Anhydrite Cement Research Articles

Experimental study on water–rock interactions during CO 2 flooding in the Tensleep Formation, Wyoming, USA

The conditions for mineral alteration and formation damage during CO 2 treatment of Tensleep sandstone reservoirs in northern Wyoming, USA, were examined through core-flooding laboratory experiments carried out under simulated reservoir conditions (80°C and 166 bars). Subsurface cores from the Tensleep sandstone, which were cemented by dolomite and anhydrite, and synthetic brines were used. The brines used were (Ca, Mg, Na)SO 4–NaCl solution (9.69 g/l total dissolved solids) for Run 1 and a 0.25 mol/l NaCl solution for Run 2. The solution used in Run 1 was saturated with respect to anhydrite at run conditions, which is characteristic of Tensleep Formation waters. Three major reactions took place during flooding, including (1) dissolution of dolomite, (2) alteration of K-feldspar to form kaolinite, and (3) precipitation (in Run 1) or dissolution (in Run 2) of anhydrite. All sample solutions remained undersaturated with respect to carbonates. The permeability of all the cores (except one used in Run 2) decreased during the experiments despite the dissolution of authigenic cement. Kaolinite crystal growth occurring in pore throats likely reduced the permeability. Application of the experimental results to reservoirs in the Tensleep Formation indicates that an injection solution will obtain saturation with respect to dolomite (and anhydrite) in the immediate vicinity of the injection well. The injection of NaCl-type water, which can be obtained from other formations, causes a greater increase in porosity than the injection of Tensleep Formation waters because of the dissolution of both dolomite and anhydrite cements.

Influence of Early Diagenesis on Reservoir Quality of Rotliegende Sandstones, Northern Netherlands

Reservoir quality of Rotliegende sandstones (Lower Permian) in the northern Netherlands was significantly affected by early diagenetic processes that are related to sedimentary facies in marginal desert/playa lake environments. In the past, burial-related products and processes have received more attention than early diagenetic processes, neglecting the effects that early diagenesis can have on reservoir quality and well productivity of Rotliegende sandstones. Shallow groundwater-related diagenesis is a major factor affecting Rotliegende reservoir sandstones. The precipitation of the main porosity-occluding cements (dolomite, anhydrite, and quartz) occurred early in the diagenetic history. Oxygen isotopes of dolomite (d18O = 18.4 to 23.4‰ SMOW) suggest that meteoric water has played a major role throughout the diagenetic history, and that precipitation of these cements occurred at low temperatures (<80°C). The strontium isotopes of dolomite cements (87Sr/86Sr = 0.70935-0.71387) and the sulfur isotopes of anhydrite cements (d34S = 6.7 to 9.7‰ CDT) rule out Permian or younger seawater as a source for these cements, and are consistent with formation from meteoric water within a continental basin. An early diagenetic origin of the cements is also suggested by the fact that the majority of sandstone samples have lost more porosity due to cementation than through compaction. Most samples with high (<20%) amounts of porosity loss via cementation were affected by early dolomite and anhydrite. The relative abundance of dolomite and anhydrite cements can be correlated with depositional environments. Wet depositional environments (e.g., interdune and fluvial sandstones) with water tables close to the surface show the highest amount of dolomite and anhydrite cements, whereas dry environments with relatively deep water tables (e.g., dune sandstones) are characterized by low amounts of cements. The precipitation of early diagenetic cements has strongly influenced the present-day porosity patterns, with depositional environments interpreted as drier showing less cements and significantly altering the porosity patterns formed during early diagenesis. Knowledge of the influence of sedimentology and paleogeography on the diagenetic patterns is a key element for improved understanding and prediction of reservoir quality in the Rotliegende sandstones of the northern Netherlands.

Thermal History Constraints from Studies of Organic Matter, Clay Minerals, Fluid inclusions, and Apatite Fission Tracks at the Ardeche Paleo-margin (BA1 Drill Hole, GPF Program), France

