Abstract: To achieve safe drilling in the ultra-thick asphalt layers of Y Oilfield, a contamination control technique was developed.By means of a HTHP sand bed filtrate test, an asphalt emulsifying test, a plugging test and an asphalt hardening test, an optimal plugging slurry formula, emulsifier dosage, chemical plugging formula and asphalt hardener were confirmed.Laboratory studies showed that the plugging while drilling technology could effectively prevent asphalt from invading into the wellbore and that adding emulsifier (RHJ-3) up to 20% (W/W) of asphalt could effectively reduce drilling fluid viscosity and shear force and keep it stable.The optimized chemical plugging slurry was well performed in permeable formations.The asphalt hardener (YHJ-5) could increase the softening point of asphalt by 75 ℃.In a practical application, the plugging while drilling technique reduced the total hydrocarbon value of asphalt layers in Well F18 from 87.98% to 23.4%, which effectively prevented asphalt influx effectively;the emulsifying and viscosity reducing technique kept drilling fluid funnel viscosity under 62 s in Well F02;the chemical plugging technique improved formation strength by 0.2 g/cm3 in Well F19;the asphalt hardening technique reduced drilling fluid volume contamination by 19.5% in Well F17;the optimized casing program and MPD drilling technique assured safe drilling through asphalt layers of 158 m in Well S03.Therefore, the new contamination control technique could solve the problems encountered when drilling in ultra-thick asphalt layers in Y Oilfield.