Introduction
Potassium, often referred to as the “lifeblood of crops”, is an essential nutrient for plant growth and is crucial for enhancing agricultural production. Lithium, due to its irreplaceable role in battery technology, has become a key component in new energy and high-tech products. From 2018 to 2022, China accounted for approximately 25% of global potash fertilizer consumption and relied on imports for over 50% of its potassium chloride supply, totaling (1 400-1 700)×104t. In 2021, the confirmed reserves of potassic salt in China were only 2.88×108 t, posing a long-term challenge to the stable supply of potassic salt [1]. The lithium resource in China primarily relies on imports because the guarantee rate of domestic lithium supply was only 27.11%, according to the lithium industry in 2023, highlighting a severe challenge to securing lithium resources [2]. With the rapid development of new energy vehicles and energy storage, the demand for lithium is increasing rapidly. Exploration and development of potassium and lithium resources is demanding and of significant importance for ensuring sustainable and healthy development of the national new energy industry and food security.
Large and super-large potassic salt deposits were primarily found in ancient marine salt basins. In China, the discovered potassic salt resources are sourced from Quaternary salt lakes [3]. The Triassic was a key transitional phase from marine to terrestrial environments in the Upper Yangtze region, where large-scale ultra-thick evaporites were deposited, making them important targets for future exploration of potassium and lithium resources[4-5]. Previous studies identified potassium-rich polyhalite (K2SO4·MgSO4·2CaSO4·2H2O) through surface investigation and confirmed that the Sichuan Basin has favorable geological conditions for potassium formation. However, polyhalite is associated with anhydrite and exhibits poor lateral continuity in shallow mineral layers, making it difficult to develop. Deep polyhalite below 1 000 m is “inactive” or “unutilized” since it is citrate-soluble rather than water-soluble. Deep potassium-rich and lithium-rich brine are typically found in deep brine reservoirs of sedimentary basins in closed and high-pressure environments, representing valuable liquid mineral resources [6]. In the late 1970s, the Second Geological Survey Division of the former China Geological Survey drilled an oil exploration well, Chuan 25, at the Huangjinkou Anticline in the northeastern Sichuan Basin and discovered black brine with a K+ content of 72 g/L at 3 267 m in the 4th and 5th members of Triassic Jialingjiang Formation (referred to as Jia 4 Member and Jia 5 Member)[7]. Exploration practices in northeastern Sichuan have confirmed both solid and liquid potassium-lithium resources in deep marine are potential targets for exploration, but overall geological understanding and exploration level remain low. Developing new and practical strategies for potassium and lithium exploration based on actual geological characteristics is a pressing issue for achieving breakthroughs in marine lithium and potassium mineral exploration in the Sichuan Basin.
In northeastern Sichuan Basin, large marine gas fields, such as Puguang and Yuanba, have been discovered. Eextensive research achievements have been conducted in marine oil and gas exploration theory, geophysical prediction, and the characteristics of carbonate-gypsum salt rock combinations [8⇓-10]. The vast volume of drilling and geophysical data accumulated from oil and gas exploration has established a solid foundation for the investigation of potassium and lithium resources. To further advance the exploration and development of potassium and lithium resources in the Sichuan Basin, following the concept of “gas-potassium-lithium integrated exploration” and using the drilling, logging, seismic and geochemical data, we studied the geological and enrichment conditions, metallogenic model of potassium-rich and lithium-rich brines and halite-type polyhalite, with the intent to clarify the exploration targets for deep marine potassium and lithium resources, and provide a scientific basis for high-quality exploration and development of marine potassium and lithium resources in China.
1. Breakthroughs to potassium and lithium exploration and geology in Jialingjiang and Leikoupo formations of northeastern Sichuan Basin
1.1. Exploration breakthroughs
Since 2010, driven by the National Key Research and Development Program of China, significant efforts have been made to explore deep marine potassium and lithium resources in the Triassic of the Puguang area. Utilizing oil and gas exploratory wells and 3D seismic data, a “gas-potassium-lithium integrated exploration” approach has been adopted.
From 2015 to 2017, No. 405 Geological Team of Sichuan Bureau of Geology & Mineral Resources drilled two exploratory potassium wells, ZK601 and ZK001, in the Huangjinkou Anticline and discovered polyhalite potash deposits and potassium-rich brine. ZK601 encountered brine with K+ concentration of 34.84 g/L and Li+ concentration of 201.5 mg/L. Since 2020, the Institute of Mineral Resources, Chinese Academy of Geological Sciences, in partnership with the Xuanhan Government, drilled Well CXD1, and discovered two industrial mineral layers of soluble halite-type polyhalite in Jia 4 Member and Jia 5 Member, with potassic salt content (KCl equivalent) ranging from 3.0% to 20.5%, and cumulative thickness of 62.8 m; No.405 Geological Team drilled Well CX2, and discovered a polyhalite layer with thickness of 91 m, and a potassium-rich and lithium-rich brine reservoir with thickness of 105.9 m, K+ concentration of 48.16 g/L, and Li+ concentration of 111.5 mg/L in Jia 4 Member and Jia 5 Member. The polyhalite in the halite-type polyhalite ore occurs as intraclastic particles distributed within the halite matrix. It is a new marine potassic salt ore, soluble in water, and easy to produce at low cost and large-scale using the water-soluble method [11].
