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Diagenetic history of calcite fractures in Vaca Muerta shales (Argentina) inferred from paired Δ47 and fluid inclusion geothermometry
... Δ 47-CDES25 = 0.655 ‰ for TV04 and Δ 47-CDES25 = 0.405 ‰ for Carrara (Mangenot et al., 2024). The error related to the standards was used (± 0.014 ‰, 1σ) for samples without 3 replicates. ...
Understanding fault activity over time provides valuable insights for reconstructing the tectonic history of an orogen, assessing seismological risks and understanding mineralization processes. In the Western Alps, one of the main controversies in existing tectonic models is the understanding of syn-orogenic extension. Seismological evidence shows widespread extensional deformation related to the reactivation of major lithospheric structures, such as the Penninic Frontal Thrust (PFT). However, the onset age and origin of extension are still debated due to the lack of suitable geochronological data. Fault hematite and calcite geochronology as well as clumped isotope data can be used to relate fluid regimes to fault activity. The analysis of calcite brecciae from extensional faults above the PFT shows that two distinct fluid regimes were present. The first regime, occurring before 2 Ma is associated with upwelling of deep fluids and is recorded by fault calcite at a temperature > 110°C. The second fluid regime is characterized by a meteoric signature and temperatures around 36 °C, representing crystallization since 2 Ma. This study presents a new model for the Miocene tectonic history of the Western Alps that combines (U-Th)/He and U-Pb geochronology on fault hematite (13.3 ± 0.8 to < 0.8 Ma) and calcite (5.3 ± 0.6 Ma). Results demonstrate a progression of extensional fault activity from east to west, from the Middle Miocene (ca. 13 Ma) to the Quaternary. The onset of extension in the inner part of the belt coincides with the development of the fold and thrust belt in the western Alpine foreland. Our new model proposes that extension occurs in the hanging wall of a large top-to-the-west thrust, known as the Alpine Frontal Thrust. This thrust, located to the west of the External Crystalline Massifs gives rise to their uplifting and extension at the rear.
... The lower section of the Vaca Muerta Formation is cut by a system of antitaxial bed-parallel fibrous veins (beef), which are generally constituted by a dark inner zone with hydrocarbon inclusions and bedperpendicular calcite fibres, and an outer zone formed of white bedoblique fibres that indicate shear stresses (Rodrigues et al., 2009;Cobbold et al., 2013;Zanella et al., 2015;Weger et al., 2019). Integrating Pressure-Volume-Temperature modelling, fluid inclusions and clumped isotopes thermometry, Mangenot et al. (2024) concluded that beef formed under fluid overpressure conditions caused by hydrocarbon generation, fluid expansion and hydraulic fracturing processes, which occurred before reaching the maximum burial and thermal conditions of the Vaca Muerta Formation. The inner part of calcite Beef from Cerro Mocho and La Yesera anticlines along the Andean fold belt front grew during the Late Cretaceous (Aptian to Campanian) as determined by U-Pb ages from 117 to 79 Ma (Cruset et al., 2021). ...
We present a geochronological and structural study of the sedimentary architecture of the Vaca Muerta-Quintuco System and Mulichinco Formation in Sierra de Reyes and La Yesera anticlines along the Andean front within the Neuquén Basin. Remote sensing mapping coupled with fieldwork has allowed us to analyse an Auquilco-related secondary detachment anticline in the Sierra de Reyes that exhibits growth strata within the Vaca Muerta-Quintuco and Mulichinco formations, with a decrease in growth rate at the top of the uppermost formation.
Numerical ages based on 87Sr/86Sr ratios of calcite shells indicate that anticline growth occurred from Late Jurassic to Early Cretaceous between 148.65 and 132.5 Ma. These ages also allowed us to correlate the Vaca Muerta-Quintuco System in Sierra de Reyes with a published NW-SE seismic transect that traverses the entire Neuquén Basin and extends to the Andean front within the study area. The correlation reveals a coeval period of compression during the growth of the Sierra de Reyes, Loma la Lata and Río Neuqu´en highs (antiforms), constraining a regional period of intraplate contraction during the latest Jurassic and Early Cretaceous in the Neuquén Basin.
We combine structural analysis of fractures with 22 U–Pb dates measured in fracture-filling carbonate cements from bed-parallel fibrous calcite veins (beef), conjugated veins and faults within the Vaca Muerta Formation along the Andean fold and thrust belt in the Neuquén Basin. The measured ages constrain accurately the relationships between overpressures caused by hydrocarbon generation and Andean compression as mechanisms for natural fracturing and vein formation.
Two generations of fibres have been identified in beef. The first one, consists of dark fibres from the inner zones, which are perpendicular to bedding and contain abundant cone-in-cone structures and hydrocarbon inclusions. U–Pb dating of these fibres yielded Early to Late Cretaceous ages from 116.7 ± 17.7 to 78.8 ± 10.2 Ma. The second generation of fibres corresponds to the outer zones and consists of white fibres oblique to bedding, indicating growth during layer-parallel shortening.
Bed-perpendicular veins cutting beef yielded Late Cretaceous-late Palaeocene dates from 72.8 ± 22.4 to 60.9 ± 10.4 Ma. Eocene ages from 52.0 ± 2.9 to 42.2 ± 18.9 Ma were measured in bed-parallel slip surfaces and reverse and strike-slip faults, whereas Miocene dates from 13.9 ± 2.6 to 6.2 ± 1.1 Ma were measured in E-W calcite veins.
U–Pb dating of veins, structural analysis of fractures and subsidence curves, indicate that beef inner zones formed in the oil window during burial of the Neuquén basin, and that tectonic stresses could enhance their formation. Beef outer zones and bed-perpendicular veins formed during E-W Late Cretaceous-late Palaeocene layer-parallel shortening. Contrarily, late Palaeocene-late Eocene bed-parallel slip surfaces and faults and Miocene E-W veins formed during NE-SW and E-W syn-to post-folding deformation, respectively. In both cases, syn-to post-folding compression occurred synchronously with forelandward migration of magmatic activity attributed to flat subduction of the Pacific slab beneath the Andes.
Assessing the thermal evolution of sedimentary basins over time is a major aspect of modern integrated basin analysis. While the behavior of clay minerals and organic matter with increasing burial is well documented in different geological and thermal settings, these methods are often limited by the temperature ranges over which they can be precisely applied and by the available material. Here, we explore the emergent Δ47 clumped isotope geospeedometry (based on the diffusional redistribution of carbon and oxygen isotopes in the carbonate lattice at elevated temperatures) to refine time‐temperature paths of carbonate rocks during their burial evolution. This study provides a reconstruction of the thermal and exhumation history of the Upper Cretaceous thrust belt series in the western subalpine massifs (Bauges and Bornes, French Alps) by a new approach combining for the first time available data from three independent geothermometers. The investigated area presents two zones affected by contrasting thermal histories. The most external zone has undergone a relatively mild thermal history (T < 70°C) and does not record any significant clay mineral diagenetic transformation. By contrast, the internal zone has experienced tectonic burial (prealpine nappes) in response to thrusting, resulting in overheating (T > 160‐180°C) that induced widespread clay mineral diagenetic transformations (progressive illitization from R0 to R1 and R3 illite‐smectite mixed‐layers), organic matter maturation (oil window) and Δ47 thermal resetting with apparent equilibrium temperatures above 160°C. The three employed geothermal indicators conjointly reveal that the investigated Upper Cretaceous rocks have suffered a wide range of burial temperatures since their deposition, with a thermal maximum locally up to 160‐180°C. High temperatures are associated with the tectonic emplacement of up to 4km of prealpine nappes in the northern part of the studied area. Finally, a forward thermal modeling using Δ47, vitrinite reflectance and clay mineral data, is attempted to precisely refine the burial and exhumation histories of this area.
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The reconstruction of past diagenetic conditions in sedimentary basins is often under-constrained. This results from both the analytical challenge of performing the required analyses on the minute sample amounts available from diagenetic mineral phases and the lack of tracers for some of the diagenetic parameters. The carbonate clumped isotopes thermometry (Δ47) opens new perspectives for unravelling the temperatures of diagenetic phases together with the source of their parent fluids, two parameters that are otherwise impossible to constrain in the absence of exploitable fluid inclusions. Here is reported the study of a large number of sedimentary and diagenetic carbonate phases (from Middle Jurassic reservoirs of the Paris Basin depocentre) by combining detailed petrographic observations with a large number of Δ47 data (n > 45) on a well-documented paragenetic sequence, including calcite and dolomite burial cements. The data reveal carbonate crystallization at temperatures between 29°C and 98°C from fluids with δ¹⁸Owater values between -7‰ and +2‰, in response to the progressive burial and uplift of the Paris Basin, throughout 165 Myr of basin evolution. Coupled to the time-temperature evolution previously estimated from thermal maturity modelling, these temperatures allow determining the timing of four successive cementation episodes. The overall dataset indicates a history of complex water mixing with a significant contribution of hypersaline waters from the Triassic aquifers migrated upward along faults during the Cretaceous subsidence of the basin. Subsequent large-scale infiltrations of meteoric waters induced a dilution of these pre-existing brines in response to the Paris Basin uplift in the Tertiary. Overall, the data presented here allow proposing an integrated approach to characterize the cementation events affecting the studied carbonate reservoir units, based on temperature, oxygen isotope composition and salinity of the parent fluids as well as on petrographic grounds.
