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The distribution of good reservoir quality and its causes is the main challenges in carbonate reservoir characterization. This study investigates the effects of diagenetic processes on the reservoir quality of the carbonate successions of the Asmari Formation, in the Marun oil field, southwest Iran. The study applies an integrated approach including core petrography, petrophysical rock typing, stable carbon and oxygen isotopes as well as major and trace elements analyses. Petrographic studies and geochemical analyses express that the Asmari limestones have been affected mainly by compaction, dissolution, recrystallization, calcite and anhydrite cementation and dolomitization. Among those diagenetic overprints, dolomitization and dissolution played an important role to enhance the reservoir quality of the formation. Moreover, four types of dolomites were recognized and the rate of dolomitization increases toward the top of the Asmari carbonate successions. Possible models for dolomitization include mixing zone, brine reflux, seepage reflux and tidal pumping of seawater. Employing Flow Zone Index and Discrete Rock Type concepts led to classification of the Asmari reservoir into seven reservoir rock types. Integrating reservoir rock typing with petrographic studies and geochemical analyses also confirms that reservoir quality of the Asmari Fm. would have been mainly controlled by diagenetic processes. Moreover, stable isotopes, trace elements and facies analyses support the idea that carbonate intervals of the Asmari Formation were deposited in a warm, shallow-water environment under a saline condition.

Background: Cisplatin is an anticancer drug used in the management of solid tumors, however, dose-related nephrotoxicity is one of its major problems. Agents having antioxidants, antiinflammatory and/or antiapoptotic activities may thus represent potential therapeutic options to avoid cisplatin-induced nephrotoxicity. Among these agents, coenzyme Q10 has several pharmacological properties including antioxidant, anti-inflammatory and/or anti-apoptotic effects. Objective: The current study aimed to examine whether coenzyme Q10 could attenuate cisplatininduced nephrotoxicity or not. Methods: 24 adult rats were randomly separated into three groups (8 rats per group). The first one was the control group, rats receiving vehicle (olive oil) intraperitoneally. The second group was Cisplatin treated group, rats were receiving 13 mg/kg of Cisplatin intraperitoneally as a single dose. The third group (Cisplatin + Coenzyme Q10), rats were receiving 13 mg/kg as a single intraperitoneal dose of Cisplatin and coenzyme Q10 daily for six consecutive days (10 mg/kg intraperitoneally). Results: Cisplatin caused significant increases in serum creatinine and severe histological lesions. Cisplatin treated group also showed a significant elevation in renal malondialdehyde concentration as a marker of oxidative stress; renal tumor necrosis factor-alpha concentration as a marker of inflammation; and Kidney injury molecule -1 concentration. Coenzyme Q10 significantly attenuated cisplatininduced nephrotoxicity through lowering serum creatinine and improving nephrotoxicity histological scores. Coenzyme Q10 also significantly reduced the renal concentration of MDA, TNF-α and KIM-1 relative to cisplatin treated group. Conclusions: Coenzyme Q10 has a potential nephroprotective effect against cisplatin-induced nephrotoxicity that was demonstrated by biochemical and histopathological analysis.

Shallow water carbonate rocks of the Lower Cretaceous Qamchuqa Formation host a significant reserve of oil in the NNE Arabian plate-Southwestern Zagros Sutured Belt in the Kurdistan Region, Iraq. Such carbonates consist of several dolostone bodies with unique and complex arrays of saddle and zebra dolomite textures especially in fractured intervals. Integration of field, core description, microstructural analysis, petrographic, stable C, O and Sr isotopes, and fluid inclusion analyses reveals two main phases of multiple fluxes of hydrothermal fluid circulation during the Zagros Orogeny at the end of Cretaceous and Cenozoic times. An early regional non-focused hydrothermal dolomitization and a subsequent fault- and fracture- controlled hydrothermal fluid flow, which led to replacement and cementation by a range of saddle dolomite and zebra textures. Several generations of matrix dolomite, saddle dolomite and calcite cement were identified. The host dolostone has been affected by shortening, folding, fracturing, and thrust faulting. Saddle dolomite pipes, in particular, were found to be associated with en-echelon folding. The morphology and areal extent of the zebra dolomite are controlled by the pore geometry of the host dolostone in relationship to fracturing and faulting. Both matrix and saddle dolomite cements have comparable, overlapping stable isotopic values ranging from 0.39‰ to 3.57‰ and −0.12 to 3.11‰ VPDB, respectively for δ13C and δ18O values range from −10.53‰ to −4.47‰ and −12.87 to −7.79‰ VPDB, respectively. 87Sr/86Sr ratios of pervasive matrix dolomite range from 0.707702 to 0.707839, fall close to late Cretaceous seawater values. Slightly more radiogenic values of saddle dolomites ranging from 0.707759 to 0.708024 resulted from mixing of basinal brines with connate waters. Isotopic signatures of later calcite cements show more radiogenic Sr ratios from 0.707755 to 0.708198 and negative δ13C values 1.76 to −7.45‰, possibly related to late diagenetic processes due to incursion of meteoric waters during subaerial exposure. Fluid inclusion data of pervasive matrix dolomite and saddle dolomites show homogenization temperatures (Th) with a wide range from 71.3 to 169.7 °C and 82.3 to 188.2 °C, and salinity between 14.4 to 25.4 and 15.6 to 27.9 wt% NaCl eq., respectively. These values demonstrate the effect of hydrothermal fluid flow of the alteration of host dolomite and formation of saddle dolomite in fractures and faults.

Diagenesis of carbonates and clastic sediments encompasses the biochemical, mechanical and chemical changes that occur in sediments after deposition and prior to low-grade metamorphism [...]

Trace and rare earth elements (REEs) are considered to be reliable indicators of chemical processes for the evolution of carbonate systems. One of the best examples of ancient carbonate successions (Berdiga Formation) is widely exposed in NE Turkey. The Lower Cretaceous limestone succession of Berdiga Formation may provide a case study that reveals the effect of ocean paleoredox conditions on diagenetic alteration. Measurement of major, trace and REEs was carried out on the Lower Cretaceous limestones of the Berdiga Formation, to reveal proxies for paleoredox conditions and early diagenetic controls on their geochemistry. Studied micritic limestone microfacies (MF-1 to MF-3) indicate deposition in the inner platform to a deep shelf or continental slope paleoenvironment during the Hauterivian-Albian. The studied limestone samples mainly exhibit low Mg-calcite characteristics with the general chemical formula of Ca98.35–99.34Mg0.66–1.65(CO3). They are mostly represented by a diagnostic REE seawater signature including (1) slight LREE depletion relative to the HREEs (ave. 0.72 of Nd/YbN and ave. 0.73 of Pr/YbN), (2) negative Ce anomalies (Ce/Ce* = 0.38–0.81, ave. 0.57), (3) positive La anomaly (La/La* = 0.21–3.02, ave. 1.75) and (4) superchondritic Y/Ho (ave. 46.26). Studied micritic limestones have predominantly low Hf (bdl to 0.5 µg/g), Sc (bdl to 2 µg/g) Th (bdl to 0.9 µg/g) contents suggesting negligible to minor shale contamination. These findings imply that micritic limestones faithfully record chemical signals of their parental and diagenetic fluids. The succession also exhibits high ratios of Eu/Eu* (1.01–1.65, ave. 1.29 corresponding to the positive Eu anomalies), Sm/Yb (1.26–2.74, ave. 1.68) and La/Yb ratios (0.68–1.35, ave. 0.9) compared to modern seawater and wide range of Y/Ho ratios (29.33–70.00, ave. 46.26) which are between seawater and hydrogenetic Fe-Mn crusts. Several lines of geochemical evidence suggest water-rock interaction between parental seawater and basaltic rocks at elevated temperatures triggered by hydrothermal activity associated with Early Cretaceous basaltic magma generation. The range of Ce/Ce* values is suggestive of mostly oxic to dysoxic paleoceanographic conditions, with a sudden change to dysoxic conditions (Ce/Ce* = 0.71–0.81), in the uppermost part of the MF-1. This is followed by an abrupt deepening paleoenvironment with a relative increase in the oxic state of the seawater and deposition of deeper water sediments (MF-2 and MF-3) above a sharp transition. The differences in microfacies characteristics and foraminifera assemblage between MF-1 and overlying facies (MF-2 and MF-3) may also confirm the change in paleoceanographic conditions. Therefore, REEs data obtained from studied limestones have the potential to contribute important information as to regional paleoceanographic conditions of Tethys during an important period in Earth history.

Strata-bound dolomite occurs in deep-seated Ordovician and Cambrian sediments within the Huron Domain of southern Ontario, Canada. Core samples were analyzed for petrographic, stable and Sr isotopic composition, and fluid inclusion microthermometry to characterize dolomitization and evaluate diagenetic fluid composition. The samples represented a range of host rocks from dolomitized limestones, dolostones, sandy dolostones and sandstones within Ordovician Black River Group and underlying Cambrian formation. The petrographic and geochemical attributes have provided a basis to gain insight on the source fluids that modified these rocks, as well as, the possible timing of formation. Evidence indicates that the formations were subjected to high temperatures (average 115 °C) that cannot be explained by burial history alone. This suggests the occurrence and migration of hydrothermal fluids within the low permeability dolomite horizons, possibly during Paleozoic orogenesis. Dolomite and calcite fracture infill isotopic and fluid inclusion data point to two possibly individual diagenetic fluid systems; i) an earlier Cambrian system that is characterized by a pronounced negative shifts in oxygen and carbon isotopic composition (δ18O average −9.0‰ for dolomite and −13.9‰ for calcite, respectively; and δ13C −3.06‰ for dolomite and −4.82‰ for calcite, respectively), a more radiogenic (Sr87/Sr86 ratios range from 0.70977 to 0.71100), warm (Th values range from 84 to 156 °C for dolomite; average 113.6 °C and 87–141 °C for calcite; average 101.5 °C) and saline signature (salinity range from 23.2 to 27.2 wt% NaCl eq for dolomite; average 24.3 for dolomite and 23.6 wt% NaCl eq for calcite); and ii) a later Ordovician system that is characterized by less negative shifts in both oxygen and carbon isotopes (δ18O average −8.7 ‰for dolomite and −7.5‰ for calcite; and δ13C average + 0.37‰ for dolomite and −0.36‰ for calcite, respectively), hypersaline (salinity range from 22.4 to 30.1 wt% NaCl eq.; average 27.0 for dolomite and 27.5 to 29.7 for calcite; average 29.2), comparable homogenization temperature (Th ranges from 85 to 132 °C for dolomite; average 109.6 °C and 66–153 °C for calcite; average 107.2 °C) and a less radiogenic (Sr87/Sr86 ratios range from 0.70818 to 0.71 000) fluid system. The observation of highly discrete, strata-bound dolomites combined with only trace quantities of saddle dolomite and its associated geochemical signature suggest that diagenesis, as a result of hydrothermal fluids, was neither pervasive in volume or extent within the north western Huron Domain.

