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Reports that on the basis of a large amount of natural gas components and the carbon isotope as well as some other analysis data in Kela 2 gas field, the geochemical characteristics, source, origin, and formation process of natural gas have been discussed. Statement that the components of gas in the field tend to be "dry", and the drying coefficient is close to 1.0; Reference to the fact that the carbon isotope tends to be heavier, for instance, the average of δ&sup13;C&sub1; is -27.36‰ and that of δ&sup13;C&sub2; is -18.5‰; Comprehensive analysis showing that humic natural gas in the Kuqa petroleum system comes mainly from Triassic and Jurassic source rocks, and the contribution of Jurassic source rocks to the pool maybe is more than that of Triassic rocks; Suggestion that the main cause that the gas tends to be dry and bears heavier isotope composition lies in the fact that Kela 2 natural gas is the accumulation of late production of humic source rocks, and it is affected by the abnormal high pressure as well; Claim that considering the hydrocarbon generating and structural history, we can regard the gas pool formation processes as twice filling and twice adjusting (destroying), that is, the filling and destroying process in the early Himalayan movement and the filling and adjusting in the late Himalayan movement.

Reports that the Kuqa depression bears not only plenty of natural gas, but also a large amount of condensate and small quantity of crude oil. Confirmation in this paper that, based on the geochemical correlation between the Jurassic and Triassic terrestrial hydrocarbon source rock, the natural gas in Kuqa depression belongs to coal-type gas and the main gas source rock is attributed to the middle to lower Jurassic coal series formation, while the main oil source rock is the upper Triassic lacustrine mudstone; Indication by the authors that Kuqa depression was slowly subsided in Mesozoic, but rapidly went down in Late Tertiary, which made the Jurassic and Triassic source rock suddenly deep-buried and rapidly evolved to high and over-mature phase since 5 Ma; Statement that the Triassic source rock is postponed to the Early Miocene during 23-12 Ma when entering the oil-generating peak, while the Jurassic is suspended to the latest 5 Ma, especially since 2.5 Ma to the dry gas-generating period, which is one of the characteristics of the source rock thermal evolution in Kuqa depression; Presentation in this paper of a two-stage trapping and late gas trapping model in Kuqa depression whose characteristics are: The main oil and gas reservoirs have different sources; Formation of the oil reservoir is early while the gas reservoir is formed lately; Reference to the fact that, during the early stage, it, mainly as oil, takes long distance lateral migration, while in the later stage, it, mainly as gas, takes the vertical migration and also has lateral migration; Formation of the trap formed in different time on the south and north sides of the depression and evolution into a distributional pattern with oil in the south part and gas in the north, also oil on the outer ring and gas on the inner ring; Reference in this paper to the fact that late trapping of the natural gas in Kuqa depression is favorable for the preservation of large gas fields.

Reports that the Kuqa depression is a foreland basin developed with Mesozoic-Triassic-Jurassic coal-bearing formation. Indication of the research results of the coal-derived hydrocarbon foreland basins in Kuqa depression that the coal-bearing formation can be the rich sources for generating gas because of their thickness and rich source rocks with gas-generating predominant kerogen; Statement that although the foreland thrust belt mainly acting in compression is very complicated, integral large structural traps can be formed; Claim that the thrust belt can act as the passage for communication with deep source rocks; Development of the high quality gypsolish and gypseous mudstone cap rock in the upper formation as the key for the formation of the large gas field; Dependence of the late formation of reservoirs in the large gas fields on the hydrocarbon-generating history controlled by the foreland basin and the developing process of foreland thrust belt.