By the end of 2011, altogether 48 giant gas fields with each proven reserves over 300×10⁸ m³ had been discovered in China. After studying on the main molecular compositions (methane, heavy hydrocarbon gases, nitrogen, carbon dioxide and hydrogen sulfide) of 1170 gas samples in these gas fields, it is found that: a). Alkane gases dominate the molecular compositions of the natural gases; the extremely low content of them have a tight relationship with the extremely high content of non-hydrocarbon gases; methane accounts for 60.1% of the alkane gases. b). Carbon dioxide has the average content of 4.09%; carbon dioxide with an extremely high content (>50%) is mainly distributed in Dongfang1–1, Changling and Songnan gas fields; highly concentrated CO₂ in the former one was crustal-derived while that in the latter two was mantle-derived. c). Nitrogen has an average content of 3.56% which is quite low; highly concentrated nitrogen (<20%) is mainly distributed in Hetianhe, Tazhong, Ledong22–1 and Dongfang1–1 gas fields which are organic-originated. d). Hydrogen sulfide has been detected in 12 giant gas fields, with the content of 4.91% in average; the Triassic is the most centralized zone for hydrogen sulfide while the Sichuan Basin has the largest number of sour gas fields and sour gas bearing zones; 7 sour gas fields of high H₂S content (2% ∼ 70%) are found which are resulted from the thermochemical sulfate reduction (TSR).

As an unconventional petroleum reservoir, the volcanic reservoir is essential for finding large-scale oil and gas reserves in deep sedimentary basins. Based on basin modeling, organic geochemistry, and organic petrology, this study evaluates the exploration potential of the Carboniferous volcanic-petroleum system in the Jimsar Sag of the southeastern Junggar Basin. The Carboniferous source rocks in the study area were developed in the Lower Carboniferous Songkharsu Formation, a set of marine–terrigenous transitional source rocks, lithologically composed of carbonaceous mudstone, mudstone, and coal. The kerogen is characterized by type II₂–III, indicating a gas-prone source rock. Carbonaceous mudstones, mudstone, and coal are classified as medium–good, medium, and poor source rocks, respectively. The hydrocarbon-generating potential is close to that of the Kelameili gas field. The Carboniferous-reservoir oil in well J15 is characterized by ¹³C-enriched stable carbon isotopes, relatively high ƩC_31–35/C₃₀ and Pr/Ph ratios, relatively low density, gammacerane index, regular C₂₉-sterane content, and undetected β-carotene, indicating a Songkharsu origin. The Carboniferous-reservoired gas from well J3301 has stable carbon isotope composition enriched in ¹³C, similar to the gas from the Kelameili gas field, and should be from the Songkharsu source rock. This set of source rocks is widely distributed in the Jimsar Sag with considerable thickness, among which thickness >100 m reaches 580 km², accounting for 38.7% of the whole sag. The Songkharsu source rocks entered the main oil-generating window at the end of the Early Jurassic. The area of Songkharsu source rocks in the Jimsar Sag with gas-generating intensity greater than 20 × 10⁸ m³/km² reaches 1,015 km², whereas that with oil-generating intensity greater than 500 × 10⁴ t/km² reaches 1,146 km². However, 13 Carboniferous volcanic lithological traps were found in the sag, covering an area of 230 km². The Carboniferous volcanic-petroleum system in the Jimsar Sag has the resource potential to form large and medium oil and gas fields, which could become a critical replacement field for volcanic oil and gas exploration in the Junggar Basin after the Klameili gas field.

After analyzing 21 samples in 12 oil and gas fields in Kuqa depression, two genetic types of condensates are identified. One type is the sapropelic condensate mainly derived from the Upper-Middle Triassic lacustrine source rocks which has a relatively higher content of C_5–7 n-alkanes (>35%) and lower methylcyclohexane content (50%), pristane/phytane ratio (1.70 ∼4.62) and carbon isotope values (−26.2‰ in average). Based on a comprehensive study on the n-alkanes, light hydrocarbons and carbon isotopes, it is found the condensates in Kuqa depression have widely experienced evaporative fractionation with a high aromaticity (>0.8) and their high molecules suffered loss. With the evaporative fractionation becoming more significant, the bulk carbon isotopic values of the condensates become heavier, indicating the multi-stages of the evaporative fractionation. The structural or lithologic traps in the Tertiary and the Jurassic tend to be favorable exploration targets for future oil exploration in Kuqa depression.

There are several sets of coal-bearing source rocks within the Lower and Middle Jurassic (J_1-2) in the southern Junggar sub-basin. To obtain a comprehensive understanding of hydrocarbon generation and the amount of potentially generated natural gas, Rock-Eval analysis was conducted on source rock samples to assess their kerogen types and quality. At the same time, maceral observation and gold-tube pyrolysis were carried out to quantify the different maceral groups and to predict the gas potential, respectively. The Rock-Eval data indicate that the investigated samples are dominated by type III kerogen and occasionally supplemented with type II kerogen. Among the six immature source rocks selected for gold-tube pyrolysis, they are vitrinite-dominated except for a dark mudstone from the Badaowan Formation that is liptinite-dominated. A positive relationship between the maximum gas yield and the liptinite content was observed. However, the accuracy of liptinite content estimation largely depends on the experience level of the organic petrographer. The optimized C_1-5 generation activation energies for the six source rocks cover a broad range between 50 and 74 kcal/mol. A relatively larger proportion of higher activation energies (74 kcal/mol) for the carbonaceous mudstone from the Badaowan Formation and also for the dark mudstone from the Sangonghe Formation implies late gas generation. Under geological conditions, the modeled gas conversion from J_1-2 source rocks in the central part of the southern Junggar sub-basin ranged from 52 to 74%, indicating good exploration potential. Notably, the computed gas generation from the dark mudstone from the Sangonghe Formation resembles that from the Xishanyao Formation. Gas generation from the Sangonghe Formation should be considered in the future.