This paper integrates major, minor, trace and rare-earth element (REE) data into C-, O-, Sr-isotopes, fluid-inclusions microthermometry of replacive matrix dolomite and vug/fracture-filling saddle dolomite and calcite of the Lower Cretaceous Qamchuqe Formation, the Kurditan Region of Iraq, in order to get further insights into the origin and geochemical evolution of their parent diagenetic fluids. Multiple generations of dolomite have been observed. These dolomites constitute replacive matrix dolomite and saddle dolomite cement, dominantly poorly ordered, non-ferroan, non-stoichiometric, with an average of 59.4 and 60 mol% CaCO3, respectively, and have low Sr, Fe and Mn concentrations. Replacive matrix dolomites display fairly comparable REE patterns to saddle dolomite, but with light REE (LREE) enrichment, and moderate low (∑REEs) contents (18.9–1111.5 and 16.6–593.4 ppb, respectively). Saddle dolomite has moderately positive Eu anomalies while replacive matrix dolomite has slightly less positive Eu anomalies and negative Dy, Ho and Yb anomalies. Both dolomites are characterized by slightly radiogenic 87Sr/86Sr ratios ranging from 0.707702 to 0.707839, overlapping, wide-ranging and negative δ18O values from −10.43 to −4.47 and −12.87 to −7.79 ‰VPDB, and δ13C values ranging from 0.38 to 3.57 and −3.45 to 3.11‰VPDB, respectively. The primary fluid inclusions microthermometry of replacive matrix and saddle dolomites yield homogenization temperatures (Th) ranging from 71.3 to 228.3 °C and 82–188 °C, respectively, and salinity values ranging between 14.4 to 25.4 and 15.6 to 27.9 wt% NaCl eq., respectively. These data suggest multiple hydrothermal fluid fluxes during and post Zagros orogeny. The geochemical data also show that these fluids derived from similar source of mixed hydrothermal and marine fluids, which caused the dolomitization of host rock and precipitation of dolomite cements.

Shallow-marine carbonates of the mid-Cretaceous Sarvak Formation are important reservoir rocks in southern Iran and the Persian Gulf region. These carbonates were deposited on the margin of the Arabian Plate and rest on the Kazhdumi Formation, which is one of the major hydrocarbon source rocks in the region. The top of the Sarvak Formation coincides with the regional Turonian unconformity. Most of the observed diagenetic features are genetically related to meteoric waters entering the Sarvak Formation during Cenomanian-Turonian and mid-Turonian uplift and the subsequent paleoexposure. Integration of field and petrographic studies and isotope geochemistry reveals the history of a variety of diagenetic processes, which include dissolution and development of secondary porosity which enhance reservoir properties of the Upper Sarvak carbonates. Various types of calcite cements were identified as the main cause for porosity loss in these carbonates. Their diagenetic environment is discussed using the geochemical data acquired as part of the present study. The δ18O and δ13C values (−12.3 to −0.6‰ and −5.8–3.6‰ VPDB, respectively) of the cements indicate precipitation from marine, meteoric and/or mixed meteoric-marine fluids. Some drusy calcite cements exhibit δ18O and δ13C values (−5.1 and 0.8‰ VPDB, respectively) and higher 87Sr/86Sr ratios, comparing to mid-Cretaceous carbonates, (i.e., 0.70747), which may indicate precipitation from meteoric waters. Lower δ18O and δ13C values (e.g. −5.1, 0.8‰ VPDB) combined with higher 87Sr/86Sr ratios (e.g., 0.70747) of some drusy calcite could confirm their precipitation from meteoric waters. Lower δ18O values of some blocky calcite cements (e.g., −12.3‰ VPDB) and matrix-replacive compression-related dolomites (i.e., −7.3 to −3.4‰ VPDB) suggest their precipitation at rising temperatures during burial. The range of δ13C values (−5.8–3.6‰ VPDB) suggests that the main source of carbon in the calcite cements was primarily marine mixed with isotopically more negative carbon from atmospheric/soil-derived CO2.

Carbonate rocks of the Pekisko Formation make up an important reservoir in west-central Alberta, especially in fields along the Pekisko subcrop edge. They represent a transgressive-regressive carbonate platform sequence comprised of upward shallowing facies, which subsequently underwent extreme erosion leading to the development of karst topography. As a result, diagenetic alteration, mainly through dolomitization and karstification, has affected reservoir characterization for most of the carbonate facies. Several generations of calcite cementation and dolomite are the result of complex diagenetic changes. Calcite cements include isopachous fibrous, equant drusy mosaic, pendant/meniscus, blocky spar, syntaxial, fibrous, and bladed. These cements formed during early and late diagenetic events; pre- syn- and post exposure in shallow and deeper burial realms. There are five types of dolomite identified in the Pekisko Formation, based on petrographic and geochemical analyses: 1) pervasive, fine to coarse crystalline, subhedral to anhedral replacive dolomite;2) void-filling, coarse crystalline, euhedral dolomite cement;3) selective, fine to coarse crystalline, euhedral to anhedral dolomite; 4) dissolution seam-associated, fine crystalline, euhedral dolomite; and 5) saddle dolomite. Dolomite types 1), 3) and 4) are interpreted to have formed early in the diagenetic history and subsequently recrystallized, whereas void-filling, coarse crystalline, euhedral dolomite and saddle dolomite formed later in deeper burial setting. Petrographic evidence for recrystallization of dolomite types, excluding void-filling and saddle dolomite, includes: etching, displayed mainly on euhedral crystals; dissolved cores on many crystals of varying shapes; non-planar crystal boundaries, exclusively in pervasive dolomites; and coarsening crystal size, evident in both pervasive and selective dolomite types. Geochemical evidence, such as pronounced negative shift in oxygen isotopes (by up to 8‰ VPDB) and enrichment of radiogenic Sr isotopes further support this interpretation. There is a definite negative trend whereby wells closest to the subcrop edge have the most negative isotopic values and those farthest away show the least depletion. This trend in δ18O isotope values points to recrystallization of the earlier formed dolomites.

Field observations indicate that tectonic compression, anticline formation and concomitant uplift events of marine Paleogene carbonates in eastern United Arab Emirates, which are related to the Zagros Orogeny, have induced brecciation, karstification, and carbonate cementation in vugs and along faults and fractures. Structural analysis, stable isotopes and fluid inclusion microthermometry are used to constrain the origin and geochemical evolution of the fluids. Fluid flow was related to two tectonic deformation phases. Initially, the flux of moderately 87 Sr-rich basinal NaCl–MgCl 2 –H 2 O brines along reactivated deep-seated strike-slip faults have resulted in the precipitation of saddle dolomite in fractures and vugs and in dolomitization of host Eocene limestones (δ 18 O V-PDB −15.8‰ to −6.2‰; homogenization temperatures of 80–115 °C and salinity of 18–25 wt.% eq. NaCl). Subsequently, compression and uplift of the anticline was associated with incursion of meteoric waters and mixing with the basinal brines, which resulted in the precipitation of blocky calcite cement (δ 18 O V-PDB −22‰ to −12‰; homogenization temperatures of 60–90 °C and salinity of 4.5–9 wt.% eq. NaCl). Saddle dolomite and surrounding blocky calcite have precipitated along the pre- and syn-folding E–W fracture system and its conjugate fracture sets. The stable isotopes coupled with fluid-inclusion micro-thermometry (homogenization temperatures of ≤50 °C and salinity of 87 Sr-rich, cool meteoric waters. This carbonate cementation history explains the presence of two correlation trends between the δ 18 O V-PDB and δ 13 C V-PDB values: (i) a negative temperature-dependent oxygen isotope fractionation trend related to burial diagenesis and to the flux of basinal brines, and (ii) positive brine-meteoric mixing trend. This integrated study approach allows better understanding of changes in fluid composition and circulation pattern during evolution of foreland basins.

Background: Nowadays, there is an accumulated data about the relation between hypomagnesemia (low Serum Mg level < 0.7 mmol/l) and diabetes. Objective: In this study, we aimed to determine the prevalence of hypomagnesemia in a carefully diagnosed diabetes patients and to show how some factors could contribute to the prevalence of low serum Mg level among the population under study. Methods: In short, 62 patients of both type 1 and 2, who attended AL-Sadir medical city/ diabetes and endocrinology centre during the period of the study were included in the study. A detailed history was taken and participants were informed verbally about the procedure of this study. Serum magnesium and creatinine were measured using standardized methods. Results: The overall prevalence of the hypomagnesemia, among diabetic population involved in this study, was 29.03 % and it was nearly similar in male (29.41 %) and female (28.57 %). The means(SD) of serum Mg level were similar in type 1 and type 2 diabetic patients. Other difference in prevalence among other characteristics has been reported. Within all participants, gender (r = -0.02), fasting blood glucose (r = -0.514) and metformin use (r = -0.014) were negatively correlated with serum Mg level, in contrast to other variables, which were positively correlated. While among type 1 DM, age (r= 0.193), serum creatinine (r= 0.031) and insulin use (r= 0.217) were positively correlated with serum Mg level. In contrast, others were negatively correlated. In type 2 DM, age (r = -0.283) and fasting blood glucose (r = -0.496) were negatively correlated with serum Mg level. On the other hand, other variables were positively correlated with serum Mg level. Conclusion: Hypomagnesemia prevalence was detected in nearly one-third of the diabetic population, which is quite high prevalence. Some factors have shown to play an essential role in this prevalence. A large study is warranted to address this issue.