Successive discoveries in the Triassic formations have confirmed a great potential of deep marine solid-liquid potassium-lithium resources in the Sichuan Basin. According to the logging and 3D seismic data from over 70 oil and gas wells, and four potassic salt exploratory wells (ZK601, ZK001, CXD1, and CX2), and following the specification of solid mineral resource estimation and geometric method, the estimated solid potassic salt resources exceed 9×108 t of KCl equivalent in Jia 4 and Jia 5 members in 586.9 km2 of Puguang area.
1.2. Geological conditions
In the northeastern Sichuan Basin, the vertical lithological sequence of the Middle to Lower Triassic strata consists of interbedded and multicyclic carbonate and gypsum salt rocks. Jia 4, Jia 5, and Lei 1 (the first member of the Leikoupo Formation) members are notable for their deposits of dolomite, anhydrite, and salt rock. From the Early to Middle Triassic, the northeastern Sichuan Basin was influenced by collision and compression of the South China Plate and subsequent Indosinian movement, which marked the end of its marine cratonic basin evolution [12]. At the same time, the southeastern part of the Jiangnan landmass underwent significant uplifting while the Longmenshan landmass subsided, resulting in the tilting of the platform from southeast to northwest, and subsequent seawater retreating westward from the ancient Yangtze landmass [13]. Surrounded by highlands, including the Daba Mountain, Longmen Mountain island chain, Qinling Mountains, and Jiangnan landmass, especially the Luzhou and Kaijiang Paleo-uplift toward the southeast, which obstructed seawater from entering the platform from the east, making the Sichuan Basin gradually transform into a semi-closed to closed inland sea platform [14]. Long-term exposure to a supratidal zone, combined with seawater intrusion and restricted supply, resulted in the formation of salt lakes in the low-lying areas for brine concentration (Fig. 1a ). Intense evaporation processes concentrated residual seawater into thick gypsum-salt rocks [15] (Fig. 1b ). Meanwhile, influenced by local positive geomorphology, shallow-shoal dolomite deposits developed, and finally, they were exposed and corroded into porous reservoirs for brine accumulation.
Fig. 1. Distribution of residual sedimentary facies (a) and lithological column of Jia 4, Jia 5, and Lei 1 members (b) of the Lower Triassic in the Sichuan Basin. GR—Natural gamma; ρ—Density. |
During the late Early Triassic, magmatic activities in the southwestern Sichuan Basin left widespread deposition of green bean rocks at the top of the Jialingjiang Formation in the northeastern Sichuan Basin, creating a regional marker layer [16]. The green bean rocks encountered in Well ZK601 in Puguang area are primarily composed of potassium feldspar, mica, quartz, and clay minerals, which exhibit significant lithium enrichment, with concentrations of 127-663 mg/L, marking it as an important source of lithium ions in deep brine [17⇓-19]. Throughout the sedimentation period of the second member of the Leikoupo Formation (referred to as Lei 2 Member) and its later stages, the Luzhou and Kaijiang Paleo-uplifts continued to rise and experienced denudation, causing the gypsum salt sedimentary center to gradually move westward. The sedimentary environment in the northeastern Sichuan Basin was almost mixed tidal flats, and with substantial input from surrounding ancient landmasses, interbedded mud-rich gypsum salt, mudstone, and dolomite deposited, destroying the reservoir and potassium mineralization.