ABSTRACT The thermal history of the Ardeche paleo-margin (southeastern France) has been reconstructed using various analytical data from minerals and organic matter and conventional interpretation. These methods comprise Tmax and vitrinite reflectance measurements on organic matter, determination of the smectite content in mixed-layer illite/smectite, fluid-inclusion microthermometry in quartz, barite, dolomite, and anhydrite, and fission-track analyses in apatite. The Balazuc (BA1) drill hole, 1730 m deep, intersects mainly Jurassic carbonate rocks, Triassic sandstones, and sulfate-rich claystones and ends in Carboniferous rocks. The present bottom-hole temperature is 70°C. Other drill holes, less than 6 km away, were studied when necessary. One of the main objectives of this paper is to ssess the agreement between the different geothermometric methods, and to check their limitations and convergence when used on carbonate- and sandstone-dominated reservoirs. In the Triassic sandstones, primary aqueous inclusions in anhydrite cement and at the boundary between the detrital quartz grain and the quartz overgrowth are one-phase brine inclusions entrapped below 70°C. The homogenization temperature determined on the later barite cement is about 100°C. Aqueous two-phase inclusions in healed microfractures in anhydrite are present at the base of the drill core near major faults. Temperatures as high as 210°C recorded for these fluid inclusions could be associated with episodic fluid injections from below through the fault zones during the Early Jurassic. The fluid inclusions in dolomite, most of them reequilibrated during burial, indicate a trapping temperature of 130-145°C at 1600 m and a temperature decrease towards the surface. This value is in good agreement with the MPTB (maximum paleotemperature of burial) method on organic matter, which converges to a maximum burial temperature of 130°C at the same depth. The apatite fission-track data agree with these temperatures and indicate that the temperature decreased below 120 ± 10°C during the Eocene. These paleotemperatures, much higher than the present ones, can be explained by the erosion of 1900 m of mainly Cretaceous sediments. This interpretation implies a high average sedimentation rate during the Cretaceous, in agreement with data determined on a regional scale. A time-temperature evolution is proposed for the formations present in the Balazuc drill hole and the eroded sedimentary rocks. The application of the dual reaction model of the transformation of smectite to illite (Velde and Vasseur 1992) shows that the data are in good agreement with the kinetic model. However, the vitrinite reflectance variation trend with depth, predicted from the Burnham and Sweeney model (1989), is different from the data variation. No definitive explanation can be proposed for the failure of aturity assessment through vitrinite reflectance in the BA1 drill hole.

The origin and distribution of porosity in the Zechsteinkalk (Upper Permian) of Hewett Field, southern North Sea

The Zechsteinkalk of the Hewett Field is characterized by extremely heterogeneous porosity distribution. The two most closely spaced wells (only 200 m apart) show one of the largest differences in average porosity (1.8 to 7.6%). In order to locate future wells and achieve 'reasonable' OGIP estimates, it is therefore necessary to model porosity distribution. Facies type has some influence on porosity development. Tidal flat muds developed at the shoreward margins of shoals have only very low porosity, whereas inter-shoal and main shoal facies have moderate porosity, on average. Extreme porosity variation within facies is caused by differential diagenesis. Porosity has been created by leaching and dolomitization and destroyed by early carbonate cementation and by anhydrite cementation. Anhydrite is volumetrically the most significant, occluding 20% (bulk volume) porosity in many intervals and completely cementing many fractures. The scale of the variability that can be derived from modelling of porosity-affecting diagenetic processes is mostly much greater than that found between closely spaced wells, particularly in the case of anhydrite cementation. In spite of this uncertainty the geological model represents an improvement on simple independent porosity contouring. The gross trends of porosity distribution, defined by the sedimentological model, have helped enhance confidence in a seismic porosity model and have helped with estimates of gas resources.

Origin and modification of Cambrian dolomites (Red Heart Dolomite and Arthur Creek Formation), Georgina Basin, central Australia

ABSTRACTThe Early to Middle Cambrian Red Heart Dolomite and lower Arthur Creek Formation of the southern portion of the Georgina Basin, Australia, is an entirely dolomitized succession of shallow‐water evaporitic mudflat and deeper‐water subtidal lithologies. Three types of dolomite have been identified and are interpreted as: (1) syndepositional dolomite; (2) regional replacement dolomite; and (3) void‐filling dolomite (cement). Syndepositional dolomite, derived from saline pore fluids developed in a sabkha environment, is a minor dolomite type with very fine crystal mosaics and has a mottled, non‐zoned cathodoluminescence. The widespread regional replacement dolomite ranges from fine‐ to medium‐crystalline forming mainly planar‐s and non‐planar‐a crystal mosaics, and displays blotchy, mottled, non‐zoned cathodoluminescence. Void‐filling dolomite commonly forms planar‐s to planar‐e, medium to very coarse crystal mosaics. Rare non‐planar‐c, very coarsely crystalline saddle dolomite also exists. Void‐filling dolomite has a successively zoned cathodoluminescence pattern from non‐, to brightly, to dully luminescent.Geochemically, the syndepositional dolomite has δ18O (PDB) values ranging between − 5.3 and − 8.6%o. Regional replacement dolomites exhibit a wide range of δ18O values from − 3.3 to − 10.9%o whereas void‐filling dolomite has δ18O values ranging from − 10.8 to − 14.3%o. All three dolomite types have similar δ13C (PDB) values, in the range between +1.7 and −1.7%o.Three initial dolomitization episodes are interpreted: (1) a sabkha stage, forming the syndepositional dolomite and dolomitizing the evaporitic mudflat lithologies; (2) a brine‐reflux stage, replacing the subtidal lithologies; and (3) a burial stage, forming the void‐filling dolomite type. Final dolomite stabilization occurred during burial, at elevated temperatures, in the presence of basinal fluids, resulting in progressive recrystallization and stabilization of the earlier‐formed syndepositional and replacement dolomites. Both textural and geochemical evolution should be taken into account when studying the origin of dolomites, based on their present geochemical composition.Sulphates are represented by very fine‐crystalline syndepositional anhydrite in association with the syndepositional dolomite, and coarse to very coarse anhydrite cement. Evaportic mudflat (sabkha) and burial environments are inferred for the origin of the former and the latter anhydrite types, respectively. Evaporite dissolution breccias, indicative of the former presence of evaporites, are common throughout the succession.

