Dolomite Cement Research Articles

Application of mapping and dating techniques in the study of ancient carbonate reservoirs: A case study of Sinian Qigebrak Formation in northwestern Tarim Basin, NW China

Ancient marine carbonates experienced complex modifications, making it difficult to identify reservoir genesis and effective porosity before hydrocarbon migration. To solve these issues, we used element mapping and carbonate mineral laser U-Pb radiometric dating techniques to study the diagenetic environments based on geochemistry and diagenesis-porosity evolution based on geochronology of the dolomite reservoir of the Sinian Qigebrak Formation, northwest Tarim Basin. Two major understandings were obtained as follows: (1) Supported by petrographic observations, the element mapping, stable isotopes, strontium isotope, and cathodoluminescence tests were performed on different phases of dolomite cements precipitated in vugs and dissolved fissures. The results show that the dolomite reservoirs of the Qigebrak Formation went through freshwater, marine, extremely shallow burial, burial and hydrothermal diagenetic environments after synsedimentary dolomitization; the reservoir spaces were mainly formed in the synsedimentary period (primary pores) and freshwater environment (supergene dissolution pores) before burial; whereas the marine, burial and hydrothermal environments caused the gradual filling of reservoir space by dolomite cements. (2) Based on the above understandings, each phase of dolomite cement precipitated in the reservoir space was dated by the U-Pb radiometric dating technique, and the diagenesis-porosity evolution curves constrained by geochronology were established. The loss of reservoir porosity mainly occurred in the early Caledonian, and during the peak period of hydrocarbon generation of Yuertusi Formation source rock, the reservoirs still maintained at a porosity of 6%–10%. The above understandings provide a certain basis for the evaluation of accumulation effectiveness of the Sinian Qigebrak Formation, northwestern Tarim Basin, and provide a case for the application of mapping and dating techniques in the study of ancient carbonate reservoirs.

Sedimentology and petrophysical analysis of pre-salt lacustrine carbonate reservoir from the Santos Basin, southeast Brazil

Lacustrine carbonate deposits mark the rift-sag sedimentation stage of the so-called pre-salt section of the Campos and Santos Basins, in the Southeast Brazilian continental shelf. These salt-sealed carbonate reservoirs form deep-water, giant and prolific oil fields that are currently responsible for approximately 70% of the Brazilian oil and gas production. In the Santos Basin, the sag stage carbonates are represented by the Aptian Barra Velha Formation, characterized by exotic textural and compositional features whose origin, development, and correlation with petrophysical properties have been broadly discussed in the last decade. A geological and petrophysical study has been performed based on core samples, plugs, and thin-sections of a well in the Barra Velha Formation from the eastern Santos Basin. The analyzed core interval was grouped into six depositional facies constituted by the association of five key textural components: (1) crystal shrubs; (2) calcite spherulites; (3) hybrid forms; (4) mud-grade carbonates and dolomite; and (5) intraclasts. The relationship between the depositional framework and the poroperm system, as evidenced by petrographic image analysis and CT-scan data, is highly complex. The depositional control over porosity is erratic, mainly due to the extensive diagenetic and possible hydrothermal overprint. Dolomite cementation, dolomitization, pervasive silicification, and dissolution processes are the main primary porosity modifiers. Petrophysical analysis indicates that intraclastic and spherulitic grainstones–rudstones, affected by matrix dissolution, may present the best reservoir properties, in certain intervals, even surpassing the shrubby framestones, which are widely regarded as the best reservoir facies. This study seeks to contribute to an emerging body of research about the Brazilian pre-salt lacustrine carbonates, broadening the geological knowledge about these deposit, while providing new data to improve hydrocarbon forecast and recovery.

Sulfate resistance of Portland dolomite cement: performance and mechanisms

This study investigated the sulfate resistance of Portland-dolomite cement (PDC) cured at different temperatures. Mortars of Portland cement (PC) and PDC with different dosages of dolomite, 10, 20, 30% by weight, were cured in water at 20 °C and 60 °C for 28 days before immersed in 50 g/L Na2SO4 solution at 20 °C. In addition to monitoring the length changes of mortars, MIP and XRD were performed to investigate microstructure and evolution of phase assemblages. Results show that PDC mortars cured at 20 °C give a poorer sulfate resistance than PC, and the higher the dosage of dolomite, the worse the sulfate resistance. On the contrary, when cured at 60 °C, PDC gives a much better sulfate resistance than PC, and the higher the dosage of dolomite, the better the sulfate resistance. The increases of critical pore size and porosity with the incorporation of dolomite lead to a poor sulfate resistance of PDC cured at 20 °C. However, for PDC cured at 60 °C, it is not the pore structure but the lack of available aluminate caused by the formation of hydrotalcite contributes to its sound sulfate resistance. The higher the dosage of dolomite from 10 to 30%, the larger amounts of hydrotalcite formation and the better the sulfate resistance. Hydrotalcite is stable in the presence of sulfate.