The Middle Devonian Lucas Formation of southwestern Ontario, Canada, forms the main hydrocarbon reservoir rocks in the region. Integrated field, petrographic, fluid inclusion and isotope geochemistry reveal a variety of diagenetic processes that include the formation of dolomite, calcite, celestine and fluorite under diverse geochemical conditions. The host carbonate strata display two types of replacive dolomite fabrics: 1) fine-crystalline matrix dolomite (5–20 μm); and 2) medium-crystalline dolomite (50–150 μm) with an average size of 100 μm. Stable oxygen and carbon isotope results for both dolomite fabrics show a wide range of values (δ18O = −9.4‰ to −3.9‰ VPDB, δ13C = 0.4‰ to 3.4‰ VPBD). For late fracture-filling calcite, δ 18O and δ 13C values range from −9.4 to −5.9‰ VPDB and −7.7‰ to −0.04‰ VPDB, respectively. The 87Sr/86Sr ratios of selected dolomite and calcite samples range from 0.70798–0.70838, preserving Middle Devonian seawater values while some samples have slightly more radiogenic ratios than contemporaneous seawater. Field and petrographic evidence and Sr isotope ratios of dolomite suggest early dolomitization from evaporated seawater in a sabkha setting (shallow seepage reflux or evaporitive pumping). However, the depletion in δ18O (varying from −3.9 to −9.4‰ VPDB), and radiogenic Sr isotope values of the Lucas Formation dolomite are considered to be the result of recrystallization, which is interpreted to occur at elevated temperatures in a burial environment. The 18O-depleted values and slightly radiogenic 87Sr/86Sr ratios of calcite cements suggest precipitation from hot, saline basinal/hydrothermal brines. In addition, the 13C-depleted calcites indicate that carbon was possibly formed by oxidation of organic matter. Fluid inclusions in some of the late calcite cements fluoresce under UV light, indicating the presence of hydrocarbons. Homogenization temperatures of these inclusions vary from 87.1°C to 171.9°C with an average value of 126°C. Average salinity of fluid inclusions is 19.7 wt. % NaCl eq. Celestine samples have lower salinities relative to calcite, averaging 14.5 wt. % NaCl eq. with an average homogenization temperature of 219.5°C. Based on the geochemical and fluid inclusion results, hot, saline basinal/hydrocarbon bearing brines originated from central part of the basin and migrated to Devonian carbonates via fractures and faults. Celestine is a product of thermochemical sulfate reduction and formed due to mixing of saline, Sr and sulfate bearing brines and meteoric waters.

The Catoche Formation of the St. George Group in eastern Laurentia (western Newfoundland) consists of early Ordovician (Arenigian) shallow marine platform carbonates (∼160 m thick), which were extensively dolomitized during the course of their diagenetic history. The dolomites occur as both replacement and pore-filling cements and are a major control on porosity distribution in the formation. The origin and diagenetic history of the Catoche dolomites at Daniel's Harbour (western Newfoundland) were analysed in comparison to equivalent successions at Port au Choix (PaC) and Port au Port Peninsula (PaP) to assay the reservoir potential of these dolomites in eastern Laurentia. Petrographic examination identified at least three generations of dolomites in the Catoche Formation, which are: (1) an early replacement sub- to eu-hedral micritic dolomite (

Bed-parallel stylolites are a widespread diagenetic feature in Lower Cretaceous limestone reservoirs, Abu Dhabi, United Arab Emirates (UAE). Diagenetic calcite, dolomite, kaolin and small amounts of pyrite, fluorite, anhydrite and sphalerite occur along and in the vicinity of the stylolites. Petrographic observations, negative δ 18 O VPDB , fluid inclusion microthermometry, and enrichment in 87 Sr suggest that these cements have precipitated from hot basinal brines, which migrated along the stylolites and genetically related microfractures (tension gashes). Fluid migration was presumably related to lateral tectonic compression events related to the foreland basin formation. The low solubility of Al 3 + in formation waters suggests that kaolin precipitation was linked to derivation of organic acids during organic matter maturation, probably in siliciclastic source rocks. The mass released from stylolitization was presumably re-precipitated as macro- and microcrystalline calcite cement in the host limestones. The flanks of the oilfield (water zone) display more frequent presence and higher amplitude of stylolites, lower porosity and permeability, higher homogenization temperatures and more radiogenic composition of carbonates compared to the crest (oil zone). This indicates that oil emplacement retards diagenesis. This study demonstrates that stylolitization plays a crucial role in fluid flow and diagenesis of carbonate reservoirs during basin evolution.

International audience; The common presence of oil seepages in dolostones is widespread in Cretaceous carbonate successions of the Kurdistan Region of Iraq. This integrated field, petrographic, chemical, stable C, O and Sr isotopes, and fluid inclusion study aims to link dolomitization to the origin and geochemical evolution of fluids and oil migration in the Upper Cretaceous Bekhme carbonates. Flux of hot basinal (hydrothermal) brines, which is suggested to have occurred during the Zagros Orogeny, resulted in dolomitization and cementation of vugs and fractures by coarse-crystalline saddle dolomite, equant calcite and anhydrite. The saddle dolomite and host dolostones have similar stable isotopic composition and formed prior to oil migration from hot (81–115 °C) basinal NaCl–MgCl2–H2O brines with salinities of 18–22 wt.% NaCl eq. The equant calcite cement, which surrounds and hence postdates saddle dolomite, has precipitated during oil migration from cooler (60–110 °C) NaCl–CaCl2–H2O brines (14–18 wt.% NaCl eq). The yellowish fluorescence color of oil inclusions in the equant calcite indicates that the oil had API gravity of 15–25° composition, which is lighter than present-day oil in the reservoirs (API of 10–17°). This difference in oil composition is attributed to oil degradation by the flux of meteoric water, which is evidenced by the low δ13C values (− 8.5‰ to − 3.9‰ VPDB) as well as by nil salinity and low temperature in fluid inclusions of late columnar calcite cement.This study demonstrates that linking fluid flux history and related diagenesis to the tectonic evolution of the basin provides important clues to the timing of oil migration, degradation and reservoir evolution.

Petrography, geochemistry (stable and radiogenic isotopes), and fluid inclusion microthermometry of matrix dolomite, fracture-filling calcite, and saddle dolomite in Ordovician to Devonian carbonates from southwestern Ontario, Canada, provide useful insights into fluid flow evolution during diagenesis. The calculated dOfluid, RREE, and REESN patterns of matrix and saddle dolomite suggest diverse fluids were involved in dolomitization and ⁄or recrystallization of dolomite. The Sr ⁄ Sr ratios of dolomite of each succession vary from values in the range of coeval seawater to values more radiogenic than corresponding seawater, which indicate diagenetic fluids were influenced by significant water ⁄ rock interaction. High salinities (22.4–26.3 wt. % NaCl + CaCl2) of Silurian and Ordovician dolomite–hosted fluid inclusions indicate involvement of saline waters from dissolution of Silurian evaporites. High fluid inclusion homogenization temperatures (>100 C) in all samples from Devonian to Ordovician show temperatures higher than maximum burial (60–90 C) of their host strata and suggest involvement of hydrothermal fluids in precipitation and ⁄or recrystallization of dolomite. A thermal anomaly over the mid-continent rift during Devonian to Mississippian time likely was the source of excess heat in the basin. Thermal buoyancy resulting from this anomaly was the driving force for migration of hydrothermal fluids through regional aquifers from the center of the Michigan Basin toward its margin. The decreasing trend of homogenization temperatures from the basin center toward its margin further supports the interpreted migration of hydrothermal fluids from the basin center toward its margin. Hydrocarbon-bearing fluid inclusions in late-stage Devonian to Ordovician calcite cements with high homogenization temperatures (>80 C) and their C-depleted values (approaching )32& PDB) indicate the close relationship between hydrothermal fluids and hydrocarbon migration.

Since the concept of petroleum migration-accumulation dynamic system (PMADS) was introduced in the 1990s, its definition, division and even denomination have long been in disagreement. This article aims to further discuss and try to clarify this concept and to explore more reasonable approaches to the division of PMADSs on the basis of our studies on petroleum migration and accumulation in some petroliferous basins of China, especially those from western China. According to the studies, we define PMADS as a relatively independent natural hydrocarbon migrating and accumulating system within a petroleum system. Its extent is supposed to cover the whole migration pathway starting from a hydrocarbon provenance throughout to an accumulation or a group of accumulations of same source. Just like petroleum systems, a petroleum migration-accumulation system is also an important natural unit for petroleum exploration and assessment in petroleum basins. Its position in a basin is right between a petroleum system and a play. A petroleum system has either a number of migration-accumulation systems or one system only. However, the key element of a petroleum system is the source of hydrocarbon; a petroleum system has a distinct hydrocarbon kitchen and is different from its neighboring ones in terms of hydrocarbon kitchens. In contrast, different migration-accumulation systems within the same petroleum system have a shared source kitchen but different characteristics of migration and accumulation to form hydrocarbon accumulations. As far as the division of petroleum migration-accumulation systems (PMASs) is concerned, 8 major approaches are proposed, including those based on pool-forming dynamic mechanisms, location of hydrocarbon accumulation relative to the source kitchen, phase states of migration and accumulation, migration patterns and directions, geneses of accumulations, reservoir pressure systems, depositional systems and facies, and integrated method, of which the integrated approach is underlined with regard to the division of PMASs in a complex petroleum system like those in West-China superimposed basins.

The Parecis Basin is a large intracratonic rift located in central Brazil and filled with Paleozoic carbonate, evaporite and siliciclastic sediments. The occurrence of gas seeps has recently attracted significant exploration interest by the Brazilian petroleum agency and by Petrobras. The continuously cored PB-01-RO well provided the first opportunity to study the depositional environments, diagenetic evolution and hydrocarbon potential of the largely unknown sedimentary successions of the Parecis Basin. The cored lithologies, belonging to the Siluro–Devonian (?) Pimenta Bueno Formation, are interpreted as deposited in playa-lake and ephemeral-stream environments. The deposits display a strong facies control on the diagenetic mineral assemblages and evolution. Diagenetic minerals in the ephemeral-stream deposits include eogenetic hematite and smectitic clay coats and quartz cement, and the mesogenetic process includes precipitation of sulfates (anhydrite and barite) and carbonates (calcite, dolomite and kutnahorite–ankerite–huntite), followed by partial dissolution of these carbonates and sulfates, and of feldspar grains. Telogenetic processes include the precipitation of hematite and kaolinite within secondary pores, and the replacement of anhydrite by gypsum. A second burial phase and mesodiagenesis is indicated by the precipitation of discrete K-feldspar crystals within moldic pores after dissolved feldspars, and by the illitization of etched, telogenetic kaolinite. The playa-lake deposits show early diagenetic dolomitization of lime mud, precipitation of anhydrite nodules and extensive silicification. The anhydrite nodules were replaced by gypsum and chalcedony during telodiagenesis. Potential source rocks are locally represented by organic shales. The fluvial sandstones show fair reservoir quality and limited compaction, as indicated by their intergranular volume, suggesting that the succession has undergone moderate burial. Potential seals for hydrocarbon accumulations are provided by the evaporites and playa-lake mudstones. The presence of an active petroleum system in the basin, as indicated by the gas seeps, suggests a good hydrocarbon exploration potential for the Parecis Basin, gas and stresses the importance of studying the depositional and diagenetic evolution of its sedimentary successions.

