Abstract: Oil and gas exploration in Tarim Basin has shifted from shallow, deep to ultra-deep formations, of which the last stage
began in the 1980s. After more than 30 years of exploration in ultra-deep formations, Kelasu Gasfield, Tabei Oilfield and Tazhong
Condensate Gas Field have been discovered, and multiple major signs found in the Cambrian subsalt. At present, the ultra-deep
oil and gas reservoirs have become an important field of increasing reserve and production, and a major exploration target in the
basin. In this paper, geological understandings of ultra-deep formations are summarized from more than 30 years’ exploration
practice. It is considered that the ultra-deep oil and gas reservoirs in Tarim Basin are mainly controlled by source rocks of Cambrian,Carboniferous-Permian and Triassic-Jurassic, which were deposited at the same geologic periods with three tectonic cycles of extension-compression of the basin, and the material basis of ultra-deep oil and gas reservoirs is universal in the world. Meanwhile,there are three peculiarities in petroleum geological conditions of ultra-deep oil and gas reservoirs in Tarim Basin. First, the basin experienced shallow burying for a long period and rapidly deep burying in the late stage. The ultra-deep source rocks are
still able to generate hydrocarbon on a large scale, which provides hydrocarbon source for ultra-deep oil and gas reservoirs. Second,the basin is a typical “cold basin”, where the ultra-deep liquid hydrocarbon is still preserved, and the main source rocks are
still in the peak of gas generation, therefore the oil and gas phases are diverse of ultra-deep formation. Third, there are great differences in hydrocarbon accumulation assemblages of ultra-deep formations in different areas, all of which can be divided into
three major assemblages. Based on the new understanding of petroleum geological conditions, a comprehensive evaluation is
conducted of each one. It is considered that the exploration potential of the lower and middle assemblages is huge, and eight favorable exploration areas are suggested. In view of the ultra-deep oil and gas exploration in new field and new area, the exploration idea of “expanding exploration in the middle assemblage and achieving breakthrough in the lower assemblage” is put forward to strive for a strategic breakthrough in exploration to lay resource foundation for the high-quality development of Tarim Oilfield Company.