Evaporite Succession Research Articles

Rare earth element signatures in the Messinian pre-evaporitic Calcare di Base formation (Northern Calabria, Italy): evidence of normal seawater deposition

The Calcare di Base (CdB), a carbonate unit deposited at the base of the evaporitic succession, is a key to understanding the paleoenvironmental conditions before the Messinian Salinity Crisis. The concentration of rare earth elements and yttrium (REE + Y) in the CdB, cropping out in the Rossano Basin, were measured with inductively coupled mass spectrometry (ICP-MS). The CdB shows coherent and reliable REE + Y patterns with typical characteristics of microbial carbonates: (1) enrichment in heavy REE (NdSN/YbSN = 0.52, SD = 0.09), (2) negative Ce and positive La anomalies, (3) marine Y/Ho ratios (57.08, SD = 3.07), (4) slightly positive Gd anomalies, and (5) tetrad effect especially in the La–Ce–Pr–Nd, and [Pm]–Sm–Eu–Gd tetrads. These features suggest that the CdB incorporated REEs in equilibrium with oxygenated seawater. They also corroborate the depositional model of Guido et al. in Palaeogeogr Palaeoclimatol Palaeoecol 255:265–283, (2007) that maintains carbonatogenesis occurred via heterotrophic microbial activity. The REE + Y signatures of the CdB support the interpretation that these sediments were deposited in normal marine condition and cannot belong to the evaporite succession.

Effects of depositional and diagenetic characteristics on carbonate reservoir quality: a case study from the South Pars gas field in the Persian Gulf

ABSTRACT The largest non-associated gas reserve of the world is hosted by the Upper Dalan–Kangan (Upper Khuff equivalent) Permo-Triassic carbonate–evaporite successions. Detailed characterization of these strata in the South Pars field has shown that the reservoir properties are a function of both sedimentary and diagenetic processes at the field scale. Facies analysis of the studied units indicates that the sediments were deposited in the inner regions of a homoclinal carbonate ramp and were subsequently subjected to shallow diagenesis and minor burial. The vertical distribution of the facies shows cyclic patterns that impact reservoir quality. The rock type classes have been grouped on the basis of the dominant pore spaces, and have enabled distinct fields to be identified. This approach has demonstrated a relationship between poroperm values and rock type groups. Diagenetic overprinting has significantly affected the reservoir properties. Although the original poroperm heterogeneities in the studied reservoir are inherited from the Upper Dalan–Kangan palaeoplatform, they have been modified strongly by diagenetic overprinting. Consequently, tentative correlation may be possible between facies types and reservoir properties based on diagenetic effects. Therefore, for precise characterization of the Upper Dalan–Kangan reservoir properties it has been necessary to integrate both the depositional history and diagenetic features.

Evidence of potash salt formation in the Pliocene Sedom Lagoon (Dead Sea Rift, Israel)

The Dead Sea is a Ca-chloride evaporitic lake close to halite saturation. Carnallite (KMgCl 3·6H 2O) is the next mineral predicted to precipitate and is currently produced in solar ponds on the shores in Israel and Jordan. The particular composition of the Dead Sea has been attributed to the contribution of Pliocene residual brines formed during the deposition of the Sedom formation that currently reappear as hydrothermal flows. The chemical composition of these returning flows suggests that Sedom brines reached the potash-forming (sylvite and/or carnallite) stage. The Sedom formation is a thick (more than 2000 m) evaporite succession made up of bedded salt rock accumulated during the Pliocene when an arm of the Mediterranean sea seeped through the Dead Sea Rift forming the Sedom Lagoon. The present study is focused on the upper part of the Sedom formation (Mearat Sedom member) where detailed petrology, bromine contents in salt, Cryo-SEM-EDS microanalysis of fluid inclusions in halite, and isotopic compositions ( δ 34S and δ 18O in sulfates, and δ 37Cl in salt) were investigated. Although halite is the main evaporite mineral, minor sylvite and carnallite occurrences in the form of early diagenetic nodules, cements and/or authigenic crystals are commonly present. The geochemical modeling of the primary fluid inclusion composition in halite points to the evaporation of seawater or mixtures of seawater and continental water, with a very minor (< 1.5%) proportion of CaCl 2-rich and MgCl 2-rich brines similar to those discharging into the present-day Dead Sea. This suggests that the refluxing of residual brines was already active in Sedom times. A cyclic evolution of the basinal brines can be deduced from the composition of fluid inclusions together with the bromine contents in halite and the sulfate isotopes. Thus, stages of dilution involving fresh marine and/or continental waters alternated with stages of high concentration when the saturation of the lagoon brine with respect to the potash salts was reached. Potash minerals could crystallize on the shores or in the marginal zones of the lagoon and could be dissolved and recycled during the subsequent dilution stages.

