塔里木盆地下奥陶统白云岩化流体来源的地球化学分析

塔里木盆地是中国重要的含油气盆地,其中奥陶系白云岩是主要储层之一,然而有关白云岩成因的问题众说纷纭,其中的关键是对于白云岩化流体性质的认识不一。元素地球化学和同位素测试分析表明,白云岩的稀土元素来源大部分继承原始灰岩,具有与海水类似的配分特征,只不过稀土元素的含量明显增高了;Sr、Ti含量和Sr/Ba比值较高,且Sr和Mn都具典型的埋藏演化特征;碳、氧同位素分布于正常海相范围内,与典型的埋藏白云岩特征类似;在局部发育一些与断裂相关的铁(锰)白云岩和鞍状白云岩,显示明显的Eu正异常,具相对较高的Fe-Mn-Ba-Si(-Zn-Pb)元素含量及Sr/Ba比值。分析结果说明塔中和塔北下奥陶统的白云岩化流体主要为正常或浓缩了的海水,白云岩形成于准同生和/或埋藏环境,局部发育与断裂相关的热液白云岩,而前人强调的大气淡水或混合水作用对白云岩的形成并不重要。

Geochemical analyses on dolomitizing fluids of Lower Ordovician carbonate reservoir in Tarim basin

Tarim basin is one of the most important hydrocarbon-containing basins in China,of which dolomite is one of the major reservoir rocks.Its reserving ability was partly determined by the dolomitization fluids.The origins and characteristics of those fluids are investigated through geological and geochemical methods,i.e.,trace elements,C and O isotopes,especially the new data of rare earth elements(REE).The results show that most of the lower Ordovician dolomites display marine-like REE and trace element features,and normal marine C isotope compositions,which may have undergone a normal burial evolution,indicating that the dolomitization fluids are normal or condensed seawater.Additionally,there are still some samples developing along fractures as Fe(Mn)-dolomites,saddle dolomites or dolomite veins,coexisting with hydrothermal pyrites,sphalerites,and silicon minerals.These samples are formed by hydrothermal fluids with typical Eu anomalies and high Fe,Mn,Ba,Si and Zn contents,and Sr/Ba ratios.Therefore,dolomites in studied area were formed mainly by syndepositional and burial dolomitization,locally by hydrothermal dolomitization,while not in mixing zone as former studies showed.