川南地区灯影组白云岩地球化学特征及流体来源

Geochemical characteristics and fluid origin of the Dengying Formation dolomites in southern Sichuan Basin

四川盆地灯影组白云岩是重要的优质油气储集岩,由于复杂的成岩演化和多期的构造作用,白云岩的成因一直存在争议。通过碳、氧同位素、微量元素等分析手段,对川南地区灯影组白云岩的地球化学特征及流体来源进行了研究。研究结果表明,该区白云岩主要类型为微晶、细晶-中晶以及含角砾白云岩。灯影组白云岩的δ^(13)C介于0.52‰~2.06‰(均值1.42‰),δ^(18)O介于-11.9‰^-2.14‰(均值-9.09‰),与同期原始碳酸盐岩δ^(13)C值(4.43‰)和δ^(18)O值(-0.62‰)相比明显偏小。结合白云岩具有高U/Th比值、低Sr值以及Eu正异常等特征,认为研究区灯影组白云岩具有埋藏白云石化特征,且在成岩后期受到了热液流体作用的改造。该区白云岩具有与海水来源白云岩相似的稀土元素配分特征以及远高于正常海水的盐度指数(Z值),表明白云岩化流体主要来源于封存在地层中的浓缩海水。

Dolomite of the Dengying Formation in southern Sichuan Basin has been studied a lot as an important kind of reservoir rocks for oil and gas.However,origin of the dolomite is still in controversy.This paper studied the geochemical characteristics and fluid origin of the dolomite in the Dengying Formation in southern Sichuan Basin through analyses of C-O isotopes and trace elements.The results show that the main types of dolomites are micritic dolomite,fine-medium dolomite and breccia-dolomite.The δ13C and δ18O values are 0.52‰-2.06‰ （PDB） and -11.9‰--2.14‰ （PDB）,respectively,which are lower than the original values of δ13C （4.43‰） and δ18O （-0.62‰） of carbonates of the same period.The dolomite has high U/Th ratio,low Sr and high Eu anomalies.It is believed that the dolomite formed in a burial diagenetic environment and was altered by hydrothermal fluid later.REE distribution patterns of the dolomite is similar to the dolomite originated from seawater.Salinity indexes（Z）of the dolomite is higher than those of normal seawater.Therefore,it can be inferred that the dolomitization fluids mainly originate from the condensed seawater trapped in the Formation.