Abstract: Based on analysis of the conditions of pool-forming and their space-matching relationship of Gucheng oilfield in Biyang Sag, the pool-forming stages are determined by using fluid inclusion technique, combined with the results of hydrocarbon generation simulation, fine oil-source correlation and tectonic evolution history, the styles of oil migration & charging and the processes of pool-charging are discussed in detail. The above results indicate that there exist three stages of oil charging: (1) In the first stage of oil charging, the large amount of low-mature oil-gas generated from the upper part of first member of Eogene Hetaoyuan Formation, slowly poured toward to Gucheng nose tectonic, and formed a certain oil-gas reservoirs;(2) In the second stage during Early Himalayan, the tectonic activity resulted in uplifting and erosion of regional strata and faults activity, meanwhile destroyed the paleo-reservoirs and oil full-charged largely and rapidly ,which generated from the upper part of third member of Hetaoyuan Formation, so drove and replaced oil/gas generated from the lower part of third member of Hetaoyuan Formation, finally formed lots of fault-block and fault-nose reservoirs, laid a foundation for the distribution framework of present Gucheng oilfield;(3) In the third stage during Later Himalayan, new tectonic movements resulted in the adjustment of oil/gas reservoirs formed in the first and second pool-forming stages, and made a few high-mature oil/gas recharge and formed present Gucheng oilfield.

Key words: reservoir-forming stage, tectonic uplifting;reservoir-forming mechanism;Gucheng oilfield;Biyang sag