The Middle Permian saline lacustrine shales are identified as the most significant source rocks in the eastern Junggar Basin. Investigating their gas-generating potential and contribution to gas accumulation holds great significance for hydrocarbon exploration. Based on logs of wells newly drilled in recent years, as well as data from petrology, organic geochemistry, semi-open pyrolysis simulation experiments, and natural gas geochemical analysis, we systematically evaluate the gas-generating potential of the Middle Permian saline lacustrine source rocks, clarify the origin of natural gas from within, and delineate favorable targets for natural gas exploration. The results indicate that in the Dongdaohaizi Sag, the thickness of the Middle Permian saline lacustrine source rocks increases gradually from its margin to center, generally exceeding 350 m and reaching a maximum of 600 m at the center. In the Fukang Sag, two zones of thick Middle Permian saline lacustrine source rocks are determined, namely the northeastern margin and the south-central portion, where the source rock thicknesses primarily range from 200 to 250 m and exceed 400 m, respectively. The Middle Permian saline lacustrine source rocks in both sags principally exhibit medium to excellent quality, with oil-prone kerogen widely developed which is characterized by considerable thicknesses, high contents of retained hydrocarbons, and generally entering the gas window stage. All these suggest the potential for large-scale gas generation. The petroliferous gas generated by the Middle Permian saline lacustrine source rocks has been discovered in the eastern Junggar Basin, distributed primarily in the Fukang Sag, Dongdaohaizi Sag, and the Fukang fault zone. Besides, the petroliferous gas generated from the Middle Permian source rocks is also found mixed with the coal-derived gas in the Kelameili gas field and the Cainan oil and gas field. The ultra-deep reservoirs in the Fukang and Dongdaohaizi sags in the eastern Junggar Basin are favorable for the exploration of petroliferous gas, which can be divided into three favorable gas exploration zones: conventional gas outside source rocks, tight gas inside source rocks, and shale gas inside source rocks.
Helium sources act as the primary element in generating helium resources. Potential helium source rocks encompass basement metamorphic rocks and various sedimentary rocks, such as mudstone, argillaceous dolomite, coal, and bauxite, in basins. Based on field geological surveys, gravity-magnetic data interpretation, core description, and major-and trace-element analyses, we investigate five suites of potential helium source rocks of two categories in the Ordos Basin and their helium-generating potential. The results show that two categories of potential helium source rocks, namely basement metamorphic rocks and sedimentary rocks, are developed in the studied basin. The helium source rocks of the basement type are found in the Archean land block and the superimposed Paleoproterozoic strata, dominated by high-grade metamorphic gneiss, granulite, marble, migmatite, and granitic gneiss, with average abundances of U and Th of 3.15×10-6 and 12.38×10-6, respectively, and a helium-generating intensity of 0.735×10-6 cm3/(a·g). On the other hand, the helium source rocks of the sedimentary type are primarily found in Changchengian metasedimentary rocks of the Mesoproterozoic strata and the Paleozoic sedimentary rocks. The Changchengian black slate is widely seen in the northern and southwestern Ordos Basin, and exhibits average abundances of U and Th of 2.36×10-6 and 8.28×10-6, respectively, and a helium-generating intensity of 0.522×10-6 cm3/(a·g). The argillaceous dolomite in the Lower Ordovician Majiagou Formation of the Lower Paleozoic strata, found in the central and eastern Ordos Basin, displays average abundances of U and Th of 1.71×10-6 and 9.80×10-6, respectively, and a helium-generating intensity of 0.487×10-6 cm3/(a·g). The Carboniferous-Permian mudstones and coals of the Upper Paleozoic strata are extensively distributed throughout the Ordos Basin. The Taiyuan Formation mudstones, among others, show average abundances of U and Th of 9.69×10-6 and 22.68×10-6, respectively, and a helium-generating intensity of 1.82×10-6 cm3/(a·g), while its coals exhibit average abundances of U and Th of 16.12×10-6 and 44.13×10-6, respectively, and a helium-generating intensity of 3.21×10-6 cm3/(a·g). The Carboniferous bauxite of the Upper Paleozoic strata, primarily occurring in the eastern and southwestern Ordos Basin, demonstrates average abundances of U and Th of 7.14×10-6 and 38.57×10-6, respectively, and a helium-generating intensity of 1.97×10-6 cm3/(a·g). Various helium source rocks are discovered in the southwestern Ordos Basin, suggesting a multi-source helium supply. Overall, this study lays the foundation for helium resource exploration in the Ordos Basin.
