Dissolution Features Research Articles

Exploring prospective chemistry teachers’ perceptions of precipitation, conception of precipitation reactions and visualization of the sub-microscopic level of precipitation reactions

In this study, how prospective chemistry teachers (PCTs) perceive precipitation and conceptualize precipitation reactions at the symbolic level was explored. Further, it was of interest to identify PCTs’ visualization of the sub-microscopic level of precipitation reactions. The sample was drawn from the Chemistry Education Department at the Education Faculty of a Turkish Public University. A total of 46 PCTs (10 in the 4th semester, 15 in the 6th semester, and 21 in the 8th semester) participated in the study. The data were collected using an instrument with three open-ended questions developed by the authors and with interviews. It was revealed that the PCTs thought about precipitation in qualitatively different ways depending on their practices of phenomena. The PCTs’ perceptions of precipitation were grouped into three issues coded as (1) reacting two salt solutions, (2) undissolved solid and (3) residue. It was found that half of the PCTs (24 of the 46 PCTs) did not use state symbols when writing the precipitation equations and more than half of them included the molecular dissolution features in their drawings. It was suggested that instruction should be to address incorporating a relation between the sub-microscopic, symbolic and macroscopic levels by using the animations.

Cenomanian-Early Campanian Carbonate Reservoir Rocks of Northwestern Iraq: Diagenesis and Porosity Development

The present work focuses on the upper Cretaceous (Cenomanian-early Campanian) carbonate successions in selected wells from northwestern Iraq. These successions are represented by Gir Bir (Cenomanian-early Turonian), Wajna (late Santonian) and Mushorah (early Campanian) Formations. The succession has affected by early burial near-surface, unconformity-related and deep burial diagenesis represented by cementation, neomorphism, dolomitization, dedolomitization, silicification, authigenesis of glauconite and pyrite, compaction, micritization, solution and porosity formation. The common porosity types are intergranular, fenestral, intercrystalline, moldic, vuggy, channel and fracture. Three porosity zones (I, II, and III) are identified depending on variation in gamma ray which reflects their shale content. The upper part of zone (II) is highly porous and regarded on the main reservoir unit in the middle and upper parts of the Gir Bir Formation. Fracture and moldic and vuggy dissolution features in addition to karstic and fissure features are responsible for the porosity increase in the fractured reservoir unit.

Allanite Petrochronology in Fresh and Retrogressed Garnet–Biotite Metapelites from the Longmen Shan (Eastern Tibet)

Linking the timing of allanite growth to metamorphic conditions in metapelites is particularly chal-lenging because of the large variety of allanite textures and chemical compositions. This studyfocuses on five garnet–biotite metapelites retrogressed to different extents, from the internal do-main of the Longmen Shan (eastern Tibet) from which few petrochronological data documentingthe metamorphic ages are available. Microstructural observations were combined with whole-rockcompositions, detailed mineral characterization, phase equilibria modelling, andin situU–Th/Pb al-lanite dating to gain insights into the allanite reactivity relative to other rock-forming minerals. Allsamples experienced similar peak temperatures of 560–600C. Allanite is the main rare earth elem-ent (REE)-bearing accessory mineral but it exhibits different textures, such as epidote rims with dis-tinct REE contents as well as late inclusion-like dissolution features. Garnet is, along with allanite,critical to reconstruct the REE budget of these rocks. In the two samples where allanite is observedas inclusions in garnet, garnet shows no textural zoning and a low Y content ( 520C). Thisrelative chronology is confirmed byin situU–Th/Pb allanite dating: pre-garnet allanite shows agesofc.200 Ma, whereas syn- to post-garnet allanite has ages ofc.180 Ma. The timing of allanite ap-pearance strongly correlates with the biotite-in reaction predicted by the models and observed inmicrostructures. In the two samples collected close to the major Wenchuan Shear Zone—deformedup to mylonitization and retrogressed under greenschist-facies conditions (3–461 kbar, 350–400C)—allanite still preserves the peak metamorphic ages (c.200 andc.180 Ma). Allanite and itsepidote rims, however, are fragmented and partly replaced by a retrograde assemblage of QzþPb-depleted (<100 ppm) monazite, providing an age<90–100 Ma. Our results allow different meta-morphic stages for the studied transect in the central Longmen Shan to be refined: a prograde pathatc.200 Ma, a thermal relaxation and exhumation fromc.180 Ma, and a late greenschist overprint.This study also shows that when allanite is the principal phase in a mineral assemblage datable byU–Th/Pb, detailed observations of allanite textures may be highly informative in constraining thetiming of its growth relative to other major mineral phases.

