On the basis of geological analysis and well log interpretation, the characteristics of high gamma ray reservoir of Yanchang formation are analyzed. The results show that high gamma ray reservoir has higher content of feldspar, thorium (Th), uranium (U) and nearly constant content of potassium (K). The characteristics of Th and U contents are just like the tuffs. Second, the results show that the development degree of high gamma ray reservoir increases from Chang6 formation to Chang 4 + 5 formation, while the development of tuffs becomes weaker. Thirdly, the well log curve characteristics of higher gamma ray, higher acoustical slowness and lower resistivity for high gamma ray reservoir are just like the tuffs. The three characteristics indicate that the characteristics of higher gamma ray might be caused by homochronous sedimentary volcano tuff ash or previous tuffs. The research results about the four-property relationships of high gamma ray reservoir show that: (1) high gamma ray reservoir can be identified by the overlap of gamma ray and spontaneous potential curves, overlap of acoustic curve and spontaneous potential curve; (2) high gamma ray reservoirs have the preferable secondary pores with strong heterogeneity; the better the physical properties are, the more obvious the differential amplitude of overlap of acoustic curve and spontaneous potential curve is; (3) the computation of differential amplitude of overlap of spontaneous potential curve and spontaneous potential curve can reveal the characteristics of reservoir oil-bearing. If the differential amplitude of overlap is obvious, the oil-bearing characteristic is better.
