Sandstone-Type Uranium Deposits in Hydrocarbon-Bearing Basins: History
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As a valuable mineral resource, uranium is extensively utilized in nuclear power generation, radiation therapy, isotope labeling, and tracing. In order to achieve energy structure diversification, reduce dependence on traditional fossil fuels, and promote the sustainable development of energy production and consumption, research on the metallogenic mechanisms and related development technologies of uranium resources has been one of the focuses of China’s energy development. Sandstone-type uranium deposits make up approximately 43% of all deposits in China, making them the most prevalent form of uranium deposit there.

  • hydrocarbon-bearing basins
  • sandstone-type uranium deposits
  • metallogenic mechanisms

1. Introduction

Uranium is a rare mineral resource that is extensively distributed throughout the Earth’s crust [1,2], but it typically exists at low concentrations, with an average abundance of about 2.7 ppm (parts per million) [3]. In addition, only a relatively small number of economically viable uranium deposits exist, and they are unevenly distributed [4,5]. Currently, global uranium production is about 54,224 tons [6] and is primarily from countries such as Kazakhstan (43%), Canada (15%), Namibia (11%), and Australia (8%) [7]. Uranium is an important unconventional energy resource primarily used for nuclear power generation [8,9,10]. Through uranium fission reactions in a nuclear reactor, enormous energy can be generated for electricity production. Nuclear power generation has the advantages of high efficiency and cleanliness [11,12], so it is particularly suitable for areas with high electricity demand, helping to reduce dependence on traditional fossil fuels, and thus reducing carbon emissions [13,14]. In addition, uranium also has other nuclear technology applications, such as radiation therapy [15,16] and isotope labeling and tracing [17,18]. The Organization for Economic Co-operation and Development (OECD) has provided information that the growing demand for clean energy transition in the future is anticipated to affect nuclear energy capacity, with the East Asia region expected to experience the largest increase in uranium demand [19,20]. To promote the growth of nuclear energy capacity and drive the increase in demand for uranium, it is crucial to acknowledge the benefits of nuclear energy in providing a secure, reliable, and predictable energy supply. Additionally, offering incentives to promote the diversification of low-carbon technologies can help to achieve these goals.
For countries with large populations and rapid economic development, such as China, ensuring energy security is crucial for sustainable development. In order to lower greenhouse gas emissions and combat climate change, the Chinese government has implemented a series of policies [21] to support the development of uranium resources, fully exploiting and utilizing domestic uranium resources to achieve energy diversification, reduce reliance on traditional fossil fuels, and promote sustainable energy production and consumption [22]. China previously aimed to adhere to a strategy referred to as the ‘three-third rule’ for its uranium supply, which involved one-third coming from domestic mining, one-third from direct international trades, and one-third from Chinese companies mining abroad. However, China’s current approach has shifted towards a portfolio approach, where uranium is sourced from various locations based on feasibility and economic viability. The objective is to maximize uranium supply security by utilizing different sources, including those outlined in the ‘three-third rule’ [23]. Therefore, in recent years, China’s uranium mining has also increased, with annual production reaching 1885 tons in 2018, contributing to roughly 4% of global uranium production and ranking among the top ten producers worldwide [24].
Sandstone-type uranium deposits, which are distinguished by their economical in-situ leaching, vast scale, and minimal environmental impact during mining, have become a key area of focus in China’s ongoing exploration of uranium resources [25]. Consequently, research into the genesis, exploration, and mining of these deposits has gained significant traction in recent years. The presence of sandstone-type uranium deposits alongside oil and gas resources frequently occurs in China’s basins [26,27,28,29], indicating a close relationship and coexistence between these resources. This can be attributed to the fact that sandstone, particularly high-porosity and permeability sandstone, functions as a storage space for oil and gas, as well as channels and storage sites for the transportation and enrichment of fluids carrying uranium [30,31,32]. In addition, the geological structures and sedimentary environments required by hydrocarbon generation and uranium mineralization share certain similarities [33]. The coexistence of hydrocarbon resources and sandstone-type uranium deposits within the same basin offers China a significant advantage in the efficient and economical exploitation of its energy resources. Developed oil and gas basins have relatively complete infrastructure and geological history data that can be directly utilized for uranium exploration and evaluation, leading to comprehensive resource utilization and a substantial improvement in economic efficiency.