REE patterns and trustworthiness of stable carbon isotopes of Salitre Formation, Irecê Basin (Neoproterozoic), São Francisco Craton

Neoproterozoic limestones have been subject to variations in their original isotopic ratios, which has led to studies to verify the reliability of isotopic data obtained in these rocks. A Brazilian example of Neoproterozoic carbonate succession is found in Irecê Basin (BA) (Salitre Formation, São Francisco Supergroup). The present research aims to evaluate the reliability of the δ13C, mainly through REE patterns of samples gathered in Upper Nova America Unit. This unit represents an intermediate section included in a thick carbonate cover that represents the Salitre Formation. X-Ray fluorescence and inductive plasma mass spectrometry (ICP-MS) techniques were used to obtain elemental geochemistry. X-ray diffraction, including the fraction < 4 μm, and microscope with coupled fluorescence were used to verify the presence of carbonate, organic matter, siliciclastic phases and their proportions. Samples are mainly calcitic and have REE patterns with enrichment of light REEs, strong positive La anomalies, slightly positive Ce anomalies, variety of Gd anomalies, absence of strong Y anomaly and Y/Ho ratio of 31.5 ± 1.5. These anomalies indicate a mixture of oceanic and fresh waters during precipitation. Ce positive anomalies indicate anoxic states of early digenetic conditions of the environment. δ13C values have little contrast around 0‰. As a result, calcareous from Nova América Unit precipitated under shallow depths on carbonate platform. High concentration of Zr and SiO2 suggests high rates of weathering on marginal belts of São Francisco craton through high energy input of detrital material by freshwater. On the other hand, low concentration of Al2O3 indicates deposition of clay minerals occurred in a mild area of the plataform. The Mn/Sr and Fe/Sr ratios did not indicate the presence of significant diagenetic or metamorphic alterations. From the petrographic point of view, the samples do not present preserved organic matter, but they are recrystallized instead, sometimes dolomitized and dissolution features are present. The results indicate that Nova América limestones were precipitated on shallow carbonate restricted platform close to the coastline prone to fresh water and detrital input, therefore, δ13C does not correspond to the isotopic composition of the global ocean.

Comprehensive study of fracture flow characteristic and feasibility of hybrid volume stimulation technique in tight fractured carbonate gas reservoir

Tight fractured carbonate reservoirs are difficult to obtain industrial oil and gas flow through conventional stimulation methods. The traditional stimulation techniques such as hydraulic fracturing and acid fracturing do not perform well in such reservoirs due to poor formation properties. Therefore, a novel stimulation technique named hybrid volume stimulation (HVS) is put forward. This technique consists of three stages, which are pad fluid fracturing, large-scale acid fracturing and proppant injection. The core idea of this technique is to create a complex fracture system with c conductivity to get further control of the reserves.In this study, laboratory tests were performed on reservoir rock samples to evaluate the stimulation effect and feasibility of HVS. Effects of pad fluid fracturing on carbonate rocks with pre-existing natural fractures were investigated. Then, features of acid dissolution on tight matrix and natural fractures in different cementing degrees were revealed. Moreover, fracture conductivity provided by different propping types was measured and the equivalent conductivity of the fracture system was calculated.The results showed that the natural fractures were extended and the induced fractures were created after pad fluid injection. Gelled acid could dissolve the cement in the partially cemented fractures and connect the fractures into one system. The propped fractures could provide enough conductivity under both high and low closure stress, while the un-propped conductivity was very limited. The acid-etched fractures might provide enough conductivity under low closure stress, however, as the closure stress increasing, it reduced to the same order of magnitude as un-propped ones. The equivalent conductivity of the fracture system was 3–4 times higher than that of the double-wing fracture, verified the feasibility and effect of HVS technique in tight fractured carbonate formations.