2. Geological characteristics of potassium and lithium deposits
2.1. Sedimentary characteristics
The Jia 4, Jia 5, and Lei 1 members in the northeastern Sichuan Basin are favorable strata for the deposition of both solid and liquid potassium-lithium resources (Fig. 1b ). Jia 4 and Jia 5 members develop two salinization cycles. The early salinization cycle deposited gypsum-dolomite flats of intertidal evaporative platforms as the sea level declined. The system is almost anhydrite or dolomite, with limy dolomite and dolomitic limestone at the bottom, and thin reservoirs as the top. Entering the late cycle, as evaporation became stronger, the sedimentary environment transitioned to evaporative platform salt lakes (Fig. 2 ), with sediment thickness of 100-500 m. The formation of potassic salts is closely related to the evaporite sedimentation process. As brine concentration increased, the vertical sedimentary sequence of evaporites included carbonates (limestone and dolomite), sulfates (gypsum), and chlorides (halite and potassic salts) in turn. Drilling data revealed thick to very thick potassium-rich polyhalite in the Puguang, Yuanba, and Tongnanba areas (Fig. 3 ). This indicates that, due to the blocking effect of the Kaijiang Paleo-uplift, seawater intermittently entered northeast Sichuan which was far from the ocean, resulting in rich K+, Mg2+ and Ca2+ after long-term evaporation and concentration of brine. Ultimately, after the ions satisfied the concentration in the hexahydric system (i.e., Na+, K+, Mg2+, Ca2+, Cl−, SO2−-H2O), primary polyhalite was deposited [20]. When the Lei 1 Member deposited, tectonic uplift was further intensified, making the Luzhou and Kaijiang underwater uplifts continue to rise, and gypsum and gypsum-bearing lagoons expanded rapidly, leaving supratidal evaporative flat deposits composed of dolomite and gypsum-salt rocks. Further denudation and pinchout created structurally controlled ancient exposure surfaces. Supergene karstification occurred in early dolomite inducing dissolution pores and fractures that provided favorable spaces for deep burial late potassium-lithium brine [21]. The potassium-rich and lithium-rich brine reservoirs in Well CXD1 are mainly residual granular dolomite and powdered crystal dolomite with abundant intragranular dissolved pores, intergranular dissolved pores, and structural fractures, and classified as fractured-porous reservoirs with low porosity and low permeability.
Fig. 2. Depositional model of evaporites in Triassic Jia 4 and Jia 5 members in the northeastern Sichuan Basin (section location shown in |
Fig. 3. Comprehensive columnar comparison profile of the salt-gathering and potassium-formation area in Fuling-Puguang-Yuanba-Tongnanba (section location shown in |
On the 2D seismic section, the lithology of Jia 4 and Jia 5 members was analyzed, and the distribution of polyhalite was revealed in the Sichuan Basin. Polyhalite exhibits zonal distribution, with five distinct potassium-rich zones identified, namely, Fuling, Guang'an, Puguang, Tongnanba, and Yuanba, generally oriented northeast-southwest (Fig. 4 ). In Fuling and Guang'an potassium-rich zones around Fengdu-Shizhu, the salt rock primarily consisting of anhydrite and thin interbedded conventional polyhalite is thin, with a central thickness of 10-20 m, and a cumulative thickness is approximately 23 m, according to logging interpretation in Well TL 2. In contrast, the other three potassium-rich zones, Puguang, Tongnanba, and Yuanba, are characterized by the development of halite-type polyhalite in wide distribution, covering approximately 12 300 km2. The halite-type polyhalite is very thick, with 90 m in Well YB16 in the Yuanba belt and 40 m in Well HB102 in the Tongnanba belt according to logging interpretation.
Fig. 4. Distribution of polyhalite thickness in salt-gathering and potassium-formation area. |
From the development of polyhalite, several characteristics are summarized as follows. (1) The barrier effect of the Kaijiang underwater uplift may be a significant reason for the sedimentary difference on two sides, causing the predominance of supratidal flat salt deposits in the northern region and intertidal restricted flat deposits in the southern region, where thin polyhalite is associated with anhydrite. (2) The ancient landform controlled the distribution of salt basins and created three large-scale salt-gathering and potassium-formation centers oriented nearly northeast in Yuanba, Tongnanba, and Puguang, where the polyhalite is thick and distributed in circular patterns. Using seismic profiles and imprint method for paleogeomorphological analysis, significant thickness variation was observed within the evaporite platform in Jia 4 and Jia 5 members. This reveals the presence of geomorphological differentiation, shown by relatively independent potassic salt basins in low-lying areas. (3) Halite-type polyhalite is mainly distributed in the Yuanba, Tongnanba, and Puguang areas, with distribution range consistent with the distribution of salt rock, particularly in the salt-gathering and potassium-formation centers where both salt rock and polyhalite are thick. The discovery of large-scale halite-type polyhalite deposits in Puguang, Tongnanba, and Yuanba in the central Sichuan Basin expanded the distribution of potassic salt ore in the Sichuan Basin and filled the gap in potassium resources in the northeastern Sichuan Basin.
2.2. Halite-type polyhalite potash deposits
2.2.1. Macroscopic characteristics.
Conventional polyhalite is generally light to dark gray and semi-transparent, with greasy luster, microcrystalline to fine crystalline structures, and sedimentary textures, and usually interbedded with anhydrite, which makes it difficult to differentiate from anhydrite [22]. In the core from Well CX2, polyhalite is white and light red, distributed as clastic particles within halite (Fig. 5a ). The particles vary in shape, nearly equiaxed circles, squares, strips, ellipsoids, and even irregular shapes. Most particles are larger than 1 mm, and some are up to 5 cm (Fig. 5b ).
