Pinnacle Reefs Research Articles

Facies models of a shallow-water carbonate ramp based on distribution of non-skeletal grains (Kimmeridgian, Spain)

The internal facies and sequence architecture of a Late Jurassic (Late Kimmeridgian) shallow carbonate ramp was reconstructed after the analysis and correlation of 17 logs located south of Teruel (northeast Spain). The studied rocks are arranged in five high-frequency sequences A–E (5–26 m thick) bounded by discontinuities traceable across the entire study area (20 × 25 km). Facies analysis across these sequences resulted in the reconstruction of three sedimentary models showing the transition from interior ramp environments (i.e., lagoon, backshoal, and shoal) to the progressively deeper foreshoal and offshore areas. Coral-microbial reefs (meter-sized patch and pinnacle reefs) have a variable development throughout the sequences, mostly in the foreshoal and offshore-proximal environments. The preferential occurrence and down-dip gradation of non-skeletal carbonate grains has been evaluated across the three models: low-energy peloidal-dominated, intermittent high-energy oolitic-dominated and high-energy oolitic–oncolitic dominated. The predominance of these non-skeletal grains in the shoal facies was mainly controlled by the hydrodynamic conditions and spatial heterogeneity of terrigenous input. The models illustrate particular cases of down-dip size-decrease of the resedimented grains (ooids, peloids, oncoids) due to storm-induced density flows. Offshore coarsening of certain particles (intraclasts, oncoids) is locally observed in the mid-ramp areas favorable for microbial activity, involving coral-microbial reef and oncoid development. The observed facies variations can be applicable to carbonate platforms including similar non-skeletal components, where outcrop conditions make the recognition of their three-dimensional distribution difficult.

Geomechanical testing and modeling of reservoir and cap rock integrity in an acid gas EOR/sequestration project, Zama, Alberta, Canada

Abstract Geomechanical testing and modeling is one of the primary characterization activities employed at the Zama acid gas EOR, CO2 sequestration, and monitoring project. Program components include laboratory tests, log analysis, field stress testing, and numerical modeling to establish the overall integrity of the system. Initial results from this program indicate that the cap rock has a high mechanical strength, high stiffness, low compressibility, and very low permeability. These results demonstrate that the injection zone and cap rock at the F-Pool pinnacle reef have suitable characteristics for the long-term containment of acid gas.

A visual data-mining methodology for seismic facies analysis: Part 2 — Application to 3D seismic data

A visual data-mining approach to unsupervised clustering analysis can be an effective tool for visualizing and understanding patterns inherent in seismic data (i.e., seismic facies). The unsupervised clustering analysis is completely data-driven, requiring no external information (e.g., well logs) to guide the seismic-trace classification. We demonstrate the application of the visual data-mining approach to seismic facies analysis on a real 3D seismic data volume. We select two stratigraphic intervals, the first including a Devonian pinnacle reef system and the second containing a Jurassic siliciclastic channel system. Both analyses show major stratigraphic features that can be defined in horizon slices or other types of visualization. However, the visual data-mining approach creates seismic facies maps with improved visual detail, distinguishing seismic trace-shape variability in the data. We also compare the facies maps with those obtained from a commercial package for seismic facies classification. Both approaches created similar facies maps, but the visual strategy better depicts subtle stratigraphic changes in the bodies being imaged, offering insight into the nature of these features.

Global and regional influences on equatorial shallow-marine carbonates during the Cenozoic