Reflecting on Gold Geomicrobiology Research: Thoughts and Considerations for Future Endeavors

Research in gold (Au) geomicrobiology has developed extensively over the last ten years, as more Au-bearing materials from around the world point towards a consistent story: That microbes interact with Au. In weathering environments, Au is mobile, taking the form of oxidized, soluble complexes or reduced, elemental Au nanoparticles. The transition of Au between aqueous and solid states is attributed to varying geochemical conditions, catalyzed in part by the biosphere. Hence, a global Au-biogeochemical-cycle was proposed. The primary focus of this mini-review is to reflect upon the biogeochemical processes that contribute to what we currently know about Au cycling. In general, the global Au-biogeochemical-cycle begins with the liberation of gold-silver particles from a primary host rock, by physical weathering. Through oxidative-complexation, inorganic and organic soluble-Au complexes are produced. However, in the presence of microbes or other reductants—e.g., clays and Fe-oxides—these Au complexes can be destabilized. The reduction of soluble Au ultimately leads to the bioprecipitation and biomineralization of Au, the product of which can aggregate into larger structures, thereby completing the Au cycle. Evidence of these processes have been “recorded” in the preservation of secondary Au structures that have been observed on Au particles from around the world. These structures—i.e., nanometer-size to micrometer-size Au dissolution and reprecipitation features—are “snap shots” of biogeochemical influences on Au, during its journey in Earth-surface environments. Therefore, microbes can have a profound effect on the occurrence of Au in natural environments, given the nutrients necessary for microbial metabolism are sustained and Au is in the system.

Manifestation of diamond sectoriality during dissolution and graphitization

We studied the morphology of dissolution and graphitization forms of sectorial natural and synthetic diamonds. Experiments on diamond dissolution (graphitization) were performed in model systems of different composition in a wide range of temperatures and pressures. It was found that, after dissolution, the raised (positive) and etched (negative) regions of the relief are formed along the outcrops of the growth sectors on the round surface of the diamond crystals. The relief patterns associated with the sectorial structure of crystals do not affect in the geometry of dissolution forms, which is mainly determined by external factors such as solvent composition, redox conditions, and process temperature. Based on the relative height of the relief elements at the outcrops of different growth sectors, it is concluded that the rate of dissolution of the growth sectors of synthetic diamonds decreases in the sequence {1 0 0}, {1 1 1}, {3 1 1}, {1 1 0}. In graphitization, the highest graphitization rate is observed in the growth sectors of {1 0 0} faces as well. As a result, deep grooves are formed on the growth sectors {1 0 0} faces. We assume that the dissolution features of various growth sectors are determined by the dependence of the rate of diamond dissolution (graphitization) on the concentration of nitrogen impurities in the growth sectors of various simple forms.

Applications of Two-Dimensional Seismic Tomography for Subsurface Cavity and Dissolution Features Detection Under Doon Valley, NW Himalaya

Applications of Two-Dimensional Seismic Tomography for Subsurface Cavity and Dissolution Features Detection Under Doon Valley, NW Himalaya

Surface dissolution features and contact twinning in natural diamonds

Surface dissolution features and contact twinning in natural diamonds

A test of the effectiveness of pore scale fluid flow simulations and constitutive equations for modelling the effects of mineral dissolution on rock permeability

Macro-scale transport properties of rocks such as permeability are bulk parameters combining the contributions of various properties only properly defined at the pore scale. Since pore-scale processes are known to modify the rock properties it is legitimate to ask if constitutive equations based on macro scale properties (e.g. porosity, permeability, formation factor, etc…) can properly describe their effect. In a previous experimental study (Lamy-Chappuis et al., 2014) we found that the effect of mineral dissolution on permeability could be much higher than predicted by such semi-empirical constitutive equations. Here we directly solve pore-scale fluid flow in high-resolution (2.5 μm) 3D models of a rock's geometry before and after mineral dissolution in order to evaluate permeability and how it is changed. This methodology is limited by the resolution of the micro-CT images used to define the rock geometry, which leads to significant overestimates of absolute permeability, but it does produce a much closer match to the change in permeability due to mineral dissolution than the constitutive equations. This is possible because the dissolution features, which enhance permeability, are large enough to be adequately resolved and produce a significant change in permeability.