Oil and gas have been found in the Upper Ordovician Lianglitag Formation carbonates in the Tahe Oilfield, Tarim Basin, NW China. This study documents the origin of diagenetic fluids by using a combination of petrology, SIMS, fluid inclusion, and radiogenic isotope analysis. Six stages of calcite cements were revealed. C1-C2 formed in marine to early burial environments. C3 has relatively low δ¹⁸OVPDB values (−8.45 ‰ to −6.50 ‰ ) and likely has a meteoric origin. Meteoric water probably fluxed into aquifers during the Early Paleozoic and Late Paleozoic uplift. C4 has δ¹⁸OVPDB values typically 3 ‰ higher than those of C3, and probably formed during shallow burial. C5 displays relatively negative δ¹⁸OVPDB values (−8.26 ‰ to −5.12 ‰ ), and the moderate-to-high fluid-inclusion temperatures imply that it precipitated in burial environments. C6 shows homogenization temperatures (up to 200°C) higher than the maximum burial and much lower salinities (<10.61 wt% NaCl), which may suggest that the fluid was deeply recycled meteoric water. The average ⁸⁷ Sr/ ⁸⁶ Sr ratios of fracture- and vug-filling calcite cements are much higher, indicative of incorporation of radiogenic Sr. Caves and fractures constitute the dominant reservoir spaces. A corresponding diagenesis-related reservoir evolution model was established that favors exploration and prediction.
Natural fractures have long been suspected as a factor in production from shale reservoirs because gas and oil production commonly exceeds the rates expected from low-porosity and low-permeability shale host rock. Many shale outcrops, cores, and image logs contain fractures or fracture traces, and microseismic event patterns associated with hydraulic-fracture stimulation have been ascribed to natural fracture reactivation. Here we review previous work, and present new core and outcrop data from 18 shale plays that reveal common types of shale fractures and their mineralization, orientation, and size patterns. A wide range of shales have a common suite of types and configurations of fractures: those at high angle to bedding, faults, bed-parallel fractures, early compacted fractures, and fractures associated with concretions. These fractures differ markedly in their prevalence and arrangement within each shale play, however, constituting different fracture stratigraphies-differences that depend on interface and mechanical properties governed by depositional, diagenetic, and structural setting. Several mechanisms may act independently or in combination to cause fracture growth, including differential compaction, local and regional stress changes associated with tectonic events, strain accommodation around large structures, catagenesis, and uplift. Fracture systems in shales are heterogeneous; they can enhance or detract from producibility, augment or reduce rock strength and the propensity to interact with hydraulic-fracture stimulation. Burial history and fracture diagenesis influence fracture attributes and may provide more infoimation for fracture prediction than is commonly appreciated. The role of microfractures in production from shale is currently poorly understood yet potentially critical; we identify a need for further work in this field and on the role of natural fractures generally.
Calcite veins at outcrop in the Mesozoic, oil-bearing Wessex Basin, UK, have been studied using field characterization, petrography, fluid inclusions and stable isotopes to help address the extent, timing and spatial and stratigraphic variability of basin-scale fluid flow. The absence of quartz shows that veins formed at low temperature without an influence of hydrothermal fluids. Carbon isotopes suggest that the majority of vein calcite was derived locally from the host rock but up to one quarter of the carbon in the vein calcite came from CO2 from petroleum source rocks. Veins become progressively enriched in source-rock-derived CO2 from the outer margin towards the middle, indicating a growing influence of external CO2. The carbon isotope data suggest large-scale migration of substantial amounts of CO2 around the whole basin. Fluid inclusion salinity data and interpreted water-δ18O data show that meteoric water penetrated deep into the western part of the basin after interacting with halite-rich evaporites in the Triassic section before entering fractured Lower and Middle Jurassic rocks. This large-scale meteoric invasion of the basin probably happened during early Cenozoic uplift. A similar approach was used to reveal that, in the eastern part of the basin close to the area that underwent most uplift, uppermost Jurassic and Cretaceous rocks underwent vein formation in the presence of marine connate water suggesting a closed system. Stratigraphically underlying Upper Jurassic mudstone and Lower Cretaceous sandstone, in the most uplifted part of the basin, contain veins that resulted from intermediate behaviour with input from saline meteoric water and marine connate waters. Thus, while source-rock-derived CO2 seems to have permeated the entire section, water movement has been more restricted. Oil-filled inclusions in vein calcite have been found within dominant E-W trending normal faults, suggesting that these may have facilitated oil migration.
This paper deals about ammonites succession and age of the Jurassic-Cretaceous boundary beds of the Neuquén and Mendoza basin, between southern parallels 35° and 40°, in west central Argentina. Twenty seven stratigraphic sections were investigated, comprising the Vaca Muerta formation and synchronics (Carrín Curá, Picún Leufú, Mulichinco formations, and Mendoza formation with is members Vaca Muerta, Chachao an Cienegüitas). The aes of this formations in certains places of the basin in correspondence with the analysed is established. An ammonites zonation based on former works complemented by an ages key that lets direct transport to the stratigraphic sections is employed. In this way, the evolution of each one of the formational units recognised is fixed, showing that these units basically respond to lithofacies according with is relative position within the basin and in wich its limits ussually cuts obliquely the time line. In the Jurassic-Cretaceous boundary it has not been observed a lithologic change that makes evident the passage from one System to another.
The diffusivity of C and O in calcite in mixed CO-HO fluid was determined over the range in xCO2 from 1.0 to about 0.2 at 700 C, 100 MPa, with selected experiments conducted at pressures to 250 MPa and temperatures of 600 and 800 C. The diffusivity of C, DC, varies little with x{sub CO}, although there is some evidence for a slight increase in DC from 5 x 10¹ to 5 x 10¹ cm²/s with decreasing x{sub CO}. Our data and those of others are consistent with a model for D{sub C} 1/f{sub CO}. Despite the large uncertainty, we observed that the diffusivity of O, D{sub O}, increases from 2 x 10¹ to 5 x 10¹ cm²/s with x{sub CO} decreasing from 1.0 to 0. There is a good correlation at 700 C between log D{sub O} and log f{sub HO} regardless of the total pressure, matching the observations of previous workers. The data are consistent with a simple two-component model for the diffusion of O in calcite, one component for diffusion in the presence of CO and one in the presence of HO: DO = D{sub O}{sup CO} + D{sub O}{sup HO} a{sub HO}. The activity of HO is relative to the fugacity at 100 MPa, 700 C. D{sub O}{sup CO} is 3.45 10¹, and D{sub O}{sup HO} is 3.8 x 10{sup -14 }cm²/s. The data indicate that the rate of diffusion of C and O in calcite is controlled by reactions at the surface of calcite. Adsorption of HO and the creation of vacancies at the surface account for the dependence of the diffusivity on the fugacity of the fluid components. There is little evidence that H itself diffuses into calcite. With this model and the values of D{sub O} in pure CO (Labotka et al. 2000) and in pure HO (Farver 1994), the value of D{sub O} is predicted over the temperature range 600-800 C and p{sub HO} up to 300 MPa, the range of the data. Calculated closure temperatures for diffusive exchange of O between calcite and fluid are reduced by about 150 C in the presence of an aqueous fluid.
The authors present a simplified version of a vitrinite maturation model, complete with sample spreadsheet, based on changes in vitrinite composition with time and temperature. The simplified model, called EASY % Râ, uses an Arrhenius first-order parallel-reaction approach with a distribution of activation energies. EASY % Râ has been calibrated to a more rigorous model of vitrinite maturation based on the chemical properties of coal vitrinite. With EASY % Râ, a profile of vitrinite reflectance vs. time can be obtained for a given stratigraphic level if the time-temperature history for that level has been estimated. When applied to multiple stratigraphic levels, EASY % Râ can be used to compute profiles of the percent of vitrinite reflectance with depth for comparison with borehole data and to optimize thermal history models. EASY % Râ can be used for vitrinite reflectance values of 0.3 to 4.5%, and for heating rates ranging from those in the laboratory (1C/week) to those in slowly subsiding geologic basins (1C/10 m.y.). Examples of model applications range from sedimentary rocks heated by an igneous intrusion to a variety of burial histories. Vitrinite maturation predicted by EASY % Râ is compared to other methods currently being used, such as the Lopatin time-temperature index, level of organic maturity, and other approaches using a single activation energy. Their model successfully estimates vitrinite reflectance due to thermal metamorphism of sedimentary rocks heated by igneous intrusions, geothermal fluids, and burial in a variety of basin settings.
