beachspit accretion from ooid sources to the north and south. Ooid smothering of reef sequences occurred during the later part of each sea-level high stand. Older Pleistocene units are increasingly recrystallized and cemented, but the youngest retains most of the depositional mineralogy and has little marine or freshwater cement. Where not protected by preexisting emergent topography, the Pleistocene leeward margin sequence comprises repeated shallowing-upwards, burrowed to layered oolitic grainstone sequences. These grainstones retain much of their depositional mineralogy and are only weakly cemented. Well-cemented soil crusts formed during sea-level low stands, separate these oolitic sequences, and form permeability barriers that are responsible for ponding brines derived from overlying Holocene salinas. These brines promoted dolomitization of Holocene storm-derived grainstones below the salinas.

Abstract The shallow subsurface at Ocean Cay on western Great Bahama Bank consists of Pleistocene oolitic sand shoals and adjacent facies that exhibit facies-dependent petrophysical heterogeneity and reveal depositional and early diagenetic controls on petrophysical properties in oolitic bank margin reservoirs. Sedimentological heterogeneity is produced by complex facies architecture associated with abrupt lateral changes, while diagenetic heterogeneity is largely the result of patchy dissolution and cementation. The investigated oolitic interval is the topmost unit in 22 tightly-spaced borings covering an area of about 1 km 2 . Typical lithofacies include cross-bedded oolitic and oolitic/skeletal/peloidal fine- to coarsegrained grainstones and bioturbated skeletal/peloidal grainstone to packstone ranging in thickness from 0.5 m to 15 m. The architecture of the cross-bedded facies is similar to the modern Cat Cay shoal configuration of bars and superimposed sand waves that are linked to tidal flows. Particularly, the juxtaposition of biotur-bated skeletal/peloidal sediment to cross-bedded oolitic/peloidal sands is mirrored in the Pleistocene section. Intense bioturbation within the skeletal/peloidal grainstone to packstone and bedding in the oolitic/pel-oidal/skeletal grainstone facies produce small-scale heterogeneity that is enhanced by diagenesis. Petrophysical data and petrographic analysis show that, in addition to facies, diagenesis controls porosity and permeability at different intensities. In particular, permeability is very sensitive to cementation and can be drastically reduced by few rims of fibrous aragonite and/or meniscus cements. Porosity in the Ocean Cay grainstones is high, ranging from 29 to 47%, and permeability ranges from 0.1 to 11500 mD. Permeability is partly controlled by bedding. The average permeability in the massive-bedded oolitic grainstone lithofacies is 2770 mD while it is only 620 mD in the laminated and cross-bedded oolitic/peloidal grainstone lithofacies. Vertical/horizontal permeability ratio decreases with increasing bedding complexity: in massive bedded oolitic grainstone k v /k h 0.34, in oolitic/peloid laminated and low-angle cross-bedded grainstone k v /k h 0.22, and in trough and tabular cross-bedded oolitic/peloid grainstone k v /k h 0.11. The relationship between permeability and porosity can be approximated by power-law functions. Values for the Archie cementation exponent range from 1.9 to 4.3 (m avg = 2.7), and the cementation exponent generally increases with increasing depth. For Ocean Cay carbonates the influence of microporosity, which is as much as 35-55% of the pore volume in these rocks, on critical water saturation is significant. Critical water saturation ranges from 36% to 95% and decreases with increasing porosity. In general, petro-physical trends for some properties in the Ocean Cay oolitic/oomoldic rocks are consistent with extension of trends for lower porosity/permeability rocks in ancient reservoirs to higher porosity and permeability.

Abstract In Budapest damage due to atmospheric pollution on many public buildings is severe. Black encrustations, white crusts and other decay features of a soft oolitic limestone have been studied in detail by using field measurements and laboratory analyses. Limestone weathering was assessed by description of weathering forms, by on-site petrophysical tests (Duroscope, Schmidt hammer, water absorption) and by laboratory mineralogical assessment and thermoanalysis (X-ray diffraction, Derivatograph). There is a clear correlation between the organic carbon content in stone and location of the site, particularly in the polluted city centre. Gypsum, which is not an indigenous mineral in the limestone, can contribute up to 70% of the crust composition and indicates the importance of air-derived SO 2 . This mineralogical change in stone composition leads to changes in physical properties, by strengthening laminar black crusts and white case hardened crusts and weakening the host rock.

Abstract The Cleveland Ironstone Field (NE England) is a major sedimentary iron orefield in which the principal ore minerals are iron silicates (berthierine, chamosite) and carbonates (siderite). The siderite in this area is known to be rich in Mg and Mn in solid solution with Fe. Although this ore assemblage would not normally be expected to give rise to acid mine drainage phenomena, a number of discrete ferruginous mine water discharges (totalling some 6.5 million litres (Ml) day −1 ) have been identified flowing from abandoned underground mine workings and old spoil heaps in the ore field. Some of these discharges are extremely acidic (pH ≥ 3.3), with total Fe reaching 1220 mg l −1 . At the point of first emergence to the surface, most of the discharges are so highly charged with dissolved CO 2 that they effervesce. Upon degassing, one of the discharges precipitates a ferroan calcite deposit, which is most unusual as a mine water discharge precipitate. All the other discharges precipitate more usual ferrihydrite and goethite ‘ochres’. The geochemistry of these waters supports the view that oxidation of pyrite in the roof strata initiates dissolution of siderite in the old workings, releasing CO 2 , Fe and Mg to solution. This chain of reactions results in these waters having SO 2− 4 as their major anion (from pyrite weathering) with Mg as the major cation (except where Fe exceeds Mg in concentration). Two of the near-coastal discharges have Na as the major cation, and elevated Cl concentrations, suggesting a sea-water component. However, SO 4 /Cl ratios suggest that sea water can account for no more than 20% of these waters. Most of the Cleveland mine waters have significant environmental impacts, ranging from ecological damage to receiving water courses to flooding problems caused by the clogging of surface sewers with mine water precipitates. A range of remedial measures are proposed.

Abstract The Mississippian Greenbrier Limestone is a major gas reservoir in the Appalachian basin, but its complex porosity patterns often deter active exploration. In southern West Virginia, the reservoir consists of oolitic tidal bars that are composites of smaller shoals. Porosity trends closely follow the ooidgrainstone facies that occupied shoal crests where coarse-grained, well-sorted ooid sand was generated with either unidirectional or bidirectional crossbeds. Nonporous packstone occurred in adjacent tidal channels, and a transitional grainstone/packstone facies of marginal porosity was situated along the flanks of the shoals. The key to drilling successful wells is in understanding the complex internal geometry of Greenbrier ooid shoals. A well penetrating the oolite with good porosity and bimodal cross-beds should be offset perpendicular to the dip directions; that is, parallel to the shoal axis. However, a well penetrating thin, porous limestone with one dominant crossbed azimuth should be offset opposite to that dip direction; that is, up the flank of the ooid shoal. Shaly interbeds characterize the edges of the shoals and mark the limit of productive wells. Schlum berger’s Formation MicroScanner log, which provides data on both lithology and cross-bedding, has proven to be a useful tool in predicting the distribution of oolite porosity.

Abstract Oolitic facies of the Ste. Genevieve Limestone are common and prolific hydrocarbon reservoirs in the Illinois basin. However, development and exploration efforts are often hampered by the inability to recognize the reservoir on wireline logs. A comparison of cores with wireline logs in northern Lawrence field, Lawrence County, Illinois, indicates that a subjective but predictable relationship exists between carbonate lithology and log character. Mapping of the oolitic facies to determine orientation, geometry, and extent can be accomplished in areas where wireline logs are the only data available.