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The Ordos Basin
The Ordos Basin is located in the western part of North China Platform, extending to the Lьliang Mountain in the east, to the Qinling Mountain in the south, to the Helanshan Mountain in the west and to the Yinshan Mountain in the north. This region straddles across five provinces including Shanxi, Shaanxi, Gansu, Ningxia and Inner Mongolia, with an area of 320000km2 (Liu et al., 2009; Sun and Liu, 2009; Sun et al., 2009).
Current regional structure in the Ordos Basin is relatively simple. Its inner part is quite stable while abundant faults are developed in its active margin. As the second largest sedimentary basin in China, the Ordos Basin is prolific in oil and natural gas resources, which distribute mainly in Mesozoic and Paleozoic, respectively. In 1989, the Central Gas Field (or called Jingbian Gas Field), the largest gas field in China, was discovered in the Ordovician in the central of Ordos Basin. The major gas beds are carbonate weathered crust of the fifth member of Majiagou Formation. For tectonic setting, it is located in the middle of Yishan Slope (Figure 1). Up to now, the proven gasbearing area reaches 4000km2, and the gas in place exceeds 4000-108 m3.
Figure 1. Regional tectonic characteristics of the Ordos Basin
Since the discovery of Central Gas Field, numerous studies have been carried out from different aspects and obtained many important results (Gao et al., 2009). However, it is still in intense debate concerning the main source of Ordovician gas reservoir. One view is that the gas is mainly or completely coal-formed gas and originated from the Upper Paleozoic coal measure source rocks (Dai et al., 1999; 2005; Guan et al., 1993; Xia et al., 1999b; Yang et al., 2009; Zhang et al., 1993), the other view is the gas is mainly self generated and self contained oil-type gas and originated from the Lower Paleozoic marine carbonate rocks (Chen, 1994, 2002; Hao et al., 1997; Hung et al., 1996; Wang et al., 2009). One of the focus questions is the hydrocarbon generation potential of Ordovician marine carbonate rocks. The Ordovician Majiagou Formation marine carbonate rock, widely distributed in the central-eastern part of Ordos Basin, is unlikely to be a large-scale efficient gas source rock, because it is formed in a turbulence and rich-oxygen shallow carbonate platform and the low organic matter abundance resulted from this environment (Sun et al., 2008; Wang et al., 2009; Xia et al., 1999a; Yang et al., 2009; Sun et al., 2010). Thereby, the Upper Ordovician Pingliang Formation in the southwestern margin of Ordos Basin caused wide attentions for its deep-water slope mudstones and marlstones. Many systematic investigation and assessment have been carried out on sedimentary environment, organic geochemistry and other aspects of the Pingliang Formation marine hydrocarbon source rocks (Chen et al., 2007; Liu et al., 2008; Sun et al., 2008; Xu and Wang, 2010). However, the Pingliang Formation is various in lithology, thickness and maturity, and the sparse outcrop and drilling result in low understanding, in addition, the region is far away from current gas rich region, whether the Pingliang Formation is the main source rocks of Ordovician gas reservoir are still being questioned. Carbon isotope composition of carbonate can well reflect the ancient marine roductivity and organic matter burial, which can be a geochemical index for the development of hydrocarbon source rocks in earth history (Hatch et al., 1987; Tenger et al., 2005; Wang et al., 1997; 1998; 2002). Therefore, the main objective of this study is to investigate carbon isotope of Ordovician marine carbonates and evaluate their hydrocarbon generation potential from the point of carbon cycle, and provide new evidence for the Lower Paleozoic natural gas exploration in the Ordos Basin.
Based on sedimentology and geochemistry data, Chen et al. (2018) constrain diagenetic processes and establish a depositional model for dolomite reservoirs of the Middle Ordovician Majiagou Formation in the Ordos Basin. The reservoir was deposited on a semi-restricted platform on a tidal flat, with the peritidal shoal facies distributed along the shoreward side of the central paleo-uplift. Destructive diagenesis (e.g. recrystallisation, cementation, compaction, and pressure solution) and mineral deposition resulted in densification of the dolomites. The physical properties of the dolomite gas reservoirs likely enhanced through dolomitization and dissolution during burial.