Fig. 5. Polyhalite cores from Jia 4 and Jia 5 members in Puguang area and their microscopic features in northeastern Sichuan. (a) Halite-type polyhalite, Well CX2, 3 768.01-3 772.03 m; (b) Local photo of halite-type polyhalite, Well CXD1, 3 050.01 m; (c) Polyhalite and anhydrite, Well ZK601, 3 573.70 m, plane-polarized light; (d) Polyhalite and gypsum, Well ZK601, 3573.70 m, cross-polarized light; (e) Polyhalite, Well ZK601, 3 472.51 m, SEM. |
2.2.2. Microscopic characteristics
Under microscope, the polyhalite from Well ZK601 appears star-like, fragmentary, and bay-like, associated with anhydrite, with no salt rock observed. The interlaid polyhalite and anhydrite are interwoven together, and the latter is metasomatized by the former, resulting in a distinct metasomatic relict texture. Under both plane-polarized light and cross-polarized light, the polyhalite crystals exhibit a subhedral to anhedral granular and columnar structure (Fig. 5c, 5d ). Observed under an electronic microscope, the polyhalite particles are almost completely granular, plate-like, and elongated shapes, and locally in foliated distribution. The boundary between the polyhalite particles and the halite or anhydrite is clear, with rare phenomena of interpenetration and replacement (Fig. 5e ).
2.2.3. Chemical composition and solubility
Chemical analyses of polyhalite samples from wells ZK601, ZK001, CXD1, and CX2 found that the K+ content ranges from 2.07% to 11.87%, with an average of 6.6%. In the halite-type polyhalite, clastic particles of polyhalite are distributed within the halite matrix. After the injection of fresh water, the halite matrix acting as a cementing material, was dissolved quickly, causing the polyhalite particles to lose support and enter the brine solution, and move randomly, further dissolving in the wate. These salt crystal-cemented clastic particles of polyhalite are water-soluble, similar to soluble potassic salts such as sylvite and carnallite [22].
2.3. Potassium-rich and lithium-rich brine
2.3.1. Chemical composition
According to the “Specifications for salt mineral exploration-Part 3: Ancient solid saline mineral” [23] and “Exploration specification of hydrogeology and engineering geology in mining areas” [24], five brine samples were collected from wells ZK601 and CX2 at different time intervals and analyzed for their chemical composition (Table 1 ). The results indicate that the salinity of the brine is 354.6-396.5 g/L, with K+ concentration of 28.92-48.15 g/L, Na+ concentration of 98.3-106.0 g/L, Mg2+ concentration of 887-1 094 mg/L, Ca2+ concentration of 9 520-12 794 mg/L, SO42− concentration of 1 032-2 294 mg/L, Cl− concentration of 202.1-210.9 g/L, and HCO3− concentration of 558-1 869 mg/L. The brine is classified as calcium chloride type. Additionally, the brine contained high levels of various trace elements, with Li+ concentration of 104.5-204.5 mg/L, Br− concentration of 875-2 980 mg/L, Rb+ concentration of 36.8-57.3 mg/L, and I− concentration of 31.67-38.00 mg/L, indicating a high comprehensive utilization value.
Table 1. Major and Trace Element Compositions of Brine Samples from Wells ZK601 and CX2 |
| Sample No. | Salinity/ (g·L−1) | Li+ (mg·L−1) | K+ (g·L−1) | Na+ (g·L−1) | Mg2+ (mg·L−1) | Ca2+ (mg·L−1) | SO42− (mg·L−1) | I− (mg·L−1) | Cl− (g·L−1) | HCO3− (mg·L−1) | Br− (mg·L−1) | Rb+ (mg·L−1) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ZK601-1 | 354.6 | 169.5 | 28.92 | 106.0 | 887 | 9 520 | 2 294 | 36.30 | 202.1 | 1 337 | 2 510 | 36.8 |
| ZK601-2 | 363.0 | 204.5 | 34.80 | 101.0 | 1077 | 11 260 | 1 946 | 31.67 | 206.0 | 667 | 2 980 | 53.0 |
| ZK601-3 | 362.2 | 201.5 | 34.84 | 102.4 | 1031 | 10 970 | 1 032 | 31.67 | 206.0 | 671 | 2 110 | 51.0 |
| CX2-1 | 396.5 | 111.5 | 48.15 | 101.0 | 1094 | 12 794 | 1 423 | 38.00 | 207.8 | 558 | 1 112 | 56.4 |
| CX2-2 | 370.0 | 104.5 | 46.15 | 98.3 | 1061 | 12 428 | 1 508 | 32.50 | 210.9 | 1 869 | 875 | 57.3 |
2.3.2. Potassium and lithium sources
Cluster analysis of brine samples from wells ZK601, C25, B2, HC1, and DW3 found a correlation coefficient of 0.945 6 between boric acid and lithium ions, indicating an association between the brine composition and green bean rocks. The green bean rocks are widely distributed in the northeastern Sichuan Basin, but the thickness varies significantly in many oil and gas wells and potassium-lithium exploration wells in the Puguang area. Green bean rocks are even absent in some wells [25], suggesting that they may have undergone dissolution. Before the precipitation of sylvine, Br− and K+ are stable components in the liquid phase. The Br−/Cl− ratio of the brine from Well ZK601 is 10 240, corresponding to the initial sodium sulfate deposition stage during seawater evaporation. The K+/Cl− ratio as an indicator of brine concentratio is 169 130, exceeding the ion concentration corresponding to the potassic salt deposition stage during seawater evaporation. The discordant increase in K+/Cl− and Br−/Cl− ratios of the potassium-rich and lithium-rich brine may be related to the lixiviation of solid potassic salt ore. Furthermore, the desulfurization coefficient (SO42−/Cl− ratio) reflects the environmental closure property for brine formation. A low desulfurization coefficient represents a well-closed environment [26]. The SO42−/Cl− ratio of the brine from Well ZK601 is 490, indicating a strongly reducing and closed environment.
