How important is carbonate dissolution in buried sandstones:evidences from petrography,porosity,experiments,and geochemical calculations

Burial dissolution of feldspar and carbonate minerals has been proposed to generate large volumes of secondary pores in subsurface reservoirs. Secondary porosity due to feldspar dissolution is ubiquitous in buried sandstones;however, extensive burial dissolution of carbonate minerals in subsurface sandstones is still debatable. In this paper, we first present four types of typical selective dissolution assemblages of feldspars and carbonate minerals developed in di erent sandstones. Under the constraints of porosity data, water–rock experiments, geochemical calculations of aggressive fluids, diagenetic mass transfer, and a review of publications on mineral dissolution in sandstone reservoirs, we argue that the hypothesis for the creation of significant volumes of secondary porosity by mesodiagenetic carbonate dissolution in subsurface sandstones is in conflict with the limited volume of aggressive fluids in rocks. In addition, no transfer mechanism supports removal of the dissolution products due to the small water volume in the subsurface reservoirs and the low mass concentration gradients in the pore water. Convincing petrographic evidence supports the view that the extensive dissolution of carbonate cements in sandstone rocks is usually associated with a high flux of deep hot fluids provided via fault systems or with meteoric freshwater during the eodiagenesis and telodiagenesis stages. The presumption of extensive mesogenetic dissolution of carbonate cements producing a significant net increase in secondary porosity should be used with careful consideration of the geological background in prediction of sandstone quality.

Role of Carbonic Anhydrase as an Activator in Carbonate Rock Dissolution and Its Implication for Atmospheric CO_(2)Sink

The conversion of CO2 into H+ and is a relatively slow reaction. Hence, its kinetics may be rate determining in carbonate rock dissolution. Carbonic anhydrase (CA), which is widespread in nature, was used to catalyze the CO2 conversion process in dissolution experiments of limestone and dolomite. It was found that the rate of dissolution increases by a factor of about 10 after the addition of CA at a high CO2 partial pressure (Pco2) for limestone and about 3 at low Pco2 for dolomite. This shows that reappraisal is necessary for the importance of chemical weathering (including carbonate rock dissolution and silicate weathering) in the atmospheric CO2 sink and the mysterious missing sink in carbon cycling. It is doubtless that previous studies of weathering underestimated weathering rates due to the ignorance of CA as an activator in weathering, thus the contribution of weathering to the atmospheric CO2 sink is also underestimated. This finding also shows the need to examine the situ distribution and activity of CA in different waters and to investigate the role of CA in weathering.

An investigation on dissolution kinetics of single sodium carbonate particle with image analysis method

Dissolution kinetics of sodium carbonate is investigated with the image analysis method at the approach of single particle.The dissolution experiments are carried out in an aqueous solution under a series of controlled temperature and p H.The selected sodium carbonate particles are all spherical with the same mass and diameter.The dissolution process is quantified with the measurement of particle diameter from dissolution images.The concentration of dissolved sodium carbonate in solvent is calculated with the measured diameter of particle.Both surface reaction model and mass transport model are implemented to determine the dissolution mechanism and quantify the dissolution rate constant at each experimental condition.According to the fitting results with both two models,it is clarified that the dissolution process at the increasing temperature is controlled by the mass transport of dissolved sodium carbonate travelling from particle surface into solvent.The dissolution process at the increasing pH is controlled by the chemical reaction on particle surface.Furthermore,the dissolution rate constant for each single spherical sodium carbonate particle is quantified and the results show that the dissolution rate constant of single spherical sodium carbonate increases significantly with the rising of temperature,but decreases with the increasing of pH conversely.

Effect of carboxymethyl cellulose on dissolution kinetics of carboxymethyl cellulose-sodium carbonate two-component tablet

Sodium carbonate and carboxymethyl cellulose powders are compressed into two-component tablets with three mass ratios,97%:3%,95%:5% and 93%:7%.The dissolution tests for two-component tablets and reference pure sodium carbonate tablets are carried out at various temperatures.The dissolution process of each tablet is measured by electrical conductivity tracking method and the concentration of dissolved sodium carbonate is quanti fied with calibrated conductivity-concentration converting equation of sodium carbonate.The quanti fied dissolution data is fitted with both surface reaction model and diffusion layer model and the results clearly show that surface reaction model is suggested as the appropriate dissolution model for all measured tablets.Therefore,it is determined that carboxymethyl cellulose is a stable element to remain the dissolution mechanism of tablet unchanged.The dissolution rate constant quanti fied with surface reaction model presents that carboxymethyl cellulose-sodium carbonate two-component tablets obtain signi ficant higher dissolution rate constant than pure sodium carbonate tablet and higher proportion of carboxymethyl cellulose leads to apparent higher dissolution rate constant.The results prove for the usage of carboxymethyl cellulose in most practical applications at a relative low-level,the effect of carboxymethyl cellulose is effective and positive for two-component tablet to enhance the dissolution process and improve dissolution rate constant and this effect is speculated coming from its dynamic physical transforming process in water including dilation and conglutination.

