Abstract: Aiming at exploring the medium-deep layers of mud diapir belt in the Yinggehai Basin, CNOOC drilled two wells at the end of 20th century. The success of the two wells uncovered the mysterious veil of the medium-deep layers of the Yinggehai Basin, and proved that 1) medium-deep layers of mud diapir belt have basic geological conditions to form gas reservoirs under high temperature and high pressure; 2) free gas reservoirs exist in the area. Thus the geologist's doubt of many years disappeared. If we could do some careful research and analysis on the controlling factors of gas reservoir forming, sizeable gas field is possible to be found in this high temperature and high pressure area.