Anhydrite Cement Research Articles

Links between anhydrite precipitation and dolomitization during cycles of sea level change: Insights from the Late Jurassic Arab Formation, Abu Dhabi, United Arab Emirates

Despite extensive research in the field of evaporative dolomitization, the close link between anhydrite precipitation and dolomitization is still poorly explored within the context of sequence stratigraphy. This comprehensive diagenetic and sequence stratigraphic study of the Late Jurassic Arab Formation, Abu Dhabi, United Arab Emirates, provides important insights into the relationship between anhydrite precipitation and dolomitization within the 2nd, 3rd, and 4th orders of relative sea-level cycles. Initial good connectivity between the inner ramp and open sea during 2nd order late transgressive-early regression cycles accounts for the limestone-dominated lithology with limited anhydrite formation. Conversely, the following progressive 2nd order fall in the relative periodic restriction of the inner platform during 2nd order early regressive cycles (early highstand systems tracts, HST) caused seepage reflux dolomitization and the precipitation of scattered to pervasive anhydrite cement in the dolostones. Variations in the extent of dolomitization were controlled by the permeability and reactivity of the precursor limestones, resulting in the formation of what is known as dolomite fingers. The most laterally extensive dolomitization during late 2nd order regression resulted in the formation of microcrystalline dolostones with nodular and chickenwire anhydrite by a combination of evaporative sabkha pumping and the seepage reflux of lagoon brines. The δ1³C and δ1⁸O of calcite and dolomite reveal the influence of degree of restriction of the inner ramp, and related extent of seawater evaporation. The lower limestone-dominated interval (3rd order HST) is characterized by lower δ1³C values (+2.0‰ to +2.5‰) owing to periodic restriction of seawater circulation, which resulted in oxidation of organic matter during aging of the seawater. This study approach provides important insights into the genetic links between interbedded platform limestones, dolostones, and anhydrites, and hence a better understanding and prediction of reservoir quality distribution and compartmentalization.

Low-energy integrated technology for processing underburnt lime into anhydrite cement

Low-energy integrated technology for processing underburnt lime into anhydrite cement

A fresh look at the Lucia classification using mud- and grain-dominated reservoirs of the Persian Gulf

Carbonate reservoir heterogeneity, a multifaceted phenomenon in the domain of subsurface exploration, plays a pivotal role in the assessment of hydrocarbon resources. The upper Dalan and Dariyan formations are significant gas and oil reservoirs in the Persian Gulf. Both of these formations are strongly heterogenous. Different methods are used to evaluate reservoir heterogeneities and determine rock types. In this research, Lucia's standard petrophysical method was used to manage and compare the heterogeneity of the upper Dalan and Dariyan formations. To investigate the factors influencing the distribution of samples within Lucia's petrophysical classes, microfacies and diagenetic processes were studied in both formations using 1483 thin sections. To determine reservoir rock types, 2080 porosity and permeability data were plotted on Lucia's petrophysical chart. The charts were plotted at multiple scales, including the whole well, reservoir units, and microfacies levels. All Lucia's classes were observed in the upper Dalan Formation. Dissolution and dolomitization in the mud-dominated microfacies have led to the increase in both porosity and permeability, resulting in the distribution of samples primarily within Lucia classes 1 and 2. On the other hand, calcite and anhydrite cements in the grain-dominated microfacies have reduced the reservoir quality and increased the number of samples in the lower parts of the Lucia's chart. In contrast to the upper Dalan, the samples from the Dariyan Formation were concentrated within class 3 and the lower part of the Lucia's chart. This is due to the high abundance of mud-dominated microfacies and the presence of high microporosity and low permeability. Finally, by comparing the results obtained from the two formations, it can be concluded that the Lucia's method is more suitable for grain-dominated formations compared to mud-dominated reservoirs. The findings of this study can help for better understanding of the relationships between carbonate textures and porosity-permeability. Results also shed light on evaluating the heterogeneity of different carbonate reservoirs.

