The main reason why the application of nuclear technology in petroleum exploration has not yet been accepted by most exploration workers is that they are not clear about the homologous distribution features of oil and gas fields and radioactive radiation. The authors hold that the disequilibrium of uranium, radium and radon as a natural radioactive series is the basic feature in the use of this technology in petroleum exploration. The invention Gamma-ray Spectral Measurement of the Equilibium Coefficient Kp and Its Embodiment of the senior author now can readily solve that problem and replace the impedient measure of normalization of uranium and potassium to thorium that had to be proposed before. Application of this impedient measure has some limitations. In areas where the surface is covered by beach or river sands, thorium minerals such as monazite may be concentrated by placering. This could result in local thorium highs that would yield local uranium and potassium lows after normalization to thorium, and these would constitute false anomalies.This paper elucidates the relation between the equilibrium coefficient Kp and petroleum exploration and points out that immediately over petroleum the radioactive equilibrium is biased with uranium, while at peripheries of the petroleum reservoir the equilibrium is biased with radium; therefore, the true uranium content immediately over petroleum is not lower but higher, and by constrast the uranium content at peripheries of the petroleum reservoir is not higher but lower. This paper further introduces the main technical requirements for the instrument used in petroleum exploration with nuclear technology and the design basis and work procedure of the petroleum-field effective equilibrium coefficient ground detector and finally introduces the applicability of the detector in petroleum exploration through the distribution features of the equilibrium coefficient Kp of five types of trap-type petroleum deposits.