2.4. Spatial superposition of potassium and lithium resources.
Due to the jointing effect of sedimentation and tectonic reconstruction, solid and liquid lithium and potassium resources are superimposed into a vertical “three-story structure” in Jia 4, Jia 5 and Lei 1 members. The lower layer consists of thin limy dolomite and dolomitic limestone brine reservoirs controlled by gypsum-dolomite flats in Jia 4 and Jia 5 members. The middle layer is located in the middle-upper part of the Jia 4 Member and Jia 5 Member, limited by the carbonate rock roof and floor, where the primary sedimentary polyhalite underwent deformation and displacement, aggregating to form halite-type polyhalite potash deposits. The upper layer is characterized by potassium-rich and lithium-rich brine reservoirs in Lei 1 Member. According to drilling data and structural analysis, the jointing effect of Yanshanian and Himalayan movements at two directions induced fracture network systems, and improved reservoir properties, making favorable conditions for brine migration and accumulation.
The halite-type polyhalite in the middle-upper part of Jia 4 and Jia 5 members is characterized by ultra-thick crumpled accumulation locally. Statistical analysis shows a positive correlation between the thickness of halite-type polyhalite and the thickness of crumpled formation. For example, the thickness of halite-type polyhalite is nearly 90 m of the 400 m-thick formation drilled in well area YB 16, and 62.8 m of the 730 m-thick formation in Well CXD1 in the Yuanba area, according to logging interpretation. In the Puguang area, in a formation less than 100 m, the cumulative thickness of polyhalite is generally less than 10 m, even no polyhalite existed in some wells.
3. Enrichment condition and mineralization model of solid and liquid potassium and lithium resources
3.1. Ternary enrichment and mineralization pattern
The fundamental geological conditions for the formation of solid and liquid phase potassium and lithium resources are the sufficient sources of potassium and lithium ions and the supply of lixiviated fluids. On the other hand, deep burial and high cost are factors controlling the high production of potassium and lithium resources in the Middle to Lower Triassic. The Sichuan Basin has undergone multiple phases of tectonic modification, resulting in fragmented ores with complex lateral and vertical variation which is a key factor limiting the enrichment and mineralization of potassium and lithium resources. Considering the presence of potassium and lithium materials, fluid supply for the lixiviating system, and multiple phases of tectonic reformation, a “ternary enrichment” theory for potassium and lithium resources was established, which sets the goal and provides theoretical support for exploration and development. Thick original polyhalite and green bean rocks with high potassium and lithium content superimposed vertically, and are material basis for the enrichment of potassium and lithium resources. Due to the relatively closure property of the Triassic, external fluid supply was limited. However, based on early original seawater, the widespread and ultra-thick gypsum provided ample fluid supply for the closed system after continuous dehydration. Multiple phases of tectonic movement modified low-permeability reservoirs, and halite-type polyhalite were deformed and thickened by external forces, consequently enrich the solid and liquid potassium and lithium resources.
3.1.1. Polyhalite and green bean rocks serve as the material basis for potassium and lithium mineralization
Lithium-rich green bean rocks are developed at the bottom of Lei 1 Member, and halite-type polyhalite is in Jia 4 and Jia 5 members. These intervals with rich potassium and lithium ions are superimposed closely and vertically, creating favorable conditions for mineralization characterized by "dual-source replenishment and proximal-source release". This setup provides a material basis for the lixiviating and enrichment of potassium and lithium ions.
Experiments on dissolving polyhalite with different water and salt solutions show that potassium element can transfer to the liquid phase, particularly in calcium chloride solution, where K+ concentration may be up to 40 g/L. This indicates that polyhalite can be dissolved under various solvent conditions when coexisting with underground brine [27]. Considering the widespread distribution of potassium-rich polyhalite and lithium-rich green bean rocks in the study area, it is inferred that during the later burial stage, a significant amount of water was slowly released from gypsum transformation, resulting in a near-source lixiviating phenomenon at high temperature and high pressure, dissolving solid potassic salt, polyhalite and green bean rocks. This process played a crucial role in increasing the potassium and lithium content in the brine.
