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The Qaidam Basin
The Qaidam Basin is the only large continental oil- and gas-bearing basin in the Qinghai–Tibet Plateau where large-scale reserves have been discovered and large-scale productions have been built. The collision and compression of India and Eurasia during the Cenozoic period and the uplift of the Tibetan Plateau are among the most important geological and tectonic events of this period, and is denoted as the “Himalaya Orogeny” by scholars (Yu et al., 2017; Zhang et al., 2013). The Qaidam Basin in the northeastern margin of the Plateau is a large continental basin that began to form in the Cenozoic era, and its formation and tectonic evolution are closely related to the Himalaya Orogeny (Burchfiel et al., 1989; Gaudemer et al., 1995). Many scholars have conducted in-depth and extensive studies on the geometric, kinematic, and dynamic characteristics of the formation and evolution of the Qaidam Basin, obtaining numerous important results.
Various formation modes have been proposed, including the extension–contraction model (Wang et al., 2004; Wang et al., 2020; Xia et al., 2001), the foreland basin model (Jia et al., 2003; Jia et al., 2022;), the basin-edge uplifting to piggyback model (Yin et al., 2008), and the crustal buckle-fold model (Fang et al., 2007). Although different scholars have debated on different dynamic models, they largely agree with each other on the structural activities of the Qaidam Basin, namely, that the uplift of the plateau was episodic and spatiotemporally variable. This is reflected by structural styles, sedimentary thickness, deposition rate, and filling structure of the basin. Research on the Qaidam basin is of great significance for the understanding of the formation of the Qinghai– Tibet Plateau, as well as the oil and gas accumulation within the basin. The Qaidam Basin is the largest Cenozoic basin formed during the uplift of the Qinghai–Tibet Plateau, and is one of the areas most directly affected by the uplift of the Qinghai–Tibet Plateau. However, previous studies on the relationship between the formation and evolution of the basin structure and the uplift of the Qinghai–Tibet Plateau, as well as the origin of the huge differences in the basin structure, are limited. In particular, research on the coupling relationship between the particularity of the formation and evolution of the Qaidam Basin and the uplift of the Qinghai–Tibet Plateau is inefficient.
Furthermore, the causes of the differential uplift, differential subsidence, and differential denudation of the basin remain unknown, and the three major depressions of the basin lack a unified genetic interpretation.
The multistage episodic uplift background of the Qinghai–Tibet Plateau results in a complex tectonic environment and unique hydrocarbon accumulation conditions in the Qaidam Basin (Fu et al., 2015; Guo et al., 2017; Liu et al., 2020). Predecessors have successively constructed a variety of oil and gas accumulation models, such as the hydrocarbon-rich sag paleotectonic reservoir, out-of-the source paleo-uplift paleo-slope reservoir, the upper late source structural reservoir, and the tight oil lithologic reservoir in the source of the hydrocarbon-rich sag (Gao et al., 2014; Guo et al., 2017; Ni et al., 2019). It is generally believed that the hydrocarbon accumulation period in the Qaidam Basin is late due to the influence of the neotectonic movement. The late structure has dual effects on oil and gas reservoirs, namely, the destructive and constructive coexistence of oil and gas accumulation (Liu et al., 2007; Qin et al., 2022). On the whole, previous research on the hydrocarbon accumulation in the Qaidam Basin has achieved fruitful results and has effectively guided exploration studies. However, in-depth investigations of the relationship between the plateau uplift and basin hydrocarbon accumulation are lacking.
In recent years, oil and gas exploration has made considerable progress in the Yingdong, Zhahaquan, and Yingxi fields in the western Qaidam Basin. Some newly produced oil has lower viscosity and density with a higher gas-oil ratio, suggesting that they may be derived from deeper strata. However, little research has been conducted to investigate the genetic type and maturity of these newly produced gases. Furthermore, a systematic analysis of natural gas is necessary to deepen the recognition of gas origin and charge history in the western Qaidam Basin.
