A New Know of Characteristics and Genesis of Reef and Bank Facies Reservoirs in Ordovician Yijianfang Formation in Tahe Oilfield

The reef and bank facies,widely developed in Middle Ordovician Yijianfang Formation in Tahe oilfield,lies in the north of break of slope in the platform.The reservoir spaces are dominated by intragranular solution pores and cast solution pores.The reservoir not only has a particular log character,but also vertically develops the high system tract of sequence,while the multi-reservoir is controlled by high frequency sequence of the high system tract.On the plane,the reservoir is distributed in four beaches of the northern break of the slope in the platform,with the center of beach controlling reservoir's advantage development.Also the porosity and permeability are both lower in Yijianfang Formation.In addition,the permeable-storage spaces are heterogeneous in the reservoir intervals.The integrated study of cores,slices,fluorescence and electronic probe leads to the conclusion that reservoir's mechanism results from the superimposition of many geologic activities.It is found that the dissolution of syngenesis meteoric water is primarily responsible for the reservoir formation while tectonic fracture is key factor.The unconformable interface of sequence may have improved the reservoir,but the effect is quite limited.