柴达木盆地英西地区渐新统热液改造型白云岩的发现及意义

Discovery and Significance of Dolomites Altered by Hydrothermal Fluid in Oligocene Reservoirs of the Yingxi Area,Qaidam Basin

通过岩心及微观岩石学特征、结构特征、微量元素和碳氧同位素地球化学特征的综合研究,首次在柴达木盆地英西地区发现了一种罕见的热液改造型白云岩储集层,但热液改造程度不强,主要包括以下几个方面:1主要储集层较为致密,主要发育泥晶结构的铁白云石和方解石,偶见钠长石、黄铁矿、重晶石和天青石等热液矿物;2碳酸盐矿物中富含FeO、MnO等,其平均质量分数分别为4.112%和0.186%,显著高于本区准同生交代成因白云石中铁锰含量,且钙、镁含量也有所升高,反映了热液流体为富含Ca^(2+),Fe^(2+),Sr^(2+),Mn^(2+)和少量Mg^(2+)等多种离子的碱性热卤水;3ΣREE具有较大的变化范围且总量较高,稀土元素配分模式具有明显的Eu负异常,代表了湖泊环境特有的热流体稀土元素组成特征,形成于偏碱性的深湖热卤水还原环境;4δ18OPDB变化范围为-6.8‰^-3.6‰,平均值为-5.2‰,介于热液白云岩和准同生交代成因白云岩之间,显示了英西地区渐新统白云岩在准同生交代成因基础上受到了一定程度的热液改造。裂缝的发育程度和与热液通道的距离远近不同而使得样品的改造程度存在一定的差异性。综上所述,热液改造型白云岩储集层的首次发现开拓了柴达木盆地英西地区深层致密油气勘探的新视野,这一研究成果对英西深层寻找优质"甜点"储层具有重要指导意义。

Through the comprehensive study of core and micro structure characteristics,petrology,trace elements and carbon and oxygen isotope geochemical characteristics,a rare dolomite reservoir altered by hydrothermal fluid is discovered for the first time in the Yingxi area of Qaidam Basin,but the hydrothermal alteration degree is not strong.The evidence mainly includes the following aspects：（1）the main reservoir rocks are dense,mainly developed with ankerite and calcite with micritic structure.Albite,pyrite,barite,celestite and other hydrothermal minerals can be found occasionally.（2）The trace elements of FeO and MnO are enriched in carbonate minerals,at the average 4.112% and 0.186% respectively.They are significantly higher than the penecontemporary metasomatism dolomites.The contents of Ca^2＋and Mg^2＋also increase,reflecting that the hydrothermal fluid is alkaline hot brine rich in Ca^2＋,Fe^2＋,Sr^2＋,Mn^2＋and a few Mg^2＋.（3）REE has a large range of changes with higher total amount.Obvious Eu negative anomaly is displayed on the REE distribution model curves.This reflects the REE composition characteristics of the lake environment specific thermal fluid.It formed in alkaline deep lake hot brine environment.（4）The change range ofδ18OPDBvalues is-6.8‰--3.6‰,with an average of-5.2‰.The values are between hydrothermal dolomite and penecontemporaneous metasomatism dolomite.This shows that the dolomite is altered by hydrothermal fluid to a certain extent based on penecontemporaneous metasomatism origin.The degree of hydrothermal modification is controlled by development degree of fracture and the distance between the sample and the hydrothermal channel.In summary,the discovery of dolomite reservoirs altered by hydrothermal fluid for the first time opens up new horizons for the deep tight oil and gas exploration of Yingxi area in Qaidam Basin.This research results can be an important guide for finding quality＂dessert＂reservoirs in the deep strata of Yingxi area.