Acid-saline-lake Systems of the Triassic Mercia Mudstone Group, County Antrim, Northern Ireland

Abstract The Triassic Mercia Mudstone Group of County Antrim, Northern Ireland, is dominated by red beds and evaporites. After only limited study, both marine and continental environments have been proposed previously. Here, we describe these rocks for the first time from core and petrographic observations to interpret depositional environments and conditions, such as water depth, salinity, and aridity. Bedded halite, bedded gypsum, displacive halite, and red siliciclastic mudstone lithologies comprise most (∼ 80%) of the 591.6-m-thick Mercia Mudstone Group in the Gaelectric Carnduff-2 core. Bedded halite consists of chevron and cornet crystals, indicating bottom-growth from shallow surface brines. Bedded-gypsum lithology is composed of halite-replaced pseudomorphs after swallowtail, bottom-growth gypsum crystals. Both bedded halite and bedded gypsum contain dissolution features and are commonly overlain by mud drapes. Bedded-halite and bedded-gypsum lithologies are interpreted to have formed in shallow saline lakes influenced by flooding and evapoconcentration. The displacive-halite lithology is composed of sub-cubic halite crystals, randomly oriented in mudstone, and represents deposition in a saline mudflat. Some mudstones contain ripple cross-lamination, dewatering structures, mudcracks, and rip-up clasts, suggesting shallow surface water and desiccation. Other mudstones are massive (structureless) and may have been deposited by wind. Both of these mudstone units were likely deposited in dry mudflats. Other red mudstones contain soil slickensides, blocky peds, and circumgranular cracks and are interpreted as paleosols. Our observations indicate that the Triassic Mercia Mudstone Group was formed by shallow perennial saline lakes and associated continental environments in an arid climate. Furthermore, lack of carbonates, lack of fossils, and paucity of organic matter suggest acid saline lakes and groundwaters. The Triassic Mercia Mudstone Group is similar to some other Pangean red beds and evaporites. Therefore, we hypothesize that the supercontinent was an arid barren landscape hosting acid-saline lakes.

Volumetric aberrancy to map subtle faults and flexures

One of the key tasks of a seismic interpreter is to map lateral changes in surfaces, not only including faults, folds, and flexures, but also incisements, diapirism, and dissolution features. Volumetrically, coherence provides rapid visualization of faults and curvature provides rapid visualization of folds and flexures. Aberrancy measures the lateral change (or gradient) of curvature along a picked or inferred surface. Aberrancy complements curvature and coherence. In normally faulted terrains, the aberrancy anomaly will track the coherence anomaly and fall between the most positive curvature anomaly defining the footwall and the most negative curvature anomaly defining the hanging wall. Aberrancy can delineate faults whose throw falls below the seismic resolution or is distributed across a suite of smaller conjugate faults that do not exhibit a coherence anomaly. Previously limited to horizon computations, we extend aberrancy to uninterpreted seismic data volumes. We apply our volumetric aberrancy calculation to a data volume acquired over the Barnett Shale gas reservoir of the Fort Worth Basin, Texas. In this area, the Barnett Shale is bound on the top by the Marble Falls Limestone and on the bottom by the Ellenburger Dolomite. Basement faulting controls karstification in the Ellenburger, resulting in the well-known “string of pearls” pattern seen on coherence images. Aberrancy delineates small karst features, which are, in many places, too smoothly varying to be detected by coherence. Equally important, aberrancy provides the azimuthal orientation of the fault and flexure anomalies.

Nature and origin of the nonsulfide zinc deposits in the Sierra Mojada District, Coahuila, Mexico: constraints from regional geology, petrography, and isotope analyses

