Abstract: The Triassic low-relief structural STM oil and gas pool in the North Tarim Basin is a condensate oil and gas pool with bottom water which produces oil and gas simultaneously.But it is difficult to determine whether it is condensate gas phase or binary phases of gas and liquid underground for lack of high pressure sampling and testing data. Predecessors judged it as condensate gas phase according to hydrocarbon features and the ratio of gas and oil.But it was found that the reservoir with 7m thick middle oil formation in discovery well Sha 29 had no obvious high-moveout responses in acoustic logging curves. The determination of phases has great influence on the evaluation,reverse caluation and development planning of the oil and gas pool.Comprehensive using the physical properties,the ratio of gas and oil,the method of component Z values,the method of Φ values,the method of factor determination,the method of well logging interpretation,the positive model of low frequency seismic responses in gas formation and its coincidence with logging responses within low-frequency ranges and at corresponding depths on the top of traps in structural maps,it is considered that the middle oil formation is an oil and gas pool with condensate gas tops and oil rings.This is proved by well Sha 51 drilled later.