Logging Identification And Evaluation Of The Triassic Potash Deposits In Sichuan Basin

The consumption of potassium of Chinese agriculture is very large,which has reached 14 million tons in 2010,almost half of the world's output of potassium fertilizer,and this figure may be 18.2 million in 2020.About 30%?50%of the potassium demand depends on long-term imports of potash,and the real figureis 45.20%in 2012.Both show that potassium resources is quite scarce in China.Potassium resourcesis mainly distributed in the northwest and southwest of China with low exploration and development,therefore,there is an urgent need for exploitation potentialities of potassium resources when implementing simultaneous exploration of oil and potassium.Potassium salt is a kind of evaporite mineral,which is deposited in salt-containing layers.It is often difficult to see in the surface of the earth and buried in the ground hundreds to thousands of meters.In Triassic of Sichuan basin,there are abundant concealed potassium resources which have the characteristics of multiple phases of salt formation and three phase poly potassium.Triassic strata is the area with most developed evaporites,wide distribution of gypsum layer and salt brine,rich salt rock and polyhalite,and some potassium and magnesium salt mineral.Potassium rich mineral which has been found so far mainly contains polyhalite and potassium-riched brine,mostly developed in the middle Triassic Lei Kou-po formation and Jialingjiang formation.However,potash ores are buried so deep within the basin with its unbalanced distribution and diverse genetic type that the study of potash ores is only qualitative with less experimental studies and less quantitative analysis.In addition,available potash ores resources and its distribution are not very clear,which cann't form a perfect evaluation system of potassium resources.Based on the theory of well logging geology,petrology and geochemistry,this papermakes the most of borehole data through simultaneous exploration of oil and potash,combining with logging data,core data and water chemistry data,to do detailed research of potassium resources,such as polyhalite and potassium-riched brine,in Triassic of Sichuan basin.Firstly,according topredecessors' understanding of Sichuan Basin,this paper divides Sichuan Basin into 5 salt area,summarizes spatial-temporal characteristics of salt strata of Sichuan Basin,screens outfavorablestructureof salt formation and potassiumformation,such as Pingluoba structure,Changshou Shuanglong structure,Xuanhan Huangjinkou structure and Guangan structure,then further analyses the types and characteristics of the salt of different structure.Secondly,depending onwater chemistry data,this paper summarizes hydrochemical feature of the brine and geochemical characteristics of water-solublerock salt of Triassic strata in Sichuan Basin,and finds that salinity of brine is 100-3 80g/L,which is very high.In the brine,the ions aremajorCl-and Na+,low SO42-and HCO3-,and the main hydrochemical type of the brine is Cl-Na.Geochemicat analysis of salt formation reveals that the contents of Na+and CL-are the highest while the contents of Ca2+and SO42-are the second,which indicates that the main mineral is halite,containing some anhydrite,and the change of K+ and Mg2+ content is related to the content of polyhalite.The paper gives much emphasis on hydrochemical characteristics coefficient(K×103/Cl,Br×103/Cl and K/Br)of favorable structure of potassium,which has instructional significance to look for potassium resources.Based on this,the paper discusses formation mechanism of potassium-riched brine and polyhalite.In the process of implementing simultaneous exploration of oil and potash,there are a great many well logging data as an indication of potash,such as GR,NGS,AC,CNL,DEN and RT.Through summarizing logging curves of variousminerals of potassium formation,there is obvious curve difference among rock salt,gypsum and polyhalite,but logging curves ofpolyhalite with impurelithologyhave no obvious difference,and it's very hard to use single well logging curve to identify potash minerals,which can usuallybring about different explanations.Seeing that study area is very large with more wells,this paper comprehensively adopts Cross Plot,Fisher Discriminance,Neural Network and Support Vector Machine Algorithm to identify lithology rapidly,then through contrasting these four methods,finds that it can effectively identify polyhalite stratum with high recognition accuracy by Neural Network and Support Vector Machine Algorithm.For brine layer,the paper proposes combination logging identification method,and considering limited data of NGS in study area,water saturation calculation method can be a better choice to identify polyhalite stratum.Based on lithologic identification,the paper establishes physical model of volume to calculate volume content of polyhalite and apparent content of K2O,and discusses the purity of polyhalite.With water chemistry data,conventional well logging data and NGS data,the paper discussses calculation and its accuracy of potassium of brine layer;through analysis of single well in favorable structure of potassium in Sichuan Basin,make sure distribution and thickness of polyhalite stratum and rich potassium brine stratum.Finally,this paper implements the distribution law of polyhalite and potassium-riched brine and their resource extent.The methods of calculating the resources of potassium-riched brine include volumetric method,finite element method and unsteady analytical method,Also using seismic attribution to predicte polyhalite.At last,an effective evaluation method of potassium resources in Triassic of Sichuan basin can be established,which will play an important role in later stage of exploration and development of potassium resources.