Salt Rhythms Characteristic Of M₅⁶ In Majiagou Formation And Its Potash Exploration Significance,North Shaanxi Salt Basin In The Ordos Basin

Potash has very important agricultural value and is one of the most scarce resources in China.The salt basin in Northern Shaanxi is located in the Middle East of the Ordos Basin.Different degrees of potassium mineralization has now been found in the sixth sub-section of the fifth section of the majiagou Formation(m^5-6)in Ordovician basin,which is the most important target area for the exploration of large marine potash deposits in China.On the basis of the data collected by the predecessors,combined with wubao 1 well?yu 9 well?suijia1 well?suitan1 well?zhenjia1 well and mitan1 well drilling core data,the characteristics of sedimentary and geochemistry are discussed in the salt rhythm of the m^5-6.The characteristics of salt rhythm have been identified,divided and contrasted,and the bromine content,the bromine/chlorine index and the potassium content in the rhythm are also tested,we find that:First,there are four salt rhythms in the east depression and the west has three,which indicates the salt formation in the east takes a longer time and is favorable for potassium formation.Also,there are four quaternary relative sea-level rise and retreat changes in the period of m^5-6.Western salt caverns are poorly closed and the salt formation time is short.In the later period of m^5-6,the transgression of the western Qilian Sea is enhanced,which is not conducive to potassium formation.Carbonate-salt group is more common in the sedimentary rhythm showing the salt formation feeds on eastern North China Sea during the m^5-6.The enhanced desalination caused many interlayers in rhythm in the late m⁵-6-6 and the influence of seawater on the western salt depression is obvious in the process of desalination due to its poor sealing property.The desalination of the western Qilian was strengthened in the late period of m^5-6.More dilute interlayers appear in the rhythm,implying the beginning of the marine transgression in the m⁵-5-5 sub-period.The bromide ion concentration and bromine chlorine/index in the east second salt show a trend of“elevation-stability-decrease”and the west second and the east first gradually reduce.However,the K concentration in the salt pot in the east of the salt pan shows better,and the Br concentration and the Br/Cl index are lower,which may be due to the re-dissolution of minerals and the input of surface water.Finally,the bromine/chlorine index and the potassium content data show that the eastern salt pit is better for potassium,especially the second salt pit,and rhythm IV₂of the m⁵-6-6 is an important potassium-containing layer.The sedimentation center of the salt basin should be located on the west side of the yu 9 well,beside the discrete uplift.Therefore,the rhythm IV₂ of the m⁵-6-6 in the west of yu 9 well is the favorable layer for finding potassium in the future.