Abstract: Drilling in deep formation in Iran Y Oilfield,which contains ultra-thick active bitumen and high differential pressure,is facing challenges such as bitumen invasion,differential sticking,difficulty in formation pressure test at 149.2 mm hole section,etc.Therefore,on the basis of laboratory study,the key properties of drilling fluid including plugging,lubricity,inhibition and slough-prevention were optimized.The bitumen invasion was controlled by selective preferential adherence and thickening and plugging technology in Kazhdumi Formation.Apart from keeping good plugging and lubrication of drilling fluids,separated layer sealing technique was applied in Fahliyan Formation to overcome the sticking caused by differential pressure.All these measures that were applied in Well FH4,FH14 and FH17,solving bitumen invasion and differential sticking are effective.There was no trouble and complicated situation happened during drilling operation in high differential pressure (28MPa) formation.