3.1.2. Primary seawater and gypsum dehydration in the lixiviating system provide fluid supply
Brine is stored in carbonate and evaporite layers, and closely related to the process of seawater evaporation and concentration [28-29]. Comparing the chemical composition of the residual brine after seawater evaporation with that of underground brine is helpful to understanding the source of the brine. Evaporation and concentration experiments using South China Sea seawater at 25 °C revealed that seven stages of salt precipitation occurred sequentially: calcite, gypsum, salt rock, mirabilite, sylvine, kainite and halite [30⇓-32]. The salinity of the brine from Well ZK601 matched that observed in the mirabilite stage of typical brine evaporation. However, it has lower Mg2+ and SO42− contents and higher K+ and Ca2+ contents, indicating that other factors also influenced the evolution of the brine. The 87Sr/86Sr ratio of the brine is 0.708 324, which falls within the range of isotopic values for samples of polysulfides, salt rock, anhydrite, and dolomite from the Jialingjiang Formation (0.708 16-0.708 37). This aligns with the analysis of Triassic seawater strontium isotopes [33], confirming that the brine is related to ancient marine sources (Fig. 6 ).
A simulation experiment on gypsum dehydration at the temperature and pressure of the target strata in the study area, revealed the process of gypsum dehydration in deep formations was investigated, and the result revealed potential groundwater supply to the underground lixiviating system. X-ray diffraction analysis demonstrated that gypsum (CaSO4·2H2O) dehydrated into semi-hydrated gypsum (CaSO4·1/2H2O) at 165°C and fully transforms into anhydrite (CaSO4) at 220 °C (Fig. 7 ). In Jia 4 and Jia 5 and Lei 1 members, anhydrite is extensively developed. For instance, in Well MB1 drilled into the Maoba structure in the Puguang area, the thickness of anhydrite reaches nearly 350 m. High-temperature and high-pressure experiments indicated that at a depth of 5 000 m, gypsum remained in semi-hydrated gypsum state, but at 6 700 m, it fully converts to anhydrite. This continuous dehydration process of gypsum provides a sustainable source of deep brine. Theoretically, the water released from gypsum dehydration accounts for approximately 48.6% of the original gypsum volume [40], making it a vital water source for brine formation.
Fig. 7. X-ray diffraction analysis of gypsum dehydration. I—diffraction intensity; θ—angle of diffraction. |
3.1.3. Multiple stages of tectonic activity is key to the enrichment of potassium and lithium resources
After the marine sedimentation of the Leikoupo Formation, the northeastern Sichuan Basin experienced the superimposed effects of tectonic movements (i.e., the Indosinian, Yanshanian and Himalayan orogenies), resulting in intense uplifting, faulting, cutting, folding and deforming. Thick gypsum and salt rocks in Jia 4 and Jia 5 members exhibit high plasticity at high temperature and pressure, so making them highly susceptible to deformation and serving as a critical boundary between middle and upper deformation. Structural evolution was reconstructed through dynamic equilibrium analysis and forward modeling techniques on seismic profiles in the Puguang area. The results indicate that the Yanshanian Movement caused a northeast uplift-sag alternating structural pattern and lateral flow of plastic strata in Jia 4 and Jia 5 members, reflected by a structural style featured by thick flanks and thin top (Fig. 8 ). Notably, the thickness of the plastic strata containing polyhalite in the flanks of anticline and the core of syncline sharply increased, leading to the redistribution of polyhalite and providing favorable conditions for local thickening. The WE compressive force caused by the Himalayan movement continuously applied to the Daba Mountain, making halite-type polyhalite break, gather and become thick.
Fig. 8. Tectonic deformation of Jia 4 and Jia 5 members in the Puguang area (section location shown in |
Typical drilling data from wells B2, Ch25, and ZK601 shows that brine reservoirs are mainly located at the top of the Jialingjiang Formation and in the Lei 1 Member in the northeastern Sichuan Basin (Fig. 9 ). Taking Well ZK601 as an example, the lithology of the brine layer primarily consists of dolomite, dolomitic limestone and dolomitic limestone, with an average porosity of 2.57% and permeability less than 0.1×10−3 μm2, indicating a low-porosity and low-permeability reservoir. During drilling operation, phenomena such as well kick, lost circulation and gas invasion were observed, and the cores showed fragmented structures and tectonic breccias, indicating the development of fractures and faults. Re-examination of oil exploratory wells reveals that the porosity and permeability of reservoirs of Jia 4 and Jia 5 and Lei 1 members in Puguang area are generally poor, with strong heterogeneity. This is unfavorable for the migration and accumulation of deep, high-salinity brine, making the development of fractures particularly critical. The enrichment of brine is often associated with regions where tectonic stress is relatively concentrated and fault-fracture systems are well developed [41-42]. Due to the influence of multi-phase tectonic movements, complex fault and fracture systems have formed, connecting similar or different brine reservoirs and facilitating the movement of fluid, thereby creating a comprehensive lixiviating system. The development of faults and fractures not only promotes the development of secondary pores but also expands the storage space for brine, significantly improving the migration conditions for brine within the reservoir and thus having an important impact on the enrichment and distribution of brine.