The Andean Fold and Thrust belt between 36° and 39°S can be divided in two sectors. The Eastern Sector corresponds to the Agrio Fold and Thrust Belt (FTB) characterized by a major exhumation during the Late Cretaceous, and minor deformation during the late Eocene and Late Miocene. The Western Sector corresponds to the main cordillera and is characterized by a complex evolution that involves periods of out-of-sequence thrusting with respect to the previously deformed outer sector, and pulses of relaxation of the compressive structure. Cretaceous uplift constituted an orogenic wedge that extended to the inner sectors of the Agrio FTB. Eocene compression was mainly concentrated within the Western Sector but may have reactivated the pre-existing structures of the Agrio FTB, such as the Cordillera del Viento. Late Miocene minor compressional deformation occurred in the retro-arc area and extended into the foreland area. This deformation event produced the closure of a short-lived intra-arc basin (Cura Mallín Basin, 25-15 Ma) at the innermost sector of the FTB. The Pliocene and Quaternary, between 37°30′ and 39°S, have been periods of relaxation of the inner part of the FTB and fossilization of the Agrio Fold and Thrust Belt.
Localization of episodic late Oligocene-Early Miocene and Pliocene to the present extensional structures in the intra- and inner retro-arc is controlled by pre-existing Jurassic half-grabens related to the formation of the Neuquén Basin. The Jurassic rift seems to be controlled by deep crustal-lithospheric discontinuities derived from a Proterozoic-Palaeozoic history of amalgamation in the area, now deeply buried under multiple episodes of Mesozoic-Tertiary synorogenic and synextensional sedimentation.
Salinities of H[sub 2]O-salt inclusions are most often determined by measuring the melting temperature of ice in the inclusion and then referring this value to an equation or table describing the relationship between salinity and freezing-point depression. Generally, data for the system H[sub 2]O-NaCl are used to determine an NaCl-equivalent salinity, owing to lack of information concerning the salts (or other electrolytes) actually contributing to the freezing-point depression. The equation most often used to determine the salinity of H[sub 2]O-salt inclusions from freezing measurements is that of Potter et al (1978), which is based on a regression of data available in the literature at that time. More recently, Hall et al (1988) experimentally redetermined the ice-melting temperatures of H[sub 2]O-NaCl-KCl solutions having compositions ranging from pure water to the ternary eutectic and to each of the two binary (H[sub 2]O-NaCl and H[sub 2]O-KCl) eutectics.
Discusses numerous inclusion geobarometers used in the past and finds that some are wrong in concept and yield grossly inaccurate pressures, others need independent geothermometer to yield an estimate of the pressure. Single inclusions can yield only data that are functions of both P and T, but pairs of inclusions trapped from immiscible fluids can yield both P and T. Inclusions of an immiscible water phase along with a silicate-melt phase are a special example in that the silicate-melt inclusion provides an independent thermometer and hence the pair can provide a geobarometer. -from Authors
Veins of solid bitumen occur in Cretaceous sandstones at the northwest margin of the Junggar Basin, China. The bitumen has a low aromaticity and a composition comparable to gilsonite. The bitumen contains abundant steranes and terpanes, and β-carotane, although most n- and i- alkanes have been removed, which is characteristic of the local crude oil. The sterane and triterpane maturity parameters show that the bitumen, local crude oil, and source rocks are all mature. Bitumen–wallrock relationships suggest that the host sandstone was not completely consolidated at the time of emplacement of the bitumen veins, although bitumen emplacement was a relatively late diagenetic event. The burial history for the northwest Junggar Basin shows that hydrocarbon generation from the assumed upper Permian source rocks commenced in late Triassic/early Jurassic times and suggests that rapid hydrocarbon generation may have resulted in overpressure contributing to the bitumen emplacement.
In the Neuquen Basin of Argentina, 'beef' (bedding-parallel veins of fibrous calcite) is widespread within Late Jurassic black mudstones. A typical vein consists of two grey inner zones and two white outer zones. The inner zones contain inclusions of wall rock and hydrocarbons. Calcite fibres are perpendicular to the margins. In the outer zones, the angle between fibre and margin varies from about 45 degrees at the vein tips to 90 degrees in the centre. Imprints of fossils are offset, proving that the fibres have grown antitaxially. We infer that the veins opened in two phases. During Phase 1, the opening was vertical, against gravity. During Phase 2, the veins resisted tectonic shortening, so that shear stresses acted at the margins. The senses of shear account for the fibre angles. At outcrop, igneous intrusive rocks have cut and metamorphosed the veins. From burial curves, Maturity calculations, growth strata, and ages of igneous intrusions, we estimate that the inner zones of the beef formed in the oil window, during the Aptian to Albian, and that the outer zones formed in the gas window, during the Cenomanian to Campanian. We infer that the beef is evidence for fluid overpressure during hydrocarbon generation.
Carbonate clumped isotopes (Δ47) have become a widely applied method for paleothermometry, with applications spanning many environmental settings over hundreds of millions of years. However, Δ47-based paleothermometry can be complicated by closure temperature-like behavior whereby C–O bonds are reset at elevated diagenetic or metamorphic temperatures, sometimes without obvious mineral alteration. Laboratory studies have constrained this phenomenon by heating well-characterized materials at various temperatures, observing temporal Δ47 evolution, and fitting results to kinetic models with prescribed C–O bond reordering mechanisms. While informative, these models are inflexible regarding the nature of isotope exchange, leading to potential uncertainties when extrapolated to geologic timescales. Here, we instead propose that observed reordering rates arise naturally from random-walk O18 diffusion through the carbonate lattice, and we develop a “disordered” kinetic framework that treats C–O bond reordering as a continuum of first-order processes occurring in parallel at different rates. We show theoretically that all previous models are specific cases of disordered kinetics; thus, our approach reconciles the transient defect/equilibrium defect and paired reaction-diffusion models. We estimate the rate coefficient distributions from published heating experiment data by finding a regularized inverse solution that best fits each Δ47 timeseries without assuming a particular functional form a priori. Resulting distributions are well-approximated as lognormal for all experiments on calcite or dolomite; aragonite experiments require more complex distributions that are consistent with a change in oxygen bonding environment during the transition to calcite. Presuming lognormal rate coefficient distributions and Arrhenius-like temperature dependence yields an underlying activation energy, E, distribution that is Gaussian with a mean value of μE=224.3±27.6 kJ mol⁻¹ and a standard deviation of σE=17.4±0.7 kJ mol⁻¹ (±1σ uncertainty; n=24) for calcite and μE=230.3±47.7 kJ mol⁻¹ and σE=14.8±2.2 kJ mol⁻¹ (n=4) for dolomite. These model results are adaptable to other minerals and may provide a basis for future experiments whereby the nature of carbonate C–O bonds is altered (e.g., by inducing mechanical strain or cation substitution). Finally, we apply our results to geologically relevant heating/cooling histories and suggest that previous models underestimate low-temperature alteration but overestimate Δ47 blocking temperatures.
The P-V-X properties of two-component fluid inclusions (FIs) are generally determined from microthermometry data using appropriate thermodynamic models (i.e., VX diagrams) and/or equations of state (EoS). However, some limitations can hamper the applicability of this technique such as the small size, low density or complex composition of the analyzed FI. Raman spectroscopy is known as the best-suited alternative method to microthermometry for the investigation of natural FIs because it can provide simultaneously non-destructive qualitative and possible quantitative analyses after specific calibrations. The present work aims to provide calibration data to directly determine the P-V-X properties of binary or ternary mixtures of CH4, CO2, and N2. The variation of spectral features as a function of composition and pressure (or density) was investigated by using Raman spectroscopy coupled with an improved High-Pressure Optical Cell (HPOC) system and a customized heating-cooling stage. From our experimental data, the relative Raman scattering cross-section (RRSCS) of CH4 (νCH4∗) was demonstrated to be constant at 7.73 ± 0.16 over the investigated range of pressure (5–600 bars) and for any composition. This parameter can thus be used for the determination of composition with an uncertainty of ~0.5 mol%. Several calibration equations were calculated for different PX domains, linking the Fermi diad splitting of CO2 (Δ) or the relative variation of the CH4 peak position (νCH4∗) to the pressure (or density) and composition of CO2-CH4, CH4-N2, and CO2-N2-CH4 mixtures at 22 and 32 °C. The pressure and density of the fluids can henceforth be directly measured from Raman spectra with an uncertainty of ~20 bars and ~0.01 g·cm⁻³, respectively. Our calibration equations were then validated on natural FIs by comparing the results obtained from Raman and microthermometry. We also interpreted the variation of the peak position of CH4 based on the change of intermolecular interaction. Finally, we discussed the applicability of the obtained calibration data into another laboratory by comparing it with the data of pure CO2 and CH4 published in literature. A small shift between calibration curves implies a systematic error which is perhaps due to the difference in the configuration or the day-to-day deviation of the instruments. Therefore, standards of well-known P-V-X properties should be regularly measured to prevent and to correct any variation or shifting of the instrumental responses.
