塔里木盆地塔中45井油藏萤石化特征及其对储层的影响

Fluoritization in Tazhong 45 reservoir: characteristics and its effect on the reservoir bed

塔中4 5井油藏是位于塔中地区中上奥陶统碳酸盐岩地层中的一个油藏,其储层中发现有大量的被原油充填的萤石脉。岩芯和镜下观察表明,萤石是通过交代方解石形成的,与萤石共生的有大量的热液矿物,如石英、黄铁矿、绿泥石等。萤石中的流体包裹体可分为两期:一期为原生包裹体,均一温度主要为2 6 0～310℃,最高达330℃,为萤石形成时的热液流体温度;另一期为次生包裹体,均一温度主要为10 0～130℃之间,含有大量有机包裹体,代表油气运移成藏的温度。矿物组合和原生包裹体均一温度特征表明萤石的形成与塔中地区热液活动有关。理论计算表明,萤石交代方解石后体积要减少33.5 % ,这将在很大程度上使储层的孔隙度增加。实际岩芯和镜下观察也发现,萤石中有大量的粒间孔隙,并被原油所充填。因此认为,塔中地区的热液活动以及萤石的沉淀在很大程度上改善了储层的储集性,使其更有利于油气的聚集。

Tazhong 45 reservoir, lying in the Middle-Upper Ordovician strata of Tazhong area, is a fluoritized carbonate reservoir. There exist many fluorite veins filled with crude oils in its reservoir bed. The well core and microscope observation indicates that fluorite is formed by replacement of calcite. There are many hydrothermal minerals accompanying fluorite, such as pyrite, quartz and chlorite. Two generations of fluid inclusions are observed in fluorite, namely the primary and the secondary inclusions. Most of the primary fluid inclusions have homogenization temperatures between 260 and 310℃, which represent the temperature of hydrothermal fluid responsible for fluorite precipitation. The secondary fluid inclusions have homogenization temperatures between 100 and 130℃, which represent the hydrocarbon charging temperatures because there are many petroleum inclusions among the secondary inclusions. Characteristics of the mineral assemblage and primary homogenization temperatures indicate that the precipitation of fluorite is related to hydrothermal activities in Tazhong area. Theoretically, the volume will be reduced by 33.5% after the replacement of calcite by fluorite, which is likely to raise the reservoir quality to a great extent. There are also numerous intergranular pores filled with hydrocarbons in fluorite. Therefore, the hydrothermal activities and the precipitation of fluorite in Tazhong area greatly improve the reservoir property and make the Tazhong 45 reservoir more suitable for hydrocarbon accumulation.