Fig. 9. Tectonic positions of brine layers in wells B2 (a), C25 (b) and ZK601(c) in the Puguang area, northeastern Sichuan Basin. T1j42—2nd sub-member of 4th Member in Lower Triassic Jialingjiang Formation; T1j51—1st sub-member of 5th Member in Lower Triassic Jialingjiang Formation; T1j52—2nd sub-member of 5th Member in Lower Triassic Jialingjiang Formation; T2l11—1st sub-member of 1st Member in Middle Triassic Leikoupo Formation; T2l12—2nd sub-member of 1st Member in Middle Triassic Leikoupo Formation; T2l13—3rd sub-member of 1st Member in Middle Triassic Leikoupo Formation; T2l21—1st sub-member of 2nd Member in Middle Triassic Leikoupo Formation; T2l22—2nd sub-member of 2nd Member in Middle Triassic Leikoupo Formation. |
3.2. Mineralization evolution model
The enrichment evolution of potassium and lithium resources in both solid and liquid phases have progressed through four distinct stages (Fig. 10 ): (1) Salt-gathering and potassium-lithium accumulation. Under the depressing influence, multiple ancient low-lying areas were created within the platform, and a relatively closed salt lake environment was formed with the help of uplifts acting as barriers, which provided space for brine accumulation. Intermittent seawater replenishment, combined with intense evaporation and concentration facilitated large salt-gathering and potassium-formation centers, as well as layered primary polyhalite deposits in the depression. Concurrently, volcanic activities produced lithium-rich green bean rocks (Fig. 10a ). (2) Initial water-rock reaction. During the burial and compaction of polyhalite, differential compaction and deformation caused local mixture of polyhalite and halite, characterized by “salt-packing potassium and salt-potassium mixture” with variable thicknesses laterally. Under deep burial conditions, high temperature and pressure caused continuous gypsum dehydration, releasing fluid that promoted the dissolution of polyhalite and water-rock reactions within the green bean rocks. This maintained a long-term lixiviating system, forming a closed system with high K+ and high Li+ brine (Fig. 10b ). (3) Transformation and aggregation. Early Yanshanian thrusting nappe tectonics created a series of northeast-trending thrust structures. Under intense tectonic modification, polyhalite layers were cut and fragmented, enhancing their plasticity and causing them to crumple and mix with halite. This ultimately led to the accumulation of thick halite-type polyhalite in the flanks of the anticline. Tectonic fragmentation also led to the development of fractures and faults, intensifying fluid activity and further increasing the concentrations of potassium and lithium in the brine (Fig. 10c ). (4) Enrichment and finalization. The Himalayan compressive force applied to the Daba Mountains caused the earlier northeast-trending structures to be superimposed and modified by late northwest-trending structures, leading to complex superimposed deformation [43]. Under continuous tectonic fragmentation, the mixing of polyhalite and halite became more uniform, and the lixiviating process continued to dissolve polyhalite and green bean rocks, resulting in increasing concentrations of potassium and lithium in the brine. Multi-phase tectonic activities produced a network of fractures, improving the permeability of the Jia 4, Jia 5, and Lei 1 reservoirs. Controlled by the groundwater hydraulic system, potassium-rich and lithium-rich brine gradually gathered in the fractured-porous reservoirs at the crest and turning points of the anticline, ultimately forming deep brine reservoirs rich in potassium and lithium (Fig. 10d ). After these four stages, the Jia 4 and Jia 5 halite-type polyhalites became more uniform and thicker, making them the primary target for potassic salt exploration. Meanwhile, the Lei 1 brine reservoirs, with better space and transport channels provided by pore-fracture networks, became the favorable target for exploration and development of potassium and lithium resources.
Fig. 10. Enrichment and mineralization model of solid and liquid potassium-lithium resources. T2l—Middle Triassic Leikoupo Formation. |
4. Exploration insights
The ternary enrichment model for deep marine potassium and lithium resources in the northeastern Sichuan Basin elucidates the conditions and controlling factors for resource formation and enrichment. Based on the review of petroleum exploration wells, the total area of five potassium-rich zones, Puguang, Tongnanba, Yuanba, Fuling and Guang’an, exceeds 20 000 km2. The breakthroughs in wells CXD1 and CX2 in the Puguang are significant advancements following major breakthroughs of gas fields and Permian shale gas exploration in Puguang area, which opened new exploration fields for potassium and lithium resources, laid a solid foundation for the integrated utilization of various resources such as natural gas, lithium and potassium, and injecting new vitality into the exploration and development of marine potassium and lithium resources in China.
