Experimental investigation on the carbon isotope fractionation of methane during gas migration by diffusion through sedimentary rocks at elevated temperature and pressure

Molecular transport (diffusion) of methane in water-saturated sedimentary rocks results in carbon isotope fractionation. In order to quantify the diffusive isotope fractionation effect and its dependence on total organic carbon (TOC) content, experimental measurements have been performed on three natural shale samples with TOC values ranging from 0.3 to 5.74%. The experiments were conducted at 90°C and fluid pressures of 9 MPa (90 bar). Based on the instantaneous and cumulative composition of the diffused methane, effective diffusion coefficients of the ¹²CH₄ and ¹³CH₄ species, respectively, have been calculated.Compared with the carbon isotopic composition of the source methane (δ¹³C₁ = −39.1‰), a significant depletion of the heavier carbon isotope (¹³C) in the diffused methane was observed for all three shales. The degree of depletion is highest during the initial non-steady state of the diffusion process. It then gradually decreases and reaches a constant difference (Δ δ = δ¹³C_diff −δ¹³C_source) when approaching the steady-state. The degree of the isotopic fractionation of methane due to molecular diffusion increases with the TOC content of the shales. The carbon isotope fractionation of methane during molecular migration results practically exclusively from differences in molecular mobility (effective diffusion coefficients) of the ¹²CH₄ and ¹³CH₄ entities. No measurable solubility fractionation was observed.The experimental isotope-specific diffusion data were used in two hypothetical scenarios to illustrate the extent of isotopic fractionation to be expected as a result of molecular transport in geological systems with shales of different TOC contents. The first scenario considers the progression of a diffusion front from a constant source (gas reservoir) into a homogeneous “semi-infinite” shale caprock over a period of 10 Ma.In the second example, gas diffusion across a 100 m caprock sequence is analyzed in terms of absolute quantities and isotope fractionation effects. The examples demonstrate that methane losses by molecular diffusion are small in comparison with the contents of commercial size gas accumulations. The degree of isotopic fractionation is related inversely to the quantity of diffused gas so that strong fractionation effects are only observed for relatively small portions of gas.The experimental data can be readily used in numerical basin analysis to examine the effects of diffusion-related isotopic fractionation on the composition of natural gas reservoirs.     

硼同位素分馏的实验理论认识和矿床地球化学研究进展

硼是一种中等挥发性元素,具有¹¹B和¹⁰B两个稳定同位素。两个同位素间高达10%的相对质量差使其在地质过程中引起高达-70‰至+75‰的硼同位素变化。硼在自然界主要与氧键合形成三配位(BO₃)和四配位(BO₄)结构,因而¹¹B和¹⁰B间同位素分馏主要受控于三配体(BO₃)和四面体(BO₄)间配分。本文综述了低温和高温地质过程的硼同位素分馏的理论和实验研究进展。在溶液中B(OH)₃和${B(OH)^{-}_{4}}$间硼同位素分馏受pH和热力学p-T条件控制,实验和理论表征获得常温常压条件下的B(OH)₃和$B(OH)^{-}_{4}$间同位素分馏系数(α_3-4)变化范围为1.019 4至1.033 3。低温条件下矿物(如碳酸盐、黏土矿物(蒙脱石和伊利石)、针铁矿、水锰矿、硼酸盐)与溶液间硼同位素分馏行为除了受p-T-pH影响外,矿物表面吸附引起的分馏效应十分显著。在中高温过程(蒙脱石伊利石化、富硼电气石和白云母矿物与热液流体,以及硅酸盐熔体与流体)中硼同位素分馏行为受到硼配位构型、化学成分以及物理化学条件的控制。随着硼同位素分馏机理研究的深入以及越来越完善的地质储库硼同位素端员特征表征,硼同位素地球化学指标可以灵敏示踪成矿物质来源、探究成矿作用与成因模式和重建成矿过程物理化学条件。目前矿床硼同位素地球化学研究的难点在于实现不同赋存相(如流体、矿物和熔体)中硼配位键合结构和硼同位素组成的精细化表征。     

An experimental study of oxygen isotope fractionation between inorganically precipitated aragonite and water at low temperatures

