Hydrochemistry,Hydrogen And Oxygen Isotope Geochemical Characteristics Of Nanyishan Oil Field Brine,Qaidam Basin

Nanyishan Oilfield brine is an enrichment area of potassium,calcium,boron,lithium,strontium,iodine and other elements,and it is a treasure house for the extraction of inorganic salts.The water chemistry type is classified as calcium chloride based on Sulin.The main characteristics are high salinity,high calcium,low magnesium and low sulfate.After evaporation and crystallization,potassium salt products and enrichment of multiple elements such as lithium,boron and iodine can be obtained.The brine has great comprehensive utilization value.Predecessors have conducted many studies on the genesis of oil field water in the western Qaidam Basin,and basically agree that the water in the western Qaidam oil field originated from atmospheric precipitation,but there are still disputes in the specific evolution process.In order to develop and utilize the brine from Nanyishan Oilfield,it is necessary to store knowledge about its genesis and evolution process.Therefore,using the combination of hydrochemical characteristics and hydrogen and oxygen isotopic geochemical characteristics,and supplemented by the XRD analysis of all-rock outcrops in various sedimentary strata,the genesis and evolution process of the brine in the Nanyishan Oilfield are analyzed,and the conclusions are:(1)XRD analysis results of all-rock outcrops in each sedimentary strata show that the strata are dominated by terrigenous clastic minerals,and the content of carbonate minerals and silicate minerals both account for more than 50%,which provides abundant oxygen isotope exchange between oilfield brines and rocks.The material basis of the magmatic rock is quite different,indicating that multiple periods of magmatic activity have occurred in the Nanyishan area.(2)Most of the brine in Nanyishan Oilfield is of calcium chloride type according to the Surin classification,indicating that the brine in Nanyishan Oilfield has good sealing properties and is in a state of deep burial.The brine of Nanyishan Oilfield has a high degree of salinity,and Na⁺and Cl^-contribute more than 90%of the salinity in all samples.The dissolution of rock salt,evaporation and concentration,alternate cation adsorption,albiteization of potash feldspar and calcium-rich plagioclase,dolomitization of calcite,and dissolution of plagioclase all occurred during the evolution of the oilfield brine,creating the present The water is rich in K⁺,Na⁺,Ca²⁺,Cl^-,and poor in Mg²⁺.The combination of sodium-chlorine coefficient and bromine-chlorine coefficient once again proves the occurrence of alternate cation adsorption.The content of SO₄^2-may be lower due to the biological reduction of oil field brine.The Br-content of the brine in the Nanyishan Oilfield is lower than the average of modern seawater and higher than the average of the salt lake brine in the Qaidam Basin,which can explain the continental origin of the oilfield brine.The high content of B³⁺,Li⁺,and Sr²⁺in oil field brine is related to the leaching of boron-rich salt-bearing salt systems and the replenishment of deep hot water.(3)The brine in the Nanyishan Oilfield originated from the intrusion of magmatic water into the syngeneic sedimentary water.The increase in the temperature of mixing with magma water greatly promotes the interaction of water and rock.During the evolution process,the isotope exchange between oilfield water and hydrocarbons also occurred,which made the oilfield water?¹⁸O.Positive bias,?D negative bias.