Nature and distribution of porosity and permeability in jurassic carbonate reservoirs of the Arabian Gulf basin

Summary The Middle-Upper Jurassic section in the Arabian Gulf basin forms one of the most prolific sequences in the world, in which an excellent combination of source, reservoir and seal rocks was developed within a major sedimentary cycle. The sequence consists of a) relatively quiet deep-water mudstone, wackestone and shale (source facies), b) shallow-water high enery grainstone and packstone (reservoir facies), and c) very shallow supratidal anhydrite (seal facies). The principal factors, which controlled the sedimentation of this sequence, are considered to have been eustatic sea-level change and epeirogenic movement of carbonate shelves. The Jurassic reservoirs of the major oil fields in this region show exceptionally high porosity up to 30% for their relatively old geologic age (some 150 million years old) and depths of burial in the range between 1,200 and more than 2,700 m. Porosity occurs most commonly as intergranular/remnant primary pore spaces, but its distribution is quite uneven and very complicated. To account for the existence of such high porosity (and permeability) in the Jurassic reservoirs, probable geological, physical and chemical factors for preserving and enhancing porosity (and permeability), such as acidic formation fluids, reduced fluid mobility, tectonic forces, ductility of intercalated beds (e.g. anhydrite), and dolomitization were examined. It has been observed in various fields in the region that oilsaturated portions of the Jurassic reservoirs tend to retain higher porosity than the surrounding water-saturated zones. Porosity preservation by hydrocarbons is possible primarily because of excess hydrocarbon pressure and of reduced mobility of water in such oil-saturated zones. To continue sediment diagenesis, a steady supply of minerals by formation water and the mobility of the water may have been essential. Because the entrapment of oil in the Jurassic reservoirs in the region is considered to have been as late as early Tertiary, some other (pre-migration) mechanisms which may have worked in the earlier geologic stages for preserving and creating porosity (and permeability) seem to be necessary.     

Microbial carbonate production in a starved basin: The crenistria Limestone of the upper Viséan German Kulm facies

The crenistria Limestone is a set of three autochthonous massive limestone beds occurring with great lithological persistence in the Kulm Facies (cd III , upper Viséan) of the eastern Rheinisches Schiefergebirge. Microfacies analysis reveals mainly minipeloidal fabrics and homogeneous micrite. Uncrushed, sediment-filled conchs of goniatites represent loci of sheltered preservation of primary carbonate textures. Calcified radiolarians are abundant, forming between 20 and 80% of total rock volume. The alleged algal genus Rectangulina is common in the crenistria Limestone. It is reinterpreted to represent the faeces of goniatites. For the first time the presence of in situ preserved sponges is reported. They can be recognized as delicate networks of microsparitic needles embedded in peloidal fabrics. Hexactinellids with primary spicule arrangements can be found embedded in homogeneous micrite. The carbonate forming the limestone beds was produced microbially during decomposition of soft tissue of the radiolarians and sponges. During the cd III , anoxia in the bottom waters of the Kulm Basin persisted for long periods due to stable density stratification of the water column under humid climatic conditions. Oxic conditions in the bottom waters during formation of the limestone are indicated by bioturbation, the presence of sponges and the high Mn-contents of the carbonate. The latter derived from reduction of Mn-oxides during microbial carbonate formation.     

