In the current mud logging gas measurement technology, the hydrocarbon gas mainly comes from the electric degasser, which churns up the drilling fluid to separate the hydrocarbon gas, and to get C₁—C₅ gaseous hydrocarbon components. The content of the hydrocarbon gas detected by this way is, on the one hand, affected by the drilling fluid, and on the other hand, due to the uncertainty of the air content, the oil and gas display is only evaluated qualitatively. The semi-permeable membrane can separate selectively the hydrocarbon components, and can be directly inserted into the drilling fluid to separate the oil and gas, and thus the hydrocarbon components of the gas and liquid can be obtained at a normal temperature. This method enjoys a number of advantages, such as, it is less affected by the drilling fluid, and it may provide a quantitative evaluation of the oil and gas content. Therefore, in order to evaluate quantitatively the hydrocarbon gas sources in the mud logging gas measurement technology, a proper semi-permeable membrane and the related apparatus for the oil and gas separation are developed. The parameters, such as the response time, the degassing efficiency, the gas attenuation, and the response characteristic of C₁—C₈ (including benzene and toluene) are studied. It is shown that the oil and gas can be detected while drilling at the well site, with a richer information about the content of the hydrocarbon detection and the hydrocarbon reservoir evaluation.