Factors influencing dissolution of carbonaceous materials in liquid Fe-Mn

Carbon dissolution from four types of metallurgical cokes and graphite was investigated by using immersion rods in a resistance furnace to clarify the influence of factors governing the rate of carbon dissolution from carbonaceous materials into Fe-Mn melts at 1550℃.The factors studied were the nmicrostructure of carbonaceous materials,roughness,porosity and the wettability between carbonaceous materials and the melt.Carbon/metal in terface was characterised by sea nning electron microscopy accompanied with energy-dispersive X-ray spectrometry to investigate the form at io n of an ash layer.The results showed that coke E had the highest dissolution rate.Surface roughness and porosity of the carbonaceous materials seemed to be dominant factors affecting the dissolution rates.Further,crystallite size did not have a significant effect on the dissolution rates.Solid/liquid wettability seemed to affect the initial stage of dissolution reaction.The dissolution mechanism was found to be both mass transfer and interfacial reactions.

Genetic mechanisms of secondary pore development zones of Es_4~x in the north zone of the Minfeng Sag in the Dongying Depression,East China

The genetic mechanisms of the secondary pore development zones in the lower part of the fourth member of the Shahejie Formation（Es_4／6x） were studied based on core observations,petrographic analysis,fluid inclusion analysis,and petrophysical measurements along with knowledge of the tectonic evolution history,organic matter thermal evolution,and hydrocarbon accumulation history.Two secondary pore development zones exist in Es_4~x,the depths of which range from 4200 to 4500 m and from 4700 to 4900 m,respectively.The reservoirs in these zones mainly consist of conglomerate in the middle fan braided channels of nearshore subaqueous fans,and the secondary pores in these reservoirs primarily originated from the dissolution of feldspars and carbonate cements.The reservoirs experienced ‘‘alkaline–acidic–alkaline–acidic–weak acidic＇＇,‘‘normal pressure–overpressure–normal pressure＇＇,and‘‘formation temperature increasing–decreasing–increasing＇＇ diagenetic environments.The diagenetic evolution sequences were ‘‘compaction/gypsum cementation/halite cementation/pyrite cementation/siderite cementation–feldspar dissolution/quartz overgrowth–carbonate cementation/quartz dissolution/feldspar overgrowth–carbonate dissolution/feldspar dissolution/quartz overgrowth–pyrite cementation and asphalt filling＇＇.Many secondary pores（fewer than the number of primary pores） were formed by feldspar dissolution during early acidic geochemical systems with organic acid when the burial depth of the reservoirs was relatively shallow.Subsequently,the pore spaces wereslightly changed because of protection from early hydrocarbon charging and fluid overpressure during deep burial.Finally,the present secondary pore development zones were formed when many primary pores were filled by asphalt and pyrite from oil cracking in deeply buried paleoreservoirs.

Deep-water carbonate dissolution in the northern South China Sea during Marine Isotope Stage 3

The production, transportation, deposition, and dissolution of carbonate profoundly form part of the global carbon cycle and affect the amount and distribution of dissolved inorganic carbon （DIC） and alkalinity （ALK）, which drive atmospheric CO2 changes during glacial/interglacial cycles. These processes may provide significant clues for better understanding of the mechanisms that control the global climate system. In this study, we calculate and analyze the foraminiferal dissolution index （FDX） and the fragmentation ratios of planktonic foraminifera for the 60-25 ka B.P. time-span, based on samples from Core 17924 and ODP Site 1144 in the northeastern South China Sea （SCS）, so as to recon- struct the deep-water carbonate dissolution during Marine Isotope Stage 3 （MIS 3）. Our analysis shows that the dissolution of carbonate increases gradually in Core 17924, whereas it remains stable at ODP Site 1144. This difference is caused by the deep-sea carbonate ion concentration （[CO32 ]） that affected the dissolution in Core 17924 where the depth of 3440 m is below the saturation horizon. However, the depth of ODP Site 1144 is 2037 m, which is above the lysocline where the water is always saturated with calcium carbonate; the dissolution is therefore less dependent of chemical changes of the seawater. The combined effect of the productivity and the deep-water chemical evolution may decrease deep-water ICO32-] and accelerate carbonate dissolution. The fall of the sea-level increased the input of DIC and ALK to the deep ocean and deepened the carbonate satu- ration depth, which caused an increase of the deep-water [CO32-]. The elevated ICO32-1 partially neutralized the reduced [CO32-] contributed by remineralization of organic matter and slowdown of thermohaline. These consequently are the fundamental reasons for the difference in dissolution rate between these two sites.