Diagenesis of Deeply Buried Eagle Mills Sandstones: Implications for Paleo-Fluid Migration and Porosity Development: ABSTRACT

ABSTRACT Eagle Mills strata (Triassic-Jurassic) unconformably overlie the Paleozoic basement complex and thus, form the basal sedimentary unit within the Gulf of Mexico Basin. Jurassic-aged basaltic dikes and sills have intruded Eagle Mills strata. Werner-Louann evaporates disconformably overlie the Eagle Mills Formation. These sedimentary and igneous rocks preserve the earliest record of Gulf of Mexico rifting. Deeply buried (15,000 to 18,000 ft/4,573 to 5,488 m) Eagle Mills sandstones have subarkosic and sublithic modal compositions. These sandstones exhibit evidence of a complex and prolonged diagenetic history, including: early chlorite cementation; mechanical compaction; quartz and feldspar overgrowths; nonferroan calcite and dolomite cementation; dissolution of framework grains; kaolinite precipitation; ferroan calcite cementation; albitization; saddle dolomite and anhydrite cementation; and pyritization. Pyrobitumens coat early cements indicating that most diagenesis post-dated hydrocarbon migration. Marked shifts in paleo-water chemistry are recorded by diagenetic phases in Eagle Mills sandstones. The Eagle Mills paragenetic sequence is indicative of progressive burial into a high-temperature (>150° C) regime where thermochemical sulfate reduction was the dominant diagenetic process. Pervasive late burial dissolution of detrital feldspars in some Eagle Mills sandstones provides direct evidence for deep-seated sourcing of metal cations which promoted diagenesis of overlying Mesozoic strata. These petrographic data support interpretations of the Mesozoic Gulf Coast Basin which imply vertical mass-transfer of diagenetic fluids along faults.

Hydrocarbon-Water-Rock Interaction: Redox Reaction as a Mechanism for Sandstone Reservoir Porosity Enhancement: ABSTRACT

Experiments evaluated the potential for and extent of oil-water-rock reactions in hydrocarbon reservoirs. Results indicate not only that significant potential exists for redox reactions between oxidized mineral phases and crude oil, but that such reactions can significantly alter porosity and permeability characteristics of an elastic hydrocarbon reservoir. The red (oxidized) sandstones used in the redox experiment initially contained 10 to 25% carbonate, anhydrite, and intergranular clay cements. Porosity ranged from 6 to 15%. The sandstones were gray or white after experimentation, and porosity increased 12 to 20% over original values, primarily due to carbonate dissolution. It is suggested that during the redox experiments, the iron oxides ({+-} sulphate) were reduced and hydrocarbon was oxidized to produce oxygenated organic compounds (e.g., organic acid anions, CO{sub 2}). These redox reaction products destabilized the carbonate cements and enhanced sandstone porosity. It is concluded that redox reactions involving crude oil and the mineral matrix of these reservoir rocks in the presence of H{sub 2}O do occur and may result in significantly enhanced porosity. Hydrocarbon emplacement and the resultant redox reactions can cause bleaching and changes in porosity and permeability. This relationship is well documented in the Wingate, White Rim, and Tensleep sandstones. The hydrocarbonmore » reservoir units are white to gray and have good porosity and permeability. The adjacent non-reservoir units are red (due to hematite staining), and have good carbonate cementation and poor porosity and permeability, confining hydrocarbon flow to the nearby reservoir units or associated fractures.« less

