咸化湖盆混积岩中双重孔隙介质及其油气储集意义

摘要

The Lower Ganchaigou Formation of Eocene in Yingxi area of Qaidam Basin is mixed sediments in saline lacustrine basin.It consists of lacustrine carbonate,detrital material and mud,and all the rocks contain many salt minerals.Based on core observation,physical property analysis,capillary pressure test,imaging logging and field emission scanning electron microscopy data,the mixed sediments contain double-porosity system.One is fracture and salt mineral dissolution pore with big pore radius, another one is crack between the salt and dolomite intergranular pore with small pore radius.The two types of pores build the “narrow lower-wide higher double-platform”structure on the capillary pressure curves.On the whole,the physical property of the rock is very poor with the characteristics of “low porosity-ultra low permeability”.The pores and throats of the rock are generally fine.Electron microprobe analysis shows that the element composition of dolomite has the geochemical characteristics of “high contents of silicon and aluminum content,and low contents of iron and manganese”.The average mass fractions of FeO and MnO in the dolomicrite are 1.322% and 0.060%,which are obviously lower than that of Fe and Mn in the hydrothermal dolomite.It is considered that the dolomicrite has a penecontemporaneous metasomatic formation mechanism.A large number of shrinkage intergranular pores are produced during the dolomitization process.This kind of matrix pore as the storage space is an important support factor of sustained and stable production of oil and gas in Yingxi area of Qaidam Basin.%柴达木盆地英西地区始新统下干柴沟组为咸化湖盆沉积的混积岩,其组分包括湖相碳酸盐、碎屑颗粒、泥质等,且岩石中含较多的盐类矿物.通过岩芯观察、物性分析、毛管压力测试、成像测井和场发射扫描电镜研究发现,混积岩具有双重孔隙介质,一类为孔径较大的裂缝和盐类矿物溶蚀孔,另一类为孔径较小的盐间缝和白云石晶间孔,共同构成了毛管压力曲线上的“低窄高宽的双平台”结构,岩石物性整体上较差,以“低孔—特低渗”为主,孔喉普遍较细.电子探针微区成分分析结果表明,白云石元素组成具有“高铝硅低铁锰”的地球化学特征,FeO 平均质量分数为1．322％, MnO 平均质量分数为0．060％,明显低于热液白云岩中的铁锰质量分数,为准同生交代成因的典型特征.白云石化过程中产生了大量收缩晶间孔,这类基质孔隙储油是支撑英西地区持续稳产的重要因素.