THE OILFIELD WATER IN TAHE AREA, TARIM BASIN —CONSTRAINTS FROM ITS ELEMENT COMPOSITION

Abstract

The wide range variation of composition of (oilfield) formation water in the different geological era are likely to represent the complex origin and evolution history of fluid. It is thus important to understand how to describing the evolution of water. In order to demonstrate the relative richness of elements in the evaporation and condensation of seawaters and effect on composition variation of oilfield water in water-rock reaction, we have studied the evolution of water in terms of oxygen, carbon isotopes and a great number of chemical components of oilfield water, combined with the comparison among the components of oilfield water or the average seawater composition or brine water undergone the evaporation and condensation of seawater or meteoric water in the dilution and weathering process. The results suggest the marine carbonate rocks -related origin of Ordovician formation water with good proxy indicator of ratio of Cl~-/Br~- (322～337),which approach to the minimum richness content in a preserved pHanerozoic marine seawater, as well as high richness of halogen elements of Br,Cl and I, volatility elements of F,Cl and B,and Iron cluster elements of V,Cr, and so on , the Carboniferous formation water is characteristic of dilution and filtration process with ratio of Cl~-/Br~- as high as 1 399.8,while the Triassic formation water contains relatively high contents of Pro-copper cluster elements and germanium(Ge),indicating a significant input of meteoric water during the late diagenetic process. the isotope records of δ ~(18)O、 δ D and~(87)Sr/~(86)Sr of formation water shows a significant feature of “metamorphic water ”with a much negative shift of δ ~(18)O of Triassic formation water. In conclusion, the composition of Ordovician formation water have been mainly controlled by evaporation and condensation of ancient seawater, undergone a series of dilution input and mixing of meteoric water in the early period of Hercynian, the Carboniferous formation water have been related to dilution and filtration as well as chemical interaction between the meteoric water and gypsum-bearing or halite rocks; while the Triassic formation water maybe derived from mixing and rock interaction with meteoric water.