The term “beef” describes bedding-parallel calcite veins found
commonly in the organic-rich matrix of unconventional resource
plays. Although some authors have interpreted beef to be an early
diagenetic feature, these calcite veins are commonly attributed to
precipitation at high temperatures and localized overpressure
during hydrocarbon generation. The temperature at which the
beef formed is thus crucial to ascertain the process of beef genesis.
We use the novel methodology of clumped isotope analysis to
constrain both the temperature at which beef forms and the
isotopic composition of fluids present during formation.
For this study, we use beef from basinal sections of the Vaca
Muerta Formation in the Neuquén Basin, where veins are commonly
up to approximately 10 cm (~4 in.) thick and are laterally
continuous over 1 km(0.6mi). The calcite veins occur in isolation
or in association with concretions and ash layers. Sequence
stratigraphic boundaries have little influence on distribution, and
only a low correlation between beef and total organic content or
beef and ash layers exists. The internal crystal structure of beef
varies largely, suggesting both syntaxial and antitaxial growth
forms. The 18O values of beef range from approximately -12‰
to -9‰, and the d13C values vary between approximately -1‰
and 1‰. The surrounding mudstone and concretion fracture fills
(calcite) show little difference isotopically when compared to the
beef itself. The ∂ 18O values of nearby concretions range from
approximately -3.5‰to 1‰, and the ∂13C values vary between
approximately 6‰ and 11‰.
Bedding-parallel, fibrous calcite veins (commonly referred to as “beefs”) are widely developed within Eocene, lacustrine, laminated organic-rich source rocks in the Dongying Depression of Bohai Bay Basin, East China. Understanding the formation of this type vein is essential to elucidate the diagenetic evolution and fluid migration in muddy source rocks. This study investigates the mechanism(s) of growth of bedding-parallel veins in hydrocarbon source rocks of the Es4 upper and the Es3 lower units in the Dongying Depression, based on systematic observations of macro- and micro-scale features both in veins and their wall-rocks, combined with X-ray diffraction and analyses of geochemical indicators. The source rocks are heterogeneous in both composition and structure. The beefs mainly occur in laminated rocks consisting of organic-rich laminae, which occur together with horizontally-continuous banded lamalginites and rare carbonate-rich laminae, overall corresponding to high TOC, low carbonate contents, and low maturity (average Ro = 0.57%). Most beefs are embedded in the neighboring lamalginites of organic-rich laminae with low clay-mineral contents. The beefs are products of diagenesis accompanying burial, and both the initial fracture opening and subsequent dilation represent early stages of hydrocarbon generation and expulsion. The presence of bedding-parallel bituminite veins and the median plane (or “parting”) is evidence for horizontally oriented hydrofracturing due to fluid overpressure. In turn, fluid overpressure reflects the generation and primary migration of hydrocarbon-bearing fluids. Hydrocarbon generation (organic matter evolution) and clay-mineral dehydration (illitization of smectite) were responsible for the fluid overpressure. Crystal growth within the beefs was a continuous process with an antitaxial growth direction, and there was no growth competition between adjacent fibres. These observations indicate that the crystal growth and fracture dilation were synchronous and growth space was limited. The force exerted by crystallizing vein fill contributed to the high fluid pressure, suggesting that fibrous vein minerals actively promoted further dilation as the veins grew. Vein dilation accompanied the migration of hydrocarbon-bearing fluids, evinced by the occurrence of primary hydrocarbon inclusions found in the fibrous calcites. Therefore, the hydrocarbon-bearing fluids were present at the time of horizontal fluid flow, from the start of fracture opening to the end of the dilation.
Dolomite clumped isotope compositions are indispensable for determining the temperatures and fluid sources of dolomitizing environments, but can be misleading if they have modified since formation. Carbonate Δ_(47) values are susceptible to resetting by recrystallization during diagenesis, and, even in the absence of dissolution and reprecipitation reactions, alteration by solid-state reordering during prolonged residences at elevated temperatures. In order to understand the potential of dolomite Δ_(47) values to preserve the conditions of dolomitization in ancient sections, the kinetic parameters of solid-state reordering in this phase must be determined. We heated mm-sized crystals of near-stochiometric dolomite in a René-type cold seal apparatus at temperatures between 409 and 717 °C for 0.1 to 450 hours. In order to prevent the decarbonation of dolomite to calcite, periclase, and CO_2 at these conditions, the system was pressurized with CO_2 to 0.45–0.8 kbar. Over the course of 31 temperature-time points and 128 individual Δ_(47) measurements of powdered dolomite crystals from these points, we observed the evolution of dolomite Δ_(47) values from the initial (unheated) composition of the crystals (0.452 ± 0.004‰, corresponding to a formation temperature of ∼145 °C) towards high-temperature equilibrium distributions. Complete re-equilbration occurred in the 563 to 717 °C experiments. As with previous heating experiments using calcite and apatite, dolomite Δ_(47) exhibited complex reordering behavior inadequately described by first-order Arrhenian-style models. Instead, we fit the data using two published models for clumped isotope reordering: the transient defect/equilibrium defect model of Henkes et al. (2014), and the exchange-diffusion model of Stolper and Eiler (2015). For both models, we found optimal reordering parameters by using global least-squares minimization algorithms and estimated uncertainties on these fits with a Monte Carlo scheme that resampled individual Δ_(47) measurements and re-fit the dataset of these new mean values. Because the exact Δ_(47)–T relationship between 250 and 800 °C is uncertain, we repeated these fitting exercises using three published high-temperature Δ_(47)–T calibrations. Regardless of calibration choice, dolomite Δ47 rate constants determined using both models are resolvably slower than those of calcite and apatite, and predict that high-grade dolomite crystals should preserve apparent equilibrium blocking temperatures of between ∼210 and 300 °C during cooling on geologic timescales. Best agreement between model predictions and natural dolomite marbles was found when using the exchange-diffusion model and the ab initio Δ_(63)–T calibration of Schauble et al. (2006), projected into the Δ_(47) reference frame by Bonifacie et al. (2017). Therefore, we recommend modeling dolomite Δ_(47) reordering using the exchange-diffusion model and this parameter set. In simple heating scenarios, the two models disagree. The transient defect/equilibrium defect model suggests that dolomite fabrics resist detectable reordering at ambient temperatures as high as 180 °C for tens of millions of years, while the exchange-diffusion model predicts incipient partial reordering perhaps as low as 150 °C. In either case, barring later recrystallization, dolomite Δ47 values should be faithful recorders of the conditions of dolomitization in sedimentary sections buried no hotter than ∼150 °C for tens of millions of years.