To determine oxygen isotope fractionation between aragonite and water, aragonite was slowly precipitated from Ca(HCO₃)₂ solution at 0 to 50°C in the presence of Mg²⁺ or SO₄²⁻. The phase compositions and morphologies of synthetic minerals were detected by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The effects of aragonite precipitation rate and excess dissolved CO₂ gas in the initial Ca(HCO₃)₂ solution on oxygen isotope fractionation between aragonite and water were investigated. For the CaCO₃ minerals slowly precipitated by the CaCO₃ or NaHCO₃ dissolution method at 0 to 50°C, the XRD and SEM analyses show that the rate of aragonite precipitation increased with temperature. Correspondingly, oxygen isotope fractionations between aragonite and water deviated progressively farther from equilibrium. Additionally, an excess of dissolved CO₂ gas in the initial Ca(HCO₃)₂ solution results in an increase in apparent oxygen isotope fractionations. As a consequence, the experimentally determined oxygen isotope fractionations at 50°C indicate disequilibrium, whereas the relatively lower fractionation values obtained at 0 and 25°C from the solution with less dissolved CO₂ gas and low precipitation rates indicate a closer approach to equilibrium. Combining the lower values at 0 and 25°C with previous data derived from a two-step overgrowth technique at 50 and 70°C, a fractionation equation for the aragonite-water system at 0 to 70°C is obtained as follows:

     

Experimental studies of oxygen and hydrogen isotope fractionations between precipitated brucite and water at low temperatures

Oxygen and hydrogen isotope fractionation factors between brucite and water were experimentally determined by chemical synthesis techniques at low temperatures of 15° to 120°C. MgCl₂, Mg3N₂, and MgO were used as reactants, respectively, to produce brucite in aqueous solutions. All of the synthesis products were identified by x-ray diffraction (XRD) for crystal structure and by scanning electron microscope (SEM) for morphology. It is observed that oxygen isotope fractionations between brucite and water are temperature dependent regardless of variations in aging time, the chemical composition, and pH value of solutions. Brucites derived from three different starting materials yielded consistent fractionations with water at the same temperatures. These suggest that oxygen isotope equilibrium has been achieved between the synthesized brucite and water, resulting in the fractionation equation of 10³lnα=1.56×10⁶/T²−14.1. When the present results for the brucite–water system are compared with those for systems of gibbsite–water and goethite–water, it suggests the following sequence of ¹⁸O-enrichment in the M−OH bonds of hydroxides: Al³⁺ − OH > Fe³⁺ − OH > Mg²⁺ − OH.Hydrogen isotope fractionations between brucite and water obtained by the different synthesis methods have also achieved equilibrium, resulting in the fractionation equation of 10³lnα=−4.88×10⁶/T²−22.5. Because of the pressure effect on hydrogen isotope fractionations between minerals and water, the present calibrations at atmospheric pressure are systematically lower than fractionations extrapolated from hydrothermal exchange experiments at high temperatures of 510° to 100°C and high pressures of 1060 to 1000 bar. Comparison of the present results with existing calibrations involving other low-temperature minerals suggests the following sequence of D-enrichment in hydroxyl-bearing minerals: Al³⁺ − OH > Mg²⁺ − OH > Fe³⁺ − OH.     

An Experimental Study of Oxygen Isotope Fractionation in the Quartz—Wolframite—Water System

Oxygen isotope fractionation was experimentally studied in the quartz-wolframite-water system from 200 to 420 °C. The starting wolframite was synthesized in aqueous solutions of Na₂WO₄ · 2H₂O + FeCl₂ · 4H₂O or MnCl₂ · 4H₂O. The starting solutions range in salinity from 0 to 10 equivalent wt.% NaCl. Experiments were conducted in a gold-lined stainless steel autoclave, with filling degrees of about 50%. The results showed no significant difference in equilibrium isotope fractionation between water and wolframite, ferberite and huebnerite at the same temperature (310 °C ). The equilibrium oxygen isotope fractionation factors of wolframite and water tend to be equal with increasing temperature above 370 °C, but to increase significantly with decreasing temperature below 370 °C: 1000 ln α_wf-H₂o= 1.03×10⁶T−2-4.91 (370 °C ±200 °C ) 1000 ln α_wf-H₂o = 0.21×10⁶T −2-2.91 (420 °C -370 °C ±) This projects was financially supported by the National Natural Science Foundation of China.     