Diagenetic evolution and associated dolomitization events in the middle Jurassic Samana Suk Formation, Lesser Himalayan Hill Ranges, NW Pakistan

The Jurassic carbonates of the Samana Suk Formation are extensively exposed in the foreland areas of Himalayas and form major reservoir of the upper Indus basin. These carbonates are composed of oolitic, pelitic fossiliferous and micritic limestone units which have been extensively modified by diagenetic alterations, particularly dolomitization. Field observations show two distinct types of dolostone geobodies (i) bedding parallel stratiform, and (ii) patchy dolostone units respectively. Bedding parallel stratiform dolostones are present in the basal part of the formation, while patchy dolostones are present at the middle and upper parts of the Samana Suk Formation. The dolomitization intensity of both geobodies increases from NW to SE in the study area. Petrographic studies reveal six phases of dolomites and three phases of calcites based on texture, crystal size and morphology. These phases are: matrix replacive dolomites (MD-I to MD-III); cementing dolomites include, replacive cementing dolomite (RD), saddle cementing dolomite (SD) and late stage cementing dolomites (CD); and calcite phases include CC-I and CC-II. XRD analyses reveal that stratiform matrix dolomites (MD-I, MD-II) are stoichiometric (51.08–51.86 mol percent of CaCO3) and contain up to 95% of the mineral dolomite. The patchy dolomite cement is non-stoichiometric (33.39–55.08 mol% of CaCO3) and contains around 65% of the mineral dolomite, whereas saddle dolomites is also non-stoichiometric (51.57 to 53.50 mol% of CaCO3) in origin. Stable isotope studies reveal non-depleted δ18O and δ13C values of matrix dolomites (MD-I, MD-II) represents coeval sea-water signatures of Jurassic carbonates, hence may have been formed by evaporative process. Dolomite cements (RD, SD) shows depleted δ18O values which represent elevated temperature, related to hydrothermal fluid source for their formation. The fracture filling calcite (CC-II) exhibits less depleted values indicative of meteoric fluids affected by shallow to moderate burial. The dedolomites shows depleted δ13C values suggests their formation from the meteoric water. Field, petrographic and geochemical studies suggest that diagenetic evolution of the Samana Suk Formation is the multistage process. In the first phase, marine diagenetic processes including marine cementation, stratiform dolomitization may have formed due to surface processes of marine water in peritidal to intertidal settings, while the second phase of diagenesis is due to burial associated processes which includes hydrothermal dolomitization occurred due to movement of magnesium rich fluids along weak planes such as fractures, faults, bedding planes and stylolites. Last stage includes formation of hydrothermal fracture filling calcites, replacive pyrites and dedolomites due to the uplift related processes.

Dating and characterizing primary gas accumulation in Precambrian dolomite reservoirs, Central Sichuan Basin, China: Insights from pyrobitumen Re-Os and dolomite U-Pb geochronology

Primary gas accumulation with trillions of cubic meters of gas volumes was discovered recently in the Precambrian Dengying Formation of the Gaoshiti-Moxi block, Central Sichuan Basin, China. In this study we investigated the pervasive saddle dolomite cements and solid bitumens within karst vugs in the Dengying dolomite reservoirs. Using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in situ U-Pb dating, we obtained a high-precision absolute age of 259.4 ± 3.0 Ma for the saddle dolomite cementation, consistent in timing with the eruption of Emeishan flood basalts (262–251 Ma). Combined with high homogenization temperatures (Th) and salinities of primary fluid inclusion, as well as rare earth element (REE) compositions, we consider that the saddle dolomite precipitation was likely related to hydrothermal activity driven by the Emeishan mantle plume.The identification of rare surviving oil inclusions and extensive solid bitumens within the dolomite vugs is indicative of the presence of paleo-oil reservoirs. These bitumens are characterized by high equivalent vitrinite reflectance (Requ), no fluorescence, and deformed fibrous textures, typical of highly mature pyrobitumens. Gas geochemistry data indicates that only minor thermochemical sulfate reduction (TSR) has occurred. Thus the pyrobitumen was interpreted to result mainly from the thermal cracking of paleo-oils, and its associated Re-Os isochron age of 154 ± 21 Ma, representing the timing of dry gas formation. Petrographical evidence suggests that the paleo-oil charge must have taken place after sparry dolomite cementation but prior to pyrobitumen generation. This, combined with coeval aqueous inclusion Th data and published thermal-burial models, constrains the time of paleo-oil emplacement to ca. 222–205 Ma, suggesting an oil-filling event related to the Indosinian Orogeny. This study documented the first chronological data constraining the complex evolution history of a petroleum system in one of the oldest primary gas reservoirs in the world.

Stable-isotope and fluid inclusion constraints on the timing of diagenetic events in the dolomitized Dolomia Principale inner platform (Norian, Southern Alps of Italy)