Age and composition of carbonate shoreface sediments, Kailua Bay, Oahu, Hawaii

The origin, age, and dynamics of carbonate sediments in Kailua Bay on Oahu, Hawaii, are described. The shoreface (from shoreline to 4 km offshore) consists of a broad (5 km²) fringing coral reef ecosystem bisected by a sinuous, shore-normal, sand-filled paleostream channel 200–300 m wide. The median grain diameter of surface sands is finest on the beach face (<0.3 mm) and increases offshore along the channel axis. Kailua sands are >90% biogenic carbonate, dominated by skeletal fragments of coralline algae (e.g. Porolithon, up to 50%) followed by the calcareous green alga Halimeda (up to 32%), coral fragments (1–24%), mollusc fragments (6–21%), and benthic foraminifera (1–10%). Sand composition and age across the shoreface are correlated to carbonate production. Corals and coralline algae, principal builders of the reef framework, are younger and more abundant in sands along the channel axis and in offshore reefal areas, while Halimeda, molluscs, and foraminifera are younger and more dominant in nearshore waters shoreward of the main region of framework building. Shoreface sediments are relatively old. Of 20 calibrated radiocarbon dates on skeletal constituents of sand, only three are younger than 500 years b.p.; six are 500–1000 years b.p.; six are 1000–2000 years b.p.; and five are 2000–5000 years b.p. Dated fine sands are older than medium to coarse sands and hence may constitute a reservoir of fossil carbonate that is distributed over the entire shoreface. Dominance of fossiliferous sand indicates long storage times for carbonate grains, which tend to decrease in size with age, such that the entire period of relative sea-level inundation (∼5000 years) is represented in the sediment. Despite an apparently healthy modern coral ecosystem, the surficial sand pool of Kailua Bay is dominated by sand reflecting an antecedent system, possibly one that existed under a +1–2 m sea-level high stand during the mid- to late Holocene. Accepted: 20 December 1999     

The carbonate factory continuum, facies mosaics and microfacies: an appraisal of some of the key concepts underpinning carbonate sedimentology

Our understanding of where and how carbonate sediments are produced and accumulate has changed considerably in recent years and a more complex framework is emerging. The earlier concept invoking a limited range of productivity-depth models has now evolved into an appreciation that there is a continuum of different types of productive sites over wide depth ranges, influenced by complex factors and not simply water depth or temperature. Studies of the nature of lithofacies ordering in the stratigraphic record, and most recent studies of the spatial distribution of Holocene environments, raise the issue that at the lithofacies scale the sedimentary record represents, in part, the product of complex and mobile facies mosaics. Many of these mosaic elements are not depth dependent and can change through time as a consequence of subtle environmental changes. As the rates of change typically exceed rates of accommodation space creation, individual sites are likely to have sediments of different environments superimposed and mixed (palimpsest). Recent studies showing the extent that dissolution is capable of skewing sediment compositions suggest that many ancient microfacies are unrepresentative of their original sediments, and there is a need for a more critical approach to interpreting microfacies in terms of identifying habitats and especially water depth. The carbonate factory is spatially and temporally highly variable and is not simply a uniform production line. This fact, coupled with the likely importance of selective early dissolution, may in part explain why accumulation rates estimated from ancient strata are lower than the production rates measured over short time periods.     

Polychaete annelids from Kuwaitian islands, Arabian Gulf, with descriptions of four new species

A systematic account of a collection of polvchaete annelids from four Kuwaitian islands, Arabian Gulf, is given together with ecological notes. The whole collection comprises 45 species, in 22 families. Of these, four species are named as new and ten are recorded as new to the fauna of the Arabian Gulf. The new species and records are: Anastrosyllis rigida, Brania balani new combination, Perinereis nigropunctata, Perinereis vancaurica, Onuphis eremita, Pkyh kubbarensis sp. nov., Scolelepis indica, Poecilochaetus serpens, Mesochaetopterus minutus, Parasclerocheilus branchiatus, Scyphoproctus aciculatus sp. nov., Clymenura annulata sp. nov., Pista unibranchiata and Telothelepus macrothoracicus sp. nov.     

Demise of reef-flat carbonate accumulation with late Holocene sea-level fall: evidence from Molokai,Hawaii

Twelve cores from the protected reef-flat of Molokai revealed that carbonate sediment accumulation, ranging from 3 mm year⁻¹ to less than 1 mm year⁻¹, ended on average 2,500 years ago. Modern sediment is present as a mobile surface veneer but is not trapped within the reef framework. This finding is consistent with the arrest of deposition at the end of the mid-Holocene highstand, known locally as the “Kapapa Stand of the Sea,” ~2 m above the present datum ca. 3,500 years ago in the main Hawaiian Islands. Subsequent erosion, non-deposition, and/or a lack of rigid binding were probable factors leading to the lack of reef-flat accumulation during the late Holocene sea-level fall. Given anticipated climate changes, increased sedimentation of reef-flat environments is to be expected as a consequence of higher sea level.     