This diagenetic study (including fieldwork, petrographic, fluid inclusion, and stable isotope investigations) deals with the outcrop of Upper Permian–Lower Triassic carbonate rocks, which are equivalent to the Khuff Formation. The studied succession, which outcrops in the Ras Al Khaimah region, northern United Arab Emirates, comprises three formations, including the Bih, the Hagil, and the Ghail formations. The study focuses on unraveling the conditions and fluid compositions encountered during diagenesis of the succession. Emphasize is also made on linking diagenesis to major stratigraphic surfaces and to highlight reservoir property evolution and heterogeneity of the studied rocks. The evolution of fluids and related diagenetic products can be summarized as follows: (1) formation of near-surface to shallow burial, fine-crystalline dolomite (dolomite matrix) through pervasive dolomitization of carbonate sediments by modified marine pore waters; (2) formation of coarse-crystalline dolomite cement by highly evolved marine pore waters (13–23 wt.% NaCl eq.) at elevated temperatures (120–208°C), and (3) calcite cementation by highly saline fluid (20–23 wt.% NaCl eq.) at high temperature (170–212°C). A final calcite cement generation has been formed by the percolation of meteoric fluids during uplift. Fracture- and vug-filling diagenetic minerals are mainly restricted to the mid-Bih breccia marker level, suggesting preferential focused fluid flow through specific stratigraphic surfaces as well as along tectonic-related structures. Reservoir properties have been evolved as result of the interplay of the original sedimentary texture and the diagenetic evolution. Porosity is higher in the Bih Formation, which is dominated by dolomitized packstones and grainstones, than in the Hagil and Ghail formations, consisting mainly of dolomitized mudstones and wackestones. Image analyses were used to quantify the visual porosity in thin sections. The highest porosity values were measured in the Bih Formation, which is characterized by significant amounts of vug- and fracture-filling cements. This feature is attributed to the increase of porosity owing to substantial dissolution of abundant intergranular and vug-filling cements. In contrast, the Hagil and Ghail formations, which consist of finer-grained rock than the Bih Formation, were less cemented, and thus, the porosity enhancement by cement dissolution was insignificant.

Petrographic, geochemical analyses (major, trace and rare-earth elements and C, O- and Sr-isotopes), and fluid-inclusion microthermometry study of vein calcites in primarily Cretaceous reservoir carbonate rocks in oilfields from Abu Dhabi, UAE, helped to unravel the formation conditions and origin of diagenetic fluids. The vein calcites have diverse crystal shapes (equant blocky, bladed and fibrous) and sizes (up to 3 cm across) and display complex cross-cutting relationships. These calcites display a wide range of stable carbon (δ13CV-PDB = − 11.1‰ to + 9.6‰) and oxygen (δ18OV-PDB = − 12.7‰ to − 2.3‰) isotopic compositions, indicating formation under diverse geological conditions. The oxygen isotopic composition suggests that the precipitation of vein calcites occurred at temperatures of ca. 30–100 °C assuming that the fluids had δ18OV-SMOW values of marine pore waters which evolved to basinal brines (i.e. − 1.2‰ and + 2‰, respectively). These inferred temperatures corroborate the fluid-inclusion microthermometry, which revealed a predominantly single, whole liquid phase that suggests entrapment temperature less than ca. 50 °C. A few two-phase fluid inclusions in equant and bladed calcite indicate precipitation at ca. 68–100 °C and salinity of ca. 3.9–10.1 wt.% equivalent NaCl). The carbon isotopic signatures suggest derivation of dissolved carbon from the host marine carbonates and, less commonly, from the degradation of organic matter by methanogenic and sulfate-reducing bacteria. The diverse sources of carbon coupled with the inferred wide range of precipitation temperatures presumably account for the lack of correlation between the carbon and oxygen isotopes. The derivation of dissolved carbon mainly from the host rocks is in good agreement with the marine Sr isotopic ratios of these calcites (87Sr/86Sr = 0.70744 to 0.70766). Rare-earth element patterns suggest significant fluid interaction with upper continental crustal rocks as evidenced by negative Eu anomalies and LREE-enriched patterns.

Hormoz Island, a salt diapir in the eastern Persian Gulf, is dominated by the InfracambrianHormoz Complex comprising both evaporites (Hormoz Salt) and carbonates, siliciclastics andvolcanic rocks. Minerals include black, white and grey dolomites, pyrite, gypsum, anhydrite, apatiteand iron oxides. Formation of some of the dolomite crystals is interpreted to be linked to theoxidation of hydrocarbons.The

Petrographic and stable isotope investigations of Oligocene-Miocene carbonates in the Asmari Formation from the Gachsaran oilfield and surrounding area in SW Iran indicate that the carbonates have been subjected to extensive diagenesis including calcite cementation and dolomitization. Diagenetic modification occurred in different diagenetic realms ranging from marine, meteoric and finally burial. Asmari carbonates were in general deposited in a ramp setting and are represented by intertidal to subtidal deposits together with lagoonal, shoal and low-energy deposits formed below normal wave base. Lithofacies include bioclastic grainstones, ooidal and bioclastic, foraminiferal and intraclastic packstones, and mudstones. Multiple episodes of calcite cementation, dolomitization and fracturing have affected these rocks to varying degrees and control porosity. Four types of dolomites have been identified: microcrystalline matrix replacement dolomite (D1); fine to medium crystalline matrix replacement dolomite (D2); coarse crystalline saddle-like dolomite cement (D3); and coarse crystalline zoned dolomite cement (D4). Microcrystalline dolomites (D1) (6–12 μm) replacing micrite, allochems and calcite cements in the mud-supported facies prior to early compaction show δ18O and δ13C values of −4.01 to +1.02‰ VPDB and −0.30 to +4.08‰ VPDB, respectively. These values are slightly depleted with respect to postulated Oligocene-Miocene marine carbonate values, suggesting their precipitation from seawater, partly altered by later fluids. The association of this type of dolomite with primary anhydrite in intertidal facies supports dolomitization by evaporative brines. Fine to medium crystalline matrix dolomites (D2) (20–60μm) occur mostly in grainstone facies and have relatively high porosities. These dolomites formed during early burial and could be considered as recrystallized forms of D1 dolomite. Their isotopic values overlap those of D1 dolomites, implying precipitation from similar early fluids, possibly altered by meteoric fluids. Coarse crystalline saddle-like dolomites (D3) (200–300 μm) partially or completely occlude fractures and vugs. The vugs developed through the dissolution of carbonate components and rarely matrix carbonates, while fractures developed during Zagros folding in late Oligocene to early Miocene times. A final diagenetic episode is represented by the precipitation of coarse crystalline planar e-s zoned dolomite (D4) (80–250 μm) that occurs in fractures and vugs and also replaces earlier dolomite and post-dates stylolitization. Fluids responsible for the formation of D3 and D4 dolomites are affected by brine enrichment and increasing temperatures due to increasing burial. Reservoir porosity is dominated by microcrystalline pore spaces in muddy, dolomitized matrix and mouldic and vuggy porosity in grainstone. Porosity was significantly enhanced by the formation of multiple fracture systems.

Stable isotopic compositions of the pedogenic calcites and calcitic dinosaur eggshells analyzed from the Seonso Formation in southern Korea provide information on the paleoenvironmental conditions that dominated the region during the Upper Cretaceous as well as on the nature of the diagenetic fluids. The δ 18 O values of the calcites recovered from the paleosols (−15.46 to −26.22 ‰ VPDB) indicate that they were precipitated and/or altered under a high temperature regime, whereas the δ 13 C values (−3.88 to −7.72 ‰ VPDB) suggest that these carbonates received contributions from hydrothermal fluids during shallow burial and possibly from C 3 -type vegetation. The δ 18 O and δ 13 C of partially to well-preserved calcite eggshells are significantly different from those of the paleosols, indicating a departure from the earlier environmental conditions. The observed shift in δ 18 O values between the dinosaur eggshells (−3.43 to −14.09 ‰) and pedogenic calcites indicates different environmental and thermal conditions and suggesting two possible scenarios; either that the timing of egg laying by the dinosaur and the soil formation were not synchronous, or that the dinosaur consumed water from the rivers and evaporated ponds. In contrast, isotopic compositions of vein calcites cross-cutting paleosols (−19.06 to −21.72 for δ 18 O, and −3.78 to −4.79 for δ 13 C) reflect their precipitation under hydrothermal conditions. The narrow range in the δ 13 C values of the eggshells (−7.04 to −8.69 ‰) reflect a uniform source for carbon; mostly representing fresh water charged with CO 2 from hydrothermal sources as dissolved carbon.

The Tournasian age Pekisko carbonates in the Normandville Field (northwestern Alberta) form waulsortian-like, bryozoan/crinoid mounds that developed in fairly deep, low energy, cool water systems, close to the ramp margin. Three main depositional environments occur: (1) crinoidal apron with wackestone, grainstone and floatstone facies; (2) mound flank with grainstone, wackestone, packstone and floatstone facies dipping ∼35°; and (3) bryozoan mound core, composed of rudstone and floatstone facies with fenestrate bryozoa, minor crinoids and carbonate mud. Local highs due to fault-bounded blocks, created from the collapse of the Devonian Peace River High, may have controlled the location of mound nucleation. Diagenesis of the bryozoan/crinoid mounds included calcite cementation, compaction, dolomitization, silicification, and hydrocarbon emplacement events. The mound core facies contains submarine fascicular optic calcite and bladed/prismatic calcite cements, and later ferroan, brightly luminescent, pore-filling blocky spar cement. The crinoid apron facies contains syntaxial cement associated with crinoids, and the ferroan blocky spar cement. The mounds are dominantly limestone; however, in one well, dolomite dominates the lower section. Four types of dolomite have been identified: partial replacive; chemical-compaction-related, pervasive dolomite and saddle dolomite cement. All dolomites are non-stoichiometric (CaCO3 mole% 56.6–62.6). The partial, zoned replacive dolomite replaces micrite and syntaxial rim calcite in mound flank and crinoid apron facies. The chemical compaction-related dolomite is found along dissolution seams and stylolites and has similar CL characteristics to the replacive dolomite. The pervasive dolomite is fabric destructive and has dull cores and bright rims in CL. Saddle dolomite (0.15 mm) has brightly-luminescent, concentric zoning and occurs in vugs and fossil pore spaces. Chemical and isotopic analysis of the bryozoan/crinoid mounds indicate that the original marine signatures in micrite, early cements, some crinoids and brachiopods have been preserved. However, carbon isotopic values for some crinoids, matrix and dolomite show more positive values compared to known Mississippian carbonate values. Recrystallization during shallow burial has reset the oxygen isotopic composition of some crinoids and micrite. Oxygen and carbon isotopic compositions of most dolomites overlap with altered crinoids and early calcite cements. However, saddle dolomites have lighter δ18O values, similar to saddle dolomites from the Devonian Wabamun Group in this area. The isotopic variations in later ferroan calcite cements show an inverted-J trend, possibly due to variable amounts of water-rock interaction. While the Sr-isotopic ratio of submarine calcite cement coincides with that of Mississippian seawater, the later ferroan calcite cement is more radiogenic, indicating a different source of fluids.