Paleozoic Fluid History of the Michigan Basin: Evidence from Dolomite Geochemistry in the Middle Ordovician St. Peter Sandstone

ABSTRACT The isotope (Sr and O) and elemental (Mg, Ca, Mn, Fe, and Sr) compositions of the various dolomites in the Middle Ordovician St. Peter Sandstone in the Michigan Basin are determined and the variations are modeled in terms of fluid-rock interaction or as mixing relations. These geochemical models, combined with the paragenetic sequence of the dolomites and late anhydrite cement, suggest the existence of at least four distinct diagenetic fluids in the St. Peter Sandstone during the Paleozoic. Fluid 1 (Sr = 30 ± 10 ppm, Sr/Ca = 0.019, 87Sr/86Sr = 0.7093 ± 1) has a composition consistent with a modified older (pre-Middle Ordovician) seawater origin, which indicates that the flow path for this fluid had a major upward component. This fluid resulted in th first and volumetrically most important burial dolomitization event, producing dolomite in both carbonate and quartz sandstone lithofacies in the St. Peter Sandstone. Fluid 2 (Sr = 30 ± 10 ppm, Sr/Ca = 0.019, 87Sr/86Sr = 0.7084 ± 1, low Fe) has a composition consistent with a modified Middle to early Late Ordovician seawater origin, suggesting a major downward component for fluid flow. Fluid 2 produced dolomite cement in the carbonate lithofacies that postdates Fluid 1 dolomite. The composition of Fluid 3 (Sr > 50 ppm, Sr/Ca = 0.044-0.110, 87Sr/86Sr = 0.710-0.711, high Fe) is best interpreted to reflect a heated, deep basinal brine that had previously interacted with the K-feldspar-rich rocks near the Cambrian-Precambrian unco formity in the Michigan Basin, indicating a major upward component for fluid flow. Fluid 3 produced dolomite cement in quartz sandstone lithofacies that postdates Fluid 1 dolomite. Fluid 4 (87Sr/86Sr = 0.7085 ± 1) resulted in precipitation of late anhydrite in fractures. The 87Sr/86Sr ratio of the anhydrite is consistent with Fluid 4 originating as a dilute fluid that interacted extensively with Silurian gypsum in the Michigan Basin; this indicates that the flow path of Fluid 4 had a major downward component. The St. Peter Sandstone was an important migration pathway for many different types of fluids in the Michigan Basin, and the proposed fluid origins indicate that both upward and downward cross-formational fluid flow occurred during the Paleozoic. Older and younger modified seawater produced most of the burial dolomite in the St. Peter Sandstone.

Late anhydrite cements mark basin inversion: isotopic and formation water evidence, Rotliegend Sandstone, North Sea

The Lower Permian Rotliegend Sandstone in the Leman Field today contains formation waters which are extremely saline (165 530–238 531 mg/l), with cation/chloride ratios indicating mixing of Zechstein marine evaporite brines with meteoric water. Ancient Rotliegend formation waters, inferred from the analyses of cements, were similarly a mixture of marine brine and meteoric water (δ 18O= +0.3 ‰ SMOW; °D= −16 ‰ SMOW). The strontium isotopic compositions of present day formation waters ( 87Sr/ 86Sr= 0.7107) are also consistent with mixing between late Permian marine brines ( 87Sr/ 86Sr= 0.707) and radiogenic strontium from a detrital source ( 87Sr/ 86Sr= 0.720–0.730). Isotopic analyses of texturally late intergranular and fracture-filling anhydrite suggest waters very similar to the present day formation waters. δ 34S values of anhydrite cements (+10‰ CDT) indicate that sulphate in the Rotliegend originated from the stratigraphically overlying Zechstein evaporites. The ( 87Sr/ 86Sr) 150 ratios of anhydrites (0.7080–0.7113), however, suggest variable mixing of strontium sourced from the overlying Zechstein evaporites with more radiogenic strontium derived from diagenetic reactions in the underlying Carboniferous shales. The oxygen isotope compositions of anhydrite reflect precipitation temperatures between 120 and 140°C from waters with a δ 18O composition similar to the present day formation waters (approximately 0‰ SMOW). These are the maximum inferred burial temperatures for the Rotliegend Sandstone in the Leman Field and would have occurred at maximum burial depths of 3.5–4 km (11 500–13 100 ft) during the Late Jurassic-Early Cretaceous at the start of basin inversion. These data indicate an unusual, short-lived, catastrophic event of cross-formational flow, through fractures which were tectonically induced by basin inversion. Pore fluids may have escaped from the overlying and underlying formations into the lower pressure Rotliegend Sandstone regional aquifer to ‘drain’ this part of the basin.