THE ZECHSTEIN Z2 HAUPTDOLOMIT PLATFORM IN THE SOUTHERN UK MID NORTH SEA HIGH AND ITS ASSOCIATED PETROLEUM PLAYS, POTENTIAL AND PROSPECTIVITY

The Mid North Sea High (MNSH) region represents one of the least explored areas for the Late Permian Zechstein Hauptdolomit play in the Southern Permian Basin although some of the first offshore wells drilled in the UK were located here. In other parts of the basin such as onshore Poland, the Hauptdolomit Formation (“Hauptdolomit”) is an active and attractive exploration target, with oil and gas production from commercial‐sized fields. In the UK, the play has been overshadowed by drilling campaigns in areas to the south of the MNSH which tested plays in the underlying Rotliegend and Carboniferous successions. However, with these areas now in decline, there is increased exploration interest in the Hauptdolomit in the MNSH region, particularly since 2019 when 3D seismic data were acquired and the first hydrocarbon discovery was made at Ossian (well 42/04‐01/1Z). Geochemical data from the latter discovery have pointed to the presence of a prolific petroleum system with the potential for Hauptdolomit reservoirs to be charged both by Zechstein‐generated oils and Carboniferous condensate/gas. With regard to hydrocarbon migration and preservation in the southern MNSH, a detailed evaluation of the effects of the Mid Miocene Unconformity has allowed for a greater understanding of the main factors controlling hydrocarbon preservation and remigration. Reservoir characterization of the Hauptdolomit play has been achieved by integrating petrographic microfacies analyses, core data and petrophysical interpretations. The most important factors controlling reservoir quality are the presence and extent of anhydrite cementation and the presence of high energy shoal facies. Thicker and coarser grained shoal facies are expected to occur along the yet‐to‐be explored Orchard platform margin where numerous prospects have been mapped and refined using recently acquired 3D seismic data.

SEDIMENTOLOGY, PALAEOGEOGRAPHY AND DIAGENESIS OF THE UPPER PERMIAN (Z2) HAUPTDOLOMIT FORMATION ON THE SOUTHERN MARGIN OF THE MID NORTH SEA HIGH AND IMPLICATIONS FOR RESERVOIR PROSPECTIVITY

This paper provides an updated understanding of the reservoir stratigraphy, sedimentology, palaeogeography and diagenesis of the Upper Permian Hauptdolomit Formation of the Zechstein Supergroup (“Hauptdolomit”) in a study area on the southern margin of the Mid North Sea High. The paper is based on the examination and description of core and cuttings data from 25 wells which were integrated with observations based on existing and new 3D seismic.Based on thin‐section petrography of cuttings and core from the wells studied, it is evident that Hauptdolomit microfacies are distributed in a relatively predictable way, and well‐defined platform interior, platform margin, slope and basin settings can be distinguished. Platform margins are typically characterised by the development of ooid shoals and, to a lesser‐extent, by microbial build‐ups. High‐energy back‐shoal settings are characterised by a more complex combination of peloid grainstones, thrombolitic and microbial build‐ups, and fine crystalline dolomites. Lower energy lagoons which developed further behind the platform margin are characterised by a variety of microfacies types; fine crystalline dolomites are common in this setting as well as peloidal facies and local microbial build‐ups. Intertidal and supratidal settings are typified by increased proportions of anhydrite and the development of laminated microbial bindstones (stromatolites). Platform margins are in general relatively steep and pass into slope and basinal settings. Only a few wells have penetrated Hauptdolomit successions deposited in a slope setting, and these successions are characterised by a range of resedimented shallow‐water facies together with low‐energy laminated dolomicrites and fine crystalline dolomites. Slope zones in the study area are interpreted from seismic data to be typically 1‐1.5 km in width. Basinal Hauptdolomit deposits have been strongly affected by post‐depositional diagenesis and are dedolomitised to variable degrees. The original depositional facies are rarely preserved.Diagenetic studies show that dolomitisation has affected almost the entire Hauptdolomit Formation throughout the study area in both basinal and platform settings. The dolomite is considered to result from seepage‐reflux processes and is an early diagenetic phase. Mouldic porosity is present in many facies types as a result of dissolution, especially in ooid grainstones, thrombolitic build‐ups and peloidal facies. The dissolution cannot be associated with any one diagenetic phase but was most likely a result of the dolomitisation process itself. Stable isotope analyses indicate that all dolomites were precipitated from Permian marine‐derived pore fluids. Fluid inclusion analyses of dolomite cements indicate that cementation continued into the burial realm. Anhydrite cementation occurs in two phases: early anhydrite precipitation was associated with dolomitisation, and can be distinguished from a later, pore‐filling cement which is highly detrimental to reservoir quality.The Hauptdolomit succession in basinal wells (and in some slope wells) in the study area has undergone significant dedolomitisation. Dedolomitisation was a shallow burial process which affected precursor dolomites, whereby excess calcium from the transition of gypsum to anhydrite during burial combined with CO2 and organic acids derived from basinal sediments. The process was triggered by excess calcium reacting with excess carbonate ions from dissolution.3D seismic volumes supplemented by numerous 2D lines were available in the study area and allowed an interpretation to be made of Hauptdolomit gross depositional settings; platform margins and base‐of‐slope polygons were mapped, with the greatest confidence in areas of 3D seismic. The basin, slope and platform settings were distinguished using seismic data integrated with the results of micro‐facies analysis and incorporating seismic‐to‐well ties. The data shows that large parts of the study area are characterised by the presence of polyhalites within the overlying (Z2) Stassfurt Halite Formation, which may create particular seismic geometries at the Hauptdolomit slope. These are interpreted to be intra‐Stassfurt Halite features, providing an alternative model to the thickened, prograded Hauptdolomit which has been suggested in previous publications.Because few wells drilled in the study area had the Hauptdolomit as the primary target, cores were limited but significant data was obtained from cuttings analyses. More than 400 thin sections were evaluated, allowing depositional models based on microfacies observations to be developed, verifying the seismic‐scale observations.