Displacive calcite in the organic-rich, fine-grained carbonates of the "Nordegg Member is characterized by textures, mineralogy and stable carbon isotopic compositions which indicate precipitation in the bacterial sulphate reducing and decarboxylation zones at burial depths less than 1 km. Oxygen isotopic compositions of displacive calcite suggest pore waters had a variable but increasing meteoric influence towards the west/southwest, indicating emergence in that area in the late Early Jurassic (?) to Middle Jurassic, probably in response to terrane accretion along the western margin of ancestral North America. Solid bitumen occurs within displacive calcite zones and also fills vertical and horizontal fractures. These were derived from the "Nordegg Member'. The fracture-filling solid bitumen is more mature than the solid bitumen associated with the diplacive calcite. In situ formation of some of the solid bitumen, from organic-rich rock fragments trapped within the displacive calcite zones, likely occurred during catagenesis. -from Authors
Constraining basin thermal history is a key part of reservoir characterization in carbonate rocks. Conventional palaeothermometric approaches cannot always be used: fluid inclusions may be reset or not present, while δ¹⁸O palaeothermometry requires an assumption on the parent fluid composition. The clumped isotope palaeothermometer, however, is a promising technique for constraining the thermal history of basins. In this study, we test if clumped isotopes record temperatures of recrystallization in deeply-buried dolomitic reservoirs, through comparison with fluid-inclusion data. The studied reservoir is the Cretaceous Pinda Formation, offshore Angola, a deeply-buried dolomitized sedimentary carbonate hydrocarbon reservoir. It provides an ideal test case as samples from industry wells are available over a relatively wide burial depth range of c. 2000-4000 m below seafloor (mbsf) and the constituent dolomites are relatively homogeneous. Across this depth range, fluid-inclusion homogenization temperatures for the Pinda Formation record a range of temperatures from c. 110 to 170°C, increasing with depth. These closely match present-day ambient well temperatures, indicating recent resetting of the fluid inclusions. Clumped isotopes, however, record temperatures significantly (c. 20-60°C) below fluid-inclusion and well temperatures for the seven samples analysed. The deepest five samples (c. 2800-3700 mbsf) record clumped isotope temperatures of around 100-120°C, interpreted to represent a deep burial recrystallization event responsible for a massive (re)dolomitization of the reservoir. The lower clumped isotope temperatures (65 and 82°C) of the shallower (2055 and 2740 mbsf) samples are interpreted to represent physical mixing of two dolomite generations due to incomplete burial recrystallization of an early shallow dolomite. Determination of temperature through clumped isotopes allows calculation of the parent fluid δ¹⁸O values. In the five deepest samples, the fluid δ¹⁸O values of 3.7-6.5‰ cluster around the modern-day porewater composition (‰), suggesting that burial dolomitization occurred in the presence of evolved brine. Mineral δ¹⁸O values of c. -7 to -4.‰ are lower than pristine Cretaceous marine dolomite and are in accordance with burial recrystallization. Clumped isotopes are therefore interpreted to record temperatures corresponding to open-system burial recrystallization events. This study shows that clumped isotopes are a valuable tool in characterizing the thermal history of deeply-buried (> 2000 m) carbonate hydrocarbon reservoirs.
In the Late Jurassic–Early Cretaceous Vaca Muerta Formation, previously unrecognized yet abundant structures constituting a new category of kinematic indicator occur within bed-parallel fibrous calcite veins (BPVs) in shale. Domal shapes result from localized shortening and thickening of BPVs and the intercalation of centimeter-thick, host-rock shale inclusions within fibrous calcite beef, forming thrust fault-bounded pop-up structures. Ellipsoidal and rounded structures show consistent orientations, lineaments of interlayered shale and fibrous calcite, and local centimeter-scale offset thrust faults that at least in some cases cut across the median line of the BPV and indicate E-W shortening. Continuity of crystal fibers shows the domal structures are contemporaneous with BPV formation and help establish timing of fibrous vein growth in the Late Cretaceous, when shortening directions were oriented E-W. Differences in the number of opening stages and the deformational style of the different BPVs indicate they may have opened at different times. The new domal kinematic indicators described in this study are small enough to be captured in core. When present in the subsurface, domal structures can be used to either infer paleostress orientation during the formation of BPVs or to orient core in cases where the paleostress is independently known.
Shales of the Upper Jurassic-lower Cretaceous Vaca Muerta Formation are the main source rock for petroleum in the Neuquén Basin, Argentina and an important unconventional exploration target. Folded Vaca Muerta Formation is well exposed in the Agrio Fold-and-Thrust belt where an arid climate and rapid erosion reveal relatively unweathered shale strata accessible along creek beds at Arroyo Mulichinco and in 10+ m-tall cliffs at Puesto. Widespread within these organic-rich shales are several cm-thick, prominent bed-parallel veins (BPVs) of fibrous calcite (beef) that are cut by multiple sets of vertical calcite lined or filled fractures having apertures unaffected by near-surface stress release. Similar, and probably contemporaneous fractures are present within horizons of interbedded dolomitic rock. Evidence that vertical fractures in BPVs and dolomitic horizons continue into shale beds suggests that in-depth analysis of vertical fractures within BPVs and dolomitic horizons allows fracture set and orientation identification and size population measurements—primarily aperture distributions—that circumvent some of the limitations of shale outcrops. At Arroyo Mulichinco, four main fracture sets are present separable by orientation and crosscutting relations. An E-W set is oldest, followed by successively younger NE-SW, NW-SE, and N-S sets. At Puesto, the E-W and N-S sets are the most prominent and show opposite cross-cutting relationships (E-W set is youngest) indicating a possible episode of younger E-W fractures. The E-W set shows the highest micro-and macrofracture intensity at both localities. The intensity of N-S micro- and macrofractures is similar at both outcrops away from faults, but macrofracture intensity increases closer to faults. While macrofracture abundance is similar in BPVs and in shale, microfractures having apertures smaller than ∼0.1 mm are mostly absent in shale and dolomitic layers but are abundant cutting BPVs. Thus, microfractures are BPV-bounded and only fractures wider than ∼0.05 mm are tall enough to cut into shale. Nevertheless, using size distributions of microfractures in BPVs that are absent in shale accurately predicts the abundance of macrofractures in nearby shale, either because microfractures in organic shale have annealed, or because of only small differences in fracture strain for fractures of different sizes across different rocks types. Microfractures in readily sampled BPVs may be a practical way to diagnose or predict attributes of macrofractures in adjacent shale.
Field, petrographic and geochemical analysis of fibrous calcite veins in the Lower Jurassic Shales-with-Beef Member in the Wessex Basin was conducted to investigate the formation mechanism of the veins. Bedding-parallel fibrous calcite veins, including beef veins and tabular cone-in-cone structures, are widespread in the black shales. The calcite veins consist of subvertical fibrous crystals and a dark median zone. The median zone contains scattered clays, pyrite microcrystals, skeletal fragments and amorphous organic matter. The veins exhibit moderate carbon isotope values, ranging from −1.515 to 2.732‰. The oxygen isotope composition ranges from −8.872 to −4.521‰, which is possibly too negative to reflect the primary porewater oxygen isotope signatures and indicates a porewater modification. It is interpreted that the veins mainly derive carbonates from seawater inorganic carbon and bioclasts. The veins formed as closed-system hydraulic fractures in overpressured cells during sediment degassing in the methanogenic zone. The shale beds with a high total organic carbon content could have generated abundant CO2, which may have resulted in either the cementing of the pores in the matrix or overpressure buildup. The skeletal fragments provide a control on the spatial distribution of veins as nuclei for calcite precipitation from supersaturated pore fluids.
To better understand potential vertical variability of the original kerogen quality in an unconventional reservoir we have carried out organic geochemical analyses on hundreds of rock and fluid samples coming from the Upper Jurassic / Lower Cretaceous Vaca Muerta Formation (Neuquén Basin, Argentina). The studied samples are derived from six wells located in different parts of the Neuquén "embayment" where the Vaca Muerta Formation reached distinct maximum thermal maturities. This natural maturity sequence ranges from the peak oil to dry gas window (VReq ~ 0.7 % to 1.8 %). Applied geochemical analyses comprise high-frequency TOC measurements on a centimeter scale, conventional Rock-Eval analyses on a decimeter scale, and meter by meter molecular and isotopic mud gas analyses. In addition we have performed molecular and isotopic analyses on several hydrocarbon liquid extracts and produced oils. Our data clearly indicate highly heterogeneous vertical distributions of organic matter contents on centimeter scales reflecting the fine shale laminations in the Vaca Muerta Formation. Despite the regional control of thermal maturity on liquid and gaseous hydrocarbon compositions we also observe at well scale some remarkable vertical compositional kerogen and fluid variability. Molecular and isotopic analyses on liquid hydrocarbon extracts document vertical differences in the source organofacies. These vertical organofacies variances are responsible for the distinct compositions/properties of the produced fluids at well scale.
The carbonate clumped isotope (or Δ47) thermometer relies on the temperature dependence of the abundance of ¹³C¹⁸O¹⁶O2²⁻ ion groups within the mineral lattice. This proxy shows tremendous promise to reconstruct sea surface temperatures (SSTs), but requires calibration of the relationship between Δ47 and calcification temperatures. Specifically, it is important to determine whether biologically-driven fractionation (the so-called “vital effect”) overprints Δ47 values, as reported in some biominerals such as the foraminifera and the coccoliths for the carbon and oxygen isotope systems. Despite their abundance in the pelagic environment, coccolithophores have not been comprehensively investigated to test the reliability of coccolith Δ47-infered temperatures. In this study, we cultured three geologically-relevant coccolith species (Emiliania huxleyi, Coccolithus pelagicus, and Calcidiscus leptoporus) at controlled temperatures between 7 and 25 ± 0.2 °C. Other variables such as pCO2, pH, alkalinity, nutrient concentrations, and salinity were kept constant at mean present-day oceanic conditions. Although cultured coccoliths exhibit substantial species-specific oxygen and carbon isotope vital effects, we found that their Δ47 composition follows a statistically indistinguishable relationship with 1/T² for all three species, indicating a lack of interspecific vital effects in coccoliths. Further, the Δ47 composition of coccolith calcite is identical to inorganic calcite precipitated at the same temperature, indicating an overall absence of clumped isotope vital effect in coccolith biominerals. From a paleoceanographic perspective, this study indicates that the Δ47 values of sedimentary coccoliths – even from highly diverse / mixed assemblages – can be analyzed to reconstruct SSTs with confidence, as such temperature estimates are not biased by taxonomic content or changing interspecies vital effects through time.
