川东北下三叠统飞仙关组鲕粒滩白云岩同位素地球化学特征*

川东北地区下三叠统飞仙关组为浅海碳酸盐岩夹泥页岩与蒸发岩序列，而在碳酸盐岩台地边缘通常发育一些白云石化的鲕粒滩。这些鲕粒滩白云岩储集层是川东北地区主要的产气层，一些学者认为该套白云岩为大气淡水与海水的混合水白云石化成因，另外一些学者将其视为回流—渗透白云石化成因。飞仙关组鲕粒滩白云岩稳定同位素氧值一般为-6.73‰～-3.65‰(PDB)，平均值为-4.89‰(PDB)（罗家寨地区为-10.81‰(PDB)），稳定同位素碳值一般为+0.57‰～+3.00‰(PDB)。对基质和孔洞中充填的鞍状白云石和亮晶白云石胶结物而言，稳定同位素⁸⁷Sr/⁸⁶Sr值为0.70735～0.70800。这些有关鲕粒滩白云岩的数据表明白云石化作用是在埋藏条件下进行的。在测定流体包裹体的均一化温度后，计算出白云石化流体稳定同位素氧成分(δ¹⁸O_白云石-δ¹⁸O_水=［3.2×10⁶ T^-2］-1.5，来自 Friedman 和 ONeil（1977）)，其平均值约为+4‰(SMOW)。根据流体稳定同位素氧、碳成分与海水蒸发时流体盐度的正相关性，计算出流体δD平均值约为+25‰(SMOW)。流体包裹体盐度测定表明，白云石化流体是一种超盐度卤水，其盐度是海水的数倍，白云石化的温度为90～130℃。由于下三叠统鲕粒滩白云岩的稳定同位素氧和碳成分与上二叠统生物礁白云岩的稳定同位素氧和碳成分类似，因此，它们的白云石化流体很可能是同一来源。然而，这一结论还有待于进一步研究。

Isotopic geochemical characteristics of oolitic bank dolostones in the Lower Triassic Feixianguan Fornation in northeastern Sichuan Province

A shallow marine carbonate rock sequence with shale and evaporate developed in the Lower Triassic Feixianguan Formation in northeastern Sichuan Province.The edge of carbonate platform commonly developed some oolitic banks,which are dolomitized.The Feixianguan Formation which are formed by oolitic bank dolostone is the most important gas reservoir in northeastern Sichuan Province. Some researchers considered them as the origin of mixing for fresh water and sea water,the others make up them into model for dolomitization by seepage-reflux.The Feixianguan oolitic bank dolostone included stable isotopic oxygen composition of -6.73‰～-3.65‰(PDB)(-10.81‰ in Luojiazhai area),the average is -4.89‰(PDB),stable isotopic carbon composition of +0.57‰～+3.00‰(PDB),for matrix and vug-filling saddle dolomite and bright dolomite cement,stable isotopic strontium composition of 0.70735～0.70800 for 87Sr/86Sr rate.These data about oolitic bank dolostone indicate that the dolomitization was formed in the buried condition.After measuring fluid inclusion homogenization temperature,dolomitization temperature and isotopic oxygen of dolomite are known,and then the isotopic oxygen composition of dolomitizing fluid can be calculated(δ18Odol-δ18Owater=3.2×106 T -2]-1.5,from Friedman and O’Neil (1977)). The average is about +4‰ (SMOW).According to the positive covariance between fluid isotopic oxygen and carbon composition and fluid salinity during evaporation of seawater,fluid δD can be calculated.It is about +25‰(SMOW).The measurement of fluid inclusion salinity suggest that dolomitizing fluid is a brines,which is about several times of seawater.Dolomitizing temperature changes from 90 ℃ to 130 ℃.As far as our knowledge goes,stable isotopic O and C composition of the Lower Triassic oolitic bank dolostone is similar to these of the uppermost Permian reef dolostone.Therefore,it is possible that their dolomitizing fluids are identified source.However,the reason for this conclusion needs to be perfected in the future.