Diagenetic Anhydrite Research Articles

Origin of sulfate-rich fluids in the Early Triassic Montney Formation, Western Canadian Sedimentary Basin

Abstract This study investigates diagenetic and geochemical processes that control regional distribution and formation of sulfate minerals (i.e., anhydrite and barite) in the Early Triassic Montney Formation in the Western Canadian Sedimentary Basin. The generation of H2S in hydrocarbon reservoirs is often associated with the dissolution of sulfate minerals, as a major source of sulfate required for sulfate-reducing reactions. The formation of pervasive late diagenetic anhydrite and barite in the high H2S zone of the Montney Formation is therefore contrary to the normal paragenetic sequence of sour gas reservoirs. Petrographic observations revealed early and late anhydrite and barite cement. The early fine-crystalline anhydrite cement is dominant in northeastern British Columbia (low H2S zone), while the late-stage coarse-crystalline cement and fracture/vug-filling anhydrite are dominant in Alberta (high H2S zone). The bulk isotopic values (δ34S: +2.9 to +24.7‰ V-CDT, δ18O: −11.2 to +15.7‰ V-SMOW) suggest that sulfate-rich fluids originated mainly from modified Triassic connate water was the origin of early anhydrite. In contrast, the SIMS isotopic values of late anhydrite (δ34S: +18.5 to +37‰ V-CDT, δ18O: +12 to +22‰ V-SMOW) and barite cement (δ34S: +23.3 to +39‰ V-CDT, δ18O: +13.2 to +18.7‰ V-SMOW) as well as fracture/vug-filling anhydrite (δ34S: +23.5 to +24.7‰ V-CDT, δ18O: +13.3 to +14.7‰ V-SMOW) from Alberta represents a mixed isotopic signature of Triassic connate water and contribution of dissolved sulfate-rich fluids derived from dissolution of Devonian evaporites. The 87Sr/86Sr isotope ratios of the fracture/vug-filling anhydrite (0.7092–0.7102) are highly radiogenic suggesting extensive water/rock interactions between sulfate-rich fluids and siliciclastic and basement rocks. The similar isotopic composition of the late anhydrite/barite and fracture/vug-filling anhydrite in western Alberta with Devonian evaporites isotopic signature, and the highly radiogenic 87Sr/86Sr ratio further supports sulfate-bearing fluids were mainly originated from underlying Devonian evaporites and migrated upwards through deep-seated faults/fractures to the Montney Formation.

Seismic interpretation and characterization of anhydrite caprocks in the Tromsø Basin, SW Barents Sea

The origin of anhydrite caprocks found in many evaporite basins is currently under debate. Existing models in the literature for anhydrite formation include rotated stratigraphic layering, rotated crestal caprock and diagenetic anhydrite. In this work, we interpret high-quality 2D seismic reflection data to understand the processes associated with anhydrite caprock formation. Our study focuses on anhydrite caprock formation in the Tromsø Basin, which represents one of several underexplored salt basins in the Arctic. We use well log data and seismic stratigraphic as well as seismic attribute analyses to characterize the ‘anhydrite’ layers associated with salt diapirs. Our results reveal that the anhydrite layers are high-amplitude reflections at the crests, which are often found close to the seabed and lateral caprocks on the flanks of salt diapirs. A third class of anhydrite layers is folded, faulted and boudinaged intrasalt stringers encased by the salt diapirs. Lateral caprocks in the Tromsø Basin are formed in response to rotated crestal caprock failure. The crestal caprocks are secondary in origin, occur at shallow depths, and are formed as insoluble residues from the dissolution of halite by the ingress of basinal fluids into the tops of diapirs. Anhydrite stringers, or floaters, originate from tectonic-related multi-dimensional salt flow. Salt diapirs in the Tromsø Basin are associated with varied anhydrite caprocks; their formation cannot be fully explained using a single model. Our work demonstrates from seismic reflection data the occurrence of caprock events that formed progressively on the flanks of salt diapirs due to local folding.