Thus, in order to standardize the temperature estimates out of different laboratories running high temperature digestion of (Ca,Mg,Fe)CO3 carbonate minerals, we recommend the use of this single Δ47-T calibration to convert Δ47CDES data into accurate and precise temperature estimates. More widely, this study extends the use of the Δ47 thermometry to studies of diagenesis and low-grade metamorphism of carbonates with unprecedented precision on temperature estimates based on Δ47 measurements — environments where many other thermometers are generally empirical or semi-quantitative.
Veins of solid bitumen (asphaltite) have been commercially exploited in the Neuquen basin, Argentina, for over 100 yr. Veins are up to 5 m wide and several kilometers in length, over a region of 15,000 km2. These veins were emplaced in fractures both parallel and at high angles to bedding, in close proximity to their source rocks in the Vaca Muerta and Agrio formations (Late Jurassic-Early Cretaceous). Two or more phases of bitumen emplacement can be recognized in several localities; structures bearing viscous oil are younger than structures having solid bitumen. Bitumen emplacement was vigorous and caused brecciation and spalling of the host rocks. The bitumen was also viscous, and supports rock debris ranging in size from sand grains up to meter-scale slabs. Brecciation, bedding-parallel injection, and wall rock impregnation suggest high fluid pressures during emplacement. High fluid pressure may have been engendered by substantial hydrocarbon generation from rich source rocks in a low-permeability sequence, and probably caused the fractures into which the bitumen migrated. The bedding-parallel veins facilitated decollement during thrusting that took place during and after bitumen emplacement. The timing of emplacement relative to thrusting and oil migration constrains bitumen emplacement to the Eocene-Oligocene.
We present a simplified version of a vitrinite maturation model, complete with sample spreadsheet, based on changes in vitrinite composition with time and temperature. The simplified model, called EASY%R[o], uses an Arrhenius first-order parallel-reaction approach with a distribution of activation energies. EASY%R[o] has been calibrated to a more rigorous model of vitrinite maturation based on the chemical properties of coal vitrinite. With EASY%R[o], a profile of vitrinite reflectance vs. time can be obtained for a given stratigraphic level if the time-temperature history for that level has been estimated. When applied to multiple stratigraphic levels, EASY%R[o] can be used to compute profiles of the percent of vitrinite reflectance with depth for comparison with borehol data and to optimize thermal history models. EASY%R[o] can be used for vitrinite reflectance values of 0.3 to 4.5%, and for heating rates ranging from those in the laboratory (1 degree C/week) to those in slowly subsiding geologic basins (1 degree C/10 m.y.). Examples of model applications range from sedimentary rocks heated by an igneous intrusion to a variety of burial histories. Vitrinite maturation predicted by EASY%Ro is compared to other methods currently being used, such as the Lopatin time-temperature index, level of organic maturity, and other approaches using a single activation energy. Our model successfully estimates vitrinite reflectance due to thermal metamorphism of sedimentary rocks heated by igneous intrusions, geothermal fluids, and burial in a variety of basin setting .
Fossil fluids from petroleum basins are present in diagenetic minerals as fluid inclusions. They are the witnesses of the original oil or gas composition and the memory of the pressure and temperature conditions at the time of fluid emplacement. Brines are always present as non-miscible phase at equilibrium with oils in reservoir. The analytical procedure is based on the individual analysis of both aqueous and petroleum inclusions. Temperature of phase transitions is measured by microthermometry, dissolved methane content of aqueous inclusions is quantified by Raman spectrometry, CH4 and CO2 content of petroleum inclusions is approximated by Fourier transform infrared spectrometry and the volume of petroleum inclusions is reconstructed by confocal scanning laser microscopy. Thermodynamic models are based on Duan and Peng-Robinson equations of state and are applied to aqueous and petroleum inclusions respectively. The intersection of the isochores of the two fluid systems gives the true pressure and temperature conditions of fluid trapping. The knowledge of the fluid pressure allows us to reconstruct the hydrodynamic evolution of reservoirs in various geodynamic regimes.
Antitaxial fibrous calcite veins (2-6 cm thick) outcrop parallel to bedding in the Bluefish Member of the Middle Devonian (Givetian) Hare Indian Formation, Norman Wells area, Northwest Territories, Canada. The Bluefish Member consists of dark brown to black laminated shales with total organic matter content ranging from 1.7-8.0 wt%. The pattern of incorporation of host-shale fragments and tiny inclusions into the fibrous calcite indicates repreated episodes of vein opening and sealing. The formation of finely crystalline non-stoichiometric Ca-rich dolomite disseminated in the shale inclusions occurred subsequent to the emplacement of fibrous calcite veins under elevated burial temperatures. -from Authors
Stable isotope and petrographic analyses of diagenetic calcite veins from British Jurassic and Lower Cretaceous shales have been used to determine the environment of precipitation of fibrous calcites. Successive growth took place by antitaxial displacive addition at the vein margins, away from primary sedimentary laminations or early diagenetic concretions. Carbon and oxygen isotopic ratios indicate a relatively late diagenetic origin for the veins after cessation of bacterial activity and considerable modification of the oxygen isotopic composition of the pore water. Vein growth was discontinuous and took place in waters of changing isotopic and trace-element composition. Changes are not uniderectional; reversals in isotopic trends indicate that precipitation took place during periods of renewed groundwater flow, tapping different sources of bicarbonate-bearing solution.- Author
Carbonate clumped-isotope geothermometry is a tool used to reconstruct formation or (re)equilibration temperatures of carbonate bearing minerals, including carbonate groups substituted into apatite. It is based on the preference for isotopologues with multiple heavy isotopes (for example,13C16O218O2- groups) to be more abundant at equilibrium than would be expected if all isotopes were randomly distributed amongst all carbonate groups. Because this preference is only a function of temperature, excesses of multiply substituted species can be used to calculate formation temperatures without knowledge of the isotopic composition of water from which the mineral precipitated or other phases with which it may have equilibrated. However, the measured temperature could be modified after mineral growth if exchange of isotopes amongst carbonate groups within the mineral has occurred through internal isotope-exchange reactions. Because these exchange reactions occur through thermally activated processes, their rates depend on temperature and increase at higher temperatures. Thus internal isotope-exchange reactions could lead to effective reequilibration at high temperatures, overprinting the original temperatures recorded during mineral growth. We measured clumped-isotope temperatures in carbonate bearing minerals (including apatites) from several carbonatites to constrain the kinetics of these internal isotope-exchange reactions. We observe two key trends for clumped-isotope temperatures in carbonatites: (i) clumped-isotope temperatures of apatites and carbonate-bearing minerals decrease with increasing intrusion depth and (ii) apatites record lower clumped-isotope temperatures than carbonate minerals from the same intrusion. We additionally conducted heating experiments at different temperatures to derive the temperature dependence for the rate constants that describe the alteration of clumped-isotope temperatures with time in calcites and apatites. We find that calcites exhibit complex kinetics as has been seen in previous studies. To quantify these results, we constructed a model that incorporates both diffusion of isotopes through the crystal lattice and isotope-exchange reactions between adjacent carbonate groups. We tested this model through comparison to previous measurements of optical calcites and brachiopods and to samples with known cooling histories and find that the model is able to reasonably capture kinetic data from previous experiments and the observed clumped-isotope temperatures of calcites assuming geologically reasonable cooling rates. A similar model for apatite over-predicts the observed clumped-isotope temperatures found in natural samples; we hypothesize this discrepancy is the result of annealing of radiation damage in our experiments, which lowers the diffusivity and rate of isotope exchange of carbonate groups compared to damaged natural samples. Finally, we constructed models to explore how heating can alter recorded clumped-isotope temperatures. Our model predicts that samples change in clumped-isotope temperatures in two stages. The first stage changes the recorded clumped isotope temperatures by <1 °C if held at 75 °C for 100 million years and by up to ∼40 °C if held at 120 °C, but the clumped-isotope temperatures does not reach ambient values through this low-temperature mechanism. A second, slower change becomes effective at temperatures above 150 °C and can take the measured clumped-isotope temperature up to the true ambient temperature. This result implies that old (hundreds of million years) samples that have only experienced mild (<100-125 °C) thermal histories could exhibit small but measurable (order 10 °C) changes in their clumped-isotope temperatures. We compared this heating model to clumped-isotope measurements from paleosol samples from the Siwalik Basin in Nepal, which were buried up to 5 km and then rapidly exhumed to the surface; these samples often do not give reasonable surface temperatures. The modeled temperatures agree with measured temperatures of these samples, suggesting that partial re-equilibration during shallow crustal burial is responsible for their elevated clumpedisotope temperatures.