The Sierra Mojada District comprises multiple types of near-surface mineral concentrations ranging from polymetallic sulfide zones, “nonsulfide Zn” (NSZ) deposits, and a silver-rich Pb carbonate deposit hosted by lower Cretaceous carbonate strata. Hypogene concentrations of Fe-Zn-Pb-Cu-Ag sulfides and sulfosalts are locally preserved and are associated with hydrothermal dolomite and silica. Alteration mineralogy and sulfur isotope data suggest primary Zn-Pb-Ag mineralization from circa 200 °C hydrothermal fluids. The NSZ deposits dominantly consist of smithsonite and hemimorphite associated with local Mn-Fe oxides. The Red Zinc Zone consists of strata-bound zones dominantly of hemimorphite that fills pores in residual and resedimented Fe oxides. The White Zinc Zone shows local dissolution features, including internal sediments interbanded with and cemented by smithsonite. Similar Pb isotopic compositions of smithsonite, hemimorphite, and cerussite to Sierra Mojada galena document that the NSZ deposits originated from polymetallic carbonate-replacement sulfide deposits, with flow of metal-bearing groundwater being controlled by local topography and structural features in this extensional terrane. Oxygen isotope values for Sierra Mojada smithsonite are relatively constant (δ18OVSMOW = 20.9 to 23.3‰) but are unusually low compared to other supergene smithsonites. Using δ18OVSMOW (− 8‰) of modern groundwater at nearby Cuatrocienegas, smithsonite formational temperatures are calculated to have been between 26 to 35 °C. Smithsonite precipitation was favored by near-neutral conditions typical of carbonate terranes, whereas hemimorphite precipitated by reaction with wallrock silica and locally, or episodically, more acidic conditions resulting from sulfide oxidation. Transition to, and stabilization of, the modern desert climate over the past 9000 years from the Late Pleistocene wetter, cooler climate of northern Mexico resulted in episodic drawdown of the water table and termination of local supergene metal mobilization.

Effects of organic ligands and background electrolytes on barite dissolution

Barite dissolution rates and features were investigated under conditions that combine a variety of organic ligands with different background electrolyte solutions. The organic ligands included low molecular weight organic compounds (LMWO) that can be produced by halophilic bacteria and are common in natural environments, as well as synthetic chelators that were previously studied in barite dissolution. Background electrolyte solutions included deionized water, 0.72 M NaCl and 2 M NaCl solutions designed to simulate freshwater, seawater, and brine fluids, respectively. Barite dissolution rates and corresponding rate constants normalized for saturation states were calculated for most experiments. Equal or lower rates and rate constants for pure LMWO relative to controls at all background electrolyte concentrations and pH values indicated that LMWO do not promote barite dissolution. Synthetic chelators showed higher rates and rate constants at pH 10 at static conditions than controls, indicating enhanced barite dissolution. Distinctive etch pit morphologies on the barite (0 0 1) face were observed in the presence of synthetic chelators, suggesting a direct contact dissolution mechanism by synthetic chelators. The effect of synthetic chelators on barite dissolution was significantly reduced in the presence of 2 M NaCl and/or at pH 6.5 due to salting out effects. Under hypersaline conditions, barite dissolution rates and rate constants by bacterial activities were up to four times higher than those of synthetic chelators. The wide range of conditions studied in this work provides insights into prediction of barite dissolution behavior in various natural environments as well as in industrial treatment procedures.

Anaerobic microbial activity affects earliest diagenetic pathways of bivalve shells

AbstractThe earliest diagenetic post‐mortem exposure of biogenic carbonates at the sea floor and in the uppermost sediment column results in the colonization of hard‐part surfaces by bacterial communities. Some of the metabolic redox processes related to these communities have the potential to alter carbonate shell properties, and hence affect earliest diagenetic pathways with significant consequences for archive data. During a three‐month in vitro study, shell subsamples of the ocean quahog Arctica islandica (Linnaeus, 1767) were incubated in natural anoxic sediment slurries and bacterial culture medium of the heterotrophic Shewanella sediminisHAW‐EB3. Bulk analyses of the liquid media from the Shewanella sediminis incubation revealed an over ten‐fold increase in total alkalinity, dissolved inorganic carbon and ΩAragonite, and the alteration of the Mg/Ca, Mg/Sr and Sr/Ca ratios relative to control incubations without cultures. Ion ratios were most affected in the incubation with anoxic sediment, depicting a 25% decrease in Mg/Ca relative to the control. Shell sample surfaces that were exposed to both incubations displayed visible surface dissolution features, and an 8 wt% loss in calcium content. No such alteration features were detected in control shells. Apparently, alteration of shell carbonate properties was induced by microbially driven decomposition of shell intercrystalline organic constituents and subsequent opening of pathways for pore fluid–crystal exchange. This study illustrates the potential influence of benthic bacterial metabolism on biogenic carbonate archives during the initial stages of diagenetic alteration within a relatively short experimental duration of only three months. These results suggest that foremost the biological effect of bacterial cation adsorption on divalent cation ratios has the potential to complicate proxy interpretation. Results shown here highlight the necessity to consider bacterial metabolic activities in marine sediments for the interpretation of palaeo‐environmental proxies from shell carbonate archives.