Effects of the Permian–Triassic boundary on reservoir characteristics of the South Pars gas field, Persian Gulf

AbstractThe Permian–Triassic boundary (PTB) is a world‐wide event characterized by the most extensive mass extinction in the history of life. In the Persian Gulf, the rock record of this time interval host one of the most important hydrocarbon reserves in the world: the South Pars Gas Field and its southern extension, the North Dome (or North Field). These carbonate and evaporite successions were sampled in eight wells for petrographic, geochemical and porosity–permeability studies. An important characteristic of the Dalan and Kangan formations is the centimetre‐scale lithological heterogeneities caused by facies changes and diagenetic imprints that led to the compartmentalization of these reservoirs. These Permian–Triassic (P‐T) sediments were deposited in a shallow marine homoclinal ramp. The PTB in this hydrocarbon field is represented by a reworked coarse‐grained intraclastic/bioclastic grainstone facies deposited during a marine transgression. Prolonged subaerial exposure in the P‐T transition caused hypersaline and meteoric diagenesis, including extensive cementation, dolomitization and some dissolution, influencing reservoir characteristics of bordering units. Both δ18O and δ13C values in this succession mirror worldwide excursions typical of other P‐T sections, with some variations due to diagenetic alterations. A pronounced decline in 87Sr/86Sr values, reflective of global seawater geochemistry for most of the Permian is evident in our data. Reservoir quality declines through the late Permian, as a result of facies change and diagenesis. The Late Permian is succeeded by a Triassic transgressive facies and decline in reservoir quality. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Detrital Modes in a Late Miocene Wedge-Top Basin, Northeastern Calabria, Italy: Compositional Record of Wedge-Top Partitioning

Abstract The composition and stratigraphic relations of sandstones and associated conglomerates derived from erosion of the northern Calabrian terranes provide important constraints for paleotectonic and paleogeographic models of the Neogene wedge-top depozone evolution of the southern Italy foreland basin system. After onset of continental collision in the southern Italy orogenic belt during late Oligocene to early Miocene, the Calabrian Arc drifted northeastward, accreting over the Adria lithosphere during mid to late Miocene time. Since the Tortonian, the northern Calabrian terranes represented an uplifted thrust belt in which along its leading edge tectonic subsidence was responsible for accumulation of a thick Tortonian to Quaternary succession, dominantly siliciclastic (up to 3300 meters thick), deposited in the wedge-top depozones (Rossano and Crotone basins). Sandstone assemblages have a homogeneous quartzofeldspathic composition in the Rossano and Crotone basins, characterized by a compositional trend, through time in response to regional tectonics. Evolution of the basins fill occurred during (1) Serravallian?–Tortonian, (2) late Tortonian to early Messinian, and (3) late Messinian. Detritus shed into those basins was derived mainly from terranes within the strongly deformed Sila Mountains allochthon, which was one of the principal structural elements of the nearby northern Calabrian terranes, consisting of Hercynian plutons intruding Paleozoic (Cambrian to Carboniferous) metasedimentary host rocks, and locally having a sedimentary cover of Mesozoic (Longobucco Group) or early Miocene (Paludi unit) clastics. Low- to medium-grade metamorphic and minor plutonic grain types (metamorphiclastic petrofacies) represent the main detritus in the earliest (Serravallian?–Tortonian) sedimentation that accompanied progressive flexural bending of the western margin of the foreland-basin system. Tortonian to early Messinian time was characterized by isolation of the Mediterranean region from the global marine realm, leading to evaporite sedimentation in the foreland in addition to intense regional and local tectonics. Compositional response to those events was a marked increase in sedimentary detritus (sedimenticlastic petrofacies) related to cannibalization of early Messinian evaporite successions; after the evaporitic events within the basins and the gradual re-establishment to the normal marine conditions, sandstone detrital modes are characterized mainly by a return of the Serravallian?–Tortonian compositional modes (i.e., metamorphic–plutoniclastic petrofacies). The post-evaporitic sedimentation corresponds to a major tectonic rearrangement on both Rossano and Crotone basins related to extrusion and inversion of the basal portion of the Serravallian?–Tortonian sequences in the intermediate zone to form an intervening Ciro structural high. In the Crotone Basin during late Messinian, deposition of the Carvane Conglomerate records an abrupt sudden arrival of distinctive sedimentary containing clasts of detritus of reddish limestone, marl, quartz arenite, siltstone and chert derived from Mesozoic to Tertiary basinal sequences of the southern Apennines domains (i.e., Sicilide Complex terranes). Detrital-mode evolution of the inner portions of the late Miocene southern Italy foreland-basin system was in response to abrupt tectonic partitioning due to continuation of accretionary processes, rapid uplift of mid-crustal blocks, and the superposition of wrench tectonics.