Coralline algal facies and their palaeoenvironments in the Late Eocene of Northern Italy (Calcare di Nago, Trento)

Summary The Calcare di Nago is a carbonate unit of Middle-Late Eocene (Bartonian and Priabonian) age which is well exposed at the north-eastern end of Lake Garda, on the western margin of the Lessini Shelf (Southern Alps). This unit is highly fossiliferous as far as the coralline red algae and large foraminifera are concerned. Corals, bryozoans, echinoderms, and molluscs are also present. The present study deals with the relationships among the coralline taxa, the coralline growth-forms, and their facies development in the Priabonian part of the type section of the Calcare di Nago. The taxonomic investigation led to the identification of 15 coralline red algal species belonging to 7 non-geniculate and 2 geniculate genera. One species of Peyssonneliacean (red alga) and one of Halimedacean (green alga) were also recognized. The quantitative and qualitative analyses based on coralline red algae and large foraminifera enabled five facies to be distinguished: Algal crust-branch rudstone, Algal/Discocyclina packstone, Coralalgal boundstone, Rhodolith mound wacke/packstone, and Rhodolith pavement. According to the coralline assemblages, coralline growth-forms, and large foraminiferal associations, the five facies reflect solid and soft substrate types. Some of these facies are dominated byin situ rhodoliths, others by reworked algal debris. In the architecture of an interpreted prograding carbonate ramp, shallow water facies are dominated by members of the subfamily Mastophoroideae, while deeper water facies are dominated by those of the subfamily Melobesioideae and family Sporolithaceae. There is a significant increase both in size and in constructional voids of the rhodoliths with depth. A concomitant decrease in algal species diversity with depth has been also recognized. LargeDiscocyclina assemblages are localized across the inner and mid ramp boundary.Pellatispira andBiplanispira are present only in the uppermost mid-ramp.Nummulites, Assilina, andSpiroclypeus are dominant together with small orthophragminids both in the mid- and uppermost outer ramp facies.     

Extrapolation of depositional geometries of the Menorcan Miocene carbonate ramp with ground-penetrating radar

The Tortonian carbonate ramp of Menorca was previously studied on the basis of outcrops along sea-cliff outcrops. These sea cliffs, in combination with inland water wells, are the basis for a facies model for the reconstruction of the internal architecture and for characterizing the internal heterogeneities of this carbonate platform. However, any such three-dimensional reconstruction is generally limited by the given geometrical arrangement of the two-dimensional outcrops and the uncertainties of correlation with the one-dimensional wells. Here, ground-penetrating radar (GPR) has been employed in order to test and refine the depositional model. Although GPR is well known for being an excellent tool for high-resolution underground studies of sedimentary systems, the application for studying carbonate rocks is still far from routine. The reason for this discrepancy is two-fold: the minor mineralogical contrast between lithologies in carbonate rocks results in subtle reflections, and, even more important, the porosity structure in carbonates is thoroughly and repeatedly changed during diagenesis, commonly across the different facies, leading to problems in predictability of the petrophysical properties. The study of the Menorcan carbonate ramp with large distance–deep penetration GPR sections demonstrates that in spite of these difficulties, GPR is a valuable tool for extrapolating information from outcrops and wells. It is useful for characterizing heterogeneities larger than outcrop scale.     

