Microanalysis of Mg isotope abundances using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS): application to hydrocarbon reservoirs

This study evaluates the potential of conventional nanosecond laser ablation inductively coupled plasma mass spectrometry (ns-LA-ICP-MS) analysis in conjunction with advanced data reduction techniques based on integrated counts per second (ICPS) summation for the in situ microanalysis of the Mg isotopic composition of dolomitized rocks from hydrocarbon reservoirs. Chemical composition of these carbonates is affected by fluid/rock interactions between infiltrating dolomitizing fluid(s) and carbonate host rock(s). Diagenetic fluids can impart a unique chemical composition, causing zonation, each zone potentially having characteristic 25Mg/24Mg and 26Mg/24Mg ratios. Three standard reference materials (SRMs), both matrix-matched and pure, were compared by solution nebulization (SN)- and LA-ICP-MS methods. LA-ICP-QMS methods were then applied to dolomites from the Deep Geologic Repository (DGR) site at Bruce (Tiverton, Ontario) and Western Canada (Alberta and British Columbia). The precision of Mg isotope ratios derived by summation of outlier removed ICPS values increased by 27.6% and 72.3% for δ25MgDSM-3 and 48% and 13.9% for δ26MgDSM-3 on JDo-1 and NIST SRM 980, respectively, over mean ICPS ratios. Both accuracy and precision were approximately 2 times higher for SN over LA, indicating the presence of matrix-related interferences (standard deviation of δ25MgDSM-3 of 3.98 and 7.54 for δ26MgDSM-3 compared to 11.6 and 4.02, respectively, for data, processed the same way on solution). All 3 SRMs were found to be unsuitable for in situ microanalysis due to isotopic and elemental heterogeneity at micrometer scales, as well as multiple interferences on all mass numbers. JDo-1 showed significant loss of precision over SN due to heterogeneity of vacuum impregnated epoxy dolomite pellets (SN \u3e LA STD dev δ25MgDSM-3: 1.88 \u3e 56.16) and (SN \u3e LA STD dev δ26MgDSM-3: 4.10 \u3e 60.39). Natural samples analyzed in situ lacking this significant precision loss is evidence of this effect

Impact of diagenesis on the spatial and temporal distribution of reservoir quality in the Jurassic Arab D and C members, offshore Abu Dhabi oilfield, United Arab Emirates

This study is based on petrographic examination (optical, scanning electron microscope, cathodo-luminescence, backscattered electron imaging, and fluorescence) of 1, 350 thin sections as well as isotopic compositions of carbonates (172 carbon and oxygen and 118 strontium isotopes), microprobe analyses, and fluid inclusion microthermometry of cored Jurassic Arab D and C members from 16 wells in a field from offshore Abu Dhabi, United Arab Emirates. The formation was deposited in a ramp with barrier islands and distal slope setting. Petrographic, stable isotopic and fluid-inclusion analyses have unraveled the impact of diagenesis on reservoir quality of Arab D and C within the framework of depositional facies, sequence stratigraphy, and burial history. Diagenetic processes include cementation by grain rim cement and syntaxial calcite overgrowths, formation of moldic porosity by dissolution of allochems, dolomitization and dolomite cementation, cementation by gypsum and anhydrite, and stylolitization. Partial eogenetic calcite and dolomite cementation has prevented porosity loss in grainstones during burial diagenesis. Dolomitization and sulphate cementation of peritidal mud are suggested to have occurred in an evaporative sabkha setting, whereas dolomitization of subtidal packstones and grainstones was driven by seepage reflux of lagoon brines formed during major falls in relative sea level. Recrystallization of dolomite occurred by hot saline waters (Th 85-100\ub0C; and salinity 14-18 wt% NaCl). Anhydrite and gypsum cements (Th 95-105\ub0C; fluid salinity 16-20 wt% NaCl), were subjected to extensive dissolution, presumably caused by thermal sulfate reduction followed by a major phase of oil emplacement. The last cement recorded was a second phase of anhydrite and gypsum (Th 95-120\ub0C; 16-22 wt% NaCl), which fills fractures associated with faults