The SE Asian carbonate record allows insight into the poorly known response of equatorial marine systems to regional and global change during the Cenozoic. There is a marked change from larger benthic foraminifera to corals as dominant shallow-marine carbonate producers in SE Asia around the Oligo-Miocene boundary. The Early Miocene acme of coral development in SE Asia lags Oligocene coral development in the Caribbean and Mediterranean, despite local tectonics providing apparently suitable habitable areas. Regional and global controls, including changing CO 2, oceanography, nutrient input and precipitation patterns are inferred to be the main cause of this lag in equatorial reefs. It is inferred that moderate, although falling level of CO 2, Ca 2+ and Ca\\Mg when combined with the reduced salinities in humid equatorial waters all contributed to reduced aragonite saturation hindering reefal development compared with warm more arid regions during the Oligocene. By the Early Miocene, atmospheric CO 2 levels had fallen to pre-industrial levels. Although this was a relative arid phase globally, in SE Asia palynological evidence indicates that the Early Miocene experienced everwet, but more stable and less seasonal conditions than periods before or after. Tectonic convergence truncated deep throughflow of cool nutrient-rich currents from the Pacific to Indian Ocean around the beginning of the Miocene, thereby directly, and perhaps indirectly (though less seasonal conditions) reducing nutrients. It is inferred that aragonitic reefs were promoted where previously the waters had been more acidic, more mesotrophic, more turbid, and less aragonite saturated. Extensive reefal development resulted in an order of magnitude expansion of shallow-carbonate areas through buildup and pinnacle reef formation in the Early Miocene. Tectonics via increased habitat partitioning and reducing distances to other coral-rich regions may also have contributed to enhanced reefal development. Declining reefal importance at the end of the Early Miocene resulted from uplift of land areas, enhanced oceanic ventilation, through thermohaline circulation and narrowing of oceanic gateways as well as increased seasonal runoff, at least in SE Asia through initiation\\intensification of the monsoons. Implications of this study are that with current anthropogenically-induced environmental changes it will be the diverse reefs of SE Asia that are likely to be amongst the first and hardest hit as oceanic aragonite saturation decreases and terrestrial nutrient runoff increases.

Seismic characteristics of a reef carbonate reservoir and implications for hydrocarbon exploration in deepwater of the Qiongdongnan Basin, northern South China Sea

Our analysis of approximately 40,000 km of multichannel 2-D seismic data, reef oil-field seismic data, and data from several boreholes led to the identification of two areas of reef carbonate reservoirs in deepwater areas (water depth ≥ 500 m) of the Qiongdongnan Basin (QDNB), northern South China Sea. High-resolution sequence stratigraphic analysis revealed that the transgressive and highstand system tracts of the mid-Miocene Meishan Formation in the Beijiao and Ledong–Lingshui Depressions developed reef carbonates. The seismic features of the reef carbonates in these two areas include chaotic bedding, intermittent internal reflections, chaotic or blank reflections, mounded reflections, and apparent amplitude anomalies, similar to the seismic characteristics of the LH11-1 reef reservoir in the Dongsha Uplift and Island Reef of the Salawati Basin, Indonesia, which house large oil fields. The impedance values of reefs in the Beijiao and Ledong–Lingshui Depressions are 8000–9000 g/cc × m/s. Impedance sections reveal that the impedance of the LH11-1 reef reservoir in the northern South China Sea is 8000–10000 g/cc × m/s, whereas that of pure limestone in BD23-1-1 is >10000 g/cc × m/s. The mid-Miocene paleogeography of the Beijiao Depression was dominated by offshore and neritic environments, with only part of the southern Beijiao uplift emergent at that time. The input of terrigenous sediments was relatively minor in this area, meaning that terrigenous source areas were insignificant in terms of the Beijiao Depression; reef carbonates were probably widely distributed throughout the depression, as with the Ledong–Lingshui Depression. The combined geological and geophysical data indicate that shelf margin atolls were well developed in the Beijiao Depression, as in the Ledong–Lingshui Depression where small-scale patch or pinnacle reefs developed. These reef carbonates are promising reservoirs, representing important targets for deepwater hydrocarbon exploration.