GEOCHEMICAL AUREOLES AROUND OIL AND GAS ACCUMULATIONS IN THE ZECHSTEIN (UPPER PERMIAN) OF POLAND: ANALYSIS OF FLUID INCLUSIONS IN HALITE AND BITUMENS IN ROCK SALT

Fluid inclusions in halite and bitumens in rock salt in Upper Permian Zechstein evaporites in West Poland were studied in locations where the evaporites lie above oil and gas reservoir rocks. The samples were taken from halite intercalated within the Basal Anhydrite; this unit lies above the Main Dolomite which serves as both source rock and reservoir. Samples came from a depth of 2.3–3.2 km. A characteristic feature of the fluid (gas‐brine) inclusions was their high methane content together with the occasional presence of bitumen globules of near‐spherical form. Geochemical analyses of the bitumen in bulk samples of rock salt (including content and distribution of n‐alkanes and isoprenoids, and carbon isotope ratios) suggest an algal origin, similar to that of the oil in the underlying source rocks. For comparison, we studied fluid inclusions in halite from Zechstein evaporites in northern Poland, where hydrocarbon accumulations do not occur in underlying strata and where mostly single‐phase (brine) inclusions with a low methane content have been recorded. However, published data indicate that similar inclusions to those occurring in the Zechstein of West Poland (comprising brine with a high methane content, bitumen films and/or oil droplets) are present in other salt‐bearing sequences, where their origin is related to the thermal degradation of organic material dispersed within the salt itself. Accordingly, such fluid inclusions in an evaporite succession can only be considered to form a geochemical aureole where the bitumens in the rock salt (including those in the fluid inclusions in halite) can be compositionally linked to those in the associated oil accumulation.

Astronomical tuning as the basis for high resolution chronostratigraphy: the intricate history of the Messinian Salinity Crisis

The Messinian Salinity Crisis (MSC) in the Mediterranean resulted from a complex interplay between tectonic gateway closure and climate evolution. The climate factor, in turn, can be separated into two components, one associated with dominantly precession controlled regional climate change and the other with dominantly obliquity related glacial cyclicity. The influence of these climate changes occurred superimposed on a long(er)-term tectonic trend. Discrimination into the various forcing factors only recently came within reach due to the development of an integrated high-resolution stratigraphy and astronomically tuned age models, both for the Messinian (pre-)evaporite successions in the Mediterranean and for benthic oxygen isotope records from the open ocean. The application of these time scales in combination with a high-resolution integrated stratigraphic approach showed that 1) the onset of the MSC proper at 5.96 Ma is not related to glacio-eustatic sealevel lowering but its timing can best be attributed to the influence of the 400-kyr eccentricity cycle on regional climate superimposed on a tectonic trend, 2) the main desiccation phase between the Lower and Upper Evaporites coincides with the twin peak glacials TG12-14 suggesting a glacio-eustatic control, 3) the beginning of the Upper Evaporites and Lago Mare phase coincides with the onset of the major deglaciation following peak glacial TG 12, indicating that glacio-eustatic sealevel rise played a role, 4) the Pliocene flooding of the Mediterranean is not related to a glacioeustatic sealevel rise, and 5) the evaporite cycles are controlled by precession induced regional climate changes rather than by obliquity forced glacio-eustatic sealevel change. Our astronomical age model for the Mediterranean Messinian is consistent with 40Ar/39Ar ages of ash layers intercalated in Messinian successions in the Mediterranean. However, the intercalibration between astronomical and 40Ar/39Ar dating has to be taken into account for a direct comparison of these ages.

The ‘Evaporiti di Monte Castello’ deposits of the Messinian Southern Apennines foreland basin (Irpinia–Daunia Mountains, Southern Italy): stratigraphic evolution and geological context

Abstract An evaporitic limestone–gypsum succession, belonging to the Evaporiti di Monte Castello Formation, is recognized in the late Messinian (Upper Miocene) of the Irpinia–Daunia Mountains in the Southern Apennines arc (Southern Italy). This unit is formed by diatomaceous marls, massive and laminated evaporitic limestones, and by primary and clastic gypsum. Detailed stratigraphical, sedimentological and strontium geochemistry data has permitted reconstruction of stratigraphic and facies relations of gypsum deposits, depositional environment and basin evolution. Genetically related gypsum lithofacies can be grouped into two facies associations. The autochthonous gypsum facies association consists of shallow water selenitic, acicular and laminated gypsum and is characterized by the absence of high-energy sedimentary structures. The redeposited clastic gypsum facies association consists of shallow- to deeper-water fine-grained laminated gypsum, gypsarenites, pebbly gypsarenites and gypsrudites, showing common features of resedimented deposits. Nodular structures occurring in the laminated gypsum lithofacies seem to be mostly related to late diagenetic processes. The sedimentary evolution during the evaporative phase was characterized by a gradual increase in salinity until gypsum precipitated; then the sedimentary conditions in the basin were characterized by almost homogenous salinity conditions, and influenced by events of gypsum reworking and resedimentation probably related to flooding episodes and local tectonic activity. The gypsum was deposited from mainly marine brines, based on their Sr isotopic compositions. This sedimentary series is an equivalent of the Lower Evaporites of other parts of the Mediterranean. The Messinian Monte Castello Formation evaporites represent an uncommon type of evaporitic succession, probably developed in a extensional setting in a basin located along the Apulian foreland ramp, in contrast with the northern Apennines and Sicilian basins (e.g. Vena del Gesso, and Caltanisetta basins), which are considered to be thrust-top basins of the Apennine–Maghrebian foreland basin system.