Facies, Depositional Environment and Reservoir Quality of an Early Cambrian Carbonate Ramp in the Tarim Basin, NW China

The Xiaoerbulake Formation in the Tarim Basin is considered one of the most important deep to ultradeep hydrocarbon reservoirs in the world. The objective of the present study is to analyze the facies, depositional environment and reservoir quality of the Early Cambrian Xiaoerbulake Formation in the Tarim Basin through integrated analysis of 120 m of cores, 3240 m of well cuttings, wireline logs and over 1100 thin sections from 17 exploration wells. Early Cambrian Xiaoerbulake Formation deposits in the Tarim Basin were deposited on a ramp setting. The ramp was occupied by seven facies associations and ten facies types ranging from the inner ramp to the outer ramp. These facies associations include tidal flat, lagoon, high-energy shoal, sabkha, inter-shoal, low-energy shoal and open shelf. Their distribution was controlled by paleogeographic patterns, sea level changes and the paleoclimate. Reservoir quality is considerably affected by facies together with diagenetic processes, including dolomitization and anhydrite cementation. High-quality reservoirs are found in the dolograinstone of high-energy shoal environments, which are favorable exploration facies in the carbonate ramp. Their pore space types consist of interparticle pores, intraparticle pores, intercrystalline pores, vuggy pores and moldic pores, with a porosity of 1.4%–7.5%. This study will help with our understanding of the stratigraphic framework, sedimentary-facies evolution and high-quality reservoir distribution of the Early Cambrian carbonate ramp in the Tarim Basin, facilitating exploration and the production of hydrocarbons from the Xiaoerbulake Formation.

Reservoir characterization of the Oligocene–Miocene siliciclastic sequences (Ghar Member of the Asmari Formation) in the northwestern Persian Gulf

This paper presents an integrated facies, diagenesis, and sequence stratigraphic analysis, and reservoir quality evaluation of the siliciclastic hydrocarbon reservoirs (Ghar Member of the Asmari Formation) in the NW Persian Gulf. Results of core descriptions, thin sections petrography, X-ray computed tomography (CT) scanning, X-ray diffraction, sieve analysis, porosity–permeability measurements, and petrophysical logs are used. Four petrofacies are recognized indicating deposition in shore-line (beach) sand bodies and supratidal to intertidal sub-environments. Calcite and anhydrite cementation, dolomitization, recrystallization, compaction, and fracturing are diagenetic processes which occurred in marine, hypersaline, and shallow burial realms. A large-scale transgressive–regressive sequence and five deepening-upward cycles are defined. Hydraulic flow units and pore-size classes are defined. The best reservoir units (hydraulic units 5 to 8 and pore-size classes 4 and 5) are formed within the well-sorted, rounded, and mature sand/sandstone facies in which intergranular pores are predominant and diagenetic imprints are negligible. These facies are located in lower-half of depositional cycles. Carbonate-dominated facies show variable reservoir properties depending on their diagenetic alterations. In these facies, dolomitization and fracturing improved the permeability. This study shows that reservoir properties of the Ghar Member are strongly facies-controlled and, therefore, are easily predictable within a sequence stratigraphic framework.

