Abstract: A series of oil reservoirs have been discovered in Guantao formation of southern gentle slope in Yellow River Mouth sag in recent years,which contains a large number of low resistivity reservoirs,and there are also low resistivity reservoirs with high capacity and good exploration potential.As the resistivity of oil reservoirs is similar to those of water formation,there will be greater difficulty in logging evaluation.In combination with cores and thin sections,from particle sizes,clay content and clay minerals,pore structure,bound water saturation,and conductive minerals these five aspects,a systematic study on genetic microscopic mechanism of low resistivity reservoirs has been carried out.Through comparison between low- resistivity reservoir interval and normal reservoir interval,the causes such as fine lithology,high clay content,complex pore structure and high irreducible water saturation have been found out.On the basis of this,macro-low resistivity geological origin of low resistivity has been discussed and the results show the low resistivity oil reservoir of Guantao formation are geologically controlled by depositional background and reservoirforming dynamic.