Geological settings
The Ordos Basin contains a gigantically thick sequence of the Early Paleozoic marine sediments, in particular of the whole Ordovician, and uniquely owns Late Ordovician strata within the North China. In the Early Paleozoic, the Ordos Basin was tectonically within an extension stage, as a basin of an epicontinental sea to epicontinental marginal sea constrained by the Qinqi Trough and Helan Aulacogen. The Ordovician strata, generally 300~1000m thick and locally up to 2000m thick, mainly expose in the margins of Ordos Basin, in the middle part of basin, it is covered by the Upper Paleozoic, Mesozoic and Cenozoic rocks (Feng et al., 1998). Controlled by the North China Sea, Qinqi Sea and Central Paleouplift during the Ordovician, there formed three sedimentary sub-regions, including the western margin, southern margin and central-eastern part of the Ordos Basin. Divisions and correlation of the Ordovician stratigraphy in the Ordos Basin are shown in Table 1.
Table 1. Divisions and correlation of the Ordovician stratigraphy in the Ordos Basin.
The Early Ordovician sediments only distribute in the eastern and southern margins of Ordos Basin. The Yeli Formation and Liangjiashan Formation comprise a series of argillaceous dolostones and siliceous dolostones, which are of restricted tidal flat deposits, reflecting an early transgression during the Ordovician. Till the Middle Ordovician, almost all of the Ordos Basin received sediments. In the central-eastern part of the Ordos Basin, the Majiagou Formation is formed by six lithological members.
The first, third and fifth members mainly consist of dolostones, gyprocks and salt rocks interbedded with a few limestones, indicating a restricted platform to evaporate platform environment. In contrast, the second, fourth and sixth members are dominated by limestones and dolostones, indicative of an open platform condition. In the southern margin of the Ordos Basin, the Majiagou Formation is mainly composed of limestones and dolostones, with limited carbonate gravity flow deposits, implying an open platform to platform slope environment. Here, the Fengfeng Formation is compared to the sixth member of the Majiagou Formation of the central-eastern part. In the western margin of the Ordos Basin, the Sandaokan Formation is dominated by dolostones, limestones and sandstones; the Zhuozishan Formation predominantly consists of massive limestones and dolomitic limestones; and as a contrast, the Kelimoli Formation is comprised of straticulate micritic limestones containing little black shales. These formations point to environmental conditions of tidal flat, open platform and deepwater slope, respectively, and in turn reflect a gradual deepening of depositional environment associated with the continuous extending of the Helan Aulacogen. To the late Ordovician, the whole North China platform was uplifted to land by the Caledonian Movement, leading to a unique depositional environment in the southwestern margin of the Ordos Basin, which is dominated by deep-water slope to trough facies. Thus, the PingliangFormation mainly contains shales, sandstones and limestones, interbedded with gravity flow and tuff deposits and rich in graptolite fossils. The Beiguoshan Formation is lithologically characterized by limestone, with shale and conglomeratic limestone, and paleontologically by coral and other fossils. The Wulalike Formation is characterized by black shale with little straticulate limestone, while the Lashizhong Formation by gray-green sandstone and shale cyclothemic deposits containing thin limestones. The Gongwusu Formation is dominated by thin limestone, shale and sandstone and abundant ichnofossils. In contrast, the Sheshan Formation mainly consists of bioclastic limestone and sandy shale.
Ordos Basin is the second largest sedimentary basin in China, with very rich oil, gas, and coal resources, and it stretches across Shaanxi, Gansu, Ningxia, Shanxi, and Inner Mongolia (Liu et al., 2009; Chen et al., 2011). Mizhi County is located in the middle-east of the Yulin City, and its underground mineral resources are mainly natural gas and salt rocks. Among them, natural gas is widely distributed in the west of the county, in addition to being present in the eastern region that has not been studied well enough; the distribution of oil and gas coal is shown in Figure 2. It has been proven that the salt layer reserves lie underground across the whole county, its thickness is 129–148 meters, and the reserves total 1600–1800 million tons. The Mizhi region, rich in salt rock, is an ideal area to study the relationship between salt rock and oil-gas.
Figure 2. Distribution of oil-gas-coal in the middle north of Ordos Basin.
Data source: Carbon isotope evidence for Ordovician marine hydrocarbon source rocks in Ordos Basin, North China. 2011 Qiang Chen, Wenhou Li, Songli Hao, Zhuo Liu and Jian Xu
Special Issue: Advances of petroleum exploration and geology research in West China’s petroliferous basins. 2018 Jian Cao, Wenxuan Hu and Xiaolin Wang
Organic geochemical characteristics of source rocks in the Mizhi region of Ordos Basin. 2012 Zhiqiang Meng, Hongguang Ji, Shenjun Qin and Cunliang Zhao
Следующий Бассейн: Jianghan