Diagenetic Origin of Bipyramidal Quartz and Hydrothermal Aragonites within the Upper Triassic Saline Succession of the Iberian Basin: Implications for Interpreting the Burial–Thermal Evolution of the Basin

Within the Upper Triassic successions in the Iberian Basin (Spain), the occurrence of both idiomorphic bipyramidal quartz crystals as well as pseudohexagonal aragonite crystals are related to mudstone and evaporite bearing sequences. Bipyramidal-euhedral quartz crystals occur commonly at widespread locations and similar idiomorphic crystals have been described in other formations and ages from Europe, America, Pakistan, and Africa. Similarly, pseudohexagonal aragonite crystals are located at three main sites in the Iberian Range and are common constituents of deposits of this age in France, Italy, and Morocco. This study presents a detailed description of the geochemical and mineralogical characteristics of the bipyramidal quartz crystals to decipher their time of formation in relation to the diagenetic evolution of the sedimentary succession in which they formed. Petrographic and scanning electron microscopy (SEM) analyses permit the separation of an inner part of quartz crystals with abundant anhydrite and organic-rich inclusions. This inner part resulted from near-surface recrystallization (silicification) of an anhydrite nodule, at temperatures that were <40 °C. Raman spectra reveal the existence of moganite and polyhalite, which reinforces the evaporitic character of the original depositional environment. The external zone of the quartz contains no anhydrite or organic inclusions and no signs of evaporites in the Raman spectra, being interpreted as quartz overgrowths formed during burial, at temperatures between 80 to 90 °C. Meanwhile, the aragonite that appears in the same Keuper deposits was precipitated during the Callovian, resulting from the mixing of hydrothermal fluids with infiltrated waters of marine origin, at temperatures ranging between 160 and 260 °C based on fluids inclusion analyses. Although both pseudohexagonal aragonite crystals and bipyramidal quartz appear within the same succession, they formed at different phases of the diagenetic and tectonic evolution of the basin: bipyramidal quartz crystals formed in eo-to mesodiagenetic environments during a rifting period at Upper Triassic times, while aragonite formed 40 Ma later as a result of hydrothermal fluids circulating through normal faults

Diagenetic history and reservoir properties of the Cenomanian-Turonian carbonates in southwestern Iran and the Persian Gulf

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.marpetgeo.2017.06.035 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/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.Funding for the project was provided by StatoilHydroNatural Science and Engineering Research Council of Canada (NSERC

Discriminating cool-water from warm-water carbonates and their diagenetic environments using element geochemistry: the Oligocene Tikorangi Formation (Taranaki Basin) and the dolomite effect

Fields portrayed within bivariate element plots have been used to distinguish between carbonates formed in warm- (tropical) water and cool- (temperate) water depositional settings. Here, element concentrations (Ca, Mg, Sr, Na, Fe, and Mn) have been determined for the carbonate fraction of bulk samples from the late Oligocene Tikorangi Formation, a subsurface, mixed dolomite-calcite, cool-water limestone sequence in Taranaki Basin, New Zealand. While the occurrence of dolomite is rare in New Zealand Cenozoic carbonates, and in cool-water carbonates more generally, the dolomite in the Tikorangi carbonates is shown to have a dramatic effect on the "traditional" positioning of cool-water limestone fields within bivariate element plots. Rare undolomitised, wholly calcitic carbonate samples in the Tikorangi Formation have the following average composition: Mg 2800 ppm; Ca 319 100 ppm; Na 800 ppm; Fe 6300 ppm; Sr 2400 ppm; and Mn 300 ppm. Tikorangi Formation dolomite-rich samples (>15% dolomite) have average values of: Mg 53 400 ppm; Ca 290 400 ppm; Na 4700 ppm; Fe 28 100 ppm; Sr 5400 ppm; and Mn 500 ppm. Element-element plots for dolomite-bearing samples show elevated Mg, Na, and Sr values compared with most other low-Mg calcite New Zealand Cenozoic limestones. The increased trace element contents are directly attributable to the trace element-enriched nature of the burial-derived dolomites, termed here the "dolomite effect". Fe levels in the Tikorangi Formation carbonates far exceed both modern and ancient cool-water and warm-water analogues, while Sr values are also higher than those in modern Tasmanian cool-water carbonates, and approach modern Bahaman warm-water carbonate values. Trace element data used in conjunction with more traditional petrographic data have aided in the diagenetic interpretation of the carbonate-dominated Tikorangi sequence. The geochemical results have been particularly useful for providing more definitive evidence for deep burial dolomitisation of the deposits under the influence of marine-modified pore fluids