Effect of H2S and Pressure Depletion on the CO2 MMP of Zama Oils

Abstract Phase behaviour and minimum miscibility pressure measurements were carried out to evaluate the potential of applying miscible CO2/H2S flooding in the Zama reefs of NE Alberta, Canada. Since there is some variability in the properties of oils from different wells or reefs, the measurements were carried out on recombined reservoir fluids from two different wells. The injection solvent will be supplied from a nearby gas plant that is expected to produce CO2/H2S streams with a range of compositions due to a number of operational factors. Accordingly, three solvent injection gases were considered: pure CO2 and CO2 containing 20 and 40 mol% H2S. Minimum miscibility pressure (MMP) measurements were carried out using the rising bubble apparatus (RBA). The MMP decreased almost linearly with the amount of H2S in the injection gas in the range of compositions studied. Some evidence of precipitation of solids was observed. Measurements were also carried out to determine the CO2 MMP of these reservoir fluids as they were depleted of gas through a differential liberation procedure. The results show that the MMP decreased with decreasing liberation pressure. Key properties of the liberated reservoir fluids were also measured and compared with those of the recombined reservoir fluids. The results show that miscible flooding with sour acid gas is feasible in this case, and could provide an excellent means of storing/sequestering these gases while improving oil recovery. Introduction Vertical gasfloods have an excellent track record for producing additional oil. For example, hydrocarbon miscible floods have been carried out in a number of Canadian reefs(1–5). Vertical gravity-stable carbon dioxide floods were reported in three US fields(4–8), also with excellent additional oil recoveries. The Zama Field(9) is located in northeastern Alberta and contains a large number (600 – 800) of relatively small pinnacle reefs with up to 1.5 ? 106 m3 (10 mmbbl) of original oil in place (OOIP). The wells selected for this study produce from the Keg River Formation. Al-Dilwe and Asghari(10) described the reservoir and its production history from the 1960's. The produced oil and gas are sour and contain an appreciable amount of carbon dioxide. The centrally located Zama Gas Plant (operated by Apache Canada Ltd.) removes the H2S and CO2 from the produced gases by amine absorption. These acid gases constitute an excellent miscible flooding agent for the field, thereby achieving the double advantage of increasing the oil recovery and providing storage capacity for these gases. Trivedi et al.(9) simulated different strategies for solvent injection to maximize oil recovery and gas storage as well as delaying breakthrough, and found that pressure maintenance is a key factor in the operation. A significant degree of acid gas breakthrough is expected to occur due to the reservoir heterogeneity represented by these Zama/Keg River pinnacles. Breakthrough will occur at H2S concentrations that are too high to process through the existing amine systems. To accommodate early breakthrough, recycle compression is to be installed in order to bypass the amine systems and re-inject acid gas back into miscible flooded pinnacles.

Petroleum systems and hydrocarbon potential analysis of the northwestern Uralsk basin, NW Kazakhstan, by utilizing 3D basin modeling methods

Gas and minor oil production from the northwestern Uralsk basin is associated with TTGF, Çaganskaya and Darinskaya fields located along a NE–SW aligning chain of barrier reefs. Most important reservoir rocks of the basin are associated with shelf, reef, pinnacle reef, atoll and bar facies of the carbonate complex, deltaic or tidal facies of the platform deposits and clastic basinal fan or slope facies. The basin comprises multiple petroleum systems, characterized by deep marine black-shales of Paleozoic which shows up to 10% TOC. The evaluation of these petroleum systems were investigated by utilizing 1-D and 3-D basin models constructed using the geological, geophysical and geochemical data. The sub-salt section of the northwestern Uralsk basin demonstrates higher hydrocarbon potential and lower exploration risk compared to the post-salt sequence despite the necessity for drilling extreme depths (e.g. over 5000 m) particularly in the Inner Zone. Various traps associated with salt tectonics favor hydrocarbon accumulations in the Artinskian/Filippovskiy, Frasnian, Bobrikovskiy/Tulskiy and Bashkirian proven reservoir rocks. Among these reservoirs, the Artinskian/Filippovskiy is the major oil bearing reservoir rock particularly in the Teplovskaya, Gremiachinskaya and Teplovskaya fields. It has good hydrocarbon charge access to multiple source rocks. Besides, Kungurian salt forms efficient seal on the top. Among the 15 investigated and modeled source rocks, the most expulsive ones are Afoninskiy, Givetian, Famennian, Tournaisian, Tulskiy, Serpukhovian and Vereiskiy, which intensively charge reservoirs of the northwestern Uralsk basin starting from Triassic. Source to trap timing risk is minimal, particularly within the Inner and Escarpment zones since traps are older. Gas and oil generation, expulsion and migration model results point out a total of 179 Mboe gas and 89 Mbbl oil potential (subsurface conditions) associated with Gremiachinskaya, W. Teplovskaya, Teplovskaya, Tokarevskaya, E. Gremiachinskaya, Ulianovskaya and Tsyganovskaya structures.