The hydrocarbon components of natural gas mainly consist of C1-C5 compounds with trace C5+ light hydrocarbon compounds. The definition of light hydrocarbon has not been unified. In early studies, light hydrocarbons refer to hydrocarbons ranging from C1 to C14; in later studies, the ones refer to hydrocarbons ranging from C5 to C10. The C5-C7 light hydrocarbons are the focus in current researches and this paper. Besides, partial light hydrocarbons would dissolve in the gas phase because natural gas can be taken as the solvent and the volatility of light hydrocarbon.
Thus, light hydrocarbons could distribute in the gas phase and liquid phase in a gas sample. Previous studies reported that separation of the oil phase and the gas phase mainly affects the aromatic content of a light hydrocarbon, with little effect on most light hydrocarbon parameters. So far, light hydrocarbons in dry gas and wet gas have been successfully analyzed Many studies have applied the light hydrocarbon as an ancillary geochemical tool to evaluate the genetic type and thermal maturity of natural gas and identify secondary alteration. Furthermore, natural gas in western Qaidam Basin is wet gas with a higher content of light hydrocarbons, which could better reflect the geochemical characteristics of natural gas.
Identification of the genetic type of natural gas is important for assessment of its sources and exploration potential. Based on sources, there are two broad categories: biogenic and abiogenic gases. Abiogenic gas includes gas from the mantle and abiogenic formation. Biogenic gas includes biogenic gas and thermogenic gas, and the latter can be further divided into coal-type gas and oil-type gas based on the type of organic matter. Many researchers have already established some geochemical parameters and classic diagrams to identify the genetic type of natural gas.
The composition of natural gas and methane carbon isotope values (C1/(C2+C3) vs. δ13C1) is significant in identifying different types of natural gas. Besides, the carbon isotope of ethane is widely used to distinguish the coal-type gas and the oil-type gas, including the cross plot of δ13C1-δ13C2-δ13C3, δ13C1-δ13CCO2, δ13C2-3-ln(C2/C3), and so on. Natural gas in the western Qaidam Basin is mainly derived from saline lacustrine depositions which are enriched in δ13C2, and Zhang et al. modified the classification criteria for coal-type gas and oil-type gas. Owing to the complex formation process of natural gas and secondary alteration, the genetic identification of different types of natural gas should be based on multiple parameters. C5-C7 light hydrocarbons can be used to provide some new information insight into natural gas generation. Therefore, a detailed study of natural gas could get a better understanding of its genetic type.
Thermal maturity has an important effect on the composition and carbon isotope of natural gas. The carbon isotope values of methane increase with increasing thermal maturity of source rocks and are adapted to study the maturity of gas. Besides, the dryness coefficient of natural gas increases with an increase of thermal maturity. Moreover, light hydrocarbons could be used to calculate the expulsion temperature of oil and gas. The ratio of 2,4-dimethylpentane to 2,3-dimethylpentane (2,4-DMC5/2,3-DMC5) was found related to temperature, and it was calibrated to expulsion temperature. Studies have reported that expulsion temperatures have positive correlations with biomarker maturity parameters and the gas-oil ratio. While little research has been conducted to evaluate the expulsion temperatures of natural gas in the western Qaidam Basin, it may provide a novel way of assessing the maturity of gas.
The homogenous temperatures of petroleum inclusions are always applied to reconstruct the petroleum charging history combining with burial history and hydrocarbon generation history. Numerous researches have been carried out, and most suggest that there are primarily two petroleum charging episodes in the western Qaidam Basin.
Because each hydrocarbon charge event has distinct contributions to reservoired petroleum, it is important to recognize that a major charge event is not only of academic significance but also related directly to the evaluation of the commercial potential in a studied area. Dieckmann et al. proposed that expulsion temperature calculated from the C7 light hydrocarbon could reflect the average temperature of hydrocarbon expulsion. Generally, the reservoired petroleum is composed of hydrocarbons from multiperiod charging and dominated by the ones from the major period, so that the expulsion temperature obtained from the C7 light hydrocarbon, which is collected from reservoired petroleum, could be considered as the generation temperature of major reservoired hydrocarbons (GTMRH). The GTMRH reflects the character of reservoired petroleum and may provide information about major petroleum charging.