Randomness or order in the occurrence and preservation of shallow-marine carbonate facies? Holocene, South Florida

This paper re-examines unsolved problems concerning the relationships between skeletal benthic communities, the skeletal carbonate sediments they produce and how these are preserved in the subsurface. Recent work based on shelf-wide datasets of modern shallow-marine carbonate sediments of South Florida suggest that the boundaries between facies occur randomly and that facies occurrence bears little relation to water depth. This is at variance with earlier work from the region that indicated facies occurrence related to different environments and which helped establish the basis for palaeoenvironmental analysis of ancient limestones.A windward-facing depositional margin of a carbonate mound in the back-reef area of the Florida Keys is used as a small-scale, case study to examine whether surface peritidal facies occur in an ordered or random fashion and whether they are depth related. Lateral facies transition analysis along transects from the shoreline to the shallow subtidal indicates that peritidal facies occur in a very well-ordered (i.e. non-random) arrangement of zones and patches. Surface facies are generally well-preserved and recognisable in the shallow subsurface and in cores through the Holocene carbonates and shoreline mangrove peats. Analysis of upward facies transitions in cores also indicates common facies trends reflecting the evolution of the sediment mound in response to rising Holocene sea level. However, even though the modern facies occur in an ordered and depth-related pattern, subsurface facies do not show a simple relation to the known sea-level curve in the area. Rather, they relate to a complex of different rates of sea-level rise, sea-floor topography, carbonate production rates, wave/storm energy input, and bioturbation.     

Diatom assemblages in superficial sediments from the Gulf of Riga,eastern Baltic Sea

Twenty one superficial sediment samples from areas of high and moderate eutrophication in the Gulf of Riga were studied with respect to siliceous microfossils, mainly diatoms.The results seem to imply that the number of taxa and the abundance of the frustules are affected by runoff from rivers and the degree of eutrophication in different parts of the Gulf. Areas with high eutrophication, e.g. the river estuaries, have diatom assemblages of varying composition, while in areas with moderate eutrophication the composition is almost constant and the influx of freshwater diatoms and littoral periphyton small. The high abundance and low diversity of brackish-marine and brackish, planktonic diatoms seem to be a result of an influx of nutrients and pollutants in the water and bottom sediments. The sea ice diatoms occurring in the Baltic waters and also in the Gulf of Riga tend to be resistant to eutrophication or are even favoured by it.     

Upper miocene carbonate ramp deposits from the southernmost part of Maiella Nountain (Abruzzo, Central Italy)

Summary The development of carbonate ramp depositional systems in the Neogene of the Mediterranean Region represents a widespread feature so far analysed in several papers. It is striking to note that the evolution of upper Miocene carbonate ramps, characterized by the presence of coralgal bioherms, highlights the events leading to the Messinian salinity crisis. The coralgal bioherms of preevaporite Messinian age exhibit fossil assemblages indicating marine waters with normal salinity, whereas stromatolitic and microbial encrustations underline the deterioration of the environment during the Messinian salinity crisis. Maiella Mountain is a broad carbonate massif located in Abruzzo (Central Italy). The late lower Oligocene-Messinian part of its stratigraphic succession consists of stacked non-tropical carbonate ramp deposits related to third and higher order sequences. The investigations performed in the southernmost portion of the massif allowed to recognize a complete fourth order carbonate depositional sequence on a homoclinal ramp of pre-evaporite Messinian age. The presence of small coralgal patch reefs and overlaying microbial encrustations is significant. A transect exhibits the stratigraphic framework of the area. The data show how local parameters play a notable role in the development of these deposits.     

