Method for determining carbonate reservoir porosity cutoff

The invention belongs to the field of oil and gas exploration and development, in particular to a method for determining carbonate reservoir porosity cutoff. By the aid of total water saturation SWT and mudstone bound water saturation SWB, accuracy of calculation results is dependent on selection of Ddry and Dwet key parameters, mineral components are analyzed according to TH and K ratios to serve as a selection basis of a dry clay density value, and a pure shale layer with regular boreholes and stable lithology, thickness and log response serves as a wet mudstone density value. The reservoir porosity cutoff is determined according to the relationship between cumulative oil and gas volume and porosity. The method is more accurate as compared with a method for calculating water saturation by a volume model to obtain reservoir oil and gas saturation. The components of clay minerals are analyzed according to the TH and K ratios obtained by natural gamma ray spectrum logging, a proper theoretical density value is selected as the dry clay density value, namely dry mudstone density, data can be more directly acquired, and final results are real and reliable.