the correlation coefficient and envelope area of porosity logging and resistivity logging is very sensitive to response characteristics of the reservoir. Relative to water, correlation coefficient of oil layer significantly increase, envelope area also increase significantly. Using variation characteristics of the reverse of the correlation coefficient and the envelope area, after reasonable scale in the same curve way, the normalization of the correlation coefficient and the area of the envelope, can form clear image of envelope curve. By computing the correlation coefficient and envelope area and combining with test data ,we can receive quantitatively and identify the relationship between oil and water layer chart. The method is used to mudstone reservoir in Y oilfield , receive a good effect, and widening the road to using logging data to identify oil layer in area.