Petrographic and Geochemical Constraints on the Formation and Diagenesis of Anhydrite Cements, Smackover Sandstones, Gulf of Mexico

ABSTRACT Anhydrite cement is common in sedimentary rocks, yet its origin is poorly understood. The high concentration of sulfate in sea water and the lack of appreciable sulfate in most other natural waters suggests that anhydrite cement may have a marine origin, but the relatively late timing of most anhydride cement tends to preclude sea water as the source of sulfate. Anhydrite cement is present in Upper Jurassic sandstones in the Gulf of Mexico as poikilotopic masses in which detrital grains are replaced and as smaller patches that have replaced single detrital grains. Anhydrite is a relatively late cement and postdates all other volumetrically significant authigenic phases, including K-feldspar, dolomite, quartz, and most calcite cements. The expected isotopic composition and trace-element concentration of marine-derived Late Jurassic anhydrite cement is predicted based on analyses of bedded anhydrite and on analyses found in the literature. The expected chemical signature is: 34S ( CDT) = +16, 18O ( SMOW) = +14, 87Sr/86Sr = 0.7069, Sr = 1500-2500 ppm, Ba 200 ppm. When geochemical analyses of Smackover anhydrite cements are compared to the predicted composition, it is apparent that most of the cement is not of marine origin. Two generations of anhydrite cement have been identified in the East Texas basin on the basis of their strontium isotopic compositions and their strontium concentrations. An early cement may have been derived from slightly modified Late Jurassic sea water, whereas a second group of cements may have precipitated later in the burial history or may represent recrystallization of the first cement. The chemistry of Louisiana and Mississippi basin anhydrite cements indicates that the sulfate was derived from dissolved bedded anhydrite and was reprecipitated in the sandstones from fluids that had undergone extensive water-rock interaction after considerable burial. This study suggests that late anhydrite cements in sedimentary rocks are most likely derived from remobilized calcium sulfate de osits.

Groundwater Flow, Late Cementation, and Petroleum Accumulation in the Permian Lyons Sandstone, Denver Basin

The gray diagenetic facies of the Permian Lyons Sandstone, which is associated with all known petroleum accumulations in the formation, formed late in the history of the Denver basin as an alteration product of the formation's red facies. The red facies that makes up most of the sandstone contains iron oxide coatings, quartz overgrowths and calcite cements. The gray facies, which occurs locally in the deep basin, is distinguished by pore-filling dolomite and anhydrite cements and by a lack of iron oxide and calcite. The dolomite and anhydrite cements overlie bitumen that was deposited by migrating oil, and hence formed after oil was first generated in the basin, late in the Cretaceous or early in the Tertiary. The isotopic composition of oxygen in the dolomite ranges to s ch light values that the cement must have formed deep in the basin in the presence of meteoric water. The gray facies likely formed in a regime of groundwater flow resulting from Laramide uplift of the Front Range during the Tertiary. In our model, saline groundwater flowed eastward through the Pennsylvanian Fountain Formation and then upwelled along the basin axis, where it discharged into the Lyons Sandstone. The saline water mixed with more dilute groundwater in the Lyons, driving a reaction that dissolved calcite and, by a common-ion effect, precipitated dolomite and anhydrite. The facies' gray color resulted from reduction of ferric oxide in the presence of migrating oil or the Fountain brine. Underlying source beds by this time had begun to generate petroleum, which migrated by buoyancy into the Lyons. The association of the gray facies with petroleum accumulations can be explai ed if the Fountain brines discharged across aquitards along the same fractures that transmitted oil. As petroleum accumulated in the Lyons, the newly formed cements prevented continued migration, as is observed in shallower strata, by sealing oil into the reservoirs from which it is produced today.