Fluid inclusion and microfabric studies on Zechstein carbonates (Ca2) and related fracture mineralizations – New insights on gas migration in the Lower Saxony Basin (Germany)

New petrographic and fluid inclusion data from core samples of Upper Permian dolomitic limestone (Hauptdolomit, Zechstein group, Stassfurt carbonate sequence) from a gas field located at the northern border of the Lower Saxony Basin (LSB) essentially improve the understanding of the basin development. The gas production at the locality is characterized by very high CO2 concentrations of 75–100% (with CH4 and N2).Samples consist of fine grained, mostly laminated and sometimes brecciated dolomitic limestone (mudstone/wackestone) from the transition zone between the shallow water zone (platform) and the upper slope. The study focuses on migration fluids, entrapped as fluid inclusions in diagenetic anhydrite, calcite, and fluorite, and in syn-diagenetic microfractures, as well as on the geochemistry of fluorite fracture mineralizations, obtained by LA-ICP-MS analysis. Fluid inclusion studies show that the diagenetic fluid was rich in H2ONaClCaCl2. Recrystallized anhydrite contains aqueous inclusions with homogenization temperatures (Th) of ca. 123 °C, but somewhat higher Th of ca. 142 °C was found for calcite cement followed by early Fluorite A with Th of 147 °C. A later Fluorite B preserves gas inclusions and brines with maximum Th of 156 °C. Fluorite B crystallized in fractures during the mobilization of CO2-bearing brines. Crossing isochores for co-genetic aqueous-carbonic and carbonic inclusions indicate fluid trapping conditions of 180–200 °C and 900–1000 bars. δ13C-isotopic ratios of gas trapped in fluid inclusions suggest an organic origin for CH4, while the CO2 is likely of inorganic origin.Basin modelling (1D) shows that the fault block structure of the respective reservoir has experienced an uplift of >1000 m since Late Cretaceous times.The fluid inclusion study allows us to, 1) model the evolution of the LSB and fluid evolution by distinguishing different fluid systems, 2) determine the appearance of CO2 in the geological record and, 3) more accurately estimate burial and uplift events in individual parts of the LSB.

Quantification and Prediction of the 3D Pore Network Evolution in Carbonate Reservoir Rocks

This study presents an integrated approach that allows the reconstruction and prediction of 3D pore structure modifications and porosity/permeability development throughout carbonate diagenesis. Reactive Pore Network Models (PNM-R) can predict changes in the transport properties of porous media, resulting from dissolution/cementation phenomena. The validity and predictability of these models however depend on the representativeness of the equivalent pore networks used and on the equations and parameters used to model the diagenetic events. The developed approach is applied to a real case of a dolostone rock of the Middle East Arab Formation. Standard 2D microscopy shows that the main process affecting the reservoir quality is dolomitisation, followed by porosity enhancement due to dolomite dissolution and secondary porosity destruction by cementation of late diagenetic anhydrite. X-ray μ -CT allows quantifying the 3D volume and distribution of the different sample constituents. Results are compared with lab measurements. Equivalent pore networks before dolomite dissolution and prior to late anhydrite precipitation are reconstructed and used to simulate the porosity, permeability characteristics at these diagenetic steps. Using these 3D pore structures, PNM-R can trace plausible porosity-permeability evolution paths between these steps. The flow conditions and reaction rates obtained by geochemical reaction path modeling can be used as reference to define PNM-R model parameters. The approach can be used in dynamic rock typing and the upscaling of petrophysical properties, necessary for reservoir modeling.

Stratigraphic relationships between the Middle Devonian Shell Lake Member and the Upper Winnipegosis reef mounds, Saskatchewan Sub-basin

ABSTRACT The spatial distribution of the Shell Lake Member and its relationship with the Upper Winnipegosis reef mounds in the Saskatchewan Sub-basin record an important aspect of the transitional history of the Elk Point Basin from fully marine to desiccation. The Shell Lake Member in the study area is composed of anhydrite and laminated dolomite, and produces a good reflection on seismic profiles. The top of many Winnipegosis reef mounds gives a seismic reflection similar to the Shell Lake Member, leading to the interpretation in the literature that the Shell Lake Member is a continuous unit overlying the Winnipegosis reef mounds in the Sub-basin. Data presented in this paper demonstrate that the Shell Lake Member is not continuous because it terminates against the fully developed reef mounds. The anhydrite on top of the reef mounds previously interpreted as the Shell Lake Member is diagenetic anhydrite that occurs as replacement of the Winnipegosis carbonate at the top of reef mounds and as cement in the vugs. This interpretation is based on differences recorded in the depositional, lithological and diagenetic features of these two units. The formation of the diagenetic anhydrite of the Winnipegosis reef mounds is distinct from the primary depositional anhydrite in the Shell Lake Member based on the lithologic and diagenetic features, spatial distribution and stratigraphic relationship of the Winnipegosis carbonate and the Shell Lake Member in the study area. The spatial relationships of various lithologic units may serve as a predictor for the location of new Winnipegosis reef mounds and potash deposits. 1 Current address: Department of Geology, Brandon University, Brandon, MN R7A 6A9 End_Page 156------------------------