Pb2+–Calcite Interactions under Far-from-Equilibrium Conditions: Formation of Micropyramids and Pseudomorphic Growth of Cerussite

The presence of impurity ions is known to significantly influence mineral surface morphology during crystal growth from aqueous solution, but knowledge on impurity ion–mineral interactions during dissolution under far-from-equilibrium conditions remains limited. Here we show that calcite (CaCO3) exhibits a rich array of dissolution features in acidic Pb-bearing solutions. During the initial stage, calcite exhibits nonclassical surface features characterized as micropyramids that developed spontaneously. Subsequent pseudomorphic growth of cerussite (PbCO3) was observed, where nucleation occurred entirely within a pore space created by dissolution at the calcite/substrate interface. Uneven growth rates yielded a cerussite shell made of lath- or dendritic-shaped crystals. The cerussite phase was separated from the calcite by pores of less than 200 nm under transmission X-ray microscopy, consistent with the interface-coupled dissolution–precipitation mechanism. These results show that impurity metal ions exer...

Stalagmites from western Thailand: preliminary investigations and challenges for palaeoenvironmental research

Locating suitable caves and stalagmites for palaeoenvironmental and palaeoclimatic studies can be challenging. Isotopic geochemical analyses, albeit commonly performed for palaeoclimatic reconstruction, are also time consuming and costly. Therefore, petrographic and non‐destructive morphological studies on speleothems are desirable to facilitate sample selection for further analysis. In this study, 20 caves were surveyed in Ban Rai district, Uthai Thani province in western Thailand. After external physical observations in the field, three stalagmite samples were collected from Tham Nam Cave to test their potential for palaeoclimatic research. Firstly, the stalagmites were scanned by X‐ray computed tomography (CT scanning) and subsequently the CT images were compared with petrographic inspections. Columnar fabrics show the highest density, whereas closed and open dendritic fabrics have medium and the lowest densities, respectively. Layers near the top and bottom of the three stalagmites were dated by U‐Th mass spectrometric techniques. All three samples were deposited between c. 87 and c. 105 ka ago; therefore, they are probably the oldest stalagmites that have been reported so far from mainland Southeast Asia. However, their physical features indicate that all the samples have suffered from postdepositional dissolution, and are unlikely to be suitable for palaeoclimatic research. The internal dissolution feature of stalagmites, however, cannot be identified by visual inspection of uncut samples. We hereby argue that CT images are useful to characterize stalagmite petrography, in particular fabric, porosity and density. Such features can be used to select the ideal plane of a stalagmite for sectioning, to maximize the chances of robust climatic reconstruction.

Recent saltmarsh foraminiferal assemblages from Iceland

This study reports for the first time boreal to subarctic intertidal foraminiferal assemblages from saltmarshes at Borgarnes and Faskrudsfjördur on Iceland. The composition of living and dead foraminiferal assemblages was investigated along transects from the tidal flat to the highest reach of halophytic plants. The foraminiferal assemblages from Borgarnes showed 18 species in the total foraminiferal assemblage of which only 7 species were recorded in the living fauna. The assemblages were dominated by agglutinated taxa, whereas 3 calcareous species were recorded, of which only Haynesina orbicularis was found in the living fauna. The distribution limit of calcifying species corresponds to the lower boundary of the lower saltmarsh vegetation zone. Furthermore, calcareous tests showed many features of dissolution, which is an indication of a carbonate corrosive environment. The species forming the dead assemblages were mainly derived from the ambient intertidal areas and were displaced by tidal currents into the saltmarsh. The foraminiferal assemblages from Faskrudsfjördur showed two species, of which only one species was recorded in the living fauna. The assemblage was dominated by the agglutinated foraminifer Trochaminita irregularis. The foraminiferal species recorded on Iceland were the same as commonly found elsewhere in Europa. Since no species was found which is endemic to North America, Iceland is considered part of the European bio province. The foraminiferal could have been immigrated to Iceland from Europe through warm water currents, migratory birds or marine traffic since the last Ice Age.