The petrographic, cathodoluminescence (CL), stable isotope, and microthermometry investigation of inner platform facies of the pervasively dolomitized carbonate platform of the Norian (Upper Triassic) Dolomia Principale (Southern Alps, N Italy) identified two major dolomite phases. An early replacement fabric-retentive dolomite (D1 and D2, respectively replacing the sedimentary facies - from fenestral mudstone/wackestone to bioclastic packstone and mudstone and stromatolites- and early marine cements) is followed by a later, vug- and fracture-filling dolomite cement phase (D3, planar-s texture).The fabric-retentive dolomitized carbonates (<4 μm−30 μm, D1) and marine fibrous cements (30 μm−250 μm, D2) exhibit dull luminescence under CL whereas the burial dolomite cement (200 μm−500 μm, D3) exhibits zoned luminescence. Microthermometric measurements of the primary two-phase fluid inclusions in D3 yielded a mean homogenization temperature of 112.4 ± 8.2 °C. Supposing a surface seawater temperature of at least 20–25 °C and a geothermal gradient of about 40 °C/km during the Early Jurassic rifting that led to the opening of the Alpine Tethys, such temperature might be reached at reasonable burial depth ranging from 2.0 km to about 2.5 km. According to the subsidence curve this depth, compatible with the temperature recorded by the fluid inclusions, was reached during the Early Jurassic, with D3 cement precipitation causing a significant reduction of the porosity.The δ13C values of the investigated dolomites (D1, D2, and D3) show a narrow range of variation (2.0–3.1‰ VPDB), pointing toward a fluid-buffered system. Differently, the δ18O values largely varies, ranging from −12.8 to +1.9‰ VPDB. In this range, D3 values cluster distinctively between −6.0 to −12.8‰ VPDB with respect to D1 and D2, which cluster from about −4 to +1.9‰ VPDB. The estimated oxygen isotope composition of the parent fluid of D1 and D2 (~0 to −5‰ VSMOW) suggests possible contributions from marine to slightly evaporated seawater, which is consistent with the arid climate and some basin restrictions suggested by earlier studies, whereas that of D3 (+1 to +5‰ VSMOW) is consistent with burial fluids.The followed integrated approach highlights the possibility to better characterize the occurrence of multiple diagenetic events in time and space, as well as their feedback effects on the porosity evolution of dolomitized carbonate reservoirs, framing the diagenetic events at regional scale.

LA-ICP-MS U-Pb geochronology and clumped isotope constraints on the formation and evolution of an ancient dolomite reservoir: The Middle Permian of northwest Sichuan Basin (SW China)

Recent advances in laser-ablation inductively coupled-plasma mass spectrometry (LA-ICP-MS) in-situ U-Pb radiometric dating and clumped isotope thermometry (Δ47) of carbonate minerals provide potential for refining the fluid flow and diagenetic history of carbonate successions. In this study, the Middle Permian dolomites proximal to the Longmenshan fold and thrust belt in northwest Sichuan Basin, southwest China, were investigated using combined U-Pb geochronology, clumped isotope thermometry, and routine isotopic (δ13C, δ18O and 87Sr/86Sr) and elemental geochemistry, in an attempt to reveal the possible relevance of carbonate diagenesis and porosity evolution to the basin-scale tectonic/fluid flow events in the framework of absolute time. Overall, formation and evolution of these dolomites temporally correlated well with major tectonic episodes of the Longmenshan fold and thrust belt. Contrary to the previously-assumed volcanic-related model, U-Pb dating and Δ47 analyses suggest a mid- to late-Triassic replacive dolomitization event (U-Pb ages of 240 ± 12 Ma to 233.8 ± 6.4 Ma) by a hot (Δ47 temperatures 88– 104 °C) basinal brine, which was likely driven by thrust-related compression of the Longmenshan fold and thrust belt during the Triassic. Replacive dolomitization was immediately followed by cementation of euhedral dolomites (U-Pb age of 228 ± 10 Ma) and blocky calcites (U-Pb ages of 224.8 ± 1.8 Ma to 213.4 ± 3.3 Ma) precipitated from the basinal brine in the same tectonic regime. Afterwards, there was long-term cessation of diagenesis that was coincident with, and thus may well be attributed to, tectono-thermal quiescence during most of the Mesozoic Era and Paleogene. Finally, saddle dolomite cements yield Miocene ages (U-Pb ages of 16.40 ± 0.74 Ma to 12.3 ± 1.2 Ma) and precipitated in pre-existing vugs, representing a hydrothermal fluid flow event (Δ47 temperatures up to 170 °C) during the reactivation of thrusting. In addition, similarities in U-Pb ages and geochemical signatures between host rocks (replacive dolomites) and the vug-lining dolomite cements suggest that most porosity in these dolomite reservoirs was inherited from the precursor limestones, which probably experienced meteoric leaching during late Permian uplift. This study highlights that there are geodynamic controls on fluid flow and consequent diagenetic evolution of carbonates in tectonically active sedimentary basins. It illustrates that burial alone in this setting is not a sufficient driver for diagenetic alteration and porosity reduction. The combination of U-Pb radiometric dating, clumped isotope thermometry and routine geochemical analyses is a useful approach in refining the diagenetic and porosity evolution history of ancient carbonate successions.

The Effect of Mineral Dissolution on the Effective Stress Law for Permeability in a Tight Sandstone

AbstractWe present flow‐through experiments to delineate the processes involved in permeability changes driven by effective stress variations and mineral cement dissolution in porous rocks. CO2‐enriched brine is injected continuously into a tight sandstone under in situ reservoir conditions for 455 hrs. Due to the dolomite cement dissolution, the bulk permeability of the sandstone specimen significantly increases, and two dissolution passages are identified near the fluid inlet by X‐ray CT imaging. Prereaction and postreaction examinations of the effective stress law for permeability suggest that after reaction, the bulk permeability is more sensitive to pore pressure changes and less sensitive to effective stress changes. These observations are corroborated by Scanning Electron Microscopy and X‐ray CT observations. This study deepens our understanding of the effect of mineral dissolution on the effective stress law for permeability, with implications for characterizing subsurface mass and energy transport, particularly during fluid injection/production into/from geologic reservoirs.

