A Study Of Geochemical Tracers For Fluids Related To Dolomite Reservoirs

Deciphering the origin of fluids involved in dolomitization and subsequent/associated diagenesis is vital towards understanding the formation of dolomite and the evolution of dolomite reservoir.For the origin of dolomite,various dolomitization models have been carried,such as Sabka-style dolomite with evaporative seawater,meteoric mixing zone dolomite and dolomitization by seepage refluxion,to solve this problem.For the evolution of dolomite reservoirs,it evolves the meteoric water leaching during contemporaneous period,diagenetic fluids corrosion during diagenesis and deep-hydrothermal fluids during post-diagenetic.Thus,the origin of these fluids is vital to better understanding the formation of dolomite and evolution of the dolomite reservoirs.However,this issue has not been well resolved till the present research now.Therefore,this study carried out major elements,trace elements(including REEs),Sr isotope and halogens on representative sections(Geshan section from Yixing,Jiangsu and Panlongdong section from Xuanhan,Sichuan)and got meaningful results.The Triassic Zhouchongcun Formation from Geshan in Yixing County,Lower Yangtze displayed excellent outcrop of carbonate ranging from limestone,through dolomitic limestone and calcite dolomite,to dolomite.In this study,major elements,minor elements,halogens and Sr isotopes were analyzed in samples from Geshan section.In addition,the variation of these geochemical tracers was described and the related mechanism was discussed,providing theoretical foundation for these tracers.The Panlongdong section outcropped dolomite layers from Permian Changxing Formation and Triassic Feixianguan Formation.It developed Bioherm and oolitic beach,which are greatly reformed by post-diagenetic fluids,are high quality reservoirs in this district.Application of seawater normalized REE patterns in Panlongdong section further demonstrate the meteoric water leaching impact and the partly alteration by hydrothermal fluids.The sample powders were dissolved by 1M acetic acid to selectively dissolve the carbonate component.Meanwhile,it can avoid the undesirable effects of leaching detrital aluminosilicates.In this situation,the ?REE in carbonates have a certain variation range,ranging from about 10 ppm to about 30 ppm,which can be used to evaluate the effect of leaching impact or impurity input.Seawater normalized REE patterns of samples from Geshan show slightly LREE enrichment and notable positive CesN,implying the enrichment of LREE relative to HREE and the enrichment of Ce during the carbonate precipitate from seawater.For the REE patterns of dolomites from Panlongdong section,it can be found that(1)the ?REE in bioclastic dolomite are the highest(mean value=32.43 ppm);(2)the ?REE in oolitic dolomite and reefal dolomite are relatively lower(about 10 ppm);(3)the ?REE in oolitic dolomite with ooid ghost are the lowest(mean value=3.35 ppm).The variation tendency of ?REE indicates the leaching impact in REEs.Furthermore,the meteoric leaching of oolitic dolomite can reduce the Ce concent and the value even under the determination limit.As a result,the positive Ce anomaly was disappeared in the seawater normalized REE patterns.However,the hydrothermal altered dolomite has obvious positive Eu anomaly.Thus,the Ce anomaly can be used to trace the leaching impact of meteoric water,while the Eu anomaly can be used to identify the influence of hydrothermal fluids.Determination of halogens(C1,Br and I)in limestone-dolomite samples were conducted using the pyrohydrolysis technique.The Cl and Br concentrations in limestone,dolomitic limestone,calcitic dolomite and dolomite are increases as the enhanced dolomitization degree.The increased tendency of Cl and Br is similar to the seawater evaporation process,implying the salinity of dolomitization fluid is relatively higher.However,the I content greatly decreased as the increased dolomitization degree.IO3-/?-system serves as the most sensitive indicator of redox processes in the ocean.Besides,IO3-is the most possible occurrence of I in carbonate.During dolomitization,IO3-was converted to I-in enhanced reducing fluid conditions.As a consequence,the I migrate from the carbonate lattice and I concentration in carbonates decreased.Thus,Cl and Br are good proxies for paleo-salinity,whereas I serves as a proxy for paleo-redox conditionsThe 87Sr/86Sr ratio in carbonate has been widely used in the research of seawater 87Sr/86Sr variation and strontium isotope stratigraphy.Dolomite with little alteration can be selected to be the proxy as no limestone exists in some strata.Determination of Sr concentrations and 87Sr/86Sr ratios in limestone-dolomite samples from Geshan section show that the Sr concentration decrease with enhancing dolomitization degree,ranging from 1358 ppm in limestone to 94 ppm in dolomite,while the 87Sr/86Sr ratios increase from 0.708019 in limestone to 0.708182 in dolomite.It indicates the obvious migration of Sr and Sr isotope fractionation during dolomitization process.The fractionation degree of Sr isotope might be about 10-4 during this process.In addition,Ca is replaced by Mg during dolomitization,the isomorphous Sr is lost simultaneously.Meanwhile,more 86Sr emigrate from the crystal,causing higher 87Sr/86Sr ratio in mineral.Thus,more atteLtion should be payed to the fractionation of Sr during dolomitization process when choosing dolomite as proxy in the research of strontium isotope stratigraphy.Besides,87Sr/86Sr ratio in dolomite is an important indicator to identify the source and characteristics of the fluids related to the formation and evolution of dolomite.It can be found that 87Sr/86Sr ratio in dolomite are higher than that in contemporaneous limestone from previous research.In this situation,the fractionation degree of Sr isotope was about 10-3 or higher,which was always attributed to dolomitization fluid with high 87Sr/86Sr ratios.