Experimental Study Of Oxgen Isotopic Fractionation In Carbonate Water-rock Interaction

Relied on the study of "Deuterium Excess Evolvement of Brines in Brine-bearing Strata in South Sichuan Basin",the project of National Fund of Natural Science,the former work revealed that the isotopic compositions and Deuterium Excess of brines in this basin have evident regularity:(1)On theδD-δ¹⁸O plot,the oxygen and hydrogen isotope compositions of brines sampled in marine sedimentary brine-bearing strata fall in the right part of the plot,and the hydrogen isotope compositions are lower than SMOW,while oxygen isotope compositions are higher than SMOW.From Sinian stratum,along Cambrian and Permian Maokou Formation up to Triassic Jialingjiang Formation,theδD value of brines gets relatively positive with strata turn to younger.(2)Brines of terrestrial deposit are derived from precipitation,the isotope compositions falling in the neighborhood of the globle meteoric water line(GMWL).(3)Brines of transition between marine and terrestrial deposit(Upper Triassic Xujiahe Formation)haveδD andδ¹⁸O values falling between the two end members of marine deposit brines and precipitation.(4)In the basin,water samples of the brine-bearing strata of different ages,have various deuterium excess(d)which evidently decrease with strata getting older.The brine-bearing strata of Triassic Leikoupo-Jialingjiang Formation,Permian Maokou Formation,Cambrian and Sinian are all carbonate rocks.Having same lithology,the strata present brines whose deuterium excess essentially change with time.All brine-bearing strata surrounding the basin or locating at the fault structures,as well as brine well long-term exploited for resources,have been visibly influenced by precipitation supply.(5)Therefore,the deuterium excesses of their brines have increased to different extents,depending on the amount of mixing precipitation. (6)Brines from different strata of Triassic have d values whose change and distribution are correlative to depth of the stratum to a certain extent.The more depth of stratum is,lower deuterium excesses of its brine is presented.In above,the relationship between deuterium excesses evolvement of brines and water-rock interaction is specially attention-getting.In order to find out the conditons of water-rock interaction in brine-bearing strata and the main influencing factors,for deeply probe the evolving mechanism of deuterium excesses,the simulating experiment of brine's water-rock interaction has been put on the study schedule.The water-rock interaction goes in water-rock system,the oxgen isotope has exchanged between walter and rock.According to dissolution-redeposition mechanism of J.R.O'Neil,this paper embarked upon analyzing the possible factors from microcosmic aspects,and designed the simulating experiment research of oxgen isotopic exchange in water and rock interaction.Take the carbonate and the different concentration halogenide solution as the object of study,the influence of temperature,homoion effect of CaCl₂,salting effect of KCl and NaCl upon oxgen isotope exchange was disccused.After simulating experiment of water-rock interaction in calcite-halide(NaCl,KCl,CaCl₂)solution and calcite-water solution,the results were obtained as follows:(1)Taking example forδ¹⁸O value in KCl solution of 10g/L,when 40℃for -12.04,when 80℃for-11.51,when 120℃for-11.30,it can be summarized that the influence of temperature on the oxygen isotopic exchange is intense.Along with temperature elevation,theδ¹⁸O value in KCl solution,NaCl solution and CaCl2 solution has the tendency of increasing gradually,this had fully proven the temperature is an important factor affecting the oxygen isotopic exchange.(2)KCl,NaCl solution has the obvious promotion to the oxygen isotopic exchange.When solution concentration is smaller than 25g/L,under the given three temperatures,the influence of NaCl solution uponδ¹⁸O value is obviously strong compared to KCl solution.But when the solution concentration achieves 25g/L,under the given three temperatures,the influence of KCl solution uponδ¹⁸O value is obviously strong compared to NaCl solution.(3)In the CaCl₂ solution,the CaCl₂ has the inhibitory action to the oxygen isotopic exchange.When the CaCl₂ solution concentration is low,regardless of any temperature,the homoion effect is obvious.But along with the temperature elevation, the influence of homoion effect of CaCl₂ gets more and more weak;when up to 120℃, the homoion effect on inhibitory action to the oxygen isotopic exchange in water-rock interaction cannot appear nearly.Here,the temperature and the salting effect are the main controlling factors.Comprehensively considered the effect of temperature,concentration,and homoion effect,salting effect,the author proposed that temperature effect mainly controlls the oxygen isotopic exchange in carbonate water-rock interaction;while the influences of concentration effect,homoion effect and salting effect are obviously weaker than that of temperature effect,and the influence of concentration effect is the weakest among them.