Palaeoproterozoic magnesite: lithological and isotopic evidence for playa/sabkha environments

Magnesite forms a series of 1- to 15-m-thick beds within the approximate to2.0 Ga (Palaeoproterozoic) Tulomozerskaya Formation, NW Fennoscandian Shield, Russia. Drillcore material together with natural exposures reveal that the 680-m-thick formation is composed of a stromatolite-dolomite-'red bed' sequence formed in a complex combination of shallow-marine and non-marine, evaporitic environments. Dolomite-collapse breccia, stromatolitic and micritic dolostones and sparry allochemical dolostones are the principal rocks hosting the magnesite beds. All dolomite lithologies are marked by delta C-13 values from +7.1 parts per thousand to +11.6 parts per thousand (V-PDB) and delta O-18 ranging from 17.4 parts per thousand to 26.3 parts per thousand (V-SMOW). Magnesite occurs in different forms: finely laminated micritic; stromatolitic magnesite; and structureless micritic, crystalline and coarsely crystalline magnesite. All varieties exhibit anomalously high delta C-13 values ranging from +9.0 parts per thousand to +11.6 parts per thousand and delta O-18 values of 20.0-25.7 parts per thousand. Laminated and structureless micritic magnesite forms as a secondary phase replacing dolomite during early diagenesis, and replaced dolomite before the major phase of burial. Crystalline and coarsely crystalline magnesite replacing micritic magnesite formed late in the diagenetic/metamorphic history. Magnesite apparently precipitated from sea water-derived brine, diluted by meteoric fluids. Magnesitization was accomplished under evaporitic conditions (sabkha to playa lake environment) proposed to be similar to the Coorong or Lake Walyungup coastal playa magnesite. Magnesite and host dolostones formed in evaporative and partly restricted environments; consequently, extremely high delta C-13 values reflect a combined contribution from both global and local carbon reservoirs. A C- 13-rich global carbon reservoir (delta C-13 at around +5 parts per thousand) is related to the perturbation of the carbon cycle at 2.0 Ga, whereas the local enhancement in C-13 (up to +12 parts per thousand) is associated with evaporative and restricted environments with high bioproductivity

Estructuras discontinuas y migración de fluidos durante la evolución tectónica paleógena-neógena de los materiales del Jurásico Superior de la Cuenca del Maestrazgo: Estudio estructural, mineralógico y geoquímico