This study investigates the depositional environment and sequence stratigraphy of the Asmari Formation (Oligocene-Miocene) in Gachsaran Area. The formation is carbonate sequence, which is laid down in the southern side edge Neotethys Ocean (Zagros area). The Asmari Formation represents sedimentation on a carbonate ramp. Seven major microfacies and three subfacies are recognized which include: MF-1) planktonic foraminifer wackestone-packstone, MF-2) nummulitic-bioclastic-corallinacean wackestone-packstone, MF-3) bioclastic grainstone, MF-4) ooid-grainstone, MF-5) bioclastic-miliolid-borelisid, MF-6) miliolid-intraclast-bioclast and MF-7) carbonate mudstones (laminated mudstone, fossiliferous mudstone and algal mudstones). This carbonate succession consists of four, thick shallowing-upward sequences. Based on shallowing and deepening trend of depositional facies and changes in the cycle stacking pattern four third-order depositional sequences were identified. With the concern of 7 microfacies kinds, five carbonate sedimentary environments are represented. These environments from basin to land: outer slope to basin and slope sited in outer ramp; shoal and shelf lagoon located between middle ramp-internal ramp; and restricted lagoon and low intertidal too supratidal set up in the internal ramp. The formation indicates a large-scale trend of decreasing accommodation upward. The lower Asmari layers was laid down on open-marine conditions and represented by a laterally equivalent shaley faices, assigned to that generally the Pabdeth formation and communicated prograding side edge to the upward gradient attitudes, while the center to the upper parts of the formation were deposited in lower energy settings with more frequent evidence of exposure and evaporitic conditions, reflecting ramp settings established behind the prograding margin. The top side of the sequence is obviously marked by an erosional surface, which is overlaid by thick tidal flats. The surface erosional shows intense staining and leaching out suggesting diagenetic alteration during the sub-aerial exposure. With petrographic methods five kinds of dolomite are differentiated in this formation: 1- dolomicrite 2- dolomicrosparite 3- sucrosic dolomite 4-cloudy center dolomite and 5-saddle dolomite.

Authigenic vivianite and siderite microconcretions were found, respectively, in hemipelagic and deltaic facies of 600-m-long BDP-98 sediment section from Lake Baikal. Textural investigations of these microconcretions show that they are typically

Elucidation of diagenetic alterations in the Petrohan Terrigenous Group (fluvial; highstand systems tract HST) sandstones and Svidol Formation (tide-dominated deltaic and tidal flat, transgressive systems tract TST and highstand systems tract HST, respectively) sandstones and calcarenite, Lower Triassic, NW Bulgaria was constrained within a sequence stratigraphic framework. Eogenetic alterations in the fluvial HST sandstones include (i) formation of grain-coating infiltrated clays as a result of percolation of mud-rich surface waters into underlying coarse-grained and permeable channel-fills and crevasse splay sandstones; (ii) formation of pseudomatrix by mechanical compaction of mud intraclasts that were incorporated into the coarse-grained channel sandstones during their lateral avulsion; and (iii) cementation by calcite (δ 18 O VPDB = − 6.5‰ to − 3‰; δ 13 C VPDB = − 5.1‰ to + 0.6‰) and dolomite (δ 18 O VPDB = − 6.1‰ to − 0.3‰; δ 13 C VPDB = − 7.2‰ to − 5.8‰) in the crevasse splay and floodplain sediments. Mesogenetic alterations that are encountered in the fluvial HST sandstones include (i) illitization of grain-coating clays, mud intraclasts, and mica, possibly because of simultaneous albitization of feldspars; (ii) cementation by calcite (δ 18 O VPDB = − 14.5‰ to − 8.4‰; δ 13 C VPDB = − 7.7‰ to + 0.6‰) and dolomite (δ 18 O VPDB = − 15.8‰ to − 5‰; δ 13 C VPDB = − 7.9‰ to + 1.5‰); and (iii) limited amounts of quartz overgrowths in the channel sandstones owing to occurrence of thick grain-coating clays. Conversely, the tide-dominated deltaic TST sandstones and the tidal flat HST calcarenite were pervasively cemented by calcite (δ 18 O VPDB = − 6.6‰ to − 3.1‰; δ 13 C VPDB = − 5.1‰ to + 0.6‰) and siderite (δ 18 O VPDB = − 7.2‰ to − 5.7‰; δ 13 C VPDB = + 0.3‰ to + 0.9‰) particularly below marine and maximum flooding surfaces, due to the presence of abundant bioclasts and prolonged residence time of the sediments under certain geochemical conditions along these surfaces. The remaining open pores were cemented during mesodiagenesis by calcite (δ 18 O VPDB = − 6.6‰ to − 3.1‰ and δ 13 C VPDB = − 5.1‰ to + 0.6‰) and dolomite (δ 18 O VPDB = − 6.6‰ to − 3.1‰ and δ 13 C VPDB = − 5.1‰ to + 0.6‰). This study shows that constructing a conceptual model for the distribution of diagenetic alterations is possible by integration of diagenesis with sequence stratigraphy. The model shows that tide-dominated deltaic TST sandstones and tidal flat HST calcarenite were pervasively cemented by carbonates during near-surface eodiagenesis, owing to the presence of abundant bioclasts. Conversely, fluvial LST sandstones remained poorly cemented during near-surface eodiagenesis due to the lack of bioclasts, but were cemented by mesogenetic calcite, dolomite and quartz overgrowths instead.

The spatial and temporal distribution of diagenetic alterations has been constrained in relationship to depositional facies and sequence stratigraphy of the Upper Ordovician glaciogenic quartzarenite sandstones in the Murzuq Basin, SW Libya, which were deposited during the Haritanian glaciation when the basin was laying along the continental margin of Gondwana. Eogenetic alterations encountered include: (i) replacement of detrital silicates, mud matrix and pseudomatrix by kaolinite in paraglacial, tide-dominated deltaic, in foreshore to shoreface (highstand systems tract; HST) and in post-glacial, Gilbert-type deltaic (lowstand systems tract; LST) sandstones, particularly below the sequence boundaries (SB). Kaolinite formation is attributed to the influx of meteoric water during relative sea level fall and basinward shift of the shoreline. (ii) Cementation by calcite ( δ 18 O VPDB = − 3.1‰ to + 1.1‰ and δ 13 C VPDB = + 1.7‰ to + 3.5‰) and Mg-rich siderite in the paraglacial, tide-dominated deltaic and foreshore to shoreface HST sandstones, in the glacial, tide-dominated estuarine (transgressive systems tract; TST) sandstones and in the post-glacial, shoreface TST sandstones is interpreted to have occurred from marine pore-waters. (iii) Cementation by Mg-poor siderite, which occurs in the post-glacial, Gilbert-type deltaic LST sandstones and in the paraglacial, tide-dominated deltaic and foreshore to shoreface HST sandstones, is interpreted to have occurred from meteoric waters during relative sea level fall and basinward shift of the shoreline. (iv) Pervasive cementation by iron oxides has occurred in the glacial, shoreface–offshore TST sandstones and post-glacial, shoreface TST sandstones immediately below the maximum flooding surfaces (MFS), which was presumably enhanced by prolonged residence time of the sediments under oxic diagenetic conditions at the seafloor. (v) Formation of grain-coating infiltrated clays mainly in the glacial, fluvial incised-valley LST sandstones and in the post-glacial, Gilbert-type deltaic LST sandstones as well as, less commonly, in the paraglacial, foreshore to shoreface HST sandstones and in the tide-dominated deltaic HST sandstones below the SBs. Mesogenetic alterations include mainly the formation of abundant quartz overgrowths in the glacial, fluvial incised-valley LST sandstones, post-glacial, Gilbert-type deltaic LST sandstones and glacial, shoreface TST sandstones, in which early carbonate cements are lacking. Illite, chlorite and albitized feldspars, which occur in small amounts, are most common in the glacial, tide-dominated estuarine TST sandstones and paraglacial, shoreface HST sandstones. This study demonstrates that the spatial and temporal distribution of diagenetic alterations and their impact on reservoir-quality evolution in glacial, paraglacial and post-glacial sandstones can be better elucidated when linked to the depositional facies and sequence stratigraphic framework.

The presence of oil shows associated with fractures provides a significant opportunity to a) unravel the type, origin and evolution of fluids involved in fracture-fills, and b) examine how they relate to oil migration. Two stages of calcite cement (C1 and C2) were distinguished in the fractures of the Eocene Armancies platform carbonates; C1 is characterised by fence-like crystals, exhibits dull red luminescence and contains abundant twin planes, inclusions and δ18O values that range from − 6.2‰ to − 4.8‰ VPDB. C2 consists of blocky clean crystals, is characterized by dark brown-red luminescence that alternates with yellow bands, and contains hydrocarbon fluid inclusions with homogenisation temperatures of approximately 120 °C. δ18O values range from − 9.6‰ to − 8.9‰ VPDB. The remaining porosity after C2 precipitation is filled with liquid oil that reached 115 °C. This would seem to indicate that free oil and fluid inclusions oil probably come from the same migration pulse. Oil migration timing was coeval with C2 and continued after calcite cementation was completed.

Elucidation of the distribution of diagenetic alterations in incised-valley sandstones within a sequence stratigraphic framework allows a better understanding of their reservoir-quality evolution during burial. Gas reservoirs of the upper Miocene Abu Madi Formation (present-day depth 3350 m, temperature 116°), the Nile Delta Basin, Egypt, consist of lowstand systems tract (LST) fluvial and transgressive systems tract (TST) estuarine sandstones deposited in an incised valley. These sandstones have a wide range of porosity (2 to 29%) and permeability (0.01 to 6071 mD), which reflect both depositional facies and diagenetic controls. Diagenetic events that influenced the reservoir-quality evolution include mainly the formation of extensive grain-rimming Fe-Mg chlorite, mechanical compaction of ductile grains, pressure dissolution of quartz, and quartz cementation. The chlorite rims, distributions of which are not clearly related to sequence stratigraphy, have preserved reservoir quality by inhibiting quartz cementation but have increased microporosity values in the reservoirs. Calcite cementation (18O = −13.5‰ to −6.0‰, δ13C = −14.3‰ to −1.0‰, and 87Sr/86Sr = 0.707124 to 0.708181), has not influenced the overall reservoir-quality evolution of the sandstones because of its localized occurrence. Other diagenetic events that have little influence on reservoir quality include the more frequent formation of kaolin in the LST fluvial sandstones and formation of pyrite in the TST estuarine sandstones. This study demonstrates the possibility of constraining and evaluating the impact of diagenetic alterations on reservoir-quality evolution in incised-valley deposits and their sequence stratigraphic distribution, and thus has an important impact on hydrocarbon exploration in such settings.