Effect of Operational Parameters on Carbon Dioxide Storage Capacity in a Heterogeneous Oil Reservoir: A Case Study

Underground storage of carbon dioxide (CO2) is attracting considerable interest worldwide as a means of avoiding continued release of CO2 from anthropogenic sources. Here, a heterogeneous oil reservoir in Alberta, Canada, was chosen for evaluating the potential use of this site for storage of a mixture of 90% CO2 + 10% H2S produced from a nearby gas plant. This reservoir produces 34° API light oil from a pinnacle reef, which is a carbonate reservoir with a depth of 4800 ft (1441 m). A fully compositional, three-dimensional (3-D) reservoir simulation model, CMG-GEM, was used to simulate various operational conditions, study the reservoir and fluid characteristics, and investigate the amount of CO2 stored and oil recovered. The results of this study show that a combination of two vertical injectors and one horizontal producer optimizes the incremental oil recovered and amount of CO2 stored. The procedure developed in this study, and the findings of this study, can be used as guidelines for designing and implementing any future CO2 injection and storage project in similar oil reservoirs.

Microbial–metazoan reefs of the terminal Proterozoic Nama Group (c. 550–543 Ma), Namibia

Thrombolite and stromatolite reefs occur at several stratigraphic levels within the terminal Proterozoic Nama Group (c. 550–543 Ma) of central and southern Namibia. The reefs form integral parts of several carbonate platforms within the Nama Group, including the Kuibis platform of the northern Nama Basin (Zaris subbasin), and Huns platform (Witputs subbasin) of the southern Nama Basin. The reefs are composed of both thrombolites and stromatolites that form laterally continuous biostromes, isolated patch reefs, and isolated pinnacle reefs ranging in scale from a metre to several kilometres in width. In the majority of cases, the reefs occur stratigraphically as an integral facies within the transgressive systems tracts of sequences making up the Kuibis and Huns platforms. This suggests that a regime of increasing accommodation was required to form well-developed reefs, though reefs also occur sporadically in highstand systems tract settings. Within a given transgressive systems tract, a regime of increasing accommodation through time favours the transition from sheet-like biostromal geometries to more isolated patch and pinnacle biohermal geometries. Similarly, increasing accommodation in space, such as a transect down depositional dip, shows a similar transition from more sheet-like geometries in updip positions to more isolated geometries in downdip positions. Reefal facies consist of thrombolitic domes, columns and mounds with well-developed internal clotted textures, in addition to stromatolitic domes, columns and mounds, with crudely to moderately well-developed internal lamination. Stromatolites are better developed in conditions of relatively low accommodation, and updip locations, under conditions of higher current velocities and greater sediment influx. Thrombolites are better developed in conditions of relatively high accommodation and low sediment influx. Both types of microbialites are intimately associated with the first calcifying metazoan organisms, which may have attached themselves to the sediment surface or otherwise lived within sheltered depressions within the rough topography created by ecologically complex mats. The appearance of thrombolitic textures during terminal Proterozoic time is consistent with colonization of cyanobacterial mats by higher algae and metazoans, which would have been an important process in generating clotted textures. Fabrics in the Nama thrombolites are well preserved and show evidence of thrombolitic mesoclots being overgrown by fibrous marine carbonate, interpreted as former aragonite. This was followed by emplacement of geopetal micrite fills, and precipitation of dolomite as an isopachous rim cement, followed by occlusion of remaining porosity by blocky calcite spar.

Well-log tomography and 3-D imaging of core and log-curve amplitudes in a Niagaran reef, Belle River Mills field, St. Clair County, Michigan, United States

Well-log tomography, a recently developed technique that generates subhorizontal slices through log curves, was used to create a series of amplitude slices from the core permeability, core porosity, and gamma-ray log curves for the wells penetrating the Silurian Brown Niagara reservoir in the Belle River Mills field. Log-curve amplitude slicing and three-dimensional (3-D) imaging of the core permeability and core porosity amplitudes show the detailed anatomy of the pinnacle reef in terms of these important reservoir properties. Although the reef has been dolomitized, the spatial distribution of permeability and porosity amplitudes in the reservoir appears to be controlled by the original pore and depositional fabrics of the wackestone, boundstone, stromatolite, and reef conglomerate rock types.Trends and patterns observed in the normalized gamma-ray log tomography approximate the trends and patterns observed in the core permeability and core porosity tomography in the Belle River Mills field. The similarities in the tomography trends and patterns establishes a potentially significant relationship that can be used to interpret log-curve amplitude patterns in other Niagaran pinnacle reef fields with log-curve data only and perhaps lead to improved sequence-stratigraphic models for these reservoirs.Well-log tomography and 3-D imaging provide complementary visualization tools that use existing data to image reservoirs and geologic formations. These imaging tools can be used to improve the design and placement of vertical, deviated, and horizontal wellbores for primary, secondary, tertiary, and gas storage operations in these reefs and other reservoirs.