Bedding-parallel fibrous veins are common worldwide in sedimentary basins, especially within strata of low permeability. The term “beef” refers to bedding-parallel veins of fibrous minerals, where the fibres are mutually parallel and have formed quasi-vertically. More complex on a smaller scale are “cone-in-cone” structures, yet these are also common within bedding-parallel veins. For both beef and cone-in-cone we have compiled a worldwide catalogue (157 localities). Typically, the veins consist mainly of white gangue minerals (for example, calcite, gypsum, or quartz), but may also contain accessory minerals of economic interest (for example, bitumen, sulphides, emerald, pitchblende or gold). Fluid inclusions may contain oil or gas.
Normally pressured reservoirs have pore pressures which are the same as a continuous column of static water from the surface. Abnormal pressures occur where the pore pressures are significantly greater than normal (overpressure) or less than normal (underpressure). Overpressured sediments are found in the subsurface of both young basins from about 1.0 to 2.0 km downwards, and in older basins, in thick sections of fine-grained sediments. The main mechanisms considered responsible for most over-pressure conditions can be grouped into three broad categories, based on the processes involved: (1) ineffective volume reduction due to imposed stress (vertical loading during burial, lateral tectonic processes) leading to disequilibrium compaction, (2) volume expansion, including porosity increases due to changes in the solid:liquid ratios of the rock, and (3) hydraulic head and hydrocarbon buoyancy. The principal mechanisms which result in large magnitude overpressure are disequilibrium compaction and fluid volume expansion during gas generation. Disequilibrium compaction results from rapid burial (high sedimentation rates) of low-permeability rocks such as shales, and is characterized on pressure vs. depth plots by a fluid retention depth where overpressure commences, and increases downwards along a gradient which can closely follow the lithostatic (overburden) gradient. Disequilibrium compaction is typical in basins with a high sedimentation rate, including Tertiary deltas and some intracratonic basins. In older basins, disequilibrium compaction generated earlier in the basin history may be preserved only in thick, fine-grained sequences, but lost by vertical/lateral leakage from rocks with relatively high permeabilities. Gas generation from secondary maturation reactions, and oil cracking in the deeper parts of sedimentary basins, can result in large fluid volume increases, although the magnitudes are uncertain. In addition, the effect of increased pressures on the reactions involved is unknown. We doubt that any of the other mechanisms involving volume change can contribute significant regional overpressure, except in very unusual conditions. Hydraulic head and hydrocarbon buoyancy are mechanisms whose contributions are generally small; however, they can be easily assessed and may be important when additive to other mechanisms. The effects of transference of overpressure generated elsewhere should always be considered, since the present pressure distribution will be strongly affected by the ability of fluids to move along lateral and vertical conduits. Naturally underpressured reservoirs (as opposed to underpressure during depletion) have not been as widely recognized, being restricted mainly to interior basins which have undergone uplift and temperature reduction. The likely principal causes are hydraulic discharge, rock dilation during erosional unroofing, and gas migration during uplift.
Production and Accumulation of Organic Matter: A Geological Perspective.- Production and Accumulation of Organic Matter The Organic Carbon Cycle.- Evolution of the Biosphere.- Biological Productivity of Modern Aquatic Environments.- Chemical Composition of the Biomass: Bacteria, Phytoplankton, Zooplankton, Higher Plants.- Sedimentary Processes and the Accumulation of Organic Matter.- The Fate of Organic Matter in Sedimentary Basins: Generation of Oil and Gas.- Diagenesis, Catagenesis and Metagenesis of Organic Matter.- Early Transformation of Organic Matter: The Diagenetic Pathway from Organisms to Geochemical Fossils and Kerogen.- Geochemical Fossils and Their Significance in Petroleum Formation.- Kerogen: Composition and Classification.- From Kerogen to Petroleum.- Formation of Gas.- Formation of Petroleum in Relation to Geological Processes. Timing of Oil and Gas Generation.- Coal and its Relation to Oil and Gas.- Oil Shales: A Kerogen-Rich Sediment with Potential Economic Value.- The Migration and Accumulation of Oil and Gas.- An Introduction to Migration and Accumulation of Oil and Gas.- Physicochemical Aspects of Primary Migration.- Geological and Geochemical Aspects of Primary Migration.- Secondary Migration and Accumulation.- Reservoir Rocks and Traps, the Sites of Oil and Gas Pools.- The Composition and Classification of Crude Oils and the Influence of Geological Factors.- Composition of Crude Oils.- Classification of Crude Oils.- Geochemical Fossils in Crude Oils and Sediments as Indicators of Depositional Environment and Geological History.- Geological Control of Petroleum Type.- Petroleum Alteration.- Heavy Oils and Tar Sands.- Oil and Gas Exploration: Application of the Principles of Petroleum Generation and Migration.- Identification of Source Rocks.- Oil and Source Rock Correlation.- Locating Petroleum Prospects: Application of Principle of Petroleum Generation and Migration - Geological Modeling.- Geochemical Modeling: A Quantitative Approach to the Evaluation of Oil and Gas Prospects.- Habitat of Petroleum.- The Distribution of World Oil and Gas Reserves and Geological-Geochemical Implications.
We describe (1) bedding-parallel veins of fibrous calcite (beef) and (2) thrust detachments, which we believe provide good evidence for fluid overpressure in source rocks for petroleum. Our examples are from the surface or subsurface of the Magallanes-Austral Basin, which lies at the southern tip of South America. There, the best source rocks for petroleum are of Early Cretaceous age. In the central parts of the basin these source rocks have become overmature, but at the eastern edge, onshore and offshore, they are today either immature or in the oil window.
The reconstruction of the entrapment conditions of geological fluids requires determining the volumetric and composition properties of the inclusions containing these fluids. In some cases, the analytical data necessary for PVTX determination cannot be obtained from microthermometry. The quantification of the dissolved gases in aqueous fluid inclusions by Raman spectroscopy, following proper calibration of the instrument and methodology, can provide alternative data. In the present study, the intensity of the Raman signal of the symmetric stretching vibrational mode of methane was calibrated in order to (1) determine CH4 concentration in pure and saline water and (2) quantify the molar fractions of H2O and CH4 in the gas phase. High-pressure optical cell (HPOC), i.e. a pressurization system connected to a silica microcapillary heated on a customized heating-cooling stage, reveals to be much more convenient and accurate than synthetic fluid inclusions. Moreover, a wide range of pressure, temperature, and salinity can be covered by this methodology. Over 1000 measurements were produced to define the calibration curves, covering the ranges in temperature, pressure and salinity of 60-180 degrees C, 30-1000 bar, and 0-4 mol.kg(-1) NaCl, respectively, which corresponds to a CH4 molality up to 0.6 mol.kg(-1). The CH4 solubility vs. CH4/H2O Raman peak area ratio was fitted by a second-order polynomial curve (R-2 = 0.996). In the gas phase, the molar fraction of H2O vs. Raman peak area ratio is fitted by a straight line (R-2 = 0.990). The calibration was applied to a set of natural fluid inclusions trapped within late quartz Alpine fissure of the external part of the Central Alps (Switzerland). The determination of CH4 concentration in the studied fluids provided valuable insight on conditions of trapping and on the pressure regimes prevailing in this low-grade metamorphic setting.
RESUMEN. Depositos de 'abanico de talud' de la Formacion Los Molles del Jurasico Inferior en Chile central. La Formacion Los Molles, del Jurasico Inferior, y expuesta en Chile Central (32°15'S/71°30'W), es una sucesion marina de 765 m de espesor de rocas sedimentarias elasticas de grano fino. La sucesion es predominantemente pelitica y fue depositada por debajo del nivel base del oleaje, por corrientes de 'contour' y turbiditicas. La acumulacion tuvo lugar en una pendiente alta, irregular e inestable, de una reducida cuenca transtensional ('pull-apart'), conformando un 'abanico de talud' ('slope apron'). Esta cuenca se habria localizado en la zona de ante-arco de un margen continental activo.