2. Metallogenic Mechanisms of Sandstone-Type Uranium Deposits in Hydrocarbon-Bearing Basins in China

Sandstone-type uranium deposits are commonly a result of a specific stage in crustal evolution. The main sources of uranium minerals that are present in China’s sandstone-type uranium deposits are sedimentary layers within the basin and nearby geological structures that contain significant concentrations of uranium minerals. The redox behavior of uranium (U6+→U4+) is a fundamental principle followed by uranium mineralization. Uranium minerals are transported and enriched in the form of U6+ and ultimately deposited in the sandstone deposits in the form of U4+ compounds. Current research among Chinese scholars suggests a consensus regarding the infiltration of fluids in the vicinity of the basin’s periphery, which leads to the formation of roll or plate uranium orebodies [109]. Some researchers have proposed that reducing fluids in deep basins are as important during the formation process of these deposits in light of recent advancements in prospecting and research [25]. Moreover, it is suggested that the basin’s tectonic movement can cause uranium-bearing fluid to vertically migrate, facilitating uranium deposits to form. Figure 1 illustrates the metallogenic mechanisms involved in the formation process of sandstone-type uranium deposits within basins containing hydrocarbon resources.
Figure 1. Metallogenic mechanisms of sandstone-type uranium deposits in hydrocarbon-bearing basins.

2.1. Fluid Action

The crucial role of fluids in uranium mineralization has been widely recognized in current research. Sandstone-type uranium deposits are formed with the significant involvement of surface water and groundwater. It has been discovered that uranium ore, oil, and gas, as well as low-temperature hydrothermal minerals, can be found in hydrocarbon-bearing basins with extensive sandstone-type uranium resources. This suggests the involvement of low-temperature hydrothermal fluids during uranium mineralization [111,112]. Therefore, this study offers a metallogenic analysis of sandstone-type uranium deposits within hydrocarbon-bearing basins, examining the effects of three fluid types: surface water and groundwater, oil and gas, and hydrothermal fluids.

2.1.1. Action of Surface Water and Groundwater

In uranium-rich hydrocarbon-bearing basins, sandstone-type uranium mineralization primarily occurs through the action of surface water and groundwater, which play a crucial role in providing the hydrodynamic force necessary for uranium mineralization. Under oxidizing conditions, surface water dissolves uranium minerals that are present within the rocks of the surrounding orogenic zone, while groundwater dissolves a significant quantity of uranium-bearing rock debris that is brought into the basin by weathering and denudation. These uranium minerals dissolved by surface water and groundwater can migrate with water flow to the tectonic slope zone, which is favorable for mineralization and enrichment. Under the influence of the siphon effect [113,114] and pulsation cycle mechanism [25], uranium-bearing fluids can promote uranium deposition in sandstone through the form of minerals or adsorptive precipitation. This occurs due to a slowing down of water flow, a decrease in oxygen content in water, and the occurrence of reduction reactions.