Experimental study of silicon isotope dynamic fractionation and its application in geology

Silicon shows no variation in its chemical valence in nature and exists mainly in the form of silicon-oxygen tetrahedra, so very small silicon isotope thermodynamic fractionation occurs and the resultant silicon isotope variation is limited. Dynamic fractionation of Si isotopes during precipitation of SiO₂ from a solution is a main factor leading to substantial variations in silicon isotopes in nature. In this experimental study, we determined the dynamic fractionation factorα for silicon isotopes during precipitation of SiO₂ from the solution. And in combination of α, a theoretical explanation is presented of the considerably low δ³⁰Si values of black smokers on modern seafloor, Archean banded magnetite-quartzite and clay minerals of weathering origin, and of clearly high δ³⁰Si values of siliceous rocks in shallow-sea carbonate platforms. This paper won the Paper of Excellence in the Second National Young Scientist Symposium on Geochemistry of Minerals and Rocks.     

硅同位素动力学分馏的实验研究及地质应用

由于硅在自然界中没有化学阶态变化，主要呈硅氧四面体形式存在，因此硅同位素的热力学分馏很小，由此产生的硅同位素变化不大。溶液中ＳｉＯ２沉淀过程的同位素动力学分馏是引起自然界硅同位素明显变化的主要原因。本文实验测定了溶液中ＳｉＯ２沉淀过程的硅同位素动力学分馏系数α，并结合所测定的硅同位素动力学分馏系数对现代海底黑烟囱、太古代条带状磁铁石英岩、风化成因粘土矿物的δ＾３０Ｓｉ值明显偏低；浅海碳酸盐台地中硅     

石英一水一盐体系氧同位素分馏作用

温度为180—550℃,盐度(wt.％)分别为0、5、25和40条件下,在高压釜内完成了由硅胶合成石英的氧同位素分馏作用实验研究,目的是了解:①盐同位素效应;②△t值对同位素分馏的影响;③温度与同位素分馏系数的关系。研究资料表明:低温条件下矿物和纯水之间同位素平衡作用不可能发生;影响含氧矿物(初)之间氧同位素平衡速率的因素包括盐度、△t值大小和温度等;我们的研究还表明,盐度对同位素分馏作用同系数无影响,即不存在所谓的“同位素盐效应”。在180—550℃温度范围内,不同盐度条件下获得的石英-水氧同位素分馏实验方程为:10001nα_(石英-水)=3.306×10~5T~(-2)—2.71。     

Experimental studies of oxygen isotope fractionation between rhodochrosite (MnCO3) and water at low temperatures

Rhodochrosite crystals were precipitated from Na-Mn-Cl-HCO₃ parent solutions following passive, forced and combined passive-to-forced CO₂ degassing methods. Forced and combined passive-to-forced CO₂ degassing produced rhodochrosite crystals with a small non-equilibrium oxygen isotope effect whereas passive CO₂ degassing protocols yielded rhodochrosite in apparent isotopic equilibrium with water. On the basis of the apparent equilibrium isotopic data, a new temperature-dependent relation is proposed for the oxygen isotope fractionation between rhodochrosite and water between 10 and 40 °C:

1000lnα_{rhodochrosite-water}=17.84±0.18(10³/T)-30.24±0.62     

Ca-Mg inter-diffusion in synthetic polycrystalline dolomite-calcite aggregate at elevated temperatures and pressure

