Abstract: C/O Logging is a main logging method that can determine oil saturation, the relationship between C/O values and water saturation, porosity and mud invasion depth was studied by simulating non-elastic scattering gamma spectroscopy under the conditions of horizontal wells. But the type, size, detection efficiency and energy resolution of the detector, and stability and processing method of gamma-ray spectrum will influence the logging data. In addition, the reaction of detector crystal induced by the neutron can change the result. The neutron energy distribution of the oil and water sandstone formation was simulated by using Monte Carlo method, under the condition of different spacing and borehole and formation fluid. It is concluded that the reaction on detector induced by neutron will reduce the distinguish ability to formation fluid according to researching the neutron response of NaI, BGO, LSO and GSO detector. The ability is different when different energy window was selected to distinguish formation by simulating the detector response spectrum of various diameters and length of NaI and BGO crystal. The C/O difference of BGO crystal is higher than NaI crystal when the energy window includes photoelectric peak, the first and the second escape peaks, but it is less affected by the different size of detector crystal. But using photoelectric peak corresponding to the energy window, the C/O difference of BGO crystal detector is much larger than NaI, but also it is increasing with the size of crystal increasing. The C/O value is greatly affected by energy drifting, while relatively a little by energy resolution.