Burial dolomitization driven by modified seawater and basal aquifer‐sourced brines: Insights from the Middle and Upper Devonian of the Western Canadian Sedimentary Basin

AbstractDolomitization in the Western Canadian Sedimentary Basin has been extensively researched, producing vast geochemical datasets. This provides a unique opportunity to assess the regional sources and flux of dolomitizing fluids on a larger scale than previous studies. A meta‐analysis was conducted on stable isotope, strontium isotope (87Sr/86Sr), fluid inclusion and lithium‐rich formation water data published over 30 years, with new petrographic, X‐ray diffraction, stable isotope and rare‐earth element (REE+Y) data. The Middle to Upper Devonian Swan Hills Formation, Leduc Formation and Wabamun Group contain replacement dolomite (RD) cross‐cut by stylolites, suggesting replacement dolomitization occurred during shallow burial. Stable isotope, REE+Y and 87Sr/86Sr data indicate RD formed from Devonian seawater, then recrystallized during burial. Apart from the Wabamun Group of the Peace River Arch (PRA), saddle dolomite cement (SDC) is more δ18O(PDB) depleted than RD, and cross‐cuts stylolites, suggesting precipitation during deep burial. SDC 87Sr/86Sr data indicate contributions from 87Sr‐rich basinal brines in the West Shale Basin (WSB) and PRA, and authigenic quartz/albite suggests basinal brines interacted with underlying clastic aquifers before ascending faults into carbonate strata. The absence of quartz/albite within dolomites of the East Shale Basin (ESB) suggests dolomitizing fluids only interacted with carbonate strata. We conclude that replacement dolomitization resulted from connate Devonian seawater circulating through aquifers and faults during shallow burial. SDC precipitated during deep burial from basinal brines sourced from basal carbonates (ESB) and clastic aquifers (WSB, PRA). Lithium‐rich formation waters suggest basinal brines originated as residual evapo‐concentrated Middle Devonian seawater that interacted with basal aquifers and ascended faults during the Antler and Laramide Orogenies. These results corroborate those of previous studies but are verified by new integrated analysis of multiple datasets. New insights emphasize the importance of basal aquifers and residual evapo‐concentrated seawater in dolomitization, which is potentially applicable to other regionally dolomitized basins.

Slope destabilization provoked by dissociation of gas hydrates in the Outer Carpathian basin during the Oligocene: Sedimentological, petrographic, isotopic and biostratigraphic record

Sedimentological, biostratigraphic, petrographic, and stable C and O isotope study was carried out on chaotic complexes hosting carbonate concretions in the uppermost Oligocene successions of the Outer Carpathians. These chaotic complexes reveal a range of sedimentary features indicative of intrabasinal, submarine mass-wasting deposition, including soft-sediment deformation, brecciation, homogenization and fluidization of the parent material. Apart from carbonate concretions, they enclose blocks of rocks having lithologies typical for the underlying Oligocene strata. The concretions exhibit extremely depleted δ13C (by down to −51.3‰) indicating anaerobic oxidation of methane, whereas their δ18O values are strongly enriched relative to coeval brackish basin water indicating dissociation of gas hydrates as the main fluid source. These data and the geotectonic context imply that the chaotic complexes formed as submarine slumps on a N-inclined paleoslope of the basin in consequence to tectonically-induced dissociation of gas hydrates. Hydrates were destabilized within the sediments due to synorogenic uplift, related to northward progradation of the accretionary prism. These slumps and hosted concretions were dated by the combination of calcareous nannoplankton and organic-walled dinoflagellate cysts. The age of concretions indicated by dinoflagellate cyst assemblages always overlaps with that of the surrounding slump matrix determined by both micropaleontological groups. Together with sedimentological and petrographic evidence, this confirms that the concretions formed in situ within the sediment shortly after deposition and prior to slumping. Hydrate dissociation, seepage of methane-charged fluids and the resultant slope destabilization was not an isolated event, but a continuous process occurring diachronously in front of N-prograding accretionary prism during Oligocene, at least from early NP23 to late NP25 biozones.The stable C and O isotope analysis of carbonate concretions was crucial for this research, because the low δ13C and high δ18O values were the only preserved indicators of anaerobic oxidation of methane and dissociation of gas hydrates in the basin, respectively. Because, the concretions are composed of a mixture of dolomite and calcite, a semiautomatic method for measuring C and O isotope composition selectively for calcite and dolomite (Baudrand et al., 2012) was applied, which provided conclusive results. These measurements were combined with detailed petrographic investigations, which enabled assessment of isotopic composition for various carbonate constituents separately, e.g. sedimentary calcite (biogenic and detrital), early and late calcite cements, dolomite cement. They showed that the concretionary microcrystalline calcite was the main cement that precipitated due to hydrate dissociation and subsequent methane oxidation. This approach can provide valuable information when applied to other authigenic carbonates composed of a mixture of dolomite and calcite.