This study measures the reaction rate of dolomite and aragonite (calcite) into Mg-calcite at 800, 850, and 900°C and 1.6 GPa. The dry synthetic dolomite-aragonite aggregate transformed very rapidly into dolomite-calcite polycrystalline aggregate while Mg-calcites formed at a relatively slow rate, becoming progressively richer in Mg with run time. We modeled the reaction progress semi-empirically by the first-order rate law. The temperature dependence of the overall transport rate of MgCO₃ into calcite can be described by the kinetic parameters (E?=?231.7 kJ/mol and A _o ?=?22.69 h^?1). Extrapolation using the Arrhenius equation to the conditions during exhumation of UHPM rocks indicates that the reaction of dolomite with aragonite into Mg-saturated calcite can be completed as the P-T path enters the Mg-calcite stability field in a geologically short time period (<1 Ky). On the other hand, the extrapolation of the rate to prograde metamorphic conditions reveals that the Mg-calcite formed from dolomitic marble in the absence of metamorphic fluid may not reach Mg-saturation until temperatures corresponding to high-grade metamorphism (e.g., >340°C and >10 My). SEM-EDS analysis of individual calcite grains shows compositional gradients of Mg in the calcite grains. The Mg-Ca inter-diffusion coefficient at 850°C is around 1.68?×?10^?14 m²/sec if diffusion is the major control of the reaction. The calculated closure temperatures for Ca-Mg inter-diffusion as a function of cooling rate and grain size reveal that Ca/Mg resetting in calcite in a dry polycrystalline carbonate aggregate (with grain size around 1 mm) may not occur at temperatures below 480°C at a geological cooling rate around 10°C/My, unless other processes, such as short-circuit interdiffusion along grain boundaries and dislocations, are involved.     

硫化物中硫同位素分馏理论计算研究进展

硫化物是重要的矿物类,通常是一个或多个金属元素与硫结合而形成硫化物.硫化物作为大多数金属的主要来源具有重大的经济利用价值,硫化物中硫同位素分馏的研究一直是同位素地球化学研究的热点.研究不同金属硫化物之间的硫同位素分馏效应,对于利用硫同位素对成矿作用过程和成矿物质来源开展地球化学示踪,具有非常重要的意义.本文结合笔者近期的工作概述了硫化物中硫同位素分馏的理论计算研究,认为虽然半经验半理论的增量方法在同位素分馏计算中存在一定的局限性,但在没有其他实用的理论计算方法时,改进的增量方法可以作为硫化物中硫同位素分馏计算的一种理论估算方法.     

A note on hydrogen isotope fractionation involving acidic and basic solutions

The dissociation constant of D₂O is significantly lower than that of H₂O, hence hydroxyl and hydronium ions have very much lower deuterium contents than the water from which they form. If the concentration of either of these ions becomes significant, its presence will effect the hydrogen isotope fractionation between any hydrogen bearing phase and the total solution.     

中酸性硅酸盐熔体-水体系氢同位素分馏的压力效应

对0．2－2000MPa条件下钠长石熔体，钾长石熔体以及0．2－150MPa条件下流纹岩熔体－－水体系的氢同位素分馏实验数据进行了筹压拟合，发现硅酸盐熔体与水之间的氢同位素分馏存在显著的压力效应，在800，1000和1200度条件下对钠长石熔体，水体系和流夺熔体－－水体系氢同位素分馏压力方程进行的等温拟合表明，只有在特定的压力条件下才可以用钠长石熔体－水体系来近似流纹岩熔体－－水体系的氢同位素分馏行为，当压力超过临界值时，硅酸盐熔体－水体系氢同位素分馏会发生变化，本文拟合的硅酸盐熔体－水体系氢同位素分馏等值线在P－T空间的形态变化特征与矿物－水体系存在较大差异，依据流纹岩熔体与水之间氢同位素分馏的压力效应，成功地模拟了美国西部Glass Creek流纹岩δD值和水含量变化规律与岩浆去气之间的关系。     

Fundamental studies on kinetic isotope effect (KIE) of hydrogen isotope fractionation in natural gas systems

Based on quantum chemistry calculations for normal octane homolytic cracking, a kinetic hydrogen isotope fractionation model for methane, ethane, and propane formation is proposed. The activation energy differences between D-substitute and non-substituted methane, ethane, and propane are 318.6, 281.7, and 280.2 cal/mol, respectively. In order to determine the effect of the entropy contribution for hydrogen isotopic substitution, a transition state for ethane bond rupture was determined based on density function theory (DFT) calculations. The kinetic isotope effect (KIE) associated with bond rupture in D and H substituted ethane results in a frequency factor ratio of 1.07. Based on the proposed mathematical model of hydrogen isotope fractionation, one can potentially quantify natural gas thermal maturity from measured hydrogen isotope values. Calculated gas maturity values determined by the proposed mathematical model using δD values in ethane from several basins in the world are in close agreement with similar predictions based on the δ¹³C composition of ethane. However, gas maturity values calculated from field data of methane and propane using both hydrogen and carbon kinetic isotopic models do not agree as closely. It is possible that δD values in methane may be affected by microbial mixing and that propane values might be more susceptible to hydrogen exchange with water or to analytical errors. Although the model used in this study is quite preliminary, the results demonstrate that kinetic isotope fractionation effects in hydrogen may be useful in quantitative models of natural gas generation, and that δD values in ethane might be more suitable for modeling than comparable values in methane and propane.     