2.1.2. Action of Oil and Gas

Uranium deposits and hydrocarbon fields within the same basin are closely intertwined. The spatial location of uranium deposits and the origin of hydrocarbon fields are strongly related. In these hydrocarbon-bearing basins, when sandstones are rich in organic materials, such as fossil plant remains, these organic materials can undergo pyrolysis reactions under the influence of pressure and temperature over an extended length of time, leading to the formation of hydrocarbons. Additionally, these organic materials can also react with uranium-bearing oxidized groundwater as a reducing agent to form uranium deposits. This is because fossil plant remains are abundant in organic matter, including soluble organic matter, fixed organic matter, and structural organic matter, with the majority of it being humus. The organic matter can react with uranium-bearing oxidized groundwater either directly through reduction with bacterial as a catalyst [115], or indirectly through the production of biogenic hydrogen sulfide [116], a reducing agent that causes uranium to precipitate for the deposit formation. Therefore, sandstones that are rich in organic materials within hydrocarbon-bearing basins often become important sites for hydrocarbon generation and uranium deposit mineralization.
However, regarding production stratigraphy and mineralization location, sandstone-type uranium deposits are substantially distinct from hydrocarbon resources. Studies have shown that oil and gas reservoirs are typically located at lower or deeper sections of the sandstone-type uranium-bearing layer within the same basin [117]. In contrast, sandstone-type uranium is primarily mineralized at the top of hydrocarbon reservoirs, especially around the edges of hydrocarbon cap rocks [115]. This up-and-down superposition location association between sandstone-type uranium deposits and hydrocarbon is a key factor in the modification effect of hydrocarbons on uranium deposits.
The spatial distribution of uranium deposits above hydrocarbon reservoirs is determined by their source rock locations and formation processes. As shown in Figure 1, hydrocarbons are mainly derived from organic-rich source rocks, usually located underground or within sedimentary basins, while uranium is derived from uranium-rich minerals in surrounding rocks or debris. Hydrocarbon formation results from the accumulation and maturation of organic matter, while sandstone-type uranium deposition involves the uranium dissolution by surface water or groundwater under oxidizing conditions, the movement of uranium-bearing fluid, and the uranium precipitation under reducing conditions. In basins containing both hydrocarbon resources and sandstone-type uranium deposits, hydrocarbons are usually transported through fractures or unconformities to the overlying or shallow sandstone-type uranium-bearing layers and laterally migrate along the intra-stratigraphic sand body toward the decompression zone [117]. When hydrocarbons encounter oxygenated uranium-bearing fluids or uranium ore bodies, post-generation modification occurs [117] for uranium deposition. Therefore, sandstone-type uranium deposits are usually located above hydrocarbon reservoirs. Chemical analysis of rock samples from drill holes [118] and fluid inclusion tests [119,120] have revealed the presence of hydrocarbon gases, including hydrogen sulfide, carbon dioxide, and methane. These results suggest that hydrocarbon gases, with the function of rapid and local reduction, which leak from oil and gas reservoirs can be utilized as an essential favorable factor to promote larger deposits to form [115]. In addition, sandstone-type uranium deposits are more prone to form within areas with poor sealing at the margins of hydrocarbon reservoirs, as these conditions are more conducive to hydrocarbon infiltration.
Hydrocarbons possess various forms of reducing effects on uranium mineralization depending on their different formation timeframes [117]. Figure 2 presents the initial formation periods of these hydrocarbon-bearing basins, together with those of hydrocarbon reservoirs and uranium deposits contained therein. Within a given basin, the hydrocarbon generation and the sandstone-type uranium deposition may vary, with the possibility of occurring before, concurrently with, and after the latter. When oil and gas are generated earlier than uranium mineralization, they infiltrate the sandstone-type uranium-bearing layer to create a large-scale reducing environment. This increases the reducing capacity within the mineralized layer, favoring the precipitation and enrichment of uranium. When oil and gas are generated almost simultaneously with uranium mineralization, their infiltration not only increases the reducing capacity but also obstructs the upward migration of oxygenated and uranium-bearing fluids. This leads to the development of a uranium mineralization equilibrium interface, which is a favorable condition for enriched uranium deposits with large scales to form. When oil and gas are generated later than uranium mineralization, their infiltration and reducing modification occur outside the already-formed uranium deposit. This reduces the possible oxidation zone surrounding the deposit and protects the formed uranium mineral body. Additionally, carbonate minerals can form both inside and outside the mining area when hydrocarbons reduce uranium-bearing fluids. Therefore, the presence of carbonate minerals can serve as a valuable indicator or clue for exploring sandstone-type uranium resources.
Figure 2. Initial formation periods of hydrocarbon-bearing basins and their internal hydrocarbon reservoirs and sandstone-type uranium deposits.

2.1.3. Action of Hydrothermal Fluids

Hydrothermal fluids form when groundwater at great depths undergoes dissolution of minerals and ions under high temperature and pressure conditions [121]. These fluids are primarily composed of water, dissolved minerals, and ions, which commonly include sodium, potassium, and iron ions.
Hydrothermal fluids found in hydrocarbon-bearing basins not only contain common ions but also CO32− and HCO3 ions, which are typically sourced from dispersed oil and gas. In a slightly acidic environment, these ions in the hydrothermal fluids can more effectively dissolve U6+. Soluble compounds such as uranyl carbonate complex ions ([UO2(CO3)3]4−) [122,123] easily form and dissolve in oxygen-containing groundwater, which contributes to the enrichment of uranium ores. These complex anions then react with reducers, such as organic carbon from dispersed oil and gas. As a result, U4+ can precipitate under reducing circumstances, which, in turn, further promotes uranium mineralization.