A giant Late Triassic–Early Jurassic evaporitic basin on the Saharan Platform, North Africa

Abstract This paper describes the Late Triassic and Early Jurassic evaporites of the Berkine/Ghadames Basin in eastern Algeria and western Libya, which is important because it forms the regional seal to a number of hydrocarbon reservoirs in the Triassic Argilo-Gréseux Inférieur. This evaporitic succession spans the time interval Norian to Late Liassic and is divided into three contrasting units (S1, S2 and S3). S1 consists of five evaporitic cycles dominated by mudstone and halite, which represent the main salt deposits of the Berkine Basin. At this time the Berkine/Ghadames basin was a restricted evaporitic basin with a barrier to the north (Medenine High) separating the basin from the developing peri-Tethys and North Atlantic. The succession thickens to the west, thins around the Hass Messouad Ridge and continues across the Maghreb into the western High Atlas of Morocco, a distance of some 1500 km. S2 is represented by the basin-wide development of a carbonate platform in Pliensbachian times. This is the product of a relative sealevel rise concomitant with the developing extension of the region and may reflect the opening of an Atlantic gateway. S3 comprises five cycles that are predominantly mudstones, fine-grained carbonates and anhydrites. Mounded structures, evident on seismic sections, may represent the development of patch reefs and the whole succession is interpreted as a shallow-marine carbonate–evaporite ramp. Similar thickness variations to those observed in Sequence 1 continue through Sequence 3 and reflect the on-going influence of differential subsidence along basement lineaments. A number of factors controlled the development of the evaporite basin. These include Late Triassic sealevel rise and flooding of a sub-sealevel basin, combined with globally warm climatic conditions and on-going extensional tectonics in the peri-Tethyan and Atlantic areas. Restricted circulation was caused by an east–west oriented sill to the north (Medenine High in central Tunisia) and the basin was episodically refilled as global sealevel rose during the Early Mesozoic and extensional tectonics continued.

Offshore evidence of polyphase erosion in the Valencia Basin (Northwestern Mediterranean): Scenario for the Messinian Salinity Crisis

The Messinian Salinity Crisis at the end of the Miocene strongly affected the physiography of the Mediterranean margins, and in particular that of the Valencia Basin and its margins. The Valencia Basin, which is an aborted Tertiary rift related to the opening of the oceanic Provençal Basin, shows evidence of two major Messinian erosion surfaces — a Basal Erosion Surface (BES) and a Top Erosion Surface (TES) — bracketing the Messinian evaporite succession; the basin's margins show only a single erosion surface, here termed the Margins Erosion Surface (MES). In examining the geometrical relationships of these erosion surfaces with the deep basin facies deposited in the abyssal Provençal Basin during the Messinian Salinity Crisis, we have been able to establish a chronology for the Messinian in the Valencia Basin. Briefly, the Basal Erosion Surface extends the Margins Erosion Surface into the Central Valencia Basin and records an early and important sea-level fall at the beginning of the Messinian Salinity Crisis. The Miocene basin series were incised by canyons, while the exposed margins underwent substantial subaerial erosion. The basin's detrital products linked to this basal erosion grade laterally into the massive Messinian Salt layer of the Provençal Basin. The overlying Upper Evaporites unit appears to be widely transgressive beyond the edge of the Messinian Salt and onlaps the Basal Erosion Surface in the Valencia Basin. This disposition suggests a progressive sea-level rise during the deposition of the Upper Evaporites, possibly due to the rapid accumulation of thousands of metres of shallow-water evaporite facies in the Provençal Basin. The Top Erosion Surface at the top of the Upper Evaporites shows a network of incisions, and we suggest that this final erosion was linked to the Lago-Mare event recorded elsewhere in the Mediterranean at the end of the Messinian Salinity Crisis.