Diagenesis and its controls on reservoir quality of the Tambar oil field, Norwegian North Sea

A study of the diagenetic evolution of the Late Jurassic sandstones of the shallow marine facies in the Tambar oil field, Norwegian North Sea was carried out to understand its controls on reservoir quality. Core samples and a set of wireline logs obtained from three wells were used to carry out petrographic studies including light optics and scanning electron microscopy, well log analysis, fluid inclusions, X-ray diffraction (XRD) for whole rock samples, clay mineral extracts and stable isotopes. The Tambar reservoir sandstones range mainly from siltstone to fine-grained sandstones, exclusively arkoses cemented mainly by microcrystalline quartz, euhedral quartz overgrowth, dolomite cement, illite and chlorite. Early diagenetic/eodigenetic minerals include pyrite, calcite, and microcrystalline quartz and late diagenetic/mesodiagenetic minerals include quartz overgrowth, dolomite, illite and chlorite. The δ18O values of dolomite cement in the Tambar sandstone reservoir range from −11.77‰ to −3.57‰ PDB, while the δ13C values of the dolomite cement are from −5.07‰ to −1.12‰ PDB. Homogenization temperature for fluid inclusions trapped in authigenic quartz fall between 130 °C and 169 °C. The reservoir quality is controlled mainly by early formed grain-coating microcrystalline quartz that precipitated from the dissolution of sponge spicules which significantly inhibits quartz overgrowth precipitation. Oil emplacement do not show significant control on reservoir quality because considerable quartz cementation has taken place before the onset of major oil charge. Only the coarse-grained sands show a correlation between quartz cement volume and water saturation. However, the interpreted temperature for dolomite cementation is significantly higher for water leg-suggested effect of oil emplacement on the late dolomite. Understanding these controls on the reservoir quality will enhance the exploration strategy for the Tambar oil field.

Petrological and geochemical characteristics of the botryoidal dolomite of Dengying Formation in the Yangtze Craton, South China: Constraints on terminal Ediacaran “dolomite seas”

Recent studies suggest that the massive Precambrian dolomite successions were likely the products of specific seawater conditions known as “dolomite seas”. However, whether these so-called “dolomite seas” existed in the terminal Ediacaran ocean has not yet been constrained. The widespread botryoidal dolomite with well-preserved sedimentary fabrics from the Upper Ediacaran Dengying Formation (ca. 551 to 545 Ma), South China can be potentially an ideal target to constrain this issue. Results showed that within the botryoidal dolomite, fibrous dolomite cements are the dominant constituent part, and either grew upward from microbialite layers or occurred in sheet cracks within the microbialites, implying a syn-sedimentary diagenesis. Two different types of fibrous cements are newly redefined: fascicular and radial slow dolomite. Well-preserved chemical and cathodoluminescent zonations and length-slow optical characters are direct evidence for syn-sedimentary primary dolomite precipitation, different with the previous alternative interpretations of mimetic dolomitization of aragonite or high-Mg calcite precursors. In contrast, microbialite matrix consists of finely crystalline dolomicrite and preserved microbial fabrics, reveling an early mimetic dolomitization of aragonitic precursors. The dolomite precipitation and mimetic dolomitization likely occurred in coeval seawater or seawater-type fluids as indicated by comparable δ13C and 87Sr/86Sr data with global changes. In particular, the presence of primary dolomite cement precipitates within botryoidal dolomite can reflect that the specific “dolomite seas”, associated with anoxic or even euxinic shallow water conditions (inferred by redox-sensitive trace elements and REEs) and increased microbial activity, also prevailed in the terminal Ediacaran ocean. However, the occurrence of “dolomite seas” was not one occasional event, but common even if not persistent in the early to terminal Neoproterozoic oceans. This study helps to improve the understanding of “dolomite problem” and ubiquitous distribution of Precambrian dolomite worldwide.

UNRAVELLING THE COMPLEXITY OF THIN (SUB‐SEISMIC) HETEROGENEOUS CARBONATE RESERVOIRS: AN INTEGRATED STUDY OF THE ALBIAN MAUDDUD FORMATION IN THE GREATER BURGAN AREA, KUWAIT