Zinc Redistribution in a Soil Developed from Limestone During Pedogenesis

The long-term redistribution of Zn in a naturally Zn-enriched soil during pedogenesis was quantified based on mass balance calculations. According to their fate, parent limestones comprised three Zn pools: bound to calcite and pyritesphalerite grains, bound to phyllosilicates and bound to goethite in the inherited phosphate nodules. Four pedological processes, i.e., carbonate dissolution, two stages of redox processes and eluviation, redistributed Zn during pedogenesis. The carbonate dissolution of limestones released Zn bound to calcite into soil solution. Due to residual enrichment, Zn concentrations in the soil are higher than those in parent limestones. Birnessite, ferrihydrite and goethite dispersed in soil horizon trapped high quantities of Zn during their formation. Afterwards, primary redox conditions induced the release of Zn and Fe into soil solution, and the subsequent individualization of Fe and Mn into Zn-rich concretions. Both processes and subsequent aging of the concretions formed induced significant exportation of Zn through the bottom water table. Secondary redox conditions promoted the weathering of Fe and Mn oxides in cements and concretions. This process caused other losses of Zn through lateral exportation in an upper water table. Concomitantly, eluviation occurred at the top of the solum. The lateral exportation of eluviated minerals through the upper water table limited illuviation. Eluviation was also responsible for Zn loss, but this Zn bound to phyllosilicates was not bioavailable.

Foraminifera in surface sediments of the Bering and Chukchi Seas and their sedimentary environment

Based on a quantitative analysis of foraminifera in 39 surface samples of the Bering andChukchi Seas, the nearly absence of planktonic foraminifera in the surface sediments can be related to the low surface primary productivity and strong carbonate dissolution in the study area. It has been revealed that the surface primary productivity, and carbonate dissolution and properties of water masses related to the water depth mainly control the distribution of benthic foraminifera. The shelf of the Chukchi Sea is dominated by the Elphidium spp. assemblage and Nonionella robusta assemblage with low foraminiferal abundance and diversity, which is controlled by the coastal water mass of the Arctic Ocean. The slope of the Bering Sea is dominated by the Uvigerina peregrina - Globobulimina affinis assemblage with abundant N. robusta, and relatively high foraminiferal abundance and diversity, which is controlled by the intermediate and deep water masses of the Pacific Ocean. However, the Bering Sea has relatively shallow carbonate lysocline and compensation depth (CCD) , at about 2 000 and 3 800 m, respectively. In addition, there exists Stetsonia arctica in the surface sediments of the upper slope in the Bering Sea, which is a typical deep-sea benthic foraminiferal species of the slope in the Arctic Ocean. This indicates that the deep water of the two seas beside the Bering Strait had ever exchanged.

^(137)Cs Redistribution in Thin Stony Soil of a Carbonate Rock Slope in Southwest China

The fallout radionuclide cesium-137(137 Cs) has been widely employed as a tracer for assessment of soil loss from thick uniform soils;however,few studies have been conducted on thin stony soils on slopes underlain by carbonate rocks which are widely distributed in karst areas.Information derived from 137 Cs measurement of soil samples collected along a carbonate rock slope with thin stony soil where neither soil erosion nor deposition occurred was used to investigate the characteristics of 137 Cs redistribution in a karst area of Southwest China.The results indicated that the 137 Cs inventories of the surface soil on the slope studied were much lower than that of the local 137 Cs reference inventory and the 137 Cs activities were much higher than those on slopes with thick uniform soils.The spatial distribution of 137 Cs inventories was characterized by considerable variation.The high 137 Cs depletion in the stony soil of the slope studied was mainly because a considerable proportion of the fallout input of 137 Cs could be lost with runoff and the dissolution of carbonate particles in the soil promoted the loss of 137 Cs.These demonstrated that the rates of soil loss could not be estimated from the degree of depletion of the 137 Cs inventory relative to the local reference inventory for the thin stony soil of the rocky slope underlain by carbonate rocks in the study area in the way that has been widely used in areas with thick uniform soils.