Abstract: A drilling technique combining PDC bit and torsional impact tool has been developed and applied in drilling through the igneous rocks of Junggar Basin which solves the technical difficulties such as high hardness,poor drillability and serious well deviation,etc in drilling through igneous rock formation.In view of the lithologic characteristics and technical difficulties of igneous rocks,PK6245MJD PDC bit for φ311.1 mm diameter borehole with strong wear resistance was developed,which combined with SLTIT type torsional impact tool suitable for PDC’s rock shear breaking mechanism,forming a composite drilling technique.The technique showed effective performance in drilling igneous rocks in Well Hashan 3.In field application,φ311.1 mm PK6245MJD PDC bit and SLTIT type torsional impact tool reached a continuous working time of 649 h with 457.50 m of drilling footage in one trip.The ROP was 1.0 m/h,and its roundtrip ROP was 0.7 m/h.Compared with the drilling situation of adjacent upper igneous rocks,at least 11 trip times and bit cost were saved.At the same time well deviation dropped from 4.2° at 2 882 m down to 1.2° at 3 310 m.So the composite drilling technique with φ311.1 mm PK6245MJD PDC bit and SLTIT type torsional impact tool is an effective way to realize optimal and fast drilling in igneous rock formations of Junggar Basin.