Only potassium and lithium resources with commercial mining value are of practical significance. In deep mining, solid potassic salt deposits must be soluble halite-type polyhalite. Conventional polyhalites in Fuling and Guang’an are difficult to solve, so they are treated as “inaccessible ore”. In contrast, thick halite-type polyhalites in Puguang, Tongnanba and Yuanba are favorable exploration targets. Tectonic activity not only aids in the redistribution and accumulation of polyhalite but also improves the physical properties of tight brine reservoirs. The tectonic deformation in Puguang area is the most significant, while that in Yuanba and Tongnanba areas is relatively weaker. From the perspective of burial depth, the halite-type polyhalite in Puguang area is shallower, and advantageous for production costs and holds strong practical significance for pilot mining and commercial development.
From the perspective of exploration and development of potassium and lithium resources, halite-type polyhalites in middle reservoirs are undoubtedly successive resources for production capacity construction of potassic salts. In the future, reasonable acid-fracturing treatments to lithium-rich and potassium-rich brine reservoirs are expected to enhance reservoir permeability and brine yield. Considering interbedded features of solid and liquid resources, highly deviated wells will be designed to penetrate brine reservoirs as thick as possible while connecting brine reservoirs with halite-type polyhalite layers to facilitate effective comprehensive collection of potassium and lithium brine.
The breakthroughs to solid and liquid potassium and lithium resources in Puguang area mark a significant step forward for the understanding of resource utilizability, potential and enrichment model, but challenges remain in precise geophysical localization and quantitative prediction of potassium and lithium contents. Future work should focus on the differences in the thickness and content of halite-type polyhalite and potassium-rich brine reservoirs. The mechanisms of mineralization and migration can be revealed by systematic sample analysis and numerical simulation. Meanwhile, advanced geophysical technologies, high-precision imaging of salt rock layers, quantitative interpretation of well logging data, and geophysical prediction of potassic-lithic resource distribution are support measures. Strengthening the integration of seismic, geological and production, enhancing collaboration between enterprise and local government, and research and development of key technologies are important for lixiviating mining and production capacity construction, and eventually realizing high-quality exploration and development of potassium and lithium resources.
5. Conclusions
The Jia 4, Jia 5 and Lei 1 members are characterized by the development of gypseous dolomite, evaporative flats, and salt lakes under the influence of late salinization cycles in the northeastern Sichuan Basin. The Kaijiang underwater uplift blocked seawater from advancing westward, leading to the development of three large-scale salt-gathering and potassium-formation centers, Puguang, Tongnanba and Yuanba, under the combined effects of subsidence and intense evaporation in Jia 4 Member and Jia 5 Member. Controlled by local positive geomorphology, and suffering from exposure and dissolution, intragranular and intergranular dissolution pores were created at the bottom of Jia 4 and Jia 5 and Lei 1 members, which are favorable spaces for brine accumulation.
The halite-type polyhalites in Jia 4 and Jia 5 members may be 90 m, and cover approximately 12 300 km2. They are associated with halite, and water-soluble for solution mining at deep burial. The brine with high potassium and lithium contents indicates high-quality potassium and lithium resources in the deep marine strata in the northeastern Sichuan Basin. The Triassic marine solid and liquid potassium and lithium resources have near sources and are stored in a “three-story structure”. Geometric estimation suggests that the potential resource amount of KCl in halite-type polyhalite exceeds 900 million tons in 586.9 km2 in Puguang, laying a solid foundation for the development and utilization of deep marine potassium and lithium resources in China.
The potassium and lithium resources in Jia 4, Jia 5 and Lei 1 members follow a “ternary enrichment” mineralization pattern. The Green bean rock at the bottom of Lei 1 Member and the polyhalite in Jia 4 and Jia 5 members provide resource basis and favorable mineralization conditions characterized by “dual-source replenishment and proximal-source release”. Under deep burial conditions, continuous dehydration of gypsum provides ample fluid supply, ensuring long-term closed lixiviation and a brine system rich in K+ and Li+. Multi-phase tectonic reconstruction not only controls the development of fracture-porous reservoirs but also causes polyhalite to break, crumple, form plastic flow mixed with halite, and finally accumulate and thicken into high-quality halite-type polyhalite. Faults and fractures promote the development of secondary pores in the brine reservoirs, improving the migration conditions and significantly impacting the enrichment of brine resources.
The mineralization process of solid and liquid potassium and lithium resources underwent four stages: salt-gathering and potassium-lithium accumulation, initial water-rock reaction, transformation and aggregation, and enrichment and finalization. After the four stages, halite-type polyhalites in Jia 4 and Jia 5 members have accumulated significantly, making them the primary potassic salt target for solution mining. Among the lithium-potassium brine reservoirs, the Lei 1 Member is featured by favorable fracture networks that are helpful to brine storage and transport. making it an important target for comprehensive exploration and development of potassium and lithium resources.