Digital Reconstruction and Stratigraphic Evolution of a Microbial-Dominated, Isolated Carbonate Platform (Terminal Proterozoic, Nama Group, Namibia)

ABSTRACT A terminal Proterozoic (ca. 550 Ma) isolated carbonate platform developed downdip on a carbonate ramp of the Nama Group of southern Namibia. The stratigraphic evolution of the platform was reconstructed from an extensive dataset that was compiled by using digital surveying technologies and traditional outcrop observations. Digital field geology techniques enabled rapid and accurate mapping over the 10 km outcrop belt, with spatial and geological information captured at scales ranging from centimeters to kilometers. Furthermore, these digital geology techniques made the reconstruction and interpretation of the development of the carbonate platform considerably more efficient and quantitative. The carbonate platform deposits encompass three accommodation cycles in which each subsequent cycle experienced progressively greater influence of a long-term accommodation increase. Aggradation and progradation during the first cycle produced a flat-topped, sheet-like platform. Deposits of the first cycle show a coarsening-upward motif grading from dominantly columnar stromatolitic thrombolites and interbedded mudstone-grainstone at the base, to dominantly massive and cross-bedded dolostones toward the top. The second cycle features aggradation and is typified by the formation of a distinct margin and development of a bucket-shaped geometry. The margin contained thrombolite mounds and domes, whereas columnar stromatolitic thrombolites dominated the platform interior. The third and last cycle records a gradual deepening trend with initial aggradation and formation of well-bedded, thin deposits in the interior and microbial mounds at the margins. While the interior drowned, the platform margins kept up with rising sea level and complex pinnacle reefs formed, which contained fused and coalesced thrombolite mounds flanked by bioclastic grainstones and collapse breccias. The top of the unit consists of a set of isolated large thrombolite mounds flanked by shale, and is interpreted to indicate the final stage of the carbonate platform before drowning. The overall geometric evolution and spatial scale of this platform, from flat-topped to bucket-shaped with elevated margins, is recorded in many Proterozoic and Phanerozoic isolated carbonate platforms. This study clearly illustrates that accommodation changes are a fundamental control on the development of carbonate platforms, regardless of whether they are formed from microbial or metazoan components, or are of Precambrian or Phanerozoic age. Significantly, the architectural elements and spatial dimensions are comparable throughout earth history.

Paleogene and Early Neogene Lacustrine Reefs in the Western Qaidam Basin, China

Abstract Typical reefs in the Paleogene and early Neogene strata of the Qaidam Basin, Tibetan Plateau, China, reveal their internal structures and sedimentation environments and consist mainly of algal reef, stromatolite reef and thrombolite reef with distinct øeef structures, fore‐reef, back‐reef and reef‐plateau. The fore‐reef is characterized by a combination of pinnacle reef, thrombolite and algal reef. The back reef is composed of stromatolite reef and algal reef. The pinnacle reefs (micro‐atoll), most of which are several tens of centimeters in diameter (whereas some exceptionally big ones are over 200 cm in diameter), and several tens of centimeter to 2 m in height, are situated on the far front‐edge of the reef; the pinnacle reef is also often of recumbent form with a gravel‐filled circular hole in the center. The algal reef is in the form of dome and irregular beds, and filled with algal detritus, ostracodes, spirorbis fossils, ooid and terrigenous debris, and worm traces; cavities and scour marks are often developed. The algal reef is gray commonly when fresh and weathers to a brown color. The lacustrine thrombolite in the Qaidam Basin is light gray or deep gray when fresh, white‐gray or brown when weathered, dense and homogeneous with abundant pores filled by oil and bitumen. Observed under the microscope, the thrombolite consists mainly of brown or brown‐black clots with a little algal debris, ooid, pellet, ostracodes, spirorbis fossils and terrigenous debris, in some cases, terrigenous debris, even gravel, is abundant. Many features of the thrombolite suggest that it is formed in a high‐energy environment. The stromatolite reefs developed on the lacustrine algal reef in the Qaidam Basin are very complex whether in shape or in internal structure. The simplest ones form laminated layers and the most complex ones have intensely branching structures. The size is also variable.