Multi-scale characterization of the incipient carbonation of peridotite

Carbonation of peridotite is a widespread process in nature, with emerging societal and environmental implications through geological storage of CO2. We studied the micro- and nano-scale structures associated with the carbonation of peridotite pervaded by a CO2-rich water and the induced changes of its hydrodynamic properties. Focused ion beam (FIB) nanotomography was coupled with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. An experimental sample of dunite core percolated by a CO2-rich fluid (pCO2=11MPa) at 160°C was compared with a naturally carbonated sample from the Atlantic Ridge (Exp. IODP 304). Results of the 8-days percolation experiment show continuous alteration of olivine into magnesite locally associated with a poorly crystalline serpentine-type mineral referred to as proto-serpentine. The experiment is also characterized by an overall decrease in rock porosity (from 7.7 to 6.4%) and permeability (from 8.5×10−16 to 1.7×10−16m2). The primary inter-granular porosity (1 to 10μm) is progressively filled with magnesite, leading to the overall decrease of permeability. At the same time, a secondary small-scale porosity (10nm to 1μm) is created at the tip of the dissolution features along the (010) cleavage planes of olivine grains. Carbonate crystals grow within these etch-pit structures behind the dissolution front. Dissolution and precipitation occur at a relative rate that maintains the secondary porosity over the timescale of the experiment (i.e. carbonate growth has to be the slowest one). This process is interpreted as an interface coupled dissolution-precipitation mechanism of olivine carbonation. Proto-serpentine is only formed in highly reduced flow areas, such as small olivine etch-pits (10 to 500nm wide), which are more isolated from CO2 input. SEM-FIB analyses show the 3D preferential orientation of the dissolution-precipitation planes within a given olivine grain. It suggests the formation of lateral connections between the etch-pits (infra-micrometric “channels”) providing access to reactants and removing reaction products at the reaction sites. The experimental results show that olivine dissolution and magnesite precipitation converge to a quasi-steady state resulting in a constant carbonation rate controlled by reaction kinetics and not by mass transfer for this flow regime. The system self-regulates, independently of the porosity change at least during this incipient stage of olivine carbonation. Similar textures are observed in natural samples collected at 170m-depth in the vicinity of veinlets driving late fluids down into the cooling oceanic lithosphere. This suggests that similar mechanisms are active in nature and can play an important role in the late, low temperature alteration stages of fractured peridotite. These results also highlight the critical role of nm-scale spatial arrangement of dissolution and precipitation features on mass transfers and on the sustainability of olivine carbonation through coupled dissolution-precipitation processes.

Discovery of a shoal-controlled karst dolomite reservoir in the Middle Permian Qixia Formation, northwestern Sichuan Basin, Southwest China

The dolomites of the Middle Permian Qixia Formation have been important targets of natural gas exploration in the Sichuan Basin for decades. However, more and more exploration and research indicate that the formation of the reservoir might be related to karstification. To testify this hypothesis, we conduct comprehensive outcrop, core, and logging analyses based on a case study in the representative northwestern Sichuan Basin, which has obtained exploration breakthroughs recently. Results show that the Qixia dolomite reservoirs are mainly developed within fine-crystalline dolomites formed by a series of diagenetic modifications, which can be further divided into three types according to the macro- and micro-occurrences of dolomites: euhedral-subhedral crystalline dolomites in the quasi-stratiform karst system (mean porosity and permeability is 3.51% and 3.11 mD, respectively), euhedral-subhedral crystalline dolomites in the leopard porphyritic karst system (mean porosity and permeability is 3.36% and 1.22 mD, respectively), and allotriomorphic mosaic crystalline dolomites with residual parent rock fabrics (mean porosity and permeability is 0.94% and 0.92 mD, respectively). Their reservoir qualities decrease along the order. The formation mechanism of the reservoir is shoal-controlled karst. The preservation of residual intergranular pores within the thin-layer grainstones of shoal facies provides favorable channels for karst water. In the vadose zone, the heterogeneous dissolution within grainstones leads to the formation of leopard porphyritic dissolution features. In the phreatic zone, the karst water flowing along the stratiform grainstones results in the formation of quasi-stratiform dissolution features. The karst system is filled with loose carbonate sands and gravels, whose reservoir properties are far superior to parent rocks, and they can provide migration channels for the hydrothermal fluids with rich Mg2+ in the burial stage. The replacement of hydrothermal fluid results in the redistribution of pores and vugs of inter-fillings within karst system and the formation of intercrystalline pores and residual vugs, but the reservoir space of parent rocks keeps the same as the original condition. Therefore, the exploration of the Qixia dolomite reservoir should be changed to shoal-controlled karst.