Abstract: 〗Through expatiated on the effect of mud invasion on formation resistivity in a case, in this paper the cause of resistive invasion in oil & gas zone is analyzed. While drilling with fresh water mud, especially the ratio of Rmf and Rw is high, massive mud invasion can make the effect on formation resistivity by desalinization of mixed fluid bigger than that by oil saturation decrease, this phenomenon is more obvious while being closer to the borehole, and so, oil and gas zone is characterized by resistive invasion. It is pointed out when drilling with fresh water mud, due to massive mud invasion into formation, oil and gas zone is likely to be characterized by resistive invasion in the case of bad borehole conditions and high mud density.

Key words: fresh water mud, low resistivity oil &, gas zone, mud invasion, resistive invasion in oil &, gas zone, well logging responce