Reef‐capping laminites in the Upper Silurian carbonate‐ to‐evaporite transition, Michigan Basin, south‐western Ontario

AbstractSilurian pinnacle reefs in the subsurface of the south‐western Ontario portion of the Michigan Basin display a variety of laminated carbonates (laminites) within predominantly muddy reef‐capping facies in the upper part of the Guelph Formation and the overlying A‐1 Carbonate of the Salina Group. Laminites, which are limestone, dolomite or partially dolomitized limestones, have a range of morphologies, from simple planar to a variety of wavy and serrated forms. Individual laminae are composed mainly of micrite, microspar or replacive dolomite, and vary internally from isopachous and continuous over the diameter of the core to non‐isopachous and often discontinuous. Clotted and peloidal micrite, sometimes defining small knobs and chambers, is interpreted as being microbial in origin and occurs within all types of laminites. Fibrous cement locally comprises laminite clasts in breccias or coats clasts in breccias, and also occurs as spherulites in the interparticle spaces in breccias. Although similar laminites have been described from elsewhere in the Michigan Basin and interpreted as caliche, travertine and abiotic subtidal stromatolites, the laminites in south‐western Ontario are most realistically regarded as microbial. The causes for the variations in morphology and characteristics of the constituent laminae are uncertain, although fluctuations in local microenvironmental conditions would have been important, set against a backdrop of an increasingly restricted overall setting. Caliche or travertine origins for these laminites are unlikely in general, except perhaps locally at the subaerial exposure surface at the tops of pinnacle reefs.

Dolomitization and recrystallization of middle Silurian reefs and platformal carbonates of the Guelph Formation, Michigan Basin, southwestern Ontario

ABSTRACT Regional study of the Guelph Formation (middle Silurian) in the subsurface of southwestern Ontario highlights the role of recrystallization during burial diagenesis in producing massive dolomites with relatively uniform petrographic features but complicated geochemistry. The dolomite is composed mainly of replacive, unimodal to polymodal mosaics characterized by uniform, dull to weak red cathodoluminescence. Whereas patch reefs on the platform and most pinnacle reefs on the upper basin slope are pervasively dolomitized, reefs on more basinward parts of the slope may be partially dolomitized, usually in their lower portions, or they remain limestone. The lithology of overlying carbonates in the lower Salina Group mimics closely that of the underlying Guelph Formation. Initial shallow burial dolomitization occurred in the Late Silurian in response to isolation and hydrological drawdown of the Michigan Basin. The basinward-decreasing dolomite trend resulted from continuous seawater flux from the middle (back reef) platform downward through Guelph limestones to the discharge areas on the slope and beyond. Reefs at the distal end of the regional paleoflow system, or those that were poorly connected to the system remained limestone. Both dolomite 13C values, which range from +1.1 to +5.0 ‰ (PDB), and 87Sr/86Sr ratios, which range from 0.70845 to 0.70910, reflect a Silurian seawater source. Values for 18O range from -5.2 to -9.7‰ (PDB) but cannot be explained by the above mechanism alone. Recrystallization likely occurred during the progressive burial of these rocks to depths of up to 2 km, and may at least in part explain the lack of correlation among dolomite crystal size, cathodoluminescence characteristics and the various geochemical parameters measured in this study. The reason for the wide spread in 18O values both geographically and stratigraphically reflects varying degrees of recrystallization as a function of macro- to micro-scale permeability and porosity differences in the precursor lithology and their control on fluid-rock interactions during recrystallization. The cause of the apparent decrease in dolomite 18O values from patch reefs on the platform (average 18O = ~ -6‰) to the lower slope (average 18O = ~ -8.5‰) is unknown, but may have resulted from a combination of a basinward increase in burial depth and a basinward increase in heat-flow, which are now only subtly preserved through the noise of multiple recrystallizations. End_Page 177------------------------