Diagenetic origin of Basal Anhydrite in the Cretaceous Maha Sarakham salt: Khorat Plateau, NE Thailand

Development of a diagenetic anhydrite bed at the base of the Cretaceous Maha Sarakham Saline Formation (the `Basal Anhydrite' member) of the Khorat Plateau in north‐eastern Thailand took place due to leaching and/or pressure dissolution of salt at the contact between an underlying active sandstone aquifer system and an overlying massive halite‐dominated evaporite sequence. Basal evaporites composed of halite with intercalated anhydrite of the latter sequence are undergoing dissolution as a result of subsurface flushing, with anhydrite produced as the insoluble residue. The result is a 1·1 m thick interval of nodular anhydrite displaying unique, basin‐wide continuity. Observed textures, petrographic features and chemical data from the anhydrite and associated authigenic minerals support the origin of the Basal Anhydrite Member as an accumulation residue from the dissolution of the Maha Sarakham salts. Petrographically, the anhydrite in this unit is made up of crystals that are blocky and recrystallized, sheared, generally elongated and broken, and is bounded at the bottom by organic‐rich stylolite surfaces. Authigenic and euhedral dolomite and calcite crystals are associated with the anhydrite. Traces of pyrite, galena and chalcopyrite are present along the stylolite surfaces suggesting supply of fresh water from the underlying sandstone at highly reducing conditions of burial. The δ34S of sulphate in the Basal Anhydrite averages 15 ‰ (CDT) and falls within the isotopic composition of the anhydrite in the Cretaceous Maha Sarakham Formation proper and the Cretaceous values of marine evaporites. Measured δ18O in dolomite range from −4·37 to −14·26‰ (PDB) suggesting a re‐equilibration of dolomite with basinal water depleted in 18O and possible recrystallization of dolomite under relatively elevated temperatures. The δ13C, however, varies from +1·57 to −2·53‰ (PDB) suggesting a contribution of carbon from oxidation of organic matter. This basal anhydrite bed, similar to basinwide beds found at the bottom of many giant evaporite sequences, has always been considered to be depositional. Here, at the base of the Maha Sarakham Formation, we demonstrate that the anhydrite is diagenetic in origin and was formed by accumulation of original anhydrite by dissolution of interbedded halite from waters circulating though the underlying aquifer: it represents an `upside‐down' caprock.

Deep‐water resedimentation of anhydrite and gypsum deposits in the Middle Miocene (Belayim Formation) of the Red Sea, Egypt

ABSTRACTThe Middle Miocene evaporites in the Red Sea rift were deposited within a complex system of fault‐bounded basins that were episodically active during sedimentation. Such a tectonic framework is known to be highly favourable to resedimentation processes. An offshore petroleum well in the north‐western Red Sea has cored, below a massive salt unit, an anhydrite‐bearing succession which provides an excellent opportunity to study the processes of gravity induced redeposition of Ca‐sulphates in a deep basin. Anhydrite deposits, interbedded with siliciclastic layers and thin halite layers, are composed of resedimented facies ranging from fine‐grained laminated sediments to coarse‐grained breccias. The components derive from the reworking of shelf sediments deposited initially in shallow water to supratidal settings on the surface and edges of structural highs bordering depressions: proximal siliciclastic deposits with interstitial anhydrite (cement patches, nodules) or gypsum and dolostones with early diagenetic anhydrite facies (nodular, chicken‐wire) formed in sabkha conditions, interstitially grown gypsum crystals and subaqueous gypsum crusts precipitated in hypersaline ponds, and diatom‐rich oozes formed in marine, shallow‐water conditions. The homogeneity of the stable isotope composition and petrography of sulphates argue for the initial crystallization of Ca‐sulphates within brines of the same origin and in closely interconnected sedimentary settings. The unconsolidated sediments redeposited as slope‐foot accumulations were carried both as anhydrite (nodules, soft masses, various fragments, individual grains or crystals released by disintegration of large masses) and gypsum (crystalline aggregates or single crystals) later converted to anhydrite during burial. Layers of chaotic breccia are interpreted as the result of seismic events, whereas the fine‐grained deposits could be related to redistribution by nepheloid layers of suspensions of finer grains released by disintegration of the soft anhydrite masses during downslope transport, or of in situ deposits removed by the turbiditic flows.