An Upper Aptian saline pan/lake system from the Brazilian equatorial margin: integration of facies and isotopes

AbstractFacies and isotope (Sr and S) data were combined in order to discuss the genesis and palaeohydrology of an Upper Aptian evaporite succession formed during the initial development of an intra‐continental rift from northern Brazil. Comparison of facies distribution between two investigated areas shows: (i) stable, well‐stratified and hypersaline lakes with periods of anoxia and closure in the eastern margin of the basin, where there was evaporite precipitation only in central lake environments; and (ii) more ephemeral conditions in the eastern margin of the basin, where a saline pan complex developed and evaporite precipitation took place mainly in marginal salinas and surrounding mudflats. In both areas, expansion/contraction cycles developed as sedimentation took place, accompanied by a decrease and then an increase in 87Sr/86Sr‐ and δ34S‐isotope values. This, combined with the wide distribution of Sr‐ and S‐isotope data within individual depositional cycles, as well as petrographic and scanning electronic microscopic (SEM) images, led to the conclusion that diagenesis was not enough to obscure the primary geochemical signature. A non‐marine brine source is suggested by 87Sr/86Sr ratios between 0·707824 and 0·709280, which are higher than those from Upper Aptian sea water (i.e. between 0·70720 and 0·70735). The δ34S value varies from 16·12 to 17·89‰ in the eastern margin of the basin, which is in disagreement with Upper Aptian marine values (13 to 16‰). Both geochemical tracers were influenced by facies characteristics, and thus a model is provided, where 87Sr/86Sr values respond to changes in expansion and contraction episodes of saline pan/lake systems.

Anatomy of a mixed marine-non-marine lowstand wedge in a ramp setting. The Record of a Barremian-Aptian complex relative sea-level fall in the central Neuquén Basin, Argentina

Abstract During the Cretaceous, western Argentina was occupied by the Neuquén Basin, a back-arc-foreland basin that was open through the proto-Andes to the Pacific Ocean in the west. The Neuquén Basin contains a thick succession of sediments that include the offshore marine deposits of the Agrio Formation. These deposits represent a time when the arc was an island chain and the Neuquén Basin was freely connected to the Pacific. This offshore marine succession is punctuated by two intervals of arid continental deposits that represent major, second-order, relative falls in sea level. In both of these cases there is no evidence of tectonic uplift or angular truncation along a basal bounding unconformity. The upper of the two lowstand wedges is characterized by a complex arragement of shallow-marine and continental deposits. Shallow-marine deposits sharply overlying offshore shales and capped by a master sequence boundary are interpreted as falling-stage deposits recording a complex relative sea-level fall. On top of a regional erosion surface, a drying-upwards succession of fluvial-aeolian deposits is developed, recording a fully non-marine stage in the evolution of the basin. These deposits are overlain by a marginal marine evaporite succession. The absence of a return to fully open-marine conditions is attributed to uplift in the Andes and marks the transition of the Neuquén Basin from a back-arc to a foreland system. This succession has important implications for the basin’s evolution and in the timing of the uplift of the Andes, is a very spectacular example of a lowstand wedge and is also a major hydrocarbon reservoir.

The importance of recycling processes in the Middle Miocene Badenian evaporite basin (Carpathian foredeep): palaeoenvironmental implications

The Middle Miocene Badenian evaporitic succession within the Carpathian foredeep (Poland) has been conventionally described as formed in a purely marine basin. Until recently, data from this basin have been used to deduce the sulphate isotopic composition of the ocean during the Miocene and to describe evolution of major elements in seawater. This paper presents a new interpretation proving major continental inputs and ongoing recycling processes. It is based on detailed petrographical/mineralogical studies, X-ray microanalysis of primary fluid inclusions in halite (Cryo-Scanning Electron Microscopy (SEM)–Energy Dispersive Microanalysis (EDS)), isotopic analysis (δ 34S and δ 18O in sulphates) and computer-based evaporation models. Two boreholes, Wojnicz intersecting the halite succession in the centre of the basin and Cieszanów borehole cutting through the gypsum units found in the margin of the basin, are used in reconstructing the general palaeohydrological evolution of the basin. Model calculations constrained from experimental data suggest recharge proportions during halite precipitation of between 20−30% seawater and 65−70% continental waters, with 5−10% of the continental waters recycling previously precipitated halite. The isotope signatures measured show average values of δ 34S CDT=+24.61‰ (±0.34) and δ 18O SMOW=+12.06‰ (±0.40) for Wojnicz borehole and δ 34S CDT=+22.74‰ (±0.42) and δ 18O SMOW=+12.66‰ (±0.38) for the Cieszanów borehole. These are consistent with an ocean having a sulphate isotope composition similar to Eocene or present day and the scenario of intrabasin recycling processes of previous marine halite successions. The present interpretation for the Carpathian foredeep evaporites argues against the use of these in deducing Miocene seawater composition.