Evidence for reversal of basin polarity during carbonate ramp development in the Mesoproterozoic Borden Basin, Baffin Island

Distribution of facies in the lower half of the Bylot Supergroup suggests overall westward deepening of the Mesoproterozoic Borden Basin. In marked contrast, the upper half of the succession records a reversal in the overall bathymetric trend, such that the eastern portion underwent relative deepening as the west experienced relative shallowing. Strata deposited during this reversal belong to the Victor Bay Formation, a ramp composed predominantly of limestone. Karsting of carbonate strata and development of an angular unconformity in the west contrast with back-stepping and drowning of the ramp in the east, followed by mantling by deep-water limestone, carbonaceous carbonate, and turbidites. Increased accommodation space during this time, via both tectonic subsidence and eustatic sea-level rise, led to a profusion of stromatolite pinnacle reefs and large biostromes. The reversal of basin polarity is best reconciled with development of a distal foreland basin superimposed on the Borden aulacogen. Crustal rethickening and uplift occurred along reactivated basement faults during an eastward-directed compressional event and could be related to thrusting of similar age and vergence in the Coppermine River Group of northwestern Canada.

Drowned reefs and antecedent karst topography, Au'au Channel, S.E. Hawaiian Islands

During the last glacial maximum (LGM), about 21,000 years ago, the Hawaiian Islands of Maui, Lanai, and Molokai were interconnected by limestone bridges, creating a super-island known as Maui-Nui. Approximately 120 m of sea-level rise during the Holocene Transgression flooded, and then drowned, these bridges separating the islands by inter-island channels. A new multibeam high-resolution bathymetric survey of the channels between the islands, coupled with observations and video-transects utilizing DeepWorker-2000 submersibles, has revealed the existence of numerous drowned reef features including concentric solution basins, solution ridges (rims), sand and sediment plains, and conical-shaped reef pinnacles. The concentric basins contain flat lagoon-like bottoms that are rimmed by steep-sided limestone walls. Undercut notches rim the basins at several depths, marking either sea-level still stands or paleo-lake levels. All of the solution basins shallower than 120 m were subaerial at the LGM, and at one stage or another may have been shallow shoreline lakes. Today, about 70 drowned reef pinnacles are scattered across the Maui–Lanai underwater bridge and all are situated in wave-sheltered positions. Most drowned during the interval between 14,000 and 10,000 years ago when sea-level rise averaged 15 mm/year. Virtually all of the surficial topography in the Au'au Channel today is a product of karst processes accentuated by marginal reef growth during the Holocene. Both the submerged basins and the drowned reefs represent an archive of sea-level and climate history in Hawaii during the late Quaternary.

Microbialites in a modern lagoonal environment: nature and distribution, Tikehau atoll (French Polynesia)

Microbialites, including stromatolitic structures, gelatinous masses and mats develop in Tikehau lagoon (Tuamotu, French Polynesia), on the flanks and slopes of numerous pinnacles and ‘motu’ islets between 0 and 25 m deep, on coral colonies, algal tufts or on lagoonal sediments. Individual stromatolitic structures consist of hemispherical domes and are produced by monospecific populations of filamentous cyanobacteria belonging to three distinctive Phormidium species, Symploca, and possibly to one new species of Schizothrix. Gelatinous masses hanging from the ceiling of caverns are produced by Phormidium laysanense. Microbial mats are well developed at all depths on pinnacle slopes and are produced by either monospecific or mixed cyanobacterial populations. The study of micro- and ultrastructure of stromatolites reveals that they result both from trapping and binding of sediments and from carbonate precipitation. In addition to phototrophic biomineralization, pervasive carbonate precipitation occurred in association with decaying organic matter. The basic constructional elements of the precipitated carbonate consist of 0.1–0.2-μm rounded bodies that form very fine anhedral to subhedral micrite; their surfaces comprise extremely small carbonate granules (20–50 nm). Gradual changes (ageing) in the ultrastructure of carbonate precipitates include the precipitation of successive euhedral calcite layers around these grains. Widespread development of microbialites in this pristine lagoonal environment results from rapid blooms of microbial benthic communities, possibly induced by temporary and local nutrient pulses. Nutrients seem to undergo a restricted circulation between the sediment and pinnacle reef frameworks. Pinnacles and reef islets are repeatedly affected by catastrophic storm events which cause redistribution of sediments and nutrients.