Paleoclimatic Controls on Sedimentation, Diagenesis, and Reservoir Quality: Lessons from Miocene Carbonates

Miocene carbonates worldwide were deposited in a wide range of paleogeographic and tectonic settings and form important reservoirs in several regions. The reservoir quality of these carbonates appears to have been primarily controlled by the prevailing paleoclimate, which exerted a major control on both the depositional patterns and diagenetic pathways. Two end members are discussed here: (1) humid, oceanic tropical-subtropical settings, and (2) arid, land-locked temperate-subtropical settings. In humid, oceanic tropical-subtropical settings (e.g., Miocene of Southeast Asia), a warming trend paralleled by generally rising sea level allowed thick coral reefs and skeletal banks to develop, most of which are seismically resolvable. These carbonates typically represent several stages or cycles of development (largely third-order) separated by discontinuities in platform growth with episodic subaerial exposure. Development of economic reservoirs is mainly related to relative sea level falls and associated meteoric diagenesis. Trends and patterns of reservoir distribution can be predicted within a sequence stratigraphic framework. Basal transgressive carbonates are mostly tight because of their relatively fine-grained textures, intense compaction, and isolation from meteoric water influence. Best reservoir quality is commonly developed beneath subaerial unconformities in highstand buildups where effects of meteoric water leaching and karstification are most intense. In contrast, in arid, land-locked, subtropical-temperate settings, unfavorable environmental conditions caused by periodically increased salinity and, probably, relatively low temperature restricted the full growth of carbonate buildups. As a result, relatively thin and narrow fringing coral reefs with small, poorly developed lagoons in rhodalgal ramps are the most common facies type in the upper Miocene of the Mediterranean and the middle Miocene of the Gulf of Suez and Red Sea, whereas the lower Miocene of the Middle East is characterized by low-energy, shallow shelfal and lagoonal carbonates. Under arid climatic conditions, recharge of fresh groundwaters is limited, thus minimizing meteoric dissolution and cementation during subaerial exposure. Instead, sea level fall and lowstand ommonly resulted in extensive evaporitic conditions. Hypersaline brines, originated within evaporitic basins or lagoons, were the main cause of the pervasive dolomitization, leaching of metastable skeletal grains, and generation of moldic, vuggy, and intercrystalline porosity. Often associated with these arid land-locked carbonates is the widespread anhydrite cement that precipitated over a wide range of burial depths. Porosity distribution in these carbonates thus depends not only on the original depositional fabrics but more importantly on the degree to which the porosity-occluding anhydrite cement was dissolved or the low-permeability carbonates were fractured. These Miocene examples illustrate the contrasting diagenetic patterns and reservoir styles controlled by paleoclimatic and paleoceanographic trends and demonstrate the importance of the often overlooked hypersaline diagenesis in arid, land-locked subtropical-temperate settings. The development of exploration and production strategies for carbonates that were deposited in different climatic settings should acknowledge and use these fundamentally different styles of diagenesis and reservoir characteristics.

Properties of anhydrite cement obtained from apatite phosphogypsum

The present work is a part of papers comprising the research on complex utilization of waste - apatite phosphogypsum (PG) obtained during the production of wet phosphoric acid from Kola apatite, on rare earth concentrate and anhydrite with simultaneous recovery of phosphate compounds. The way of PG utilization into anhydrite II was divided into two stages: 1. 1. Recovery of rare earth elements from PG by leaching in 12 wt pct sulphuric acid solution, 2. 2. Conversion of PG after leaching into anhydrite II in 50 wt pct sulphuric acid solution, 3. 2. Conversion of PG after leaching into anhydrite II in 50 wt pct sulphuric acid solution. The physical properties of the obtained anhydrite II as effect of potassium sulphate on the setting time and mechanical properties of anhydrite cement have been determined. Compressive strengths of 40 MPa and more have been achieved after 28 days, when 1.0 to 1.5 wt pct of K 2SO 4 had been added as accelerator. The research confirmed that the hardening process of discussed cement is connected with the formation of syngenite.

Tectonic and diagenetically induced seal failure within the south-western Great Bahamas Bank

A deep (> 6 km) well, the Doubloon Saxon 1 well, drilled in early 1986 on the Great Bahamas Bank, shows evidence of massive seal failure through tectonic and post-tectonic diagenetic alteration. Seal failure occurred through a hydrodynamically induced chemical alteration of a carbonate (limestone, dolomite)-evaporite (anhydrite, halite) sequence into a carbonate (dolomite and dedolomite) sequence. Evidence for this alteration includes a thick (3.5 km) fractured dolomite sequence interbedded with dedolomites. Fracture origins include solution collapse (of original evaporites as shown by anhydrite cement on fracture surfaces) and cavernous porosity, and by tectonic fracturing (a major reverse fault penetrated with > 1 km repeat). Additional evidence includes a temperature regression and geochemical anomalies above and below the proposed altered sequence. It is concluded that massive seal failure is a contributing factor in limiting the petroleum potential of portions of the south-western Bahamas Platform.

