Late Triassic‐Early Jurassic abnormal thermal event constrained by zircon fission track dating and vitrinite reflectance in Xishan coalfield, Qinshui Basin, central North China

Abstract

Xishan coalfield, Shanxi, is located in the northwest of the Qinshui Basin, central North China. It is notable for its varieties of coal rank ranging from high volatile bituminous coals to anthracite as well as having abundant coalbed methane resources. Zircon fission track (ZFT) analyses were carried out on the zircons in 2 Upper Carboniferous and 5 Lower‐Middle Permian sandstones, and vitrinite reflectance of Late Carboniferous and Early Permian coals were measured to determine the timing of thermal events and maximum paleo‐temperatures, which were responsible for coal maturation and coalbed methane generation. Maximum paleo‐temperatures calculated from vitrinite reflectance values reached to about 232 and 223 °C in Late Carboniferous and Early Permian coals, respectively, and the estimated paleo‐temperature gradient was 11.84 °C/100 m, representing an intensive abnormal thermal event. Results of the ZFT dating indicated that 5 samples failed the χ2‐test and 2 samples passed the test. The decomposition results of the 5 samples divided their age populations into 3 periods: (a) older ages (537, 584, and 802 Ma) than sandstones ages, (b) close to or slightly older than their depositional ages (289, 301, and 331 Ma), and (c) younger than the depositional ages (181–215). The 2 samples that passed χ2‐test yield the central ages of 168 ± 7 Ma and 190 ± 8 Ma, respectively, younger than the deposition age. The close to or older ages than the sandstones depositional ages represent the tectonothermal events occurring in their source areas; the younger ages indicate the existence of the postdepositional tectonothermal event. The agreement of the partly annealing temperature zone (210–300 °C) of zircon fission tracks with the calculated maximum paleo‐temperatures from vitrinite reflectance suggests a Late Triassic‐Early Jurassic abnormal thermal event with the formation time of the present coal rank being 181–215 Ma, rather than a unique intrusion at 95–135 Ma on the western margin of coalfield as previously believed. Combined with other ZFT ages regionally, this abnormal event also occurred in the southern as well as the northern parts of the Qinshui Basin. The Late Triassic‐Early Jurassic intensive extension in the North China Craton is the geodynamic setting of this tectonothermal event.

Characteristics and the Model of Thermal Evolution and Gas Generation of Late Paleozoic Coal in the Qinshui Basin, Based on Hydrous Pyrolysis

“…The Qinshui Basin is located in the southeast of the Shanxi Province, covering an area of about 26,000 km 2 (Figure ). It is rich in coal and coalbed methane resources, and it is the only large basin in China to develop coalbed methane on an industrial scale. − Structurally, it is located in the central part of the North China Craton and is generally a large-scale complex syncline with axial NNE–SSW. , After a series of tectonic influence, especially during the Yanshanian and the Himalayan period, the Qinshui basin formed a complex structural setting. − …”

Developmental characteristics and distribution law of fractures in a tight sandstone reservoir in a low‐amplitude tectonic zone, eastern Ordos Basin, China

“…important effects on fracture development characteristics. This effect is mainly reflected in the mineral composition, rock brittleness, sedimentary facies, and paleo-tectonic stress environment Lai, Wang, Wang, Cao, Li, Pang, Zhou, et al, 2018b;Lv, Wang, Li, Liu, & Li, 2017;Sun, Zeng, Xia, Zhu, & Liu, 2017;Y. Wang, Zhu, Chen, & Li, 2014;Yin, Zhao, Wu, & Ding, 2018).…”

“…Sedimentation systems and low‐amplitude structures both have important effects on fracture development characteristics. This effect is mainly reflected in the mineral composition, rock brittleness, sedimentary facies, and paleo‐tectonic stress environment (Han et al, ; Lai, Wang, Wang, Cao, Li, Pang, Zhou, et al, ; Lv, Wang, Li, Liu, & Li, ; Sun, Zeng, Xia, Zhu, & Liu, ; Y. Wang, Zhu, Chen, & Li, ; Yin, Zhao, Wu, & Ding, ). The study on the fracture development in low‐amplitude structural areas and the effects on the accumulation of tight sandstone gas is a hot and difficult spot in petroleum geology.…”

Mesozoic Tectono-Thermal Event of the Qinshui Basin, Central North China Craton: Insights From Illite Crystallinity and Vitrinite Reflectance

“…To date, many petroleum geologists have focused on the tectonic uplift history, thermal evolution history, and coalbed methane of the Qinshui basin, suggesting that the Qinshui Basin was affected by the destruction of the NCC, which caused a tectonic thermal event in the basin during the Early Cretaceous, prompting the rapid over-maturation of source rocks and the production of coalbed methane (Ren et al, 2005;Sun et al, 2005;Ren et al, 2006;Sun et al, 2006;Wei et al, 2007;Meng et al, 2015aMeng et al, , 2015bSun et al, 2018;Meng et al, 2019;Yu et al, 2020a;Ren et al, 2020). It has been demonstrated by hydrocarbon explorations that a large amount of coalbed methane was preserved in the north and south parts of the basin.…”