In the western part of the Maestrat Basin (Iberian Range) dolomite and calclte hydrothermal cements fill fractures and "chimney" structures In Tithonian-Berriasian limestones. The timing of fracture cementation is considered in the context of the structural history of the Alpine Orogeny. Three generations of structures filled with carbonate cements were recognized: A) Fractures formed during the Late Eocene- Miocene compressional stage were filled with calcite cement (-8.8%o d180 and +0.8 d13C VPDB) from a fluid of uncertain origin. B) Extensional fractures and C) "pipe- haped" structures developed during the Miocene-Pliocene extensional phase, are filled with four generations of carbonate cements: i) rhombic dolomites; 2) saddle dolomite characterized by high salinity (21.5 to 23.5 % wt. eq. NaCI), high temperatures (Th 110-155BC) and isotopically light d180 ratios (-11.5 to -11.3%o VPDB); 3) calcite replacing (dedolomite) saddle and rhombic dolomite with variable d180 and d13C ratios (-12.2 to -6.8%o d180; -4.4 to +0.2%o d13C), and 4) calcite cement (110-125gC and 160-2609C), with depleted oxygen isotope ratios (-13.4 t o -10 %o d180) and positive carbon ratios (+0.1 to +1.8 %o d13C). These data indicate an hydrothermal, saline and dolomitizing fluid linked to the lixiviation of the Triassic-Liassic evaporites, followed by an input of meteoric water (dedolomitization), which evolved to hydrothermal calcite

Multiphase carbonate cementation related to fractures in the Upper Jurassic limestones, Maestrat Basin (Iberian Range, Spain)

In the western part of the Penyagolosa subbasin (Maestrat Basin, Spain), carbonate cementation occludes fractures and infills stylolites in Tithonian-Berriasian limestones. Field relationships, petrography, cathodoluminesence and geochemical analyses (microprobe, fluid inclusions, oxygen, carbon and strontium isotopes) of the carbonate cements show that the paragenetic sequence includes (A) calcite cements in echelon tension gashes (- 11.37 ‰ δ18O VPDB). 03) Scarce isolated rhombic dolomite replacement cement. (C) Saddle dolomite replacement cement with fluid inclusions that are characterized by high salinity (21.5 to 23.5% wt. eq. NaC1), high temperatures (Th 110-155 °C) and similar negative values of oxygen isotopes (- 11.27 ‰ δ18 O VPDB). (D) Calcite replacing (dedolomite) saddle and rhombic dolomite (- 8.61 to - 6.76 ‰ δ18 O VPDB and -4.38 to + 0.07 ‰ δ13C VPDB). (E) Calcite cement filling vertical fractures. They have the highest Th (160-260 °C), negative values of oxygen isotopes (- 9.97 to -13.44 ‰ δ18 O VPDB). (F) Calcite cement filling bed-parallel stylolites ( - 8.81‰ δ18 O VPDB). This paragenetic sequence reflects multiple phases of fracture-controlled carbonate cements. The first stage calcite is related to syn-sedimentary rifting of the Late Jurassic-Early Cretaceous and progressive burial depth. The later phases of dolomite and calcite in vertical veins are considered hydrothermal in origin and indicate a mix of saline waters, possibly derived from the underlying Triassic and Liassic evaporites, with deep circulating meteoric water with higher temperature than the surrounding rocks and related to the regional Alpine compression

Carbonate-cemented stylolites and fractures in the Upper Jurassic limestones of the Eastern Iberian Range, Spain: A record of palaeofluids composition and thermal history

Dolomite and calcite cements fill open stylolites, fractures and bpipe-shapedQ 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-utting 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 (δ18O VPDB-8.8x and δ13C VPDB+0.8). B) The second generation represented by subvertical extensional fractures and the third generation by bpipe-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 (Th 110–155 ºC) and low δ18O values VPDB (-11.5‰ to - 11.3‰ ); 3) calcitized saddle and rhombic dolomite with variable d18O VPDB -12.2‰ to -6.8‰ and δ13C VPDB -4.4‰ to +0.2‰ , and 4) two phases of calcite cements with moderate to high temperatures (Th 110–125 ºC, 15 to 19.7 wt.% eq. NaCl and 160–198 ºC, 5.5 to 9.5 wt.% eq. NaCl), low δ18O VPDB values (-13.4‰ to -10‰ ) and positive carbon values (δ13C VPDB+0.1x to +1.8x). 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 bpipe-shapedQ 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 8C 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