Abstract: Rich in limestones,Triassic and Permian in Pengshui Block in southeastern Chongqing are prone to circulation lost through cavities;filtrating loss and intermittent severe circulation lost are frequently encountered during drilling through the target formation,Silurian Longmaxi,which is rich in fractures.From drilling practice of Well PY HF-1,PY 2HF,PY 3HF and PY 4HF,the features and reasons of circulation lost in this block were analyzed and leak prevention and plugging techniques and specific operation procedure were presented in this paper.Analysis showed that air drilling would be favorable for leak prevention and sealing in the upper formations of Pengshui Block,while leak sealing and layered sealing were suitable for lower formations in this area which has extensive and irregular thief zones.Further analysis indicated that better understanding on the geologic conditions,optimized casing program,drilling mode,and proper mud density could lower the incidence of circulation lost.Proper plugging formula,plugging agent evaluation,scientific plugging measures,and well specified drilling operation are the major means to solve circulation loss issues.