Diagenesis and Fluid Evolution of Deeply Buried Permian (Rotliegende) Gas Reservoirs, Northwest Germany

Depositional environment and tectonic setting were important in the diagenesis and evolution of reservoir properties in the Rotliegende sequence of the North German Basin. Facies belts paralleling the edge of a central saline lake controlled the distribution of early and shallow burial cements. Lake shoreline sands with radial chlorite cement show the best reservoir properties in the study area. Juxtaposition of Rotliegende deposits against either Carboniferous Coal Measures or Late Permian (Zechstein) evaporites by faulting resulted in cross-formational fluid exchange. The introduction of fluids from Carboniferous Coal Measures into Rotliegende reservoirs produced intense clay cementation, significantly reducing rock permeabilities. Influx of Zechstein fluids favored pre ipitation of late carbonate and anhydrite cements. Cross-formational and fault-related fluid flow was enhanced during periods of fault activity.

Fracture Characterization and Diagenesis in the Clipper Field, Sole Pit Basin, Southern North Sea

The Clipper field in the Sole Pit basin produces from tight Leman sandstones of the Rotliegende Group (Lower Permian). The reservoir consists of aeolian sediments. Gas production comes from open natural fractures and dune slipface sands with highly variable rates. The effects of fractures and diagenesis on reservoir quality were investigated. Three fracture networks have been observed in two highly deviated cored wells. Fault-related fractures occur close to, and parallel with, seismically mapped faults. Fold-related fractures occur as two sets of conjugate fractures, with the local maximum compressive stresses ([sigma][sub 1]) trending northeast-southwest and northwest-southeast, respectively. The dominant fracture types are cataclastic and dilational shear fractures. The cataclastic shear fractures were reopened and both fracture types are partially filled by silica, carbonate, and anhydrite cements. The main cement types within the sandstone matrix include dolomite, silica, anhydrite, illite, and ferroan carbonates. Early carbonate cements precipitated during initial burial from a mixture of Rotliegende groundwater and marine pore-fluids from the higher temperatures from Zechstein-derived pore fluids. Pore-filling and fracture-related ferroan carbonate and silica cement precipitated between temperatures of 100-150[degrees]C from isotopically evolved pore fluid. Integration of these data with the burial history and regional geological data reveal that the fault-related fracturesmore » formed during the formation of the Sole Pit rift basin in the Middle to Late Jurassic. The fold-related fractures formed during the Late Cretaceous inversion. The open fractures that contribute to production are associated with the inversion-related deformation. Modeling of these fracture networks, calibrated against available well data, can be used to define areas with high shear fracture density and assist development of fields in the Sole Pit basin.« less

Inversion of Reservoir Quality: An Example from the Triassic Buntsandstein, Offshore Netherlands

Sandstones from the Triassic Main Buntsandstein form a major gas reservoir in the Netherlands offshore. The sequence is dominated by siliciclastics deposited in an arid continental setting, and include dune, interdune, sheetsand, and fluvial sandstones. Reduction in reservoir quality is caused primarily by dolomite, halite, and anhydrite cementation, with minor authigenic illite and chlorite. Integration of petrographic and isotopic data has allowed the origins and relative timing of the different cements to be constrained. The carbon and oxygen isotopic composition of dolomite ([delta][sup 13]C = -3.76 to -9.3%, [delta][sup 18]O = -3 to +2.9% PDB) combined with strontium isotopic data (0.7091 to 0.7109 [sup 86]Sr/[sup 87]Sr) suggest that is precipitated from meteoric groundwater. Halite and anhydrite formed from a mixture of meteoric water and saline fluids expelled from underlying evaporites and claystones. Sulfur isotopic data (+4.2 to +12.1 CDT) support6 this interpretation for the origin of the anhydrite. Precipitation of the major authigenic minerals occurred during early diagenesis, prior to burial depths of 500 m. Cementation and groundwater flow followed the zones of highest permeability and caused an inversion of reservoir quality. Sandstones with the highest depositional porosity and permeability (i.e., dune sandstones) are the most cemented, and have poorermore » reservoir quality compared to the fluvial and interdune sandstones, which originally had lower depositional porosity/permeability. Formation of authigenic illite and chlorite occurred during burial and has significantly reduced permeability. Information on the depositional settings and paleogeography, combined with expected groundwater flow, has helped define potential exploration areas of reduced reservoir quality as a result of extensive early cementation.« less