Dolomite and calcite cements fill open stylolites, fractures and “pipe-shaped” structures related to faulting in Tithonian–Berriasian limestones of the Maestrat Basin in the Iberian Range (Spain) Due to the grater susceptibility of the dolomitised limestones to brittle fracturing during the Alpine tectonism, their location and distribution may have important implications for hydrocarbon prospectively within the studied region (Iberian Range). Three generations of structures were recognised (open stylolites, extensional fractures and faults) based on field observations, cross-cutting relationships and cement mineralogy. Petrographic, cathodoluminesence and geochemical analyses (electron microprobe, fluid inclusion, oxygen, carbon and strontium isotopes) of the carbonates helped unravel the origin and evolution of the fluids, from which these carbonates have been precipitated. These cements occur in the following structures: A) The first generation NNE trending fractures formed during the Late Eocene–Miocene compressional stage were filled by calcite cement ( δ 18 O VPDB-8.8‰ and δ 13 C VPDB + 0.8). B) The second generation represented by subvertical extensional fractures and the third generation by “pipe-shaped” structures, which are considered to be formed during the Miocene–Pliocene extensional stage, contain four carbonate cement phases : 1) isolated rhombic dolomites; 2) saddle dolomite with fluid inclusions characterised by high salinity (21.5 to 23.5 wt.% eq. NaCl), radiogenic Sr-enriched (0.70796 to 0.70857) in relation to the marine standard, high temperatures ( T h 110–155 °C) and low δ 18 O values VPDB (− 11.5‰ to − 11.3‰); 3) calcitized saddle and rhombic dolomite with variable δ 18 O VPDB − 12.2‰ to − 6.8‰ and δ 13 C VPDB − 4.4‰ to + 0.2‰, and 4) two phases of calcite cements with moderate to high temperatures ( T h 110–125 °C, 15 to 19.7 wt.% eq. NaCl and 160–198 °C, 5.5 to 9.5 wt.% eq. NaCl), low δ 18 O VPDB values (− 13.4‰ to − 10‰) and positive carbon values ( δ 13 C VPDB + 0.1‰ to + 1.8‰). The diagenetic fluid is interpreted to be initially hot, saline, hydrothermal (temperature higher than the estimate of the ambient temperature) and Sr-enriched (0.70800). The dolomitisation event was followed by an input of meteoric water, which was related to the extensional stage. Finally, late calcite cement precipitated from fluids, which had initially moderate salinity and moderate to high temperature and later evolved to a lower temperature and higher salinity fluid (both Sr enriched). These fluids were probably derived from the progressive mixing of Late Triassic evaporitic brines with descending meteoric water that migrated via fractures during the Miocene–Pliocene Alpine extensional stage. Major vug-filling calcite cement is probably late in timing and related to hydrothermal karst associated with the “pipe-shaped” structure. C) Bed-parallel stylolites and subvertical extensional fractures, containing idiomorphic quartz crystals for which make it difficult to deduce the timing of its precipitation. Idiomorphic quartz has homogenisation temperatures of 140–180 °C and salinities of 13.6 wt.% eq. NaCl. The high homogenisation temperature of this quartz and its association with dickite suggest precipitation from hydrothermal fluid.

Linking siliciclastic diagenesis to sequence stratigraphy allows a better understanding of the parameters controlling the spatial and temporal distribution of diagenetic alterations, and hence of reservoir quality. A study of the coal-bearing, alluvial, deltaic, estuarine and shallow-marine sandstones of the Rio Bonito Formation, early Permian, Parana Basin (southern Brazil), reveals that the distribution of diagenetic alterations and of related reservoir quality evolution can be constrained within a sequence stratigraphic framework. Calcite, dolomite, siderite, kaolinite and pyrite cementation is consistently linked to sequence and parasequence boundaries, transgressive and maximum flooding surfaces and is systematically distributed within lowstand, transgressive and highstand systems tracts. Diagenesis of coal layers at parasequence boundaries has promoted the formation of stratabound calcite (detectable in resistivity wire line logs), concretionary pyrite and kaolinite and of silicate grain dissolution in sandstones located above and below these boundaries, particularly in the transgressive systems tract. Meteoric water diagenesis caused grain dissolution and the formation of kaolinite in sandstones below sequence boundaries and in lowstand systems tract sandstones. Carbonate bioclasts and low sedimentation rates in lag deposits at parasequence boundaries, transgressive and maximum flooding surfaces favoured the formation of grain-rimming siderite. The results of this study are relevant to the exploration of coal-bed methane and other coal-bearing reservoirs, where it is crucial to unravel and predict the distribution and quality of reservoirs and compartments.

The distribution of diagenetic alterations in the fluvial, deltaic and shallow marine, arkosic to subarkosic sandstones (average Q72F26L02) of the Mullaghmore Formation (Carboniferous, NW Ireland) can be predicted within a sequence stratigraphic framework. Eogenetic calcite (18OPDB = -13.3‰ to -6.5‰, 13CPDB = -3.0 to +3.4‰, and 87Sr/86Sr = 0.706721 to 0.709227) and ferron dolomite (FeCO3 = 8-12 mol%; 18OPDB = -14.2‰ to -7.8‰, 13CPDB = -1.4‰ to -1.0‰, and 87Sr/86Sr = 0.709051 to 0.709167) occur in bioclast-rich, transgressive lag deposits at parasequence boundaries and transgressive surfaces, and in wave-influenced, deltaic, highstand systems tract (HST) deposits. Mesogenetic illite, chlorite, baroque dolomite (FeCO3 = 16 mol%; 18OPDB = -14.2‰ to -12.7‰, 13CPDB = -3.8‰ to -1.0‰), quartz, and calcite (18OPDB = -15.7‰ to -12.5‰, 13CPDB = -5.8‰ to -3.7‰, and 87Sr/86Sr = 0.709016 to 0.709122) were formed mainly in the bioclast-poor deposits, which were not pervasively cemented by carbonates during near-surface eodiagenesis. These deposits include fluvial, incised-valley sandstones of lowstand systems tract (LST), and fluvial-dominated, deltaic sandstones of transgressive systems tract (TST) and HST. Illite is the dominant diagenetic clay mineral in the fluvial, incised-valley sandstones of LST, possibly because of simultaneous albitization of K-feldspars. Conversely, chlorite, dominates in the fluvial-dominated, deltaic sandstones of TST and HST, because of the presence of suitable precursor clays. The integration of diagenesis into sequence stratigraphic framework of clastic sequences should improve the ability to predict the spatial and temporal distribution of diagenetic alterations and related reservoir-quality modifications of sandstone deposits.

Petrographic and geochemical studies of the Middle Devonian (Givetian) Sulphur Point Formation in the vicinity of the Rainbow South Field, northwestern Alberta, reveal that dolomitization was a direct result of precipitation by chemically distinct fluids, and that recrystallization of these dolomites significantly altered their original chemical signatures. Sulphur Point carbonates were deposited in a restricted peritidal environment. Lithofacies include grainstones, sparsely fossiliferous packstones, mudstones, algal mudstones, and intraclast breccia mudstones. Multiple episodes of calcite cementation and dolomitization have affected these rocks to varying degrees. Five dolomite types were identified: 1) dolomicrite, 2) fine-crystalline matrix dolomite, 3) medium-crystalline matrix dolomite, 4) saddle dolomite and 5) fracture-lining dolomite. Dolomicrite (2-20 µm) replaced both micrite and calcite cement in the mud-supported facies before early compaction. A trend toward more negative 18O values of -9.22 to -3.10‰ Vienna Pee Dee Belemnite (VPDB) with respect to postulated Middle Devonian marine carbonate values suggests that dolomicrite was recrystallized by later fluids. Geochemical modelling of the isotope and trace element trends in the dolomicrite support this interpretation. Both fine- and medium-crystalline matrix dolomites (40-200 µm) are usually fabric destructive. However, some intervals have retained lamination and algal structures. Matrix dolomite was formed during intermediate burial, as suggested by its association with dissolution seams, high Fe and Mn concentrations, and 18O values of -12.20 to -8.34‰ VPDB. This evidence, in addition to the presence of high salinity fluid inclusions (~ 18 wt% NaCl equivalent), indicates that matrix dolomite was precipitated by basinal fluids between the Mississippian and Late Jurassic. The precipitation of saddle dolomite (0.5-2.0 mm) is genetically related to fractures and breccia zones where it partially to completely occludes the fractures, breccias and vugs that were developed through the dissolution of the earlier matrix dolomites. Geochemical and petrographic evidence suggests that saddle dolomite was precipitated from a hot, slightly saline (10.5 to 13.3 wt% NaCl equivalent), calcium-rich fluid that was funnelled upward along faults and fractures that developed during the Late Cretaceous to early Tertiary Laramide Orogeny. Strontium isotope modelling confirms that saddle dolomite was precipitated from a two-component hydrothermal fluid incorporating a significant quantity of Middle Devonian brines and radiogenic basement fluids. Fracture-lining dolomite (0.2-1.0 mm) was the last dolomite phase to precipitate, and is intimately associated with blocky calcite, quartz, sulphide mineralization and pyrobitumen. Isotopic and fluid inclusion evidence imply precipitation from slightly saline brines (~ 8 wt% NaCl equivalent) at elevated temperatures. Extremely low Fe and Mn concentrations, negative 13C values (~ -5‰ VPDB), and significant volumes of H2S gas suggest that fracture-lining dolomite was precipitated from syn- to post-Laramide fluids during thermochemical sulphate reduction. RESUME Les etudes petrographiques et geochimiques de la Formation de Sulphur Point du Devonien moyen (Givetien) aux environs du champ de Rainbow Sud, dans le nord-ouest de I' Alberta, revelent que la dolomitisation a ete le resultat direct de la precipitation par des fluides distincts chimiquement et que la recristallisation de ces dolomites a altere significativement leurs signatures chimiques originales. Les carbonates de Sulphur Point ont ete deposes dans un environement peritidal restreint. Les lithofacies comprennent des grainstones, des packstones a fossiles epars, des mudstones, des mudstones alguaries et des mudstones a breche d'intraclastes. Les episodes multiples de cimentation de calcite et de End_Page 262------------------------ dolomitisation ont affecte ces roches a des degres divers. Cinq types de dolomite ont ete identifies: 1) la dolomicrite, 2) la dolomite a matrice de cristaux fins, 3) la dolomite a matrice de cristaux moyens, 4) la dolomite en selle, 5) la dolomite de garniture de fracture. La dolomicrite (2-20 µm) a remplace a la fois les ciments de micrite et de calcite dans les facies a base de boue avant la compaction precoce. Une tendance vers des valeurs de 18O de -9,22 a -3,10‰ VPDB par rapport aux valeurs postulees pour les carbonates marins du Devonien moyen, suggere que la dolomicrite a ete recristallisee par les fluides posterieurs. La modelisation geochimique des tendances des elements isotopiques et en traces dans la dolomicrite supporte cette interpretation. Les dolomites a matrice de cristaux fins et moyens (40-200 mm) sont toutes deux habituellement destructrices de fabriques. Toutefois, quelques intervalles ont conserve des laminations et des structures alguaires. Les dolomites a matrice ont ete formees durant l'enfouissement intermediaire, comme suggere par son association avec des veines de dissolution, de hautes concentrations en Fe et Mn, et des valeurs de d18O de -12,20 a -8.34 0/00 VPDB. Cette evidence, en plus de la presence d'inclusions fluides a forte salinite (~ 8 0/0 poids eq NaCl), indiquent que la dolomite a matrice a ete precipitee par des fluides de bassin entre le Mississippien et le Jurassique superieur. La precipitation de dolomite en selle (0,5 a 2,0 mm) est reliee genetiquement aux zones de fractures et de breches ouelle occlut partiellement ou completement les fractures, les breches et les cavites qui se sont developpees par la dissolution des dolomites anterieures a matrice. Les evidences geochimiques et petrographiques suggerent que la dolomite en selle a ete precipitee a partir d'un fluide chaud, legerement salin (10,5 a 13,3 % poids eq. NaCl), riche en calcium qui s'est engouffre vers le haut le long de failles et de fractures qui se sont developpees durant l'orogene des Laramides du Cretace superieur au Tertiaire inferieur. La modelisation de l'isotope du Strontium confirme que la dolomite en selle a ete precipitee a partir d'un fluide hydrothermal a deux composants, incorporant une quantite significative de saumures du Devonien moyen et des fluides radiogeniques du socle. La dolomite de garniture de fracture (0,2-1,0 mm) a ete la derniere phase de dolomite a precipiter, et est intimement associee a la calcite en bloc, au quartz, a une mineralisation en sulfure et du pyrobitume. Les evidences tirees des isotopes et des inclusions fluides impliquent une precipitation a partir de saumures legerement salines ( ~ 8 0/0 poids eq. NaCl) a temperatures elevees. Des concentrations extremement faibles en Fe et Mn, des valeurs negatives en 13C (~ -5 ‰ VPDB), et des volumes significatifs de gaz H2S suggerent que la dolomite de garniture de fracture a ete precipitee a partir de fluides syn- a post-Laramide durant la reduction thermochimique de sulfates. Traduit par Lynn Gagnon

