Abstract: The testing results of trace elements in the northern margin of Qaidam basin show that contents of Sc, Ti, Cr, Ni, Zn, Cs, La, Tb, Ho, Lu in semi-deep lake facies are obviously higher than those in the other sedimentary settings; the Cr, Co, Cu, Ni, Cs, La and La series elements have positive correlations with Ti and Fe, but negative correlations with K, Si, Al; the Ni/Co ratio (4.22) and δCe value (3.9) from Well Yu-25 indicate that the Middle Jurassic is reducing environment. The vertical changes of the trace elements show that the Middle Jurassic in Well Yu-25 experiences two up-down base level cycles. Based on the comparison of trace elements between test samples and standard samples, the sandstone in the northern margin of Qaidam basin is believed to be basic-neutral provenance. It is suggested that trace element is of great significance in sedimentary environment analysis, searching for source rocks and oil-gas exploration.

Key words: Qaidam basin, trace element, sedimentary environment, oil-gas geology, significance