The Zechsteinkalk Reservoir of the Hewett Field, Southern North Sea

The Zechsteinkalk of Hewett field is characterized by complex porosity and permeability distribution. Facies and diagenesis together control porosity distribution, while production rates are enhanced by subseismic [open quotes]megafractures.[open quotes] Highest porosities occur in the upper part of the sequence where dolomitized shoal and perishoal oolitic and peloidal grainstones predominate. Intershoal and main shoal facies show no significant differences in average porosity. Tidal-flat muds developed at the shoreward margin of shoals have only very low porosity. Extreme porosity variation within perishoal facies is caused by differential diagenesis. Porosity has been created by dolomitization and leaching but has been occluded by early carbonate cementation and anhydrite cementation. Anhydrite is volumetrically the most significant effect, destroying 20% porosity in many intervals and completely cementing many fractures. Its distribution is highly heterogeneous. Models for the origin of the anhydrite explain and predict gross distribution trends, but superimposed smaller scale trends are related to depositional grain size and the local development of evaporitic environment over subaerially exposed grainstones. Cementation took place in at least three phases, pre-, syn-, and postfracturing. Natural fracturing occurs throughout the field. Fractures are typically subvertical, planar, dilational, and completely or partially cemented by anhydrite. The cementation history of themore » fractures indicates formation prior to the development of the structure. Although fracture aperture porosity occurs in a significant proportion of fractures recorded in core (30-40%), DST results indicate fracture-enhanced flow from only a small number of fractured zones where individual fractures are sufficiently closely spaced to form an interconnected system or [open quotes]megafracture.[close quotes] The megafractures trend northwest-southwest oblique to the primary fracture trend.« less

Reservoir heterogeneity in the Vanguard Field, UKCS

Abstract The Rotliegendes Group in the Vanguard Field, UKCS, consists of a series of aeolian, fluvial, and reworked aeolian sediments, organized into four broadly correlatable lithostratigraphical units. Within these units, reservoir properties are strongly linked to lithofacies, with aeolian dune sandstones possessing the best porosity and permeability. A comparison of production log spinner profiles (PLT) with detailed sedimenttological logs from three development wells shows that gas production is intimately associated with specific lithofacies and their spatial organization. Although dune sandstones in the upper section of the Rotliegendes have good petrophysical characteristics, they do not contribute significantly to production because they are contained within laterally extensive and relatively impermeable interdune sandstones. Most production comes from 15–30 feet thick and highly permeable (100–1000 mD) dune foreset intervals deposited near the bases of large aeolian dunes. Significant interwell variation in production rates and in porosity exists for all lithofacies due to the variable extent of diagensis. Petrographical studies, combined with whole-rock geochemical analysis, have outlined a diagenetic sequence for the field. Two end-member mineralogical assemblages are identified which are closely linked to reservoir quality. High porosity wells contain a chlorite-K-feldspar-quartz-dolomite assemblage while low porosity wells contain an illite-quartz-ankerite assemblage. Empirical evidence shows that porosity loss is associated with conversion of K-feldspar, chlorite and dolomite to illite and ankerite, and is due to compaction rather than cementation. Post-depositional faults acted as conduits for fluid which altered mineralogy, removed soluble components and, in one case, introduced anhydrite cement.

Role of organics and methane in sulfide ore formation, exemplified by Kupferschiefer Cu–Ag deposits, Poland

Organic material has been involved in the formation of the Kupferschiefer Cu–Ag-sulfide deposits in Poland in several different ways and at several stages in their development. During sedimentation, a mixture of continental humic material and marine algal material was concentrated in the euxinic, peri-marine Kupferschiefer black shale environment (∼6% C org). Having the black shale directly on the continental redbeds below effected a strong redox gradient across which the ore is located. Early diagenetic pyrite-(marcasite) framboids and nodules were formed through bacteriogenic reduction of sulfate from seawater-derived pore waters. Metal zones and variations in organic composition indicate that upwelling oxidized ore fluids were reduced by organic material and Fe-sulfides in the shale, inducing the precipitation of Cu-sulfides. A network of natural hydrofractures, formed by generation of hydrocarbon-related gases in the shale, allowed the ore fluids to penetrate the otherwise impermeable caprock and concentrate sulfides. Natural gases also migrated up from Carboniferous coal source beds and accumulated in the C org-free eolian sandstone below the Kupferschiefer shale. Geologic and cathode luminescence evidence indicates that the CH 4 reacted with anhydrite cement to form distinctive calcite cements and H 2S which precipitated Cu-sulfide cements, although δ 13C- and δ 34S-values are inconclusive. Quartz replacement concomitant with sulfide formation in the Kupferschiefer and Proterozoic Nonesuch ores (Michigan, U.S.A.), and in Athabasca oil sands (Canada), provides field support that silica-organic complexing is a mechanism that accelerates dissolution and increases solubility of quartz at low temperature and neutral pH.