We present an empirical calibration of the carbonate clumped isotope thermometer based on mollusk and brachiopod shells from natural and controlled environments spanning water temperatures of −1.0 to 29.5 °C. The clumped isotope data (Δ47) are normalized to CO2 gases with equilibrium distributions of clumped isotopologues at high temperature (1000 °C) and low temperature (27 or 30 °C), and thus the calibration is unique in being directly referenced to a carbon dioxide equilibrium reference frame (Dennis et al., 2011, Defining an absolute reference frame for clumped isotope studies of CO2, Geochimica et Cosmochimica Acta, 75, 7117–7131). The shell clumped isotope data define the following relation as a function of temperature (in kelvin):Δ47=0.0327×106/T2+0.3286(r2=0.84).The temperature sensitivity (slope) of this relation is lower than those based on corals, fish otoliths, foraminifera, and coccoliths, but is similar to theoretical predictions for calcite based on lattice dynamics calculations. We find no convincing methodological or biological explanations for the difference in temperature sensitivity between this calibration and the previous calibrations, and suggest that the discrepancy might represent real but unknown differences in mineral–DIC clumped isotope fractionation between mollusks/brachiopods and other taxa. Nevertheless, revised analytical methods similar to those used in this study are now in wide use, and it will be important to develop calibrations for other taxonomic groups using these updated methods, with analyses directly referenced to the carbon dioxide equilibrium reference frame.
Carbonate clumped isotope thermometry is based on the preference of 13C and 18O to form bonds with each other. At elevated temperatures such bond ordering is susceptible to resetting by diffusion of C and O through the solid mineral lattice. This type of bond reordering has the potential to obscure primary paleoclimate information, but could also provide a basis for reconstructing shallow crustal temperatures and cooling rates. We determined Arrhenius parameters for solid-state reordering of C–O bonds in two different calcites through a series of laboratory heating experiments. We find that the calcites have different susceptibilities to solid-state reordering. Reaction progress follows a first order rate law in both calcites, but only after an initial period of non-first order reaction that we suggest relates to annealing of nonequilibrium defects when the calcites are first heated to experimental temperature. We show that the apparent equilibrium temperature equations (or “closure temperature” equations) for carbonate clumped isotope reordering are analogous Dodson’s equations for first order loss of daughter isotopes. For each calcite, the sensitivity of apparent equilibrium temperature to cooling rate is sufficiently high for inference of cooling rates within a factor of ∼5 or better for cooling rates ranging from tens of degrees per day to a few degrees per million years. However, because the calcites have different susceptibilities to reordering, each calcite defines its own cooling rate–apparent equilibrium temperature relationship.
Marine shales and marls of the Valanginian-Hauterivian Agrio Formation have been studied at five localities in order to assess lateral variations over a 100 km S-N, shelf to basin transition. The two main organic-rich intervals at the base of the Pilmatue Member (Valanginian) and the base of Agua de la Mula Member (late early Hauterivian) have been characterized using a combination of bulk organic chemistry and palynofacies. Except for the former at the southern end of the transect, both intervals have mean total organic carbon (TOC) contents of 2-3% and are dominated by marine amorphous organic matter, suggesting a similar dysoxic genetic organic facies. The mean hydrogen indices determined from the slope of S2 v. TOC are 174 in the Pilmatue Member, but 387 in the basal part of the Agua de la Mula Member, a difference that mainly reflects the range in thermal maturity (late v. early oil window, respectively). Significant lateral variation occurs in the Pilmatue Member, with dark organic-rich intervals being rare in the south but dominant at the northern (distal) end of the transect; this trend is matched by a progressive increase in the peak or mean carbonate-free TOC and hydrogen indices, the latter reaching 6% and 297, respectively, near Estancia Pampa Tril. The bulk of the Agua de la Mula Member in the south is developed in organic-poor oxic facies, with a predominance of terrestrial phytoclasts and type IV kerogen, but dysoxic-anoxic conditions apparently predominate in the northern area. Valanginian-Hauterivian black shale facies appear generally rare on a global basis, but their occurrence can be related to the combination of the progressive rise in sea level during the Early Cretaceous and locally more restricted conditions.
Fluid inclusions study and multi-techniques quantitative analyses have been coupled to reconstitute PVTX conditions of aqueous and petroleum fluid entrapment of a local Haltenbanken area (well 6507/2-2) from the Garn Formation. Unusual low temperature behaviour showing the liquid portion separating into two liquids has been described. This behaviour is typical of a gas condensate/volatile mixture with high methane content and heavy alkanes. The variable liquid–vapour ratio, compositions and morphologies of all petroleum inclusions can be interpreted as a result of a combination of heterogeneous trapping and necking down. Thus, all petroleum inclusions can be related to a single petroleum source. Three main episodes of fluid entrapment can be distinguished. The first one corresponds to the water trapped within detrital quartz microfractures, at around 50°C and 50bar; the second to the petroleum fluid arrival in the biphase field of a critical system around 100–120°C and 190–230bar. The third one to the main trapping of present-day petroleum inclusions during the recrystallisation of quartz cements (around 110–160°C and 230–280bar) within the last few million years of a short and relatively rapid burial.
Synthetic fluid inclusions were experimentally produced by equilibrating small fractured prisms of quartz with aqueous solutions at temperatures from 300° to 700°C and pressures of 1000, 2000 and 3000 bar. Solution compositions included: 0.500, 2.000 and 4.500 molal NaCl; 0.500, 2.000 and 4.500 molal KCl; 0.50 and 2.00 molal CaCl2; and H2O. The homogenization temperatures of the synthetic fluid inclusions were analyzed by microthermometry. Plots of homogenization temperatures as a function of experimental temperature and pressure indicate that lines of constant homogenization temperature are linear and intersect the liquid-vapor curve at the homogenization temperature. For each of the four chemical systems, a relatively simple function was developed by which the homogenization temperature can be related to the temperature (°C) and pressure (bar) of inclusion formation and the composition (m) of the trapped fluid:m where m is the molality; Th is the homogenization temperature; and a1, a2, a3 and a4 are constants fit to the data sets of each of the four chemical systems. Ten-parameter polynomial regressions are given for the densities of the solutions on their liquid-vapor surfaces as functions of temperature and composition. These functions combined with the above equations permit calculation of the density as a function of temperature, pressure, solute and solute concentration in the supercritical region. A function is also given which permits calculation of isochores for fluids containing more than one solute. The results of these experiments compare favorably to previously published ones.
The diffusion rates of carbon and oxygen in two calcite crystals of different Mn contents have been studied between 500° and 800° C in a CO2-H2O atmosphere (PCO2=1-5 bars, PH2O=0.02-24 bars) labeled with 13C and 18O. Isotope concentration gradients within annealed specimens were measured using a secondary ion microprobe by depth profiling parallel and perpendicular to the c axis. Despite the anisotropic structure of calcite, the diffusion of carbon and oxygen are both very nearly isotropic. Least-squares fitting of the carbon data to an Arrhenius relation gives an activation energy of 87±2 kcal/mole, with D0 terms dependent only slightly upon direction: 1
, 2
. These results are in close agreement with previous determinations. Results for oxygen diffusion, however, give D values much larger than those previously reported for dry conditions; at 650° to 800° C the D values are two orders of magnitude larger. The diffusion of oxygen, unlike carbon, is strongly dependent on water pressure, as well as Mn content, and does not fit an Arrhenius relation over the entire temperature range. On the basis of these observations and considerations of the defect chemistry of calcite, it is proposed that carbon migrates as a Frenkel pair. The diffusion of oxygen, however, appears to be more complicated and may depend upon several simultaneous mechanisms.
Veins are common features in rocks and extremely useful structures to determine stress, strain, pressure, temperature, fluid composition and fluid origin during their formation. Here we provide an overview of the origin and terminology of veins. Contrary to the classical tripartite division of veins into syntaxial (inward growth), antitaxial (outward growth) and stretching veins (no consistent growth direction), we emphasise a continuum between syntaxial and stretching veins that form from the crack-seal process, as opposed to antitaxial veins that grow without the presence of an open fracture during growth. Through an overview of geochemical methods that can be applied to veins we also address the potential, but so far little-investigated link between microstructure and geochemistry. There are basically four mechanisms with increasing transport rates and concomitant decreasing fluid–rock interaction: (1) diffusion of dissolved matter through stagnant pore fluid; (2) flow of fluid with dissolved matter through pores; (3) flow of fluid with dissolved matter through fractures and (4) movement of fractures together with the contained fluid and dissolved matter (mobile hydrofractures). A vein system is rarely the product of a single transport and mineral precipitation mechanism, as these vary strongly both in space and time within a single system.