柴达木盆地西部地区新近系藻灰岩沉积特征及形成机制

柴达木盆地西部地区（简称“柴西地区”）新近系主要为陆源沉积岩，湖相碳酸盐岩亦广泛发育，其中藻灰岩的储集性和含油性最好。为进一步探究柴西地区新近系藻灰岩沉积特征及形成机制，主要通过详细的岩心和岩石薄片观察分析，对油泉子、黄瓜峁、南翼山、大风山等地新近系的藻灰岩进行研究。研究表明:1）柴西地区新近系藻灰岩主要有层纹石灰岩、叠层石灰岩和凝块石灰岩3种类型，以凝块石灰岩最发育。层纹石灰岩中纹层呈较连续且相互平行的水平纹层状，单个纹层最大厚度一般不超过0.5 m；叠层石灰岩主要有柱状叠层石灰岩和锥状叠层石灰岩2种类型，柱状叠层石在岩心上呈高宽比较小的灌木状，锥状叠层石则呈倾斜于地层面的指型密集簇状，叠层石最大单层厚度仅为0.25 m；凝块石灰岩在岩心上表现为表面粗糙的团块状，显微镜下藻凝块内部显微组构为不均匀的云雾状和海绵状。此外，有较多叠层石灰岩和凝块石灰岩呈不规则的砾屑状产出，且发育有明显的同生变形沉积构造。2）柴西地区新近系层纹石灰岩均为原地沉积，而叠层石灰岩和凝块石灰岩除原地沉积外，以原地垮塌沉积为主，以近原地滑塌沉积为次。3）柴西地区新近系藻灰岩同生改造有2个主要控制因素，一是藻灰岩与其基底灰质泥岩岩性硬度的差异，二是以底流为主、波浪为辅的水动力，次要因素为地震的振动、浅湖斜坡的坡度以及叠层石和藻凝块的倾斜生长等。

Sedimentary Characteristics and Formation Mechanisms of Neogene Algal Limestone in Western Qaidam Basin

The Neogene in the western Qaidam Basin is mainly composed of terrigenous sedimentary rocks； lacustrine carbonate rocks are also widely developed. Of these， algal limestones have good properties for both oil reserves and saturation. The sedimentary characteristics and formation mechanism of the Neogene algal limestones in the Youquanzi， Huangguamao， Nanyishan， Dafengshan etc. areas were studied mainly by detailed core observation and slice analysis. It was found that they could be classified into three types: （1） microbial laminated limestones （maximum thickness generally no more than 0.5 m， in horizontal， continuous and parallel laminations）； （2） columnar stromatolitic limestones （maximum single layer thickness 0.25 m） appearing either as shrublike structures with small height?width ratio， or as conical， dense finger-like clusters inclined to the strata； and （3） thrombolitic limestones， appearing in the cores as block-shaped with a rough surface， and with unevenly cloudy and spongy fabrics when viewed under a microscope. （4） Syn-deformational structures occur in both the stromatolitic and thrombolitic limestones， distributed either in a breccia-like fashion in some of the lime mudstones in an isolated “floating” state， or as mixed accumulations of differently sized irregular strips. （5） The microbial laminated limestone has all been deposited in situ. However， most of the stromatolitic and thrombolitic limestones are mainly collapsed in situ deposits， with some slumped deposits nearby. （6） The syn-deformational structures have resulted from two main influences: the different hardness of the limestones and basement lime mudstones， and the hydrodynamics generated chiefly by underflow， and secondly by wave action. Other factors include， among others， seismic activity， slope angles in shallow lakes and the inclined growth of stromatolites and algal thrombolites.