2.2. Geological Structure Effect

Tectonic events in geology refer to the deformation and movement of the Earth’s plates, which can cause crustal movements and the formation and evolution of basins. These processes can result in phenomena such as basin uplift, subsidence, and deformation, which significantly affect hydrocarbon generation, transportation, and accumulation, as well as uranium deposition.
Faults, as the primary geological structural units, are of vital importance during the developmental and evolutionary processes of hydrocarbon-bearing basins, facilitating both vertical and horizontal migration and the movement of materials. They are able to control the distribution status of sand bodies within these basins and serve as a fundamental area during sandstone-type uranium accumulation [26]. Along with the primary structures, secondary structural units such as depressions and uplifts have the capability to further enhance the abundance of sandstone-type uranium deposits. This is because these units dominate the structural slope zones within the basins, which are the most favorable areas for the accumulation of sand, thereby contributing to the subsequent uranium accumulation for enrichment [124]. Geological processes such as folding and dissolution create favorable storage spaces within the sand bodies, characterized by pores, fractures, and solution cavities, which make it easier for uranium minerals to accumulate. Moreover, basin subsidence causes a significant inflow of groundwater, which promotes the dissolution of uranium in uranium-bearing rocks by groundwater, leading to uranium element enrichment. On the other hand, basin uplift creates a reducing environment for uranium-bearing groundwater, resulting in the conversion of U6+ to U4+ and precipitation in the form of a compound. Overall, tectonic events and the formation of secondary structures lead to regional differences in movement within the basins, resulting in the large-scale migration and seepage of groundwater and hydrocarbon-related fluids. These processes enhance the effects of groundwater migration and hydrocarbon reduction on uranium mineralization, further promoting the enrichment and mineralization of uranium minerals.
In China, the formation of certain uranium-rich hydrocarbon-bearing basins, such as Tarim Basin, Ordos Basin, and Sichuan Basin, is also influenced by magmatic activities [125]. The uranium-rich geological bodies within the magmatic zones surrounding these basins also significantly contribute to uranium mineralization at vast scales. These geological bodies are primarily composed of uranium-rich granites [126], which provide substantial material for uranium deposition in the uranium-bearing rock series of basins. When these granites intrude into sandstone or other uranium-bearing rocks, the uranium element in the granite is released through chemical reactions [127,128], and then reacts with the surrounding sandstone or other rocks to form uranium minerals. These minerals are further transported and enriched, ultimately forming uranium deposits.

2.3. Sedimentary Environment Effect

Figure 2 illustrates that the formation of these hydrocarbon-bearing basins (or their host rocks) generally occurred earlier than their internal uranium mineralization. For non-hydrocarbon-bearing basins, there is no established correlation between basin formation and uranium mineralization. This is due to the fact that the formation time of a basin is determined by its geological background and structure. Typically, crustal movements and tectonic activities such as fault movements, uplift, and subsidence can induce basin formation. However, hydrocarbon-bearing basins usually have abundant sediment sources and suitable sedimentary environments, in addition to experiencing crustal movements and tectonic activities, which provide sufficient reserve space for the generation and enrichment of hydrocarbons. This abundance of sediment sources and suitable sedimentary environments also create favorable circumstances for uranium deposition. Thus, these factors contribute to the suitability of China’s major hydrocarbon-bearing basins for sandstone-type uranium deposition.
Sandstone serves as the primary medium for the transportation of uranium mineralizing fluids, including uranium-bearing fluids, oil and gas, and hydrothermal fluids. Additionally, it facilitates the accumulation and storage of uranium minerals. The quality of sandstone is governed by its internal heterogeneity and properties, as well as material composition. Sandstones with higher porosity and permeability are better suited for the transport and enrichment of uranium mineralizing fluids. Moreover, the presence of quartz within sandstones promotes a stronger adsorption capacity, thus making quartz-rich sandstones more conducive to the process of uranium mineralization. Mudstone is also a significant accompanying rock [130], and its clay minerals promote uranium deposition through adsorption. Additionally, some hydrocarbon-bearing basins contain carbonate rock, shale, and coal sedimentary rock types. The components of calcite and dolomite in carbonate rocks also employ adsorption [131] to facilitate uranium mineralization, whereas shale and coal seams primarily rely on the reduction effect of their associated organic matter and sulfides to promote uranium enrichment and mineralization.

This entry is adapted from the peer-reviewed paper 10.3390/eng4020098

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