Sedimentary facies from the modern coral reefs,Jordan Gulf of Aqaba,Red Sea

Summary Sixty-three sediment samples collected from the modern fringing reefs off the Jordan coast (Gulf of Aqaba, Red Sea) have been analysed in order to determine variations of composition and texture by using correspondence factor analysis. From the shore seawards, the following physiographic zones are recognized: beach; shallower backreef zone; reef flat zone; forereef zone including sandy or coral-built slopes. Eight sediment facies and subfacies can be recognized on the basis of total component composition and foraminiferal assemblages and four sediment facies can be recognized using grain-size data. Wellsorted, fine to medium, quartzofeldspathic sands (terrigenous facies) occur on beaches and outer sandy slopes close to wadi mouths. Backreef areas exhibit relatively well-sorted fine sands of terrigenous-coral and Miliolidae-Soritidae facies. Poorly sorted, coarse sands of coral-coral-line algal and Homotremid facies characterize reef flats and the upper parts of coral-built forereef areas, which respecitively display an Amphistegina-Spirolina subfacies and an Acervulina one. Poorly sorted, medium sands of coralmolluscan-foraminiferal (Amphistegina-Acervulina) facies are restricted to the lower parts of the forereef zone. Latcral limits of the various biofacies coincide with the distribution of the related organic communities.     

Geochemical associations,risk assessment,and source identification of selected metals in sediments from the Suez Gulf,Egypt

Sequential extraction procedure has been carried out to scrutinize the geochemical association of Cadmium (Cd), Copper (Cu), Iron (Fe), Lead (Pb), and Manganese (Mn) with the main sedimentary phases (exchangeable, carbonate, Fe-Mn oxides, organic, and residual fraction) at 10 marine sediment samples collected from the Suez Gulf. Global contamination factor was used to identify the pollution hotspots, which indicated the existence of intensive metal contamination at stations close to the northern part of the Gulf, especially for Cd, Cu, and Pb. The potential ecological risk indices: Potential ecological risk index, risk assessment code, and modified risk assessment code revealed that there are potential hazards to marine biota caused by Cd, Cu, and Pb due to the increased levels of these metals in the labile fractions, which are vulnerable to changes in environmental conditions. This work is a good basis for further studies about metal fractionation in the Suez Gulf. It is also very helpful for government decisions about the development of the Gulf.     

Bathymetric distribution and diversity of deep water polychaetous annelids in the Sigsbee Basin, northwestern Gulf of Mexico

The deep water polychaete fauna is analyzed in this study particularly regarding its composition and variations with depth in the Sigsbee Basin, northwestern region of the Gulf of Mexico. Samples were taken at 10 stations along a bathymetric gradient with depth ranges from 200 to 3760 m with a USNEL (0.25 m²) corer. A total of 287 individuals were identified, from 21 families and 65 species. The most important families, both in terms of abundance and species richness, were: Paraonidae (65.4 ind./0.25 m², 9 spp.), Cirratulidae (28.93 ind./0.25 m², 7 spp.) and Spionidae (18.07 ind./0.25 m², 7 spp.). In general, density tended to decrease with depth with minima at around 2000 m, although two abundance peaks were detected at 3700 and 3760 m, making the pattern seem an inverted parabolic curve. The Shannon-Wiener diversity values varied from 0.54–0.92 at around 2000 m to 3.39 at 3620 m and 3.34 at 3760 m. These results contrast with what is already reported from the North Atlantic and the Tropical Pacific deep benthic communities, where highest diversities are found at 2000 m. Faunal changes evaluated through Beta Diversity (0.08–0.1) and the low similarity found between the stations, emphasized the high variability in the composition of the fauna in the Sigsee Basin, meaning that the faunal composition is practically different at all the sampling stations. Ten species are newly recorded for the Mexican fauna.     

A new genus and species of galeommatoid bivalve (Mollusca: Bivalvia: Montacutidae) from Kuwait Bay,Arabian Gulf

Larissamya comta gen. n., sp. n., is described from Kuwait Bay, northern Arabian Gulf. The hinge is identical to that seen in Kurtiella and Montacutona but the spined dorsal margin is unique. Anatomically Larissamya differs from Kurtiella in retaining both gill demibranchs and from both Kurtiella and Montacutona in having greatly unequal adductor muscles. Co-occurrence and density data suggest that L. comta may be associated with the ophuroid Amphiura fasciata.

http://www.zoobank.org/urn:lsid:zoobank.org:pub:ECCB79FC-6EB2-4693-B1D4A014CB17C6D8     

Modern sediments and Pleistocene reefs from isolated carbonate platforms (Iles Eparses,SW Indian Ocean): A preliminary study