Linking sedimentary properties to mechanical characteristics of carbonate reservoir rock: An example from central Persian Gulf

A comprehensive understanding of mechanical properties of reservoir rocks can help in all stages of hydrocarbon and geothermal exploration, field development, and Carbon Capture Storage (CCS). In this study, the sedimentological properties of a carbonate reservior have been studied and the effect these properties have on mechanical parameters such as Uniaxial Compressive Strength (UCS), Young's modulus (E), Cohesion (C) and Internal friction angle (φ) of the rock at ambient (20 °C) and reservoir (90 °C) temperature conditions. These carbonates were grouped in three units (A, B, and C) based on a statistical analysis of petrophysical logs and core analysis. The results show that changes in mechanical properties are mainly influenced by diagenetic processes. The two main influencing overprints in the studied samples are dolomitization and anhydrite cementation, which strongly affect porosity, the dominant pore type and mineralogy which in turn are the most important influencing factors on mechanical properties. Although samples with 25–35% anhydrite (Unit A) show the highest strength, this parameter strongly reduces under high temperature conditions. The effect of dolomitization on rock texture and on porosity as the main strength-controlling factor is also very apparent. Therefore, samples with crystalline carbonate texture (Unit C) show the lowest UCS, E, C and φ values.

Lithium and brine geochemistry in the Qianjiang Formation of the Jianghan Basin, central China

The Li-enriched oilfield brine is a very important lithium resource. It has gained much attention and become the target of active Li surveys with the growing global demand for Li. However, only little is known about their feature and nature. In the study, hydrochemical data from 155 oil wells tapping the Eocene to Lower Oligocene Qianjiang Formation of the Jianghan Basin, central China indicate that the brines are of the Na–Cl or Na–Ca–Cl type and are characterized by highly variable Li contents of 7.56 to 150 mg/L, with Mg/Li ratios less than 11.65. High Na/Cl and Cl/Br molar ratios indicate distinct contributions from halite dissolution. The Ca excess, Na deficit and Ca/Mg and Ca/Sr molar ratios in the brines imply multiple diagenetic processes, including halite dissolution, dolomitization, albitization and calcite or anhydrite cementation. The lithium contents of these brines have a weak relationship with the salinity and a negative correlation with Cl/Br ratios, possibly indicating that these Qianjiang oilfield brines have been diluted by secondary brines derived from halite dissolution. The spatial distribution patterns for Li and B concentrations of the brines are different from those for salinity and Br contents and show a geographic pattern, indicating that Li enrichment in the Qianjiang brines is likely connected with geothermal sources associated with volcanic activity.

Exceptional preservation in Quaternary Atacama Desert Tufas: Evidence for increased groundwater and surface water in the Calama Basin, Atacama, Chile

AbstractExceptionally well‐preserved tufas located west of Calama, Atacama Desert, Chile, designated Santa Juana tufas, record episodic wetter conditions, relative to today, over the past 500,000 years. Globally, tufa architecture and depositional details are poorly understood as most described tufas have been degraded by weathering and erosion. In the hyperarid Atacama, post‐depositional alteration is negligible, therefore, the exceptional preservation of Santa Juana tufas documented in this study provides new information about tufa facies and their complex interactions. Santa Juana facies include microbial stromatolites, phytoherms, cascadestone, flowstone and porous limestone. Phytoherms, consisting of former plant stems coated with calcite, developed in channels, within pools, and along spring discharge aprons. Cascadestone, representing former waterfalls, preserves microbial filaments and delicate V‐shaped calcite crystals. Flowstone lines shallow subvertical to subhorizontal channels, representing sites of rapidly sluicing water flow. Porous limestone, containing sparse calcite and/or gypsum and anhydrite cement crystals, represents detrital accumulations. Stable isotope results, coupled with U/Th ages, show that by the Quaternary, relative to the Neogene, groundwater was less supercharged with volcanogenic CO2 so degassing was moderated. The δ18O ratios from Miocene–Pliocene palustrine and lacustrine freshwater carbonates that underlie Santa Juana tufas indicate significant evaporation, but the tufa δ18O signal indicates a less evaporative trend due to shorter atmosphere exposure time. Biological fractionation in δ13C is largely masked by the region's volcanogenic carbon footprint, although tufa petrography shows well‐preserved microbial filaments and laminations. The range of tufa ages in this study shows that there were wetter time periods within the drainage basin headwater area in the Quaternary, but that by the late Pleistocene to early Holocene, aridity to hyperaridity became established. The lack of diagenesis or alteration within the Santa Juana tufas indicates that there has been minimal rainfall since their deposition.

