Abstract: The dense oil reservoir in Jimusaer sag is characterized by compact, ultra?low permeability, thick, thin sand?shale interbed, etc.Owing to many restrictions of the reservoir condition and modification process, etc., using the conventional fracturing modification is difficult to realize the commercial exploitation of such a reservoir. This paper investigates the geological characteristics of the reservoir and its difficulty for modification, introduces the successful experiences for development of dense reservoirs abroad, and proposes a new idea of composite modification of“large?scale fracturing plus volume fracturing”for the reservoir with very poor petrophysical property and without natural productivity by depletion drive, comprehensively taking account of complex fractures and high flow?conductive major long fracture in order to enlarge fracturing volume and improve single well production. For this purpose, two approaches of horizontal well segment fracturing and vertical layered fracturing in the dense oil reservoir in Jimusaer sag were performed, and a series of tools, techniques and technologies were optimized and equipped in 2012, with the cumulative modification of 6 wells and 35 layers in the field, from which the commercial oil flows were obtained. The horizontal well’s fracturing was performed by 15 levels, the production after fracturing reached71.79 m3/d, amounting to about 7 times that of vertical wells by layered fracturing approach. These successful pilot tests have become marks of major breakthrough for exploration of the dense oil reservoir in Junggar basin, meanwhile, provided significant experiences for late large?scale development of such reservoirs