The cementation exponent, m, is a major factor of uncertainty in the calculation of hydrocarbon/water saturation in heterogeneous carbonate reservoirs. Hydrocarbon saturations as high as 70 or 80% calculated with the conventional value m=2, may disappear completely with m values of 3 and 4, which are quite common in carbonate rocks. Laboratory data obtained in heterogeneous carbonate reservoirs often show a wide scatter. Constant (average) m values are usually used in spite of such scatter, and data points with m values much higher than the average value are often rejected to obtain straight-line trends. This paper presents the results of a study of the relationship between variable m values measured on core plugs and detailed carbonate rock types. The scatter in the data is analyzed in terms of rock type. The authors conclude that the high m values are often representative for specific rock types and that these values should not be rejected but applied selectively in log analysis over those intervals where these rock types occur.