Isolated carbonate platforms occur throughout the geologic record, from Archean to present. Although the respective roles of tectonics, sediment supply and sea-level changes in the stratigraphical architecture of these systems are relatively well constrained, the details of the nature and controls on the variability of sedimentological patterns between and within individual geomorphologic units on platforms have been barely investigated. This study aims at describing and comparing geomorphological and sedimentological features of surficial sediments and fossil reefs from three isolated carbonate platforms located in the SW Indian Ocean (Glorieuses, Juan de Nova and Europa). These carbonate platforms are relatively small and lack continuous reef margins, which have developed only on windward sides. Field observations, petrographic characterization and grain-size analyses are used to illustrate the spatial patterns of sediment accumulation on these platforms. The internal parts of both Glorieuses and Juan de Nova platforms are blanketed by sand dunes with medium to coarse sands with numerous reef pinnacles. Skeletal components including coral, green algae, and benthic foraminifera fragments prevail in these sediments. Europa platform exhibits a similar skeletal assemblage dominated by coral fragments, with the absence of wave-driven sedimentary bodies. Fossil reefs from the Last interglacial (125,000 years BP) occur on the three platforms. At Glorieuses, a succession of drowned terraces detected on seismic lines is interpreted as reflecting the last deglacial sea-level rise initiated 20,000 years ago. These findings highlight the high potential of these platforms to study past sea-level changes and the related reef response, which remain poorly documented in the SW Indian Ocean.     

Microbial-Activated Sediment Traps Associated with Oncolite Formation Along a Peritidal Beach,Northern Arabian (Persian) Gulf,Kuwait

Identification of microbial communities within shoreline sediments and sediment precipitates from the Tigris-Euphrates delta (northern Kuwait) were determined by microscopic/nanoscopic studies, and by molecular analysis. Oncolites are syn-diagenetic carbonate precipitates that are surviving in a shallow subtidal to intertidal siliciclastic environment with periodically excessive hydraulic energy, extreme salinity (up to 47 per mil), and high concentrations of organic matter. X-ray diffraction techniques reveal that oncolite cortices are predominantly composed of calcite, quartz, halite and dolomite, associated with minor fractions of clay minerals. Quantitative analysis of the Corey Shape Factor reveals distinct morphological populations but with local overlap. A plot of the Equivalent Diameter vs. Corey Shape Factor provided the best indicator of the morphological relationships within the total oncolite population, indicating a hydrodynamically controlled morphological distribution defining intertidal and subtidal oncolite classes. Direct microscopic examination of the samples indicates that diatoms are the most abundant eukaryotic algae in subtidal sediments and within actively precipitating carbonate cements, especially the genus Navicula. In contrast, filamentous cyanobacteria from the genus Anabaena are most abundant in the intertidal zone sediments. The PCR-DGGE of the 16SrRNA gene of the cyanobacteria shows a higher diversity for this genus of bacteria in all sediment samples and that the cyanobacterial population in the diagenetically precipitating oncolites are closely related to the population found in the subtidal sediments. Dunaliella viridis dominates the culturable algae obtained from the four tidal zones. Our results indicate that a range of microbial populations are actively contributing to the formation of microbially-induced sedimentary structures in the extreme conditions of the southern Tigris-Euphrates delta.     

The response of solar radiation in Jubail,Saudi Arabia,to smoke from oil field fires in Kuwait

The relative monthly solar radiation in Jubail Industrial City, Saudi Arabia (300 km southeast of Kuwait City) was reduced to 75% of the recent 10-year mean value by the torching of the first 50 of many oil wells and production facilities in Kuwait in mid-January 1991. The value was reduced further when an additional 600 wells were ignited in late February. Solar radiation continued at 55–65% of normal levels during March to August, when 341 oil wells were still burning. Recovery was rapid as the fires in oil fields located directly upwind of Jubail were extinguished, with the solar radiation reaching 95% of the long-term mean in October.