Sedimentological and petrophysical heterogeneities controls on reservoir characterization of the Upper Triassic shallow marine carbonate Kurra Chine Formation, Northern Iraq: Integration of outcrop and subsurface data

The lack of knowledge about the relationship between carbonate sedimentary facies and pore system modifiers in northwestern and northern Iraq severely restricts carbonate oil exploration in these areas. This research aims to highlight the significance and influence of sedimentological and petrophysical heterogeneity and their impact on reservoir characterization of the Kurra Chine Formation, which have been studied in oil wells at Ain Zalah (AZ-29), Butmah (Bm-15), and Kand (K-1), in addition to the Sararu outcrop in northern Iraq. In this study, integrated methods that incorporate thin section petrography, well logs, statistical techniques and core analysis were used for the investigation. The study revealed that the Kurra Chine Formation in the studied sections mainly consists of limestone, dolomite, shale, and anhydrite, the last of which is absent in the Sararu outcrop. Its thickness in previous outcrop and wells was 848 m, 1136 m, 985 m, and 1149 m for Sararu, AZ-29, Bm-15, and K-1, respectively. Petrographically, five main microfacies types and nine submicrofacies were identified in the Kurra Chine carbonates of the studied sections. The percentages and distribution of these submicrofacies in the study area are diverse and show their heterogeneity. Diagenetic processes like micritization, dolomitization, cementation, neomorphism, compaction, stylolitization, solution, pyritization, and silicification resulted in increasing heterogeneity in the formation. They played a significant role in improving or reducing the reservoir characterizations of the formation. Dolomitization and dissolution processes enhance the petrophysical properties of the studied formation, especially the medium-to coarse-grained crystalline idiotopic dolomite. On the other hand, cementation, compaction, and neomorphism reduce the pore volume of the Kurra Chine Formation rocks. Qualitatively, porosity in the Kurra Chine Formation was divided into two main groups: interparticle (intergrain and intercrystal) and vuggy pores. The interparticle pore space is subdivided into three petrophysical classes: Class 1, Class 2, and Class 3. Class 2 represents the most common interparticle porosity type and is very important because it contains bituminous material. Moldic and intragrain pores are the most abundant separate vug pores within formation successions. The touching vug pores are represented by fractures, microfractures, and fenestrals. Quantitatively, three types of porosity have been calculated for the formation using conventional well logs: effective, interparticle, and separated vuggy porosity. The Butmah-15 well reservoir property outperformed the other two wells in terms of reservoir property indicators, particularly in the middle of the studied formation at depths ranging from 3089 to 3399 m. Porosity in this depth interval is interparticle, with values ranging between 0 and 0.35, while permeability values range from zero mD in beds cemented by anhydrite cement to 6196 mD within MF6 in limestone beds that are rich in interparticle and fracture porosity. The AZ-29 and K-1 wells have poor reservoir properties, especially effective porosity and permeability. This research provides a better understanding of the relationship between carbonate sedimentary facies and pore system modifiers of shallow marine environment carbonates, and it can aid in reservoir evaluation for better exploitation of these valuable resources in Iraq and elsewhere in carbonate sedimentary basins.

Low energy synthesis of anhydrite cement from waste lime mud

AbstractBackgroundThe re‐use of waste is the most important aspect of sustainable development for human health and environmental protection. Millions of tons of hazardous spent sulfuric acid and lime mud are produced by water treatment facilities every year. Unlike other types of waste, lime mud can sometimes be used for the thermal regeneration of lime. However, this approach requires a high energy consumption and, thus, cannot be considered a sustainable or green process. Compared to the latest research, the method presented here allowed for the production of anhydrite in one step at very low temperatures and in a short time while also bypassing the stage of gypsum phase formation.ResultsCalcium sulfate anhydrite was synthesized from the water treatment waste of lime mud by a low‐temperature method at temperatures of 25 and 45 °С. The resulting samples were composed of anhydrite with different crystal morphologies. This was confirmed by XRD, DTA, SEM, TEM, and thermochemical calculations. Dependences of the influence of the shape of crystals on the main technological properties of anhydrite cement and the influence of additives on the activation of the binding properties were established.ConclusionThe research shows that different types of calcium sulfate anhydrite require different additives and in different amounts. Introducing the optimal composition of additives‐activators made it possible to obtain anhydrite cement with a water‐anhydrite ratio of 0.25, an onset of setting no earlier than 40 min, an end of setting no later than 12 h, and a strength of 28.8 MPa. © 2022 The Authors. Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).