Early diagenetic Mn-Fe carbonates, occurring in thin layers, nodules, and burrows, were recovered from Cenozoic silts and clays from the Arctic Ocean and the Norwegian-Greenland Sea during Ocean Drilling Program Leg 151. At Sites 909 (Fram Strait; Miocene-Quaternary) and 911 (Yermak Plateau; Pliocene-Quaternary), fine-crystalline siderite, enriched in Ca and/or Mg, is the predominant carbonate. Variable δ13C values (-21.8 to +7.9‰ PDB) suggest that dissolved carbon was derived from the Fe-reduction suboxic zone, the oxidation of marine organic matter in the bacterial sulfate-reduction zone, and the early stages of methanogenesis. The range of δ18O values for siderite (-6.3 to +5.1‰ PDB) indicates precipitation over a temperature range of 4-56°C during successive burial. At Site 913 (East Greenland Margin; Eocene-Quaternary), concentrically zoned microspherules and rhombohedra of ferroan rhodochrosite and manganoan siderite, 20-600 μm in diameter, are the main authigenic carbonates. Although they have low δ13C values (-22.3 to -13.8‰ PDB), elevated pore-water sulfate concentrations indicate low sulfate-reducing activity and suggest that dissolved carbon may have been partially derived from thermogenic methane. The variable δ18O values for rhodochrosite (-11.9 to +1.5‰ PDB) and siderite (-11.4 to -10.3‰ PDB) suggest precipitation at elevated temperatures (60-100°C) during progressive burial. The close association with barite cement suggests that carbonate precipitation at Site 913 may have been influenced by hydrothermal fluids. However, active circulation of hydrothermal fluids is precluded by the occurrence of geochemical gradients in Site 913 pore waters. Pore waters at the three sites reflect various evolutionary pathways that are partly due to variations in the extent of interaction with volcanic ash layers and the underlying basaltic crust, which led to an overall decrease in Mg2+, K+, and δ18O and an increase in Ca2+ with depth. However, pore waters at Site 913 have evolved more extensively to Na-Ca-Cl brines. The sources of freshwater that cause the downhole decrease in Cl- and Na+ concentrations at Sites 909 and 913 are poorly constrained. The possible influence of meteoric-water incursion is suggested by the δD-δ18O relationship of pore waters and the low δ18O values of Site 913 authigenic carbonates.

The Lower Cretaceous and Miocene sequences of the NW African passive continental margin consist of siliciclastic, volcaniclastic and hybrid sediments. These sediments contain a variety of diagenetic carbonates associated with zeolites, smectite clays and pyrite, reflecting the detrital mineralogical composition and conditions which prevailed during opening of the North Atlantic. In the Lower Cretaceous siliciclastic sediments, siderite ( −6‰ to +0.7‰ δ 18 O PDB , −19.6‰ to +0.6‰ δ 13 C PDB ) was precipitated as thin layers and nodules from modified marine porewaters with input of dissolved carbon from the alteration of organic matter. Microcrystalline dolomite layers, lenses, nodules and disseminated crystals ( −3.0‰ to +2.5‰ δ 18 O PDB , −7.2‰ to +4.9‰ δ 13 C PDB ) predominate in slump and debris-flow deposits within the Lower Miocene sequence. During the opening of the Atlantic, volcanic activity in the Canary Islands area resulted in input of volcaniclastic sediments to the Middle and Upper Miocene sequences. Calcite is the dominant diagenetic carbonate in the siliciclastic-bioclastic-volcaniclastic hybrid and in the volcaniclastic sediments, which commonly contain pore-rimming smectite. Diagenetic calcite ( −22‰ to +1.6‰ δ 18 O PDB , −35.7‰ to +0.8‰ δ 13 CPDB ) was precipitated due to the interaction of volcaniclastic and bioclastic grains with marine porewaters. Phillipsite is confined to the alteration of volcaniclastic sediments, whereas clinoptilolite is widely disseminated, occurring essentially within foraminiferal chambers, and formed due to the dissolution of biogenic silica.

An integrated approach combining CSFe relationships, stable isotopic compositions, and lithofacies characterization was utilized to constrain the palaeoenvironmental and early diagenetic conditions of Middle-Upper Devonian (Eifelian-Frasnian) mudrocks from the Mackenzie Mountains, Northwest Territories, Canada. These rocks include the Hare Indian Formation (informally subdivided into the lower Bluefish Member and the Hare Indian Upper Member), Carcajou Marker and Canol Formation. The Bluefish Member is dominated by black, laminated, organic-rich shales (TOC = 0.35–10.34 wt.%; av. 5.83 wt.) with moderate degrees of pyritization (DOP) of 0.34–0.67 (av. 0.55). These mudrocks were deposited in dysoxic marine bottom-waters that became progressively more oxygenated with time. Variations in TOC, DOP and organic matter δ 13CPDB values (−29.7% to −19.9%; av. −27.2%) are attributed to repeated clastic dilution and increased input of terrestrial organic matter in association with shallowing-upward ramp-clinothem cycles. Pyrite δ 34SCDT values (−32.7% to −18.8%; av. −24.9%) indicate an open system, bacteriogenic seawater-sulphate reduction. Conversely, the overlying Hare Indian Upper Member, characterized by clinothem facies, is composed of grey to green mudstone with minor argillaceous limestones and considerably less organic matter contents (TOC = 0.28–2.99 wt.%; δ 13C = −29.5% to −22.5%). Deposition occurred in oxic to slightly dysoxic waters (DOP = 0.20–0.54; δ 34S = −23.0% to −20.9%), depending on the palaeotopographic location along the depositional slope. A rapid rise in sea level drowned the carbonate ‘ramp’ member of the Ramparts Formation and produced the thin, organic-rich Carcajou Marker. Bottom-water stagnation that resulted from subdued ramp palaeotopography produced anoxic sea bottom. Black, laminated, organic-rich shales from the Canol Formation (TOC = 1.37–6.68 wt.%) are very similar to those of the Bluefish Member, and are likewise basinal sediments. However, TOC, DOP and organic-matter δ 13CPDB values (−29.1% to −20.8%; av. −26.2%) do not show pronounced variations and indicate that low-energy, quietwater conditions persisted over relatively long, uninterrupted periods of time. High DOP values (0.72–0.93) throughout the Canol Formation suggest that deposition occurred in anoxic bottom-waters, but as basin-fill conditions continued there was a shift to a dysoxic environment (DOP = 0.55–0.65), which grades into nearshore and offshore sequences of the overlying Imperial Formation. In contrast with the Hare Indian Formation, much heavier δ 34SCDT values of pyrite in Canol mudrocks (−11.1% to +5.3%; av. −3.1%) point to bacterial sulphate reduction in a closed to semi-closed system with respect to seawater sulphate.

Siderite (delta(18)O(PDB) = -9.3 to - 5.7 parts per thousand) and Fe-dolomite/ankerite (delta(18)O(PDB) = -13.7 to -7.7 parts per thousand) occur as texturally early diagenetic, pre-compactional ce ...

Fibrous calcite veins are ubiquitous throughout the thinly bedded, organic-rich Upper Triassic marine mdrocks of the Queen Charlotte Islands and their lateral equivalents on Vancouver Island. These veins show variable and complex morphologies and can be grouped into several types: (a) simple; (b) anastomosing or composite; (c) boxwork; and (4) polygonal network oriented normal to bedding. Field, petrographic, and geochemical evidence suggest that vein opening, resulting from hydraulic fracturing due to elevated pore-fluid pressures, was an early phenomenon and occurred prior to significant compaction of the host sediments. Calcite fibers in the veins are up to 30 mm long and commonly oriented perpendicular to the wall but locally display conical structures. Fibrous calcites, with the exception of those in boxwork veins, are generally non-ferroan and dull to very weakly orange luminescent. The boxwork calcites are ferroan, zoned and show dull luminescence with some bright rims. δ 18 O values range from −8.2 to −21.6‰ (PDB) and δ 13 C values range from 2.0 to −4.4‰ (PDB). Although some variations are present among the different morphological types of calcite veins, oxygen and carbon isotopic values display important variations when compared geographically. The most depleted oxygen and carbon isotopic values are those of boxwork calcite and they are associated with areas where the effects of early Mesozoic plutonism were most severe. Precipitation of boxwork fibrous calcites is interpreted to have been related to hydrothermal discharge into unconsolidated host sediment, rather than to later burial. Although the hydrothermal influence on the formation of vein calcite is related to geological events specific to the Wrangellia Terrain, this study provides an alternative mechanism for the generation of fibrous calcite veins and demonstrates the local importance of hydrothermal input in the evolution of pore-water chemistry.

