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Helium gas is a scarce but important strategic resource, which is usually associated with natural gas. Presently, only one extra-large helium-rich gas field has been found in China: the Hetianhe Gas Field in the Tarim Basin. This paper reports a new example, the Dongsheng Gas Field (DGF) in the Ordos Basin. In this study, 92 natural gas samples from the DGF were analyzed for helium content and isotope composition using isotope mass spectrometry. The natural gas samples were found to have an average helium content of 0.133%, with 65 (70.7%) of the samples having a helium content of 0.1% or more. Based on the proven natural gas reserves of the DGF, the proven geological helium reserves were calculated to be 1.96×108m3, suggesting that it represents the first extra-large helium-rich natural gas reservoir to be hosted in tight sandstone in China. The 3He/4He ratios of 5 natural gas samples from the DGF are within the range of 3.03×10−8−3.44×10−8. Therefore, the helium in the gas field is thought to be of typical crustal origin and to have formed in the granitic basement that is rich in uranium and thorium. The accumulation of helium-rich natural gas was controlled by regional tectonic activities. Activity along the fault connecting the reservoir with the basement caused release of the helium gas, which entered the overlying strata along the fault and accumulated with conventional hydrocarbon gas. Based on the structural background and the distribution of helium source rocks in the Ordos Basin, the main helium source rocks with high exploration potential are located in deep strata within the north and middle parts of the basin.

Based on the analysis of the natural gas samples of 146 wells from four major petroliferous basins of China, the source and distribution of mercury in natural gas in major petroliferous basins of China were identified. Studies have shown that the mercury concentration of natural gas in petroliferous basins of China varies widely, ranging from 0.01 to 4050 µg/m3. The gas well with the highest mercury concentration is in the Xushen gas field in the Songliao Basin. The mercury concentration in the craton basin is relatively low, and the secondary gas reservoir in the Tarim Basin has the lowest mercury concentration. In the rift basin, due to the presence of deep faults which cut the basement and mixing effects of deep fluids, the mercury concentration in natural gas may be abnormally high, e.g. the Xushen gas field in the Songliao Basin. In relatively stable basins where deep and large faults do not develop, mercury is primarily of organic origin. In rift basins where deep fluids are mixed with gas reservoirs, mercury in natural gas may mainly come from the deep fluids and is characterized by abnormally high mercury concentration and R/Ra values.

We found for the first time enriched liquid mercury beads in a thermal simulation experiment on humic kerogen extracted from coal, which provided some kind of direct evidence that mercury can be released from coal measure during maturation. In the simulation experiment, mass distribution of liquid mercury beads is between 0.0083 and 0.2242 g; their content in simulated gas ranges from 372.5 to 2776.3 ng/m3; and their yield was from 0.3102 × 10–3to 7.4312 × 10–3g/g sample. Along with this thermal simulation experiment and previous studies, three genetic models of mercury in gas reservoirs are summarized: source-rock controlling type, fault-controlling type, and source-rock/fault joint controlling type. Mercury in source-rock-controlling gas reservoirs is mainly derived from source rocks, which are generally coal measures. Mercury in fault-controlling gas reservoirs is mainly derived from deep geologic fluids, with relatively small contribution from source rocks. The transitional source-rock/fault joint controlling type mainly develops in tectonic activity zones where source rocks can be either sapropelic or humic. Liquid mercury collected for the first time in a simulation experiment has important theoretical significance for the exploration and development of natural gas, as well as important practical significance for the prevention of mercury accidents in natural gas exploration and production.