Sedimentology and sequence stratigraphy of automated hydraulic flow units – The Permian Upper Dalan Formation, Persian Gulf

The Upper Dalan (Khuff-equivalent) Formation constitutes the principal reservoir of the giant gas fields in the Persian Gulf Superbasin. A sedimentological and sequence stratigraphic analysis was conducted on selected cores from the South Pars, Kish, and Lavan gas fields, offshore Iran, to evaluate a recently proposed method for automatic reservoir zonation, and discuss the predictability of such defined zones. The succession consists of nine evaporite-carbonate lithofacies grouped into three shallow-marine facies associations (shoal, lagoon, and tidal flat) that were deposited on a low-gradient homoclinal ramp. Lithofacies are stacked into two complete long-term (3rd-order?) transgressive-regressive depositional sequences. Sequence boundaries were defined by facies stacking patterns and presence of evaporites and meteoric diagenetic features. The reservoir quality was improved by both early-stage dolomitization and dissolution, whereas pervasive pore-filling anhydrite cementation, compaction, and late-stage over-dolomitization reduced the reservoir quality. Whereas dolomitization overall slightly affected porosity, it significantly increased the permeability in mud-dominated lithofacies. Fibrous and bladed calcite rim cements, as well as micritization of the grain-dominated lagoon and shoal lithofacies, prevented porosity reduction during the early- and late-stage burial by building a stronger framework. Core-plug porosity and permeability measurements were used to calculate the Winland R35, Reservoir Quality Index (RQI), and Flow-Zone Indicator (FZI) values. A novel, fully automated approach, was used to effectively identify the hydraulic flow units (HFUs). The HFUs are sedimentologically distinct units with characteristic combinations of the original rock texture and the subsequent diagenetic overprint, and their subsurface position within the sequence stratigraphic framework may be predicted.

Influence of substrate morphology on reef diagenesis: Examples from the Zechstein Limestone formation (SE Wolsztyn Ridge, W Poland)

Reefs may be formed in the middle parts of sedimentary basins and grow on tectonically uplifted substrate blocks. In this paper, the strata of the Central European reef system of the Upper Permian Zechstein Limestone (Ca1) formation located in West Poland are evaluated. The formation has produced natural gas since the early nineties of the past century. The reefs grew on an uplifted horst built of Carboniferous rocks, known as the Wolsztyn Ridge. As individual sub-blocks originally showed diversified altitudes, four zones suspected to represent initial sedimentation conditions are distinguished: (1) basin zone, located close to the margins of the WR; (2) lower reef zone; (3) transitional zone and (4) the upper reef zone. Clear diagenetic differences between particular zones are observed. The basin zone strata are characterized by a strongly recrystallized mosaic of dolomite but show a limited extent of anhydrite cementation. The lower reef zone rocks, covered by thick Zechstein evaporites have been dolomitized and anhydritized most intensively, probably by means of the downward seepage of heavy brines, as suggested by the stable isotope data. It is assumed that the process was focused on the calm-water facies due to their low primary porosity and permeability. This mechanism was also probably responsible for the dolomitization of Ca1 strata in the remaining zones, where lower δ18O values in dolomite reflect some temperature increase on the final isotopic signature of rocks. Transitional zone carbonates appear to be almost purely calcitic with only a minor dolomite admixture. They contain various types of calcite cement, including marine and meteoric-phreatic phases, reflecting an intermediate position of the substrate and optimal reef growth conditions. Marine phases are also commonly recognized in the upper reef zone. Owing to a significant elevation of the substrate in this zone, the corresponding strata might have experienced some impact of karstification, presumably taking place well before the deposition of the Zechstein evaporites. It is concluded that the morphology of the substrate in tectonically-influenced areas strongly controls the reef diagenesis, and thus can affect the distribution of hydrocarbon accumulations.