Conditions of ore mineral formation in certain Zambian Copperbelt deposits with special reference to the role of cobalt

Studies of the compositions of coexisting sulphide assemblages from certain Zambian Copperbelt deposits and of their textural relations and host rock environments have been undertaken by routine microscopy and using electron microprobe analysis. Special attention has been paid to sulphide assemblages containing cobalt. Using experimental data on the systems Cu-Co-S, Cu-Fe-Co-S and on sulphidation equilibria, together with the available information on equilibria involving gangue minerals and some new calculations, an attempt has been made to define the chemical conditions of ore formation and/or re-equilibration in several deposits (Baluba, Chibuluma West and Chibuluma, Chambeshi S.E.). If a re-equilibration temperature of 300°C is assumed the range of aS2 prevailing during final formation of the assemblages was of the order 10−7 – 10−9 atm, more rarely dropping to 10−11 – 10−12 atm. If magnetite is present, and this is considered very unlikely in most cases, aO2 may have reached as high as 10−35 atm but it is generally likely to have been around 10−50 atm or even lower. Values of aCO2 are assumed to have been of the order of 10−0.5 atm. Detailed studies of drill hole samples from the Chambeshi S.E. deposit suggest an important link between the petrology of the host rocks and the sulphide mineral chemistry. In particular, the control exerted on sulphide composition by aS2 may have been related to the availability of sulphate in the form of diagenetic anhydrite or in sulphate-rich interstitial waters. The distinctive distribution of cobalt in the ores appears related to the distribution of amphibolite bodies and in turn to rift fault systems in the Basement which allowed upward movement of fluids enriched in magmatically derived cobalt.

Plant-controlled supratidal anhydrite from Al-Khiran, Kuwait

We report here that in the supratidal zone of the Al-Khiran sabkha, diagenetic anhydrite occurs exclusively in association with the living halophyte Halocnemum strobilaceum (Pallas) M .B. This anhydrite has a hair cream-like consistency and the bulk of it has formed as pseudomorphs after gypsum crystals. On the death of the plant at the end of its life cycle, the anhydrite is hydrated back to gypsum. This two-way transformation results in a series of characteristic textures, whose preservation could be due to the comparative ‘youth’ of the sabkha, which is in the very earliest stages of evaporite diagenesis.

Diagenesis of Keg River Bioherms, Rainbow Lake, Alberta: ABSTRACT

The Keg River bioherms consist mainly of mechanically deposited carbonate sediments and of stromatolites. True skeletal boundstone is rare. The salinity of the contemporaneous sea fluctuated repeatedly between normal marine and highly supersaline. Accordingly, fossiliferous marine carbonates are intercalated with essentially nonfossiliferous evaporitic carbonates. Parts of these sediments were intermittently at or above sea level. Intensive penecontemporaneous cementation by high magnesium calcite and, possibly, aragonite occurred on the bioherms at times of near-normal salinity. Unconsolidated fossiliferous carbonate sediments and soft algal mats became solidly lithified, wave-resistant rocks. The bioherms grew as cementation welded most loose carbonate particles firmly to the reef body. Abundant early fractures opened after cementation. These fractures commonly were filled with internal sediment of evaporitic facies. Early diagenetic anhydrite is present in evaporitic carbonates mainly as replacement, displacement nodules, and pore cement. Dolomitization of bioherms occurred mainly during evaporitic episodes at subtidal to supratidal sites. Unconsolidated evaporitic carbonate sediments selectively were dolomi ized completely and lithified. The dolomitization produced finely crystalline dolomite in evaporitic carbonates and medium to coarsely crystalline dolomite in previously lithified fossiliferous carbonates. Temporary subaerial exposure caused little or no cementation by meteoric low-magnesium calcite. Late diagenetic processes after effective burial include: (1) conversion of all metastable calcium carbonate to low-magnesium calcite; (2) cementation and replacement by minor amounts of anhydrite, calcite, and dolomite; (3) moderate fracturing; (4) weak stylolitization; and (5) deposition of large amounts of carbonaceous material in pores. End_of_Article - Last_Page 868------------