The importance of recycling processes in the Middle Miocene Badenian evaporite basin (Carpathian foredeep): palaeoenvironmental implications

The Middle Miocene Badenian evaporitic succession within the Carpathian foredeep (Poland) has been conventionally described as formed in a purely marine basin. Until recently, data from this basin have been used to deduce the sulphate isotopic composition of the ocean during the Miocene and to describe evolution of major elements in seawater. This paper presents a new interpretation proving major continental inputs and ongoing recycling processes. It is based on detailed petrographical/mineralogical studies, X-ray microanalysis of primary fluid inclusions in halite (Cryo-Scanning Electron Microscopy (SEM)–Energy Dispersive Microanalysis (EDS)), isotopic analysis (δ34S and δ18O in sulphates) and computer-based evaporation models. Two boreholes, Wojnicz intersecting the halite succession in the centre of the basin and Cieszanów borehole cutting through the gypsum units found in the margin of the basin, are used in reconstructing the general palaeohydrological evolution of the basin. Model calculations constrained from experimental data suggest recharge proportions during halite precipitation of between 20−30% seawater and 65−70% continental waters, with 5−10% of the continental waters recycling previously precipitated halite. The isotope signatures measured show average values of δ34SCDT=+24.61‰ (±0.34) and δ18OSMOW=+12.06‰ (±0.40) for Wojnicz borehole and δ34SCDT=+22.74‰ (±0.42) and δ18OSMOW=+12.66‰ (±0.38) for the Cieszanów borehole. These are consistent with an ocean having a sulphate isotope composition similar to Eocene or present day and the scenario of intrabasin recycling processes of previous marine halite successions. The present interpretation for the Carpathian foredeep evaporites argues against the use of these in deducing Miocene seawater composition.

Middle to Upper Jurassic Saudi Arabian carbonate petroleum reservoirs: biostratigraphy, micropalaeontology and palaeoenvironments

ABSTRACTRecent work has improved understanding of the benthic foraminiferal stratigraphic and palaeoenvironmental ranges of the Middle to Upper Jurassic reservoir-containing carbonates of Saudi Arabia. The entire Jurassic succession includes the Marrat, Dhruma, Tuwaiq Mountain, Hanifa, Jubaila and Arab formations that terminate with a succession of evaporites, the final, thickest unit of which is termed the Hith Formation. This study focuses on selected carbonate members studied from the Dhruma Formation and above, and includes the Lower Fadhili, Upper Fadhili, Hanifa and Arab-D reservoirs. The Hadriya and Manifa reservoirs are not considered. An ascending order of tiered deep-to shallow-marine foraminiferal assemblages has been determined for each formation and applied to distinguish both long- and short-term palaeobathymetric variations. The Lenticulina-Nodosaria-spicule dominated assemblage characterises the deepest mud-dominated successions in all formations. The consistent presence of Kurnubia and Nautiloculina species suggests only moderately deep conditions, considered to be below fair-weather wave base and shelfal. A foraminiferally-depleted succession then follows that is characterised by encrusting and domed sclerosponges, including Burgundia species, in the Tuwaiq Mountain, Hanifa and Jubaila formations. This assemblage is followed, in the Hanifa and upper Jubaila formations, by a biofacies dominated by fragments of the branched sclerosponge Cladocoropsis mirabilis, together with Kurnubia and Nautiloculina species and a variety of indeterminate simple miliolids. Pseudocyclammina lituus, Alveosepta powersi/jacardi and Redmondoides lugeoni are present within this assemblage. A slightly shallower, possibly lagoon-influenced assemblage is developed in the Hanifa and Arab formations that include Cladocoropsis mirabilis, Kurnubia and Nautiloculina species and the dasyclad algae Clypeina sulcata and Heteroporella jaffrezoi. A further shallower assemblage, found only in the upper Arab-D Member, is characterised by the presence of Mangashtia viennoti, Clypeina sulcata and Cladocoropsis mirabilis. This assemblage is gradually supplemented by “Pfenderina salernitana” and is interpreted as slightly shallower conditions in the upper Arab-D. A very shallow assemblage in the uppermost Arab-D is characterised by the presence of Trocholina alpina, which is then followed by an intertidal assemblage of cerithid gastropods and felted calcareous algae in which foraminifera are typically absent.These various microbiofacies have provided depositional and potential reservoir stratification. A phenomenon termed “palaeobathymetric compression” has been observed in which depositional cycles are enhanced by rapidly shallowing upwards tiered biofacies that encompass less than 3m of sediment thickness but represent in excess of 20m of water depth reduction. This is attributed to short-term rapid lowering of sea level, and may be considered as the microfaunal signals of high frequency forced regressions.

Sedimentology of the Miocene evaporitic succession in the north of Çankiri-Çorum basin, central Anatolia, Turkey