Influence of anhydritisation on the reservoir quality of the Butmah Formation in north-western Iraq

Carbonates and evaporites in the Butmah Formation (Lower Jurassic) were deposited in restricted marine (inner ramp) environments in north-western Iraq. The carbonates are composed of ooidal, peloidal and fossiliferous limestone in shoal and lagoon facies, while syndepositional dolomite with early anhydrite cement is found in tidal flat facies. Study of anhydrite intervals shows different structures represented by bedded, massive, chicken-wire and nodular structures which consist petrographically of fibrous, equant, felted, spare crystal, needle and lath shape textures. Two phases of anhydrite cementation were recognised in this study: Phase I is represented by syndepositional anhydrite cement that affected the reservoir quality of the tidal flat facies, and Phase II as late anhydrite cement that affected the reservoir quality of the lagoon and shoal facies. Extensive petrophysical measurements were carried out and have shown that differences in lithology, anhydrite cement type, and the degree of anhydritisation control the reservoir quality of the Butmah Formation. The Butmah Formation was divided into three rock types according to differences in their petrophysical properties and the anhydrite cement fraction. Quantitative and qualitative observations combine to indicate that post-depositional uplift may have affected the study area during the Middle-Late Jurassic and tended to dissolve some of the anhydrite cement in the tidal flat facies, creating sulfate-rich brines, and then movement of those brines toward the lagoon and shoal facies led to saturation and deposition as late anhydrite cementation (Phase II). This mechanism has affected the reservoir quality of the Butmah Formation considerably, altering its petrophysical properties.

The diagenetic history of the giant Lacq gas field, witness to the apto-albian rifting and the Pyrenean orogeny, revealed by fluid and basin modeling

The link between fluid circulation schemes and basin histories remain one of the most valuable tools to understand the diagenetic evolution of petroleum reservoirs. This study proposes a diagenetic model for the Deep Lacq reservoir constrained by petrography, geochemistry, fluid inclusions studies and basin modeling analyses. Drill cores penetrating the 3000 m deep Late Jurassic to Early Cretaceous “Deep Lacq” reservoirs, in the Aquitaine basin SW France, were used to investigate how the geodynamic evolution of the basin influenced the diagenesis of petroleum reservoirs. This basin experienced a phase of rifting during the Lower Cretaceous followed by inversion and compression during the Paleogene. Petrographic and geochemical results indicate that early diagenesis involved bacterial activity and early dolomitization. Fluid thermodynamic modeling, coupled with the reconstruction of the basin history provided insight into the timing of diagenetic fluid circulations. Burial diagenesis involved a rift-related dolomitization episode linked mainly to the recrystallization of earlier dolomites and the circulation of deep hot fluids connected to the Triassic evaporites. The dolomitizing fluids circulated at temperatures close to 136–144 °C and pressures of 338–386 bars, during the Aptian (120-116 Ma). During the post-rift uplift and its subsequent thermal re-equilibration, multiple fluid pulses precipitated coarse blocky calcites. These Ca-rich fluids circulated between 109 and 97 Ma, at high-pressure conditions at around of 704–766 bars and with temperatures of 146–156 °C. The circulation of another fluid pulse that resulted in the precipitation of anhydrite cements is linked to the onset of the Pyrenean orogeny. No evidence that support in-situ TSR reactions were found, which suggests migration of TSR-related H2S from a kitchen in the deeper parts of the basin. This multi-modeling approach shows how the detailed thermodynamic analyses of fluid inclusions can add valuable constraints on a diagenetic model and eventually link it to the larger-scale basin's history.

Assessment of CO2 adsorption capacity in Wollastonite using atomistic simulation

The paper deals with the understanding of the CO2 capacity of adsorption through the anhydrite cement compound, i.e., CaO·SiO2 structure. The Wollastonite structure is prepared with various CaO/SiO2 ratios for the atomistic simulations. Subsequently, simulations of binary adsorption of H2O and CO2 have been performed to arrive at C–S–H structure. The CaO/SiO2 ratio is varied from 0.66 to less than 1.5 in the present study. The interlayer spacing between CaO sheets is considered as 13.5 Å. The impact of the electrostatic charges has been explored using available CLAY_FF and CSH_FF potentials. The outcomes have been successfully examined and compared with experimental results. To understand the capacity adsorption of CO2 and selectivity effect on C–S–H forming, numerical experiments have been performed using CO2 concentrations from 10 ppm up to 1,000,000 ppm at 20 °C. These numerical experiments allow understanding of the selectivity of H2O and CO2 in binary mixture adsorption, and the capacity of CO2 adsorption in the Wollastonite atomic structure. According to the physisorption simulations, the CO2 adsorption capacity depends not only on CO2 concentration but also the Wollastonite structure. The possible CaCO3 precipitation and C–S–H structure are addressed.