REEFS IN THE BASINAL FACIES OF THE ZECHSTEIN LIMESTONE (UPPER PERMIAN) OF WESTERN POLAND: A NEW GAS PLAY

Zechstein (Upper Permian) reefs have long been known to occur along the margins of the Polish Zechstein Basin where they host important gas reserves. During the last decade, reefal build‐ups have also been found in the centre of the basin on pre‐Zechstein highs such as the Wolsztyn Ridge in Western Poland. Here, early 2D seismic and subsequent more detailed 3D seismic studies and integrated geophysical and geological interpretations have led to a number of major gas discoveries. The reef complexes in the Koscian– Wielichowo region are thought to have recoverable gas reserves of more than 32B cu m.The basin‐centre reefs have been identified on the basis of palaeogeographic analysis and from the seismo‐acoustic properties of the Zechstein Limestone of which they are composed. 3D seismic profiles show that the reefs have irregular shapes in map view. Drilling results indicate that their thickness may exceed 80m, but characteristically their thickness decreases sharply at their margins corresponding to a facies change.In the Wolsztyn Ridge area, the Zechstein Limestone can be divided into five lithological units which are capped by the Werra Anhydrite. Four of these units reflect deposition in subaqueous environments whereas the uppermost unit originated in a very shallow‐water or temporarily subaerial setting. Typical “ecological” bryozoan‐stromatolitic reefs occur in the middle and upper (but not the uppermost) parts of the Zechstein Limestone, while the lower parts are dominated by bioclastic packstones and grains tones. Average reservoir porosity is 10–20%. The permeability varies significantly (0–400 mD) but values of a few tens of mD are common.The occurrence of intraclastic breccia in many boreholes suggests that the reefs may have developed on local palaeoslopes which are generally of tectonic origin. Blocks of contemporaneously lithifiedZechstein Limestone as well as steeply‐dipping laminar sheets are common in the reefs of the Wolsztyn Ridge and have also previously been recorded in the English Zechstein. Palaeoslopes elsewhere in the Wolsztyn Ridge and possibly also in other parts of the Variscan externides may host similar reefal build‐ups which could be of exploration potential.

Physiography and Late Quaternary-Holocene Processes of Northeastern Gulf of Mexico Outer Continental Shelf off Mississippi and Alabama

High-resolution multibeam mapping of the mid- and outer continental shelf and upper slope off Mississippi and Alabama reveals a complex bathymetry that reflects conditions during the last eustatic rise and the present high stand of sea level. The most prominent bathymetric features are pinnacles and hardgrounds that are scattered throughout the area. These features generally stand <10 m above the surrounding>seafloor, cover large areas, and display a variety of morphologies. Almost all the reef pinnacles and hardgrounds have zones of high acoustic backscatter on their summits and on the seafloor immediately adjacent to their southwest walls. In addition, they also have erosional moats on the seafloor to the southwest. Large fields of bedforms are scattered throughout the mapped area. The asymmetries and orientations of the bedforms suggest that they were formed by excursions of the northeast-flowing Loop Current. In contrast, the pervasive ponding of sediment on the northeast sides of bathymetric highs indicates that one of the predominant directions of sediment transport has been to the south and southwest. The shelf break is a zone of numerous landslides of various sizes and complexities. The morphology of several landslide scars indicates that some of the failures occurred recently. One large reef-capped salt dome was mapped in the area, surrounded by a large field of pockmarks. Fields of pockmarks are also scattered on the shelf. The growth and demise of the reefs are related to the fluctuating transgression of eustatic sea level during the last deglaciation. Two episodes of reef drowning are correlated with the increased rates of sea-level rise during documented melt-water pulses; the first occurred from 14.8 to 14.2 ka and the second from 11.8 to 11.2 ka. Rates of sea-level rise exceeded the maximum growth rate of hermatypic corals only during these two intervals since the last glacial maximum and thus drowned the coral communities.

ABSTRACT: Natural Gas Storage in a Southeastern Michigan Pinnacle Reef

ABSTRACT: Natural Gas Storage in a Southeastern Michigan Pinnacle Reef