The upper Miocene non-marine sediments of the Cankiri-Corum basin in central Anatolia, have both evaporitic and non-evaporitic successions. These sediments were deposited in an evaporitic lake which had temporary episodes of palustrine conditions in response to seasonal or climatic changes. The successions show different facies such as sulfates, carbonates and siliciclastics. The sulfates comprise primary, reworked and diagenetic gypsum. The primary deposits are predominantly laminated gypsum, bedded gypsum and selenite. The reworked (detrital) gypsum comprises gypsite, gypsarenite, gypsrudite and breccias. The diagenetic type comprises micro- and macrogypsum nodules. The carbonates mainly include clayey limestone, oolitic limestone and dolomite. The siliciclastics comprise red beds and both channel and non-channel, conglomerates and mudstones. Laminated gypsum, composed of alternating gypsum and dolomite, was a result of environmental schizohalinity. Bedded gypsum was precipitated in the deeper part of the lake during high evaporation periods. Chevron-type selenite crystals formed on saline mud flats during the times of aridity, whereas the discoidal-type seen in the organic-rich mudstones occurred in the gypsiferous marshes during the times of humidity. Reworked (detrital) gypsum dominates the lake margin. These formed during periodic wet episodes that caused reworking of primary gypsum. Gypsum nodules occurred as both early and late diagenetic products. Carbonates and siliciclastics were deposited during the freshening periods of the lake. Climatic or seasonal changes were the main causes of the depositional styles of the upper Miocene evaporitic and non-evaporitic lacustrine deposits in Cankiri-Corum basin. Additionally, the transition upward from alluvial to lake environment implies an important change in drainage patterns that likely occurred as a result of marginal fault activity.

The sequence stratigraphy, sedimentology, and economic importance of evaporite–carbonate transitions: a review

Abstract World-class hydrocarbon accumulations occur in many ancient evaporite-related basins. Seals and traps of such accumulations are, in many cases, controlled by the stratigraphic distribution of carbonate–evaporite facies transitions. Evaporites may occur in each of the systems tracts within depositional sequences. Thick evaporite successions are best developed during sea level lowstands due to evaporative drawdown. Type 1 lowstand evaporite systems are characterized by thick wedges that fill basin centers, and onlap basin margins. Very thick successions (i.e. saline giants) represent 2nd-order supersequence set (20–50 m.y.) lowstand systems that cap basin fills, and provide the ultimate top seals for the hydrocarbons contained within such basins. Where slope carbonate buildups occur, lowstand evaporites that onlap and overlap these buildups show a lateral facies mosaic directly related to the paleo-relief of the buildups. This facies mosaic, as exemplified in the Silurian of the Michigan basin, ranges from nodular mosaic anhydrite of supratidal sabkha origin deposited over the crests of the buildups, to downslope subaqueous facies of bedded massive/mosaic anhydrite and allochthonous dolomite–anhydrite breccias. Facies transitions near the updip onlap edges of evaporite wedges can provide lateral seals to hydrocarbons. Porous dolomites at the updip edges of lowstand evaporites will trap hydrocarbons where they onlap nonporous platform slope deposits. The Desert Creek Member of the Paradox Formation illustrates this transition. On the margins of the giant Aneth oil field in southeastern Utah, separate downdip oil pools have accumulated where dolomudstones and dolowackestones with microcrystalline porosity onlap the underlying highstand platform slope. Where lowstand carbonate units exist in arid basins, the updip facies change from carbonates to evaporite-rich facies can also provide traps for hydrocarbons. The change from porous dolomites composed of high-energy, shallow water grainstones and packstones to nonporous evaporitic lagoonal dolomite and sabkha anhydrite occurs in the Upper Permian San Andres/Grayburg sequences of the Permian basin. This facies change provides the trap for secondary oil pools on the basinward flanks of fields that are productive from highstand facies identical to the lowstand dolograinstones. Type 2 lowstand systems, like the Smackover Limestone of the Gulf of Mexico, show a similar relationship. Commonly, these evaporite systems are a facies mosaic of salina and sabkha evaporites admixed with wadi siliciclastics. They overlie and seal highstand carbonate platforms containing reservoir facies of shoalwater nonskeletal and skeletal grainstones. Further basinward these evaporites change facies into similar porous platform facies, and contain separate hydrocarbon traps. Transgressions in arid settings over underfilled platforms (e.g. Zechstein (Permian) of Europe; Ferry Lake Anhydrite (Cretaceous), Gulf of Mexico) can result in deposition of alternating cyclic carbonates and evaporites in broad, shallow subaqueous hypersaline environments. Evaporites include bedded and palmate gypsum layers. Mudstones and wackestones are deposited in mesosaline, shallow subtidal to low intertidal environments during periodic flooding of the platform interior. Highstand systems tracts are characterized by thick successions of m-scale, brining upward parasequences in platform interior settings. The Seven Rivers Formation (Guadalupian) of the Permian basin typifies this transition. An intertonguing of carbonate and sulfates is interpreted to occur in a broad, shallow subaqueous hypersaline shelf lagoon behind the main restricting shelf-edge carbonate complex. Underlying paleodepositional highs appear to control the position of the initial facies transition. Periodic flooding of the shelf interior results in widespread carbonate deposition comprised of mesosaline, skeletal-poor peloid dolowackestones/mudstones. Progressive restriction due to active carbonate deposition and/or an environment of net evaporation causes brining upward and deposition of lagoonal gypsum. Condensed sections of organic-rich black lime mudstones occur in basinal areas seaward of the transgressive and highstand carbonate platforms and have sourced significant quantities of hydrocarbons.