The Albian Mauddud Formation is a prolific reservoir in Kuwait and nearby countries such as Iraq and Iran but has received far less attention than the under‐ and overlying units (the Aptian Shu'aiba and Cenomanian Mishrif Formations). Detailed reservoir characterization studies of the formation are required to support field development and improved / enhanced oil recovery (EOR) programmes. In this study, 26 wells penetrating the Mauddud Formation within the Greater Burgan area of Kuwait (Burgan and neighbouring fields) were investigated, integrating the logging of 910 ft of core with petrographic investigations of 113 stained and impregnated thin sections. In the Greater Burgan area, the Mauddud Formation can be divided into a lower Clastic Member and an upper Carbonate Member which is the main focus of this paper. The primary objective of the study was to present a new characterization of this thin, heterogeneous carbonate reservoir by integrating facies analysis and sequence stratigraphy with a detailed petrographic investigation. A second objective was to identify the relative importance of depositional characteristics and diagenesis on the distribution of reservoir properties.Sandstones in the Clastic Member of the Mauddud Formation were deposited on a delta which passed laterally to the north and east into a carbonate platform. During subsequent regional flooding, increased carbonate production resulted in the development of a larger‐scale carbonate platform covering the entire study area. The Burgan field area was part of the proximal zone of this carbonate platform. A number of depositional environments were identified by integrating core and thin section data. These range from outer platform to mid‐ and inner platform, the latter including both high‐ and low‐energy settings (shoal, shoreline; and lagoonal respectively). Mud‐supported textures characteristic of low‐energy inner‐platform and mid‐ to outer‐platform settings are volumetrically dominant in the Mauddud Carbonate Member.Sequence stratigraphic analysis suggests that the Mauddud Carbonate Member is part of a major regressive phase (or highstand systems tract) of a third‐order sequence, with the regional‐scale K110 MFS positioned close to the transition with the underlying Clastic Member. Two 4th order transgressive – regressive (TR) cycles or sequences, M1 and M2, were identified within the Carbonate Member. The top‐Mauddud surface corresponds to a sequence boundary with long‐lasting subaerial exposure during the latest Albian and is characterized by both micro‐ and macroscopic karst features (calcite dissolution vugs and recrystallization in thin sections; and cavities in cores). This study demonstrates that the Burgan field area experienced significant uplift, with increased differential erosion and/or non‐deposition of the upper M2 TR cycle towards the southwest.Eogenetic marine and meteoric calcite cements partially fill macropores close to the tops of the TR cycles and remaining macropores were occluded by shallow‐burial calcite cements. Cementation by ferroan dolomite cement, which resulted from the increased overburden and associated chemical compaction, has almost entirely occluded residual macropores in the lower part of the Mauddud Carbonate Member, close to the underlying siliciclastic deposits of the Clastic Member. As a consequence, porosity is mostly preserved in the middle part of the Carbonate Member. The predominance of mud‐supported textures in these intervals together with the effects of diagenesis explain the widespread occurrence of microporosity within both micritic matrix and grains. In the Greater Burgan area, meteoric diagenesis associated with subaerial exposure(s) of the top‐Mauddud preserved and/or enhanced micrite microporosity. Plug porosity may reach 35% or more, but permeability does not exceed 100 mD.The Mauddud Carbonate Member in the Greater Burgan area is a thin, microporous reservoir, with a dual porosity component which is linked to karst alteration and not to tectonic fracturing. The distribution of reservoir properties results from a combination of primary depositional characteristics and a multiphase diagenetic overprint. This study will guide the future development of the Mauddud reservoir in the Greater Burgan area. It also demonstrates the importance of an integrated approach to constrain sub‐seismic heterogeneities in carbonate reservoirs.

A comparative study of two Mississippian dolostone reservoirs in the Volga-Ural Basin, Russia

A significant proportion of the remaining oil reserves worldwide is concentrated in dolostone rock bodies. The present paper focuses on Tournaisian and Visean limestones and geologically younger diagenetic dolomite units in two mature oil fields of the Volga-Ural Basin, Russia. In contrast to many dolostone-hosted hydrocarbon reservoirs worldwide, the Tournaisian and Visean dolostones in the Demkinskoye and the Onbiyskoye oil fields plot near the low end of the porosity–permeability range. Replacive and porosity-destructive burial dolomitization has deteriorated the reservoir properties of these units. Dolomitization is related to the ascent of hot (170–215 °C) and saline (22–24 wt%) fluids as based on fluid inclusion data. This temperature range points to an abnormally high temperature background thermal regime of the Volga-Ural basin. The onset of burial dolomitization predates oil charge as documented by the fact that oil-bearing inclusions are only present in the outermost zones of porosity-occluding dolomite cements. Isotope geochemical data, dolomite petrography and fluid inclusion data reveal important similarities between hydrothermal dolostone units in the Demkinskoye and the Onbiyskoye oil fields. Based on these data and the detailed cement stratigraphy presented here, it is argued that dolomite bodies in both oil fields formed as the result of one main, Alpine tectonic event pushing hot and saline dolomitizing brines ahead of ascending hydrocarbons.

Diagenetic evolution of the Eocene ramp carbonates (a paradigm from the Nile Valley, Egypt): Petrographical and geochemical attributes

Carbonate rocks of east Nile Valley in Egypt have been investigated as a paradigm for revealing the diagenetic evolution of the Eocene ramp carbonates. The integration of petrographical, elemental geochemical and stable isotopic data revealed that the studied carbonates have been influenced by a sequence of diagenetic processes which are included in three diagenetic episodes (i.e. eogenesis, mesogenesis and telogenesis) and are related to four diagenetic environments (i.e. marine-phreatic, meteoric-phreatic, burial and uplifting environments). The paragenetic sequence of the studied carbonates commences with the eodiagenetic episode, which includes the marine- and meteoric-phreatic diagenetic environments. The marine-phreatic diagenetic environment comprises micritization, glauconitization, pyritization, marine cementation (fibrous calcite and syntaxial overgrowth rim cements), early dolomitization (formation of fine-crystalline matrix-replacive dolomites) and early stages of mechanical compaction before significant burial. The meteoric-phreatic diagenetic environment includes the neomorphism, dissolution, meteoric cementation (earlier phases of sparry calcite cements) and precipitation of megaquartz in the void spaces of the peritidal carbonates. During the mesogenesis episode, the studied carbonates were exposed to burial diagenesis. The shallow burial diagenesis resulted in the creation of point-tangenetial contacts between the allochemical constituents and the precipitation of a medium-coarse crystalline dolomite cement filling the dissolved fossils and dissolution pores. The deep burial diagenetic processes embody the chemical compaction (condensed-fitted interpeneteration and pressure-dissolution fabrics), fracturing and burial cementation. The telodiagenesis represents the last episode in the diagenetic history of the studied carbonates. It witnessed the fracturing, meteoric-vadose cementation and hematitization processes.

