柴西地区始新统湖相白云岩储层地球化学特征及形成机理

近年来柴西地区始新统致密碳酸盐岩中的油气勘探获得重大突破,其中白云岩为最优质的储集岩。通过岩石学、结构学和地球化学等多方面的综合研究，详细描述和论证了柴西地区两种湖相白云岩的成因及特征：一种是数量较少的原生生物成因的藻云岩，电子探针能谱面扫描分析结果显示其Mg元素富集区与藻纹层分布高度吻合，充分显示了其微生物成因特征，因微生物活动能够克服动力学障碍，为白云石形成营造出有利的微环境；另一种为研究区广泛分布的准同生交代成因的泥晶白云岩，偏光显微镜观察和X射线衍射分析结果显示，其中均含有碎屑颗粒和泥质组分，由周缘辫状三角洲带来的碎屑物质与其混积而成，电子探针岩石组分分析结果显示，其FeO的平均质量分数为0.942%，MnO的平均质量分数为0.052%，明显低于热液白云岩，且富含SiO2和Al2O3，充分显示其准同生交代成因特征。岩石地球化学数据显示：泥晶白云岩微量元素蜘蛛网图较为离散，表明了混积的湖相白云岩的物质来源的多样性，其∑REE平均值小于平均上地壳∑REE值(146.37×10-6)，但远大于球粒陨石∑REE值(2.56×10-6)，且不具有Eu负异常。碳同位素负偏移主要受其自储的有机质氧化解体产生的CO2和周缘辫状三角洲供给的地表水影响，增加了其轻碳组分；氧同位素发生正偏移主要受咸化湖盆沉积环境影响，蒸发作用使得湖水δ18O增加，虽然周缘河流水系能带来部分轻氧组分，但收效甚微。这揭示了准同生交代成因的白云岩与热液造成氧同位素偏负截然相反。泥晶白云岩在形成过程中产生大量弥散性晶间孔，孔隙半径小而数量众多是形成“中孔特低渗”型储层的主要原因，这类孔隙抗压实能力强,为研究区最好的油气储集体。这一研究成果对柴达木盆地深层致密碳酸盐岩的油气勘探具有重大意义。

The geochemical characteristics and formation mechanism of the Eocene lacustrine dolomite reservoirs in the Western Qaidam

In recent years, important breakthrough has been made in exploring petroleum from dense carbonate reservoirs of Eocene Epoch in Western Qaidam Basin. Among the carbonate rocks in the reservoirs, dolomite proves to be the high quality reservoir rock type. By the comprehensive petrologic, textural and geochemical studies, we describe and demonstrate the origin and characteristics of the two sorts of lacustrine dolomites from western Qaidam Basin. One is the protistic algal dolomite, which is rare in this area. The analytical results of electron probe energy spectrum scan showed that the Mg enriched zone and the distribution of algal lamination are highly consistent,which, therefore,strongly indicates its microbiological characteristics because the microbial activity can overcome dynamic obstacles and create a favorable microenvironment for the formation of dolomite. Another is the penecontemporaneous metasomatic micritic dolomite, which are widely distributed in the study area. The results of the observation with polarizing microscope and X ray diffraction analysis showed that most of the dolomicrites contain detrital grains and argillaceous components, which is due to the mixed deposition of detritus that brought from the peripheral braided delta. The results of electron microprobe analyses on rock components showed that the average content of FeO is 0.942% and that of MnO is 0.052%. These data are significantly lower than those of hydrothermal dolomites. And the SiO2 and Al2O3 contents are also higher, which fully show a penecontemporaneous metasomatic origin. The discrete rare elements spider diagram shows the diversity of the source material of this mixedly deposited lacustrine dolomite. The ∑REE average value is less than that of the upper crust ∑REE(146.37×10-6), but much larger than the value of chondrite ∑REE(2.56×10-6) and without Eu negative anomaly. Carbon isotope negative offset is primarily impacted by the CO2 produced from the oxidation of reservoir organic matter and the surface water provided by the peripheral braided delta, which increased its light carbon components. The oxygen isotope positive offset is mainly impacted by the saline lacustrine basin depositional environment. Evaporation makes δ18O increase in lake water. Though the surrounding river system could bring forth some light oxygen components, it caused little success. It obviously reveals that penecontemporaneous metasomatic dolomite is opposite to the hydrothermal ones which caused the oxygen isotope positive offset. During its formation process dolomicrite generated a lot of disseminated intergranular pores, on account of intergranular diffusion process. And the pore radii are small while the number of them are numerous, which is the major cause for the forming of “medium porosity extra low permeability” type reservoir. This type of pore has a strong ability to resist compaction. It is the best oil and gas reservoirs in study area. The research results have great significance for the exploration of oil and gas in the deep seated dense carbonate rocks in Qaidam Basin.