Evaporitic Subtidal Stromatolites Produced by in situ Precipitation: Textures, Facies Associations, and Temporal Significance

ABSTRACT The transition between carbonate platforms or isolated carbonate buildups and overlying evaporites commonly is marked by assemblages of stromatolites and interlaminated carbonates and evaporites. Stromatolites display lamination textures that vary from peloidal and discontinuous on a scale of a millimeter to a few centimeters, to isopachous and continuously laminated on a scale of a centimeter to a few meters. The isopachous lamination texture may be composed of either: (1) micritic or radial-fibrous calcite, or (2) dolomite. Isopachous stromatolitic laminae are remarkably uniform, varying little in thickness over a given lateral distance, in contrast to stromatolites formed of peloidal laminae, which show marked variation in thickness over an equivalent lateral distance. These isopachous textures are uncommon on most open-marine carbonate platforms and apparently developed in transitional carbonate-to-evaporite settings because of increasing temperature, salinity, and anoxia related to water stratification, which would have created ecologic restriction and an opportunity for stromatolite growth. Stromatolites with isopachous lamination are here interpreted to have formed as a result of in situ precipitation of sea-floor-encrusting calcite and possibly dolomite, whereas the stromatolites composed of peloidal, discontinuous lamination are inferred to have formed by trapping and binding of loose carbonate sediment in microbial mats. While the presence of microbes in almost all near-surface environments nullifies use of the term abiotic to describe most precipitated minerals, we interpret growth of the isopachous stromatolites to have been dominated by chemogenic precipitation in the absence of microbial mats, and the growth of peloidal stromatolites to have been controlled by sedimentation in the presence of microbial mats. These transitional stromatolite facies are best developed atop Proterozoic and Paleozoic carbonate platforms that underlie major evaporite successions. However, inspection of Jurassic and younger evaporite basins, such as the Messinian of the Mediterranean region, shows that stromatolites with thin, isopachous lamination and radial-fibrous textures, though present, are rare. Instead, these facies may have been replaced by stromatolites with peloidal, clastic textures and by low-diversity diatomaceous and coccolith mudstones. Accumulation of the mudstones would have imposed two important effects: (1) Production of coccoliths would have helped extract calcium carbonate from seawater, thus lowering the growth potential for precipitation of sea-floor-encrusting stromatolites. (2) Settling of both coccoliths and diatoms would have created a sediment flux to the sea floor, which would have served to impede growth of precipitated stromatolites because of smothering of growing crystals.

Distribution pattern of hydrogen sulphide-bearing gas in the former Soviet Union

H 2 S is an unwelcome accompaniment to natural hydrocarbon gas but, in sufficient volumes, it can provide an economic source of sulphur, termed a ‘gaseous sulphur’ accumulation. Such accumulations are an important component of the sulphur reserves of Russia and other countries. A comparative study of the distribution of economic gaseous sulphur accumulations, together with native sulphur deposits, in the Amu Dar’ya, Volga–Urals, North Caspian and other basins of the former Soviet Union and elsewhere shows that they occur only within large hydrocarbon basins and are restricted to thick and regionally persistent evaporite–carbonate successions. The lithological and geochemical controls and various types of sulphur paragenesis result in a zoned series of accumulations, from gaseous sulphur to sulphur-rich oils to native sulphur, which can be traced from the deep central regions of hydrocarbon-bearing basins with sulphate–evaporite successions towards the shallow basin margins.

Evaporative systems and diagenetic patterns in the Calatayud Basin (Miocene, central Spain)

This paper concerns the evaporite units, depositional systems, cyclicity, diagenetic products and anhydritization patterns of the Calatayud Basin (nonmarine, Miocene, central Spain). In outcrop, the sulphate minerals of these shallow lacustrine evaporites consist of primary and secondary gypsum, the latter originating from the replacement of anhydrite and glauberite. In the evaporative systems of this basin, gypsiferous marshes of low salinity can be distinguished from central, saline lakes of higher salinity. In the gypsiferous marsh facies, the dominant, massive, bioturbated gypsum was partly replaced by synsedimentary chert nodules and siliceous crusts. In the saline lake facies, either cycles of gypsiferous lutite‐laminated gypsarenite or irregular alternations of laminated gypsum, nodular and banded glauberite, thenardite and nodular anhydrite precipitated. Early replacement of part of the glauberite by anhydrite also occurred. Episodes of subaerial exposure are represented by: (1) pedogenic carbonates (with nodular magnesite) and gypsiferous crusts composed of poikilitic crystals; and (2) nodular anhydrite, which formed in a sabkha. Additionally, meganodular anhydrite occurs, which presumably precipitated from ascending, highly saline solutions. The timing of anhydritization was mainly controlled by the salinity of the pore solutions, and occurred from the onset of deposition to moderate burial. Locally, a thick (&gt;200 m) sequence of gypsum cycles developed, which was probably controlled by climatic variation. A trend of upward‐decreasing salinity is deduced from the base to the top of the evaporite succession.