The Middle Muschelkalk (Middle Triassic) of the Catalan Coastal Range (north-east Spain) comprises sandstone, mudstone, anhydrite and minor carbonate layers. Interbedded sandstones and mudstones which are dominant in the north-eastern parts of the basin are terminal alluvial fan deposits. South-westward in the basin, the rocks become dominated by interbedded evaporites and mudstones deposited in sabkha/mudflat environments. The diagenetic and pore water evolution patterns of the Middle Muschelkalk suggest a strong facies control. During eodiagenesis, formation of microdolomite, anhydrite, baryte, magnesite, K-feldspar and mixed-layer chlorite/smectite was favoured within and adjacent to the sabkha/mudflat facies, whereas calcite, haematite, mixed-layer illite/smectite and quartz formed mainly in the alluvial facies. Low δ18OSMOW values for microdolomite (+23.7 to +28.4%) and K-feldspar overgrowths (+17.3 to +17.7%) suggest either low-temperature, isotopic disequilibrium or precipitation from low-18O porewaters. Low-18O waters might have developed, at least in part, during low-temperature alteration of volcanic rock fragments. During mesodiagenesis, precipitation of quartz overgrowths and coarse dolomite occurred in the alluvial sandstones, whereas recrystallization of microdolomite was dominant in the sabkha/mudflat facies. The isotopic compositions of these mesogenetic phases reflect increasing temperature during burial. Upon uplift and erosion, telogenetic calcite and trace haematite precipitated in fractures and replaced dolomite. The isotopic composition of the calcite (δ18OSMOW=+21.5 to +25.6%o; δ13C= 7.7 to - 5.6%o) and presence of haematite indicate infiltration of meteoric waters.

This diagenetic study (including fieldwork, petrographie, fluid inclusion, and stable isotope investigations) deals with the outcrop of Upper Permian-Lower Triassic carbonate rocks, which are equivalent to the Khuff Formation. The studied succession, which outcrops in the Ras A1 Khaimah region, northern United Arab Emirates, comprises three formations, including the Bih, the Hagil, and the Ghail formations. The study focuses on unraveling the conditions and fluid compositions encountered during diagenesis of the succession. Emphasize is also made on linking diagenesis to major stratigraphie surfaces and to highlight reservoir property evolution and heterogeneity of the studied. The evolution of fluids and related diagenetic products can be summarized as follows: (1) formation of near-surface to shallow burial, fine-crystalline dolomite (dolomite matrix) through pervasive dolomitization of carbonate sediments by modified marine pore waters; (2) formation of coarse- crystalline dolomite cement by highly evolved marine pore waters (13-23 wt.% NaCl eq.) at elevated temperatures (120-208°C), and (3) calcite cementation by highly saline fluid (20-23 wt.% NaCl eq.) at high temperature (170-212°C). A final calcite cement generation has been formed by the percolation of meteoric fluids during uplift. Fracture- and vug-filling diagenetic minerals are mainly restricted to the mid-Bih breccia marker level, suggesting preferential focused fluid flow through specific stratigraphie surfaces as well as along tectonic-related structures. Reservoir properties have been evolved as result of the interplay of the original sedimentary texture and the diagenetic evolution. Porosity is higher in the Bih Formation, which is dominated by dolomitized packstones and grainstones, than in the Hagil and Ghail formations, consisting mainly of dolomitized mudstones and wackestones. Image analyses were used to quantify the visual porosity in thin sections. The highest porosity values were measured in the Bih Formation, which is characterized by significant amounts of vug- and fracture-filling cements. This feature is attributed to the increase of porosity owing to substantial dissolution of abundant intergranular and vug-filling cements. In contrast, the Hagil and Ghail formations, which consist of finer-grained rock than the Bih Formation, were less cemented, and thus, the porosity enhancement by cement dissolution was insignificant.

The fluvial Triassic reservoir subarkoses and arkoses (2409·5–2519·45 m) of the El Borma oilfield, southern Tunisia, were subjected to cementation by haematite, anatase, infiltrated clays, kaolinite and K-feldspar at shallow burial depths from meteoric waters. Subsequently, basinal brines controlled the diagenetic evolution of the sandstones and resulted initially in the precipitation of quartz overgrowths, magnesian siderite, minor ferroan magnesite and anhydrite. The enrichment of siderite in 12C isotope (δ13CPDB= - 14·5 to - 9‰) results from derivation of carbon from the thermal decarboxylation of organic matter. During further burial, the precipitation of dickite and pervasive transformation of kaolinite into dickite occurred, followed by the formation of microcrystalline K-feldspar and quartz, chlorite and illite, prior to the emplacement of oil. Present day formation waters are Na-Ca-Cl brines evolved by the evaporation of seawater and water/mineral interaction and are in equilibrium with the deep burial (≤ 3·1 km) minerals. These waters are suggested to be derived from the underlying Silurian and Devonian dolomitic mudstones.

Dark-gray to black cryptocrystalline francolite occurs as nodules (∼ 2–8 cm in diameter) which are embedded in organic-matter-rich mudstones of the Visingso Group (Upper Proterozoic, southern Sweden). The δ13CPDB values (−18.4 to + 0.98‰) of structural-CO2, complemented by studies of the petrographic character and strontium isotopic composition of francolite, indicate precipitation in the suboxic and sulfate-reduction zones. Francolite is intergrown with berthierine that has formed in the suboxic Fe-reduction zone. Calcite enriched in Mn and, to a lesser extent Fe, occurs as fracture- and void-fillings, and as replacement of francolite. Mn- and Mg-rich calcian siderites have precipitated as pseudomorphs after bacteria in the zone of microbial methanogenesis. The carbon ( δ 13 C PDB =−0.75 to +7.04‰ ) and oxygen ( δ 18 O PDB =−14.21 to −10.58‰ ) isotopes of calcite suggest derivation of carbonate ions from microbial methanogenesis and from thermal decarboxylation of organic matter at temperatures of 60° to 80°C. Some of the void- and fracture-filling calcites ( δ 13 C = −20.8 to −18.63‰ δ 18 O = −5.46 to −5.21‰ ) have, however, precipitated at lower temperatures (∼ 25–30°C) in the sulfate-reduction zone.

Petrographic and geochemical data suggest that dolomite cement in shallow-marine sandstones of the Triassic Buntsandstein in the Ateca horst of the Iberian Range, Spain, has precipitated in the zone of mixing between phreatic meteoric and marine waters. Dolomite or rhombohedric crystals consisting of concentric bands of alternating syntaxial dolomite and calcite have filled isolated voids (350 μ–12 mm in diameter). Subsequently, limpid dolomite followed by sparry calcite precipitated in void center as the latest cements. The carbonates have low δ 13 C PDB (−9.5 to −6.0%) and δ 18 O PDB (−8.1 to −3.2%.) values and are enriched in radiogenic strontium ( 87 Sr / 86 Sr = 0.710112 to 0.720293) indicating a dominantly meteoric component in the porewaters. The overall sequence of carbonate cementation suggests precipitation from porewaters that evolved in the mixing zone with successively increased meteoric influence, and finally to fully meteoric probably due to a major sea-level fall.

Slightly hydrothermally altered sediments were recovered from a sedimented rift valley with active hydrothermal discharge. The sediment consists mainly of terrigenous material with a low content of biogenic components. In spite of the alteration, glacial and non-glacial periods are still discernable based on lithology, mineralogy, carbonate content and bulk sediment geochemistry. In the vicinity of the core studied, extremely high heat flow anomalies and the presence of active black smokers suggest a rapid discharge of hydrothermal fluids along discrete pathways such as coarse-grained turbidite layers or faults. In the biogenic-rich fine-grained sediments, diffuse transport can supply the components and allow the time necessary for the formation of authigenic phases such as carbonate concretions, gypsum, barite, often Mg-rich amorphous silica and Mg-rich silicates (stevensite). Mg-rich amorphous silica could be the precursor of the Mg-rich silicates commonly found associated with hydrothermal deposits. Carbon and oxygen isotopic results on the carbonate nodules suggest (1) that the carbon in the nodules could be partly derived from methane which would have been oxidized at the oxic-anoxic boundary and in the sulphate reduction zone and (2) that successive cycles of hydrothermal fluid migration through the sediment could explain the variability in the results.

Petrographic, chemical and stable isotope investigations have been made on calcite, dolomite, and Fe-dolomite/ankerite from Permian-Triassic clastic sequences from the Iberian Range (Spain). Calcite and dolomite cements have precipitated before the Fe-dolomite/ankerite. Calcite, however, has also formed by calcitization of dolomite due to uplift. The negative δ13CPDB values (−9.51 to −2.13%) in these carbonates suggest derivation of carbon from oxidation of organic matter in the early carbonates and from decarboxylation of organic matter in the late Fe-dolomite/ankerite. Early diagenetic dolomitic cements have heavier δ13C(> −3.54%), higher Mn/(Mn + Fe) and Mn/(Mn + Mg) and lower Fe/(Fe + Mg) ratios than later diagenetic dolomitic cements. The latters are also characterized by lighter δ18OPDB (> −10.64%) than the early cements due to increase in temperature and to pore water-mineral reactions.

Selective chemical separation techniques for isotopic analysis of coexisting calcite, dolomite, siderite and magnesite have been investigated in this study. Comparison of reaction rates of all pure carbonate minerals with 100% phosphoric acid at 25° and 50°C demonstrated that a uniformly fine grain size (

The distribution of diagenetic alterations in Upper Jurassic, siliciclastic shoreface sediments from NW France has been linked to the sequence stratigraphic framework. Calcite cement in mudrocks and sandstones of the transgressive (TST) and lower part of the highstand (HST) systems tracts is microcrystalline and occurs as continuously cemented layers and stratabound concretions. The average delta(18)O(V-PDB) (-2.6%omicron) and Sr-87/Sr-86 (0.7078) compositions of microcrystalline calcite indicate precipitation from largely marine pore waters. >Calcite cement in sandstones of the forced regressive wedge (FRWST) and lowstand (LST) systems tracts is poikilotopic and occurs mainly as stratabound concretions. Complete dissolution of the carbonate grains and concomitant precipitation of poikilotopic calcite cement with low average delta(18)O (-5.3%omicron) and radiogenie Sr-isotope (0.70882) signatures suggest incursion of meteoric waters into sandstones during relative sea-level lowstand. The poorly lithified sandstones interbedded with sandstones cemented by poikilotopic calcite concretions display evidence of diagenesis under episodes of and to semiarid paleoclimate, including: (i) partial cementation by opal, chalcedony, gypsum, and minor vadose calcite cement, (ii) mechanically infiltrated clays and Fe-oxides, and (iii) secondary porosity owing to partial dissolution of carbonate grains. The integration of diagenesis into sequence stratigraphy allows better elucidation and prediction of the spatial and temporal distribution of diagenetic alterations and related reservoir-quality modifications in shoreface sediments