Integrating petrophysical attributes with saturation data in a geological framework, Permian–Triassic reservoirs of the central Persian Gulf

In this research, an integrated geological and petrophysical approach was applied to understanding the most critical factors affecting the water saturation exponent in the Permian–Triassic Kangan and Dalan carbonate formations in the central Persian Gulf in Iran. Economically, water saturation has a fundamental role in reservoir characterisation. A total of 8 sedimentary facies were recognised with multiple diagenetic phases. Diagenetic processes include marine cementation, moldic dissolution, early dolomitisation, recrystallisation, anhydrite cementation, compaction, and fracturing, which have influenced reservoir properties. Four third-order sequences were distinguished in the study area accompanied by nine fourth-order sequences. Petrographic analysis, scanning electron microscopy, petrophysical core analysis, wireline log data as well as mercury injection tests integrated through flow zone indicator, Lucia classes, and Lorenz plot approaches. Consequently, 4 integrated rock types (IRTs) were identified with particular depositional and diagenetic features. IRT1 with dominant interparticle pore type and strong porosity–permeability correlation (average 9.1% porosity and 27 mD permeability, R2 = 0.98), has a high saturation exponent (n = 2.2). IRT4 with grain-to mud-dominated facies, low porosities (average 1.4%), and a mean value of 4.54 mD permeability has the lowest saturation exponent (n = 1.07). Moldic dissolution with low pore connectivity is the dominant pore type in IRT2 and IRT3 which have a medium and relatively high saturation exponent, respectively (IRT2 n = 1.53 and IRT3 n = 1.78). Accordingly, a conceptual model was suggested for saturation exponent variations in a high-frequency sequence stratigraphic framework. In order to evaluate the heterogeneity of the studied formations and its impact on the reservoir properties, the characteristics of the determined rock units were studied. This study integrates microscopic to macroscopic geological and petrophysical factors to understand the saturation exponent distribution in a heterogeneous carbonate reservoir, leading to obtaining a more accurate water saturation estimation of such reservoirs.

Reservoir quality estimation using a new ternary diagram approach applied to carbonate formations in north-western Iraq

A new reservoir quality ternary plot (RQTP) of effective porosity, shale volume, and matrix is presented in this study. We show it to be a useful tool for first-order estimation of the petrophysical zones and reservoir classes of each unit within a reservoir. Subsequently, we combine the RQTP results with permeability and fracturing intensity data in carbonate rocks to provide a better overall characterisation of reservoir quality. The approach has been applied to the Butmah Formation, a thick variable carbonate succession of Liassic (Lower Jurassic) rocks in north-western Iraq. The RQTP approach divides carbonate reservoirs into classes according to: (i) a measure of porosity, (ii) the fraction of shale, and (iii) the fraction of non-shale matrix. The outcome of applying this model to the Butmah Formation indicates that the best reservoir quality is identified in Unit 4, which consists of fine to medium dolomite rocks. These rocks are not associated with anhydrite cement or dissolved later due to late dissolution, presenting as clean carbonate with complex pore network heterogeneity. These types of rocks were classified as Rc2 and Rc3 using the RQTP Model. By contrast, the worst reservoir qualities (Rc7) were identified in Unit 1 which is composed of cemented limestone that shows low pore network heterogeneity (predominantly uniform pore sizes), low porosity, and poor permeability.

Digital rock physics approach to simulate hydraulic effects of anhydrite cement in Bentheim sandstone

Abstract. Cementation of potential reservoir rocks is a geological risk, which may strongly reduce the productivity and injectivity of a reservoir, and hence prevent utilisation of the geologic subsurface, as it was the case for the geothermal well of Allermöhe, Germany. Several field, laboratory and numerical studies examined the observed anhydrite cementation to understand the underlying processes and permeability evolution of the sandstone. In the present study, a digital rock physics approach is used to calculate the permeability variation of a highly resolved three-dimensional model of a Bentheim sandstone. Porosity-permeability relations are determined for reaction- and transport-controlled precipitation regimes, whereby the experimentally observed strong decrease in permeability can be approximated by the transport-limited precipitation assuming mineral growth in regions of high flow velocities. It is characterised by a predominant clogging of pore throats, resulting in a drastic reduction in connectivity of the pore network and can be quantified by a power law with an exponent above ten. Since the location of precipitation within the pore space is crucial for the hydraulic rock properties at the macro scale, the determined porosity-permeability relations should be accounted for in large-scale numerical simulation models to improve their predictive capabilities.