Dolomite cements in Cenomanian continental sand deposits: Time evolution and significance (Zoovch Ovoo U-deposit, East Gobi Basin, Mongolia)

Shallow buried unconsolidated sands generally provide very little information about diagenesis as most detrital minerals remain unchanged. However, in rare cases carbonate cemented nodules or sandstone layers may occur inside unconsolidated series. These cements could help to reconstruct the chronology of events from early to late phase stages of diagenesis. The Late Cretaceous sequence of the Zoovch Ovoo depocenter in East Gobi Basin is represented by 600 m of clastic deposits. The 60–80 m of Cenomanian unconsolidated sands and clays, deposited in alluvial-deltaic to lacustrine settings, compose the upper part of the post-riftmedium-grained siliciclastic reservoir, the Sainshand Formation which hosts uranium roll-front systems. Dolomite cemented sandstone layers with 10–20 cm thickness occur among the unconsolidated rock facies. Calcite is absent from this formation, but is present only in the overlying, also outcropping, Bayanshiree Formation. Samples from the dolomite cemented sandstone layers were investigated in detail to uncover their origin and diagenetic history. Four dolomite cement types were recognized that indicate recrystallization episodes and were classified based on the size and shape of the crystals, namely: (i)microcrystalline dolomite frequently associated with siderite, (ii) euhedral dolomite also associated with siderite, (iii) subhedral dolomite and (iv) finally anhedral dolomite. Their REE content varies significantly from dolomite I to IV, in particular by a strong depletion in LREE about 30 times. The contrasted precipitation of calcite in Bayanshiree and dolomite cements in Sainshand formations could be attributed to different Mg/Ca ratio of the circulation fluids in the two aquifers. Both carbonates display however rather homogenous oxygen and carbon isotopic compositions for δ18O (−10 ± 1‰ V-PDB) and δ13C (−7 ± 1‰ V-PDB). The δ18O values are interpreted as inherited from typical meteoric waters quite close to present day waters. The δ13C values indicate a mixed source of both organic and mineral carbon. All data taken into account, a full paragenetic succession was constructed. It includes the evolution of dolomite during burial diagenesis and the effects of the oxidizing roll-front uranium rich waters in the system. The latter induces partial dissolution followed by precipitation of a dolomite phase typical of roll-front zones. Carbonate cements can be thus considered as the best and rather unique geochemical indicators for the recognition of a burial history framework to paleofluid circulations and fluid-rock interactions in these intracontinental series of unconsolidated sands.

Rapid formation of gigantic spherical dolomite concretion in marine sediments

Spherical carbonate concretions are present in sedimentary strata of varying geological ages worldwide. Recent studies reveal that calcium carbonate concretions form very rapidly around dead organisms after burial in the seabed. However, the formation mechanism of spherical dolomite concretions in marine sediments, particularly the carbon source and the reason for their spherical shape, are still moderately known. This study aims to elucidate their formation process and diagenetic evolution through the characterization of the structure, mineralogical composition, and geochemistry in and around a concretion. Here, detailed studies were conducted on a gigantic dolomite concretion approximately 170 cm in diameter, which formed in tuffaceous fine sandstone of the Morozaki Group in Chita Peninsula, Japan. The Ca and Mg distributions in and around the concretion show that it rapidly formed by outward diffusion of bicarbonate from the carbon source in its center. The δ13C values ranging from +4.4‰ to +7.5‰ and large volume of dolomite cement indicate that the dolomite concretion formed at shallow depth from methanogenic organic matter decomposition during rapid sedimentation. Heulandite occurred only in the surrounding rock matrix comprising altered volcanic glasses because of high temperature during deep burial up to 2–4 km depth. This gigantic dolomite concretion properly preserves its evolution and changes in the superimposed post-depositional environment. This study shows that gaining a better understanding of spherical dolomite concretions can potentially help reveal the burial process of sediments during early diagenesis.