Experimental investigation of the effects of temperature and ionic strength on Mo isotope fractionation during adsorption to manganese oxides

The Mo stable isotope system is being applied to study changes in ocean redox. Such applications implicitly assume that Mo isotope fractionation in aqueous systems is relatively insensitive to frequently changing environmental variables such as temperature (T) and ionic strength (I). A major driver of fractionation is the adsorption of Mo to Mn oxyhydroxide surfaces [Barling J. and Anbar A. D. (2004) Molybdenum isotope fractionation during adsorption by manganese oxides. Earth Planet. Sci. Lett.217(3-4), 315-329]. Here, we report the results of experiments that determine the extent to which Mo isotope fractionation during adsorption of Mo to the Mn oxyhydroxide mineral birnessite is sensitive to T and I. The results are compared to new predictions from quantum chemical computations. We measured fractionation from 1 to 50 °C at I = 0.1 m and found that Δ^97/95Mo_{dissolved-adsorbed} varies from 1.9‰ to 1.6‰ over this temperature range. Experiments were also performed at 25 °C in synthetic seawater (I = 0.7); fractionation at this condition was the same within analytical error as in low ionic strength experiments. These findings confirm that the Mo isotope fractionation during adsorption to Mn oxyhydroxides is relatively insensitive to variations and T and I over environmentally relevant ranges. To relate these findings to potential mechanisms of Mo isotope fractionation, we also report results for density functional theory computations of the fractionation between and various possible structures of molybdic acid as a function of temperature. Because no plausible species fractionates from with a magnitude matching the experiments, we are left with three possibilities to explain the fractionation: (1) solvation effects on the vibrational frequencies of aqueous species considered thus far are significant, such that our calculations in vacuo yield inaccurate fractionations; (2) a trace aqueous species not yet considered fractionates from and then adsorbs to birnessite; or (3) a surface complex not present in solution forms on birnessite in which Mo is not tetrahedrally coordinated. Our findings help validate assumptions underlying paleoceanographic applications of the Mo isotope system and also lead us closer to understanding the mechanism of isotope fractionation during adsorption of Mo to Mn oxyhydroxides.     

Experimental studies of oxygen isotope fractionation in the carbonic acid system at 15°, 25°, and 40°C

In light of recent studies that show oxygen isotope fractionation in carbonate minerals to be a function of HCO₃⁻ and CO₃²⁻ concentrations, the oxygen isotope fractionation and exchange between water and components of the carbonic acid system (HCO₃⁻, CO₃²⁻, and CO_2(aq)) were investigated at 15°, 25°, and 40°C. To investigate oxygen isotope exchange between HCO₃⁻, CO₃²⁻, and H₂O, NaHCO₃ solutions were prepared and the pH was adjusted over a range of 2 to 12 by the addition of small amounts of HCl or NaOH. After thermal, chemical, and isotopic equilibrium was attained, BaCl₂ was added to the NaHCO₃ solutions. This resulted in immediate BaCO₃ precipitation; thus, recording the isotopic composition of the dissolved inorganic carbon (DIC). Data from experiments at 15°, 25°, and 40°C (1 atm) show that the oxygen isotope fractionation between HCO₃⁻ and H₂O as a function of temperature is governed by the equation:

     

Experimental calibration of oxygen isotope fractionation between quartz and zircon

We report the results of an experimental calibration of oxygen isotope fractionation between quartz and zircon. Data were collected from 700 to 1000 °C, 10–20 kbar, and in some experiments the oxygen fugacity was buffered at the fayalite–magnetite–quartz equilibrium. Oxygen isotope fractionation shows no clear dependence on oxygen fugacity or pressure. Unexpectedly, some high-temperature data (900–1000 °C) show evidence for disequilibrium oxygen isotope partitioning. This is based in part on ion microprobe data from these samples that indicate some high-temperature quartz grains may be isotopically zoned. Excluding data that probably represent non-equilibrium conditions, our preferred calibration for oxygen isotope fractionation between quartz and zircon can be described by:

This relationship can be used to calculate fractionation factors between zircon and other minerals. In addition, results have been used to calculate WR/melt–zircon fractionations during magma differentiation. Modeling demonstrates that silicic magmas show relatively small changes in δ¹⁸O values during differentiation, though late-stage mafic residuals capable of zircon saturation contain elevated δ¹⁸O values. However, residuals also have larger predicted melt–zircon fractionations meaning zircons will not record enriched δ¹⁸O values generally attributed to a granitic protolith. These results agree with data from natural samples if the zircon fractionation factor presented here or from natural studies is applied.     

Oxygen isotope salt effects at high pressure and high temperature and the calibration of oxygen isotope geothermometers

The influence of NaCl, CaCl₂, and dissolved minerals on the oxygen isotope fractionation in mineral-water systems at high pressure and high temperature was studied experimentally. The salt effects of NaCl (up to 37 molal) and 5-molal CaCl₂ on the oxygen isotope fractionation between quartz and water and between calcite and water were measured at 5 and 15 kbar at temperatures from 300 to 750°C. CaCl₂ has a larger influence than NaCl on the isotopic fractionation between quartz and water. Although NaCl systematically changes the isotopic fractionation between quartz and water, it has no influence on the isotopic fractionation between calcite and water. This difference in the apparent oxygen isotope salt effects of NaCl must relate to the use of different minerals as reference phases. The term oxygen isotope salt effect is expanded here to encompass the effects of dissolved minerals on the fractionations between minerals and aqueous fluids. The oxygen isotope salt effects of dissolved quartz, calcite, and phlogopite at 15 kbar and 750°C were measured in the three-phase systems quartz-calcite-water and phlogopite-calcite-water. Under these conditions, the oxygen isotope salt effects of the three dissolved minerals range from ∼0.7 to 2.1‰. In both three-phase hydrothermal systems, the equilibrium fractionation factors between the pairs of minerals are the same as those obtained by anhydrous direct exchange between each pair of minerals, proving that the use of carbonate as exchange medium provides correct isotopic fractionations for a mineral pair.When the oxygen isotope salt effects of two minerals are different, the use of water as an indirect exchange medium will give erroneous fractionations between the two minerals. The isotope salt effect of a dissolved mineral is also the main reason for the observation that the experimentally calibrated oxygen isotope fractionations between a mineral and water are systematically 1.5 to 2‰ more positive than the results of theoretical calculations. Dissolved minerals greatly affect the isotopic fractionation in mineral-water systems at high pressure and high temperature. If the presence of a solute changes the solubility of a mineral, the real oxygen isotope salt effect of the solute at high pressure and high temperature cannot be correctly derived by using the mineral as reference phase.     

Experimental and theoretical study on the effect of unsteady flow on the fracturing pressure in hydraulic fracturing test

Reasonable determination of formation fracturing pressure concerns the stable operation of underground fluid injection projects. In this work, we studied the effect of unsteady flow on fracturing pressure. Hydraulic fracturing tests on low permeable sandstone were conducted with the injection rate between 0.1 and 2.0 ml/min. Then, the fracturing pressure prediction models for hollow cylinder under both unsteady flow and steady flow conditions were deduced. Finally, the effect of unsteady flow on the fracturing pressure was studied based on the experimental result and several influence factors. It was shown that fracturing pressure increased with the elevated pressurization rate in the tests, while the slope of the variation curve decreases. The model considering unsteady flow can reflect the variation tendency of fracturing pressures in experiments, while fracturing pressures from the model considering steady flow are invariant with different pressurization rates. Fracturing pressure decreases with the elevated rock permeability and increases with the elevated fluid viscosity, and these two effects are actually generated by the unsteady flow. Whether to consider the unsteady flow has no significant influence on the effect of rock tensile strength on fracturing pressure when the tensile strength is very low. However, when the tensile strength is high, the effect of unsteady flow cannot be neglected.