塔里木盆地塔河油区油田水元素组成与形成

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

塔河油区的不同地质时代的油田水成分差异较大、成因复杂。该文根据对塔河油区油田水的常规成分、微量元素和同位素特征统计分析,并将不同地质时代的油田水与原始海水平均含量、氯化钠蒸发阶段中海水的含量相比反映其相对的浓缩程度,同时与淡水平均含量及水岩反应结果来推测淡水淋滤等水岩反应作用程度。研究表明,塔河奥陶系中的油田水中Cl-/Br-为322～337,接近残余海水中卤素元素中溴富集的低限值,同时相对富集卤素元素或挥发份组分和铁族元素中的铬、钒,因而具有典型海相碳酸盐岩中的油田水特征;而石炭系中油田水中为Cl-/Br-为1399.8,具有贫溴的溶滤水特征;三叠系碎屑岩中油田水中相对富集了亲铜元素和高含量锗。上述油田水中大多均具有"变质水"的δ18O、δD值,三叠系油田水δ18O负偏更明显,指示了后期可能受淡水改造;奥陶系油田水主要受原始海水控制,早期存在大气水淋滤-淋溶作用,石炭系油田水经过含膏层过滤反应及大气水的淋滤;而三叠系油田水则经大气淡水的淋滤-水岩等复杂的地质过程而形成。

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.