Geological background
The Qaidam Basin is located in the northwestern region of the Qinghai Province, at the northeastern edge of the Tibetan Plateau (Figure 1), and is located at the junction of the paleo-Asian tectonic domain and the Paleo-Tethys–Himalayan tectonic domain. It is surrounded by the Altun, Qilian, and Kunlun Mountains with an area of approximately 130,000 km2. It is a Cenozoic terrestrial basin formed during the collision and subduction of the Indian plate with respect to the Asian plate and accompanied by the uplift of the plateau. It is the largest Cenozoic basin on the Qinghai–Tibet Plateau and the basin with the largest oil and gas production and reserves (Fu et al., 2016; Liu et al., 2020).
Figure 1. Study area and tectonic background.
Previous studies have classified the tectonic units of the Qaidam Basin in terms of numerous aspects, such as the basement properties and fluctuation characteristics, the tectonic and sedimentary evolution, and the fault distribution and evolution, gradually forming a variety of schemes.
Based on the distribution of the basin exploration target layers, the basin can be divided into three primary tectonic units, namely the western Qaidam uplift, the northern margin uplift, and the Sanhu Depression, and 12 secondary tectonic units (Chen et al., 2019; Fu, 2010; Yuan et al.,2011).
Predecessors divided the tectonic evolution of the Qaidam Basin into two cycles and four stages of evolution. The two major cycles are the trough block cycle developed in the transitional margin rift in the late Paleozoic and the orogenic basin cycle developed in the superimposition of the fault depression to the compression depression in the Mesozoic and Cenozoic eras. The four stages of evolution are the late Paleozoic back-arc rifting, Mesozoic extensional faulting, early Mesozoic-Cenozoic intracontinental faulting and depression, and late Cenozoic intracontinental strike-slip compression. The formation of the present-day tectonic framework in Qaidam is mainly controlled by the latter two periods (Sun et al., 2005). Since the Cenozoic, the basin has undergone several basin evolution stages, including the initial Paleocene-Eocene fault depression, the strong compression from the Oligocene to early Miocene, and the strong
shrinkage of the uplift after the late Miocene, while some scholars also believe that the strikeslip movement is not negligible for the formation of the Qaidam Basin. However, there is little difference in the period division of the evolutionary stages. Different basin evolution models such as the synclinal model, the eastward extrusion model, and the extruded escape model have been proposed by different studies (Fang et al., 2007; Jolivet et al., 2003; Wang et al., 2006; Yin et al., 2008).
Various types of sedimentary systems such as braid river deltas, fan deltas, and alluvial fans have developed in the basin (Su et al., 2015), and the Paleogene Lulehe Formation, the lower Ganchagou Formation, the upper Ganchagou Formation in the upper Neogene, the lower Youshashan Formation, the upper Youshashan Formation, the Shizigou Formation, and the Qigequan Formation were deposited from the bottom up in the Cenozoic (Yi et al., 2011), with a thickness of up to 1.8 km (Figure 2).
Figure 2. Comprehensive histogram of the Cenozoic strata in the Qaidam Basin.
The formation and evolution of tectonics in the Qaidam Basin vary significantly in time and space from the pre-mountain basin margin to the inner basin, from the south to the north, from the east to the west, and from the deep to the shallow areas. These spatial and temporal characteristics of the tectonic deposits in the Qaidam Basin are precisely a response to the phased, shifting, and uneven uplift of the Qinghai–Tibet Plateau.
Data source: Cenozoic structural characteristics and petroleum geological significance of the Qaidam Basin, 2023. Xu Zeng, Jian Li, Jixian Tian, Bo Wang, Fei Zhou, Chenxi Wang, Haidong Cui, Zhu Haihua
Geochemical Characteristics of Natural Gas and Hydrocarbon Charge History in the Western Qaidam Basin, Northwest China, 2020. Kefei Chen, Shixin Zhou, Jing Li, Chen Zhang, Zexiang Sun, Pengpeng Li, Bingkun Meng
Следующий Бассейн: Bowen - Surat