Application of laser in-situ U-Pb dating to reconstruct the reservoir porosity evolution in the Cambrian Xiaoerbulake Formation, Tarim Basin 激光铀铅同位素定年技术在塔里木盆地肖尔布拉克组储层孔隙演化研究中的应用

The Laser Ablation (LA)Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) technique, co-developed by the CNPC Key Laboratory of Carbonate Reservoir and School of Earth and Environmental Sciences, The University of Queensland, has huge potential to overcome the difficulty in sampling and dating ultra-low concentration U-Pb minerals in ancient marine carbonates that are relatively smaller in diameter in diagenetic fabric, which was untenable by the conventional isotope dilution (ID) methods. The in-situ analysis by LA combined with MC-ICP-MS has been applied to the study on the diagenesis and porosity evolution of the Cambrian Xiaoerbulake reservoir, Tarim Basin.By dating of dolomitic and calcitic cements from filled vugs, matrix pores and fractures in different stages, we suggested that the burial diagenetic history of dolomite reservoir in the Cambrian Xiaoerblake Formation mainly features the filling of primary pores and vugs generated under karstification. The filling of vugs mainly occurred during the early Caledonian, while the filling of matrix pores mainly occurred in the middle Caledonian and Indosinian. Fractures activated during the Caledonian and Indosinian served as the channels for hydrothermal migration, providing the materials to cement the vugs and pores; while the residual vugs, pores and fractures with no filling of dolomite, calcite and siliceous cements were the main reservoir space. The diagenesis and porosity evolution history of dolomite reservoir in the Cambrian Xiaoerbulake Formation, Tarim Basin, was hence established. The understandings mentioned above are highly consistent with the tectonic-burial history and basin thermal history in the study area, which may be effective tools to date the ancient marine carbonate reservoir cements, to reconstruct diagenesis and porosity evolution history, and to evaluate effective pores before hydrocarbon migration in the ancient marine carbonates.

Thermal convection dolomitization induced by the Emeishan Large Igneous Province

Abstract The mechanism of dolomitization has long been debated due to complex sedimentary and diagenetic conditions. Large-scale dolomite bodies were recently discovered in the Permian Cisuralian Qixia Formation (284–273 Ma) in the northwestern Sichuan Basin, providing an opportunity to understand the dolomitization associated with the Emeishan Large Igneous Province (ELIP). Traditionally, the dolomites in the Sichuan Basin have been interpreted as hydrothermal dolomitization because of the hydrothermal impacts from the ELIP. Here, we report new petrographic and geochemical data on the Permian dolomites, which we grouped into two categories (matrix dolomites or Md1–Md3 and cement dolomites or Cd), that support thermal convection dolomitization. The petrographic and geochemical features (δ13C, δ18O, REE, Mn/Sr ratios, 87Sr/86Sr ratios) suggest that Md1 were formed in penecontemporaneous seawater, whereas Md2, Md3, and Cd were formed in fluids resembling Permian seawater during shallow burial stage, of which both dolomitizing fluids are non-hydrothermal. High formation temperature inferred from fluid inclusion is consistent with the low δ18O values of the Md2, Md3 and Cd, suggesting that the ELIP event may have provided the heat for the dolomitization in the Qixia carbonate platform. The formation of the widespread dolomite bodies was facilitated by the rapid convection exchange between warm pore seawater in the Qixia strata of the platform margin and cold water from outside the slope. Therefore, we demonstrate that the dolomites in the northwestern Sichuan Basin are mainly due to thermal convection dolomitization in the shallow burial stage. This study provides important implications for the mechanism of dolomite formation linked to abnormal geothermal activity.

Exotic Cu-mineralization in Triassic red beds from Navas de San Juan (Jaén, Spain)

The Navas de San Juan copper deposit is located at the southeast edge of the Iberian massif (Spain). It occurs within Triassic continental red beds that represent an undeformed, horizontal to sub-horizontal sequence, overlying unconformably folded Paleozoic metasedimentary rocks and, locally, granitic rocks. The red beds consist essentially of siliciclastic detrital materials including sandstone, silt, and clay, with some thin layers of micritic limestones and edaphic carbonates. This sequence was formed in a sedimentary framework typical of a low sinuosity fluvial system of braided type. The copper mineralization is hosted by a fine-grained sandstone (arkose) with abundant plant debris and detrital grains cemented by dolomite, which was deposited in a secondary fluvial channel.The copper mineralization includes chrysocolla, azurite and malachite with a stratabound morphology. Chrysocolla occurs unevenly in the sandy levels of the mineralized horizon replacing feldspar grains, coating detrital grains and dolomitic cement, and filling pores and cavities. Copper carbonates appear essentially inside or near the organic matter-rich zones filling cavities and veinlets. They also replace more or less limonitized plant debris preserving the original cellular structure of organic material. Azurite and malachite occur in different layers or zones without any evidence of reaction between them.The δ13C-values of azurite and malachite are indicative of a carbon source derived from the decomposition of C3 plant debris. The δ 18O values of the solutions in equilibrium with copper carbonates are consistent with solutions of meteoric origin. The differences between the values for azurite and malachite would reflect variations of the climatic conditions, probably related to seasonal changes in the hydrological parameters and arid degree of the system.Chrysocolla formation was due to the neutralization of copper solutions during the hydrolysis and replacement of the feldspars, with additional precipitation of chrysocolla in interstitial spaces, cavities and microfractures. Copper carbonates derived from the interaction between the neutralized copper solutions and the CO2 released by the decomposition of organic matter under oxidizing conditions. Based on geological, mineralogical and petrographic criteria, in conjunction with isotopic data, the Navas de San Juan deposit is interpreted to represent an exotic copper mineralization. The Cu-bearing vein mineralizations located in the Variscan basement are inferred as the possible copper source.