Overpressure is extensively distributed in the Paleogene Dongying Formation in the southwestern sub-sag of the Bozhong Sag, Bohai Bay Basin. However, there is a lack of a clear understanding of the pressure distribution, the formation mechanisms of overpressure, and the effects of overpressure on hydrocarbon accumulation in this formation. In this study, we explore the present-day pressure in the formation and its spatial distribution using measured formation pressure and logs. Based on a comprehensive analysis of log curves and the cross plots of sonic velocity vs. density, we analyze the genetic types of overpressure in the formation. Furthermore, through basin modeling, we reconstruct the paleopressure evolution during hydrocarbon accumulation to assess the effects of the formation paleopressure on hydrocarbon accumulation. Key findings are as follows: (1)The Dongying Formation exhibits a complete overpressure system vertically. Horizontally, the overpressure is distributed around hydrocarbon-generating sub-sags, intensifying closer to the sag center; (2)The genetic type of overpressure shifts from predominant undercompaction in shallow strata to a combination of undercompaction and hydrocarbon generation-induced expansion in deep layers; (3)The overpressure within the source rocks of the Dongying Formation, including the 3rd member and the lower sub-member of the 2nd member, offers a sufficient dynamic for hydrocarbon migration. Furthermore, it forms seals for the underlying overpressured fluids, preventing the upward escape of hydrocarbons. It is proposed that ancient buried hills represent favorable targets for future deep and ultra-deep hydrocarbon exploration in the southeastern sub-sag of the Bozhong Sag.
The Ordos Basin is renowned for its rich natural gas resources, which exhibit a high helium content in some areas. Therefore, there is an urgent need to thoroughly investigate the geochemical characteristics and enrichment factors of helium-bearing natural gas. Based on the experiments, analysis, and tests of the compositions and isotopes of natural gas samples, along with geological conditions, we examine the distribution, geochemical characteristics, and controlling factors of helium-bearing natural gas in the Ordos Basin. Key findings are as follows: (1)The natural gas in the basin is dominated by hydrocarbon gases, with a helium content ranging from 0.016 % to 0.487 %(average:0.060 %); (2)The carbon isotope values of methane in the helium-bearing natural gas within the basin range from -53.88 ‰ to -29.23 ‰,and those of methane, ethane, propane, and butane indicate an organic origin of the hydrocarbon gases; (3)The helium-bearing natural gas in the Ordos Basin exhibits 3He/4He ratios ranging from 20.10×10-9 to 120.00×10-9, with an average of 42.00×10-9, and R/Ra ratios varying between 0.014 and 0.085, with an average of 0.030. These ratios represent crust-derived helium characteristics, which are unaffected by the genetic type and maturity of naturalgas; (4)The natural gas with a high helium content is primarily distributed in the Carboniferous-Permian strata of the Upper Paleozoic of Dongsheng gas field (north of the basin), Qingyang gas field (southwest of the basin) and Huanglong gas field (southeast of the basin). This distribution is closely associated with factors like ancient and modern tectonic locations,basement faults, and the intensities of helium and hydrocarbon generation. Based on the geochemical characteristics of helium-bearing natural gas, we identify helium-rich-to-moderate, low-helium, and helium-deficient areas in the Ordos Basin.
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