Open AccessArticle

Stages and Evolution of Strike-Slip Faults of the Ultra-Deep-Burial Ordovician Strata in Fuman Oilfield, Tarim Basin: Evidence from U-Pb Geochronology of Siliceous Minerals

Chao Yao

¹,

Zhanfeng Qiao

^2,3

Xiao Luo

¹,

Tianfu Zhang

^2,3,*,

Bing Li

¹,

Shaoying Chang

^2,3,

Zhenyu Zhang

¹ and

Jiajun Chen

Tarim Oilfield Company, PetroChina, Korla 841000, China

Hangzhou Research Institute of Geology, PetroChina, Hangzhou 310023, China

State Energy Key Laboratory of Carbonate Oil and Gas, Hangzhou 310023, China

Author to whom correspondence should be addressed.

Minerals 2025, 15(3), 270; https://doi.org/10.3390/min15030270

Submission received: 20 January 2025 / Revised: 26 February 2025 / Accepted: 4 March 2025 / Published: 6 March 2025

(This article belongs to the Special Issue Deformation, Diagenesis, and Reservoir in Fault Damage Zone)

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Abstract

Siliceous minerals with the property of resistance to diagenetic alteration precipitate during the migration of hydrothermal fluids through strike-slip faults and the interaction of these fluids with host rocks during fault activity. Based on petrological analyses and U-Pb dating of siliceous minerals, the stages of strike-slip faulting of the ultra-deep-burial Ordovician in the Fuman oilfield were subdivided and their evolutionary process was discussed in combination with seismic interpretation. The results reveal the following: (1) the strike-slip faults contain hydrothermal siliceous minerals, including cryptocrystalline silica, crystalline silica, and radial silica. (2) Based on the twelve U-Pb ages of siliceous minerals (ranging from 458 ± 78 Ma to 174 ± 35 Ma) and five U-Pb ages of calcite, the activity of the strike-slip faults was divided into six stages: the Middle Caledonian, Late Caledonian, Early Hercynian, Middle Hercynian, Late Hercynian, and Yanshanian, corresponding to twelve siliceous U-Pb ages ranging from 458 ± 78 Ma to 174 ± 35 Ma, and five calcitic U-Pb ages. The Late Caledonian and Early Hercynian were the main periods of strike-slip fault activity, while the Late Hercynian period marked the final period of the fault system. (3) Later-stage faults inherited and developed from pre-existing faults. Steep linear strike-slip faults formed during the Middle and Late Caledonian movements. During the Late Hercynian and Yanshanian movements, mid-shallow faults, branch faults, and shallow echelon faults developed on the foundation of these linear faults. The methods and results of this study can guide future hydrocarbon exploration in the Fuman oilfield and can be applied to areas with similar tectonic backgrounds.

Keywords:

siliceous mineral; U-Pb dating; strike-slip fault; ultra-deep; Tarim Basin

1. Introduction

Strike-slip faults are crucial targets for hydrocarbon exploration, serving not only as pathways for hydrocarbon migration but also as reservoirs for hydrocarbon accumulation. Recently, the discovery of a number of highly productive hydrocarbon wells controlled by ultra-deep strike-slip faults worldwide [1,2,3,4] has further highlighted their importance in hydrocarbon exploration and development. Understanding the stages and evolutionary processes of these faults is essential, as they provide the foundation for reconstructing the history and architectural framework of strike-slip fault systems.

Traditionally, methods such as precise seismic profile interpretation [5,6,7,8], modeling experiments [9,10,11], and numerical simulations [12,13] have been widely used to study strike-slip faults. However, the quality of seismic data is strongly affected by high temperatures, high pressure, and complex geological formations, while the interpretation results of ultra-deep-burial strata and seismic profiles depend on the subjective speculations of the interpreter, which can lead to significant differences in the interpretation of the same seismic data. Fluid–rock interactions associated with strike-slip faulting could result in the formation of a variety of minerals in faults [14,15], which can be used to infer the timing and evolution of fault activity [16,17,18,19]. Chronological analysis of fault gouge illite and carbonate formed in faults, such as Sm–Nd [20,21], K–Ar [22,23,24,25,26,27], ⁴⁰Ar–³⁹Ar [21], and Rb–Sr [28,29], and U-Pb dating [30,31,32,33,34], can provide direct insights into the periods of fault activity and their evolution, especially when combined with the consideration of regional tectonic movements and specific geological events. For example, Middleton et al. [21] employed ⁴⁰Ar–³⁹Ar, ⁸⁷Rb–⁸⁷Sr, and ¹⁴⁷Sm–¹⁴³Nd dating to analyze the thermal and tectonic history of the Warburton–Cooper Basin in central Australia. They dated granite intrusions and authigenic illite near fault zones, revealing that fault development was associated with regional extensional tectonics during three active periods: the Carboniferous (323.3 ± 9.4 Ma), Late Triassic (201.7 ± 9.3 Ma), and Cretaceous (128–86 Ma). Uysal et al. [22] used K-Ar isotopic dating to determine the formation age of authigenic illite in the North Anatolian Fault Zone, Turkey, as 57 Ma, linking its development to strike-slip or extensional processes following the closure of the Neo-Tethys Ocean.. Qiao et al. [31] conducted U–Pb isotopic dating on calcite fillings within Ordovician karst caves, vugs, and fractures in the Halahatang Oilfield of the Tabei Uplift. By integrating these ages with the tectonic history of the region, they identified seven distinct tectonic events that modified the Ordovician fracture-cavity reservoirs in the Halahatang Oilfield. However, the isotopic dating of authigenic illite and calcite in faults has two limitations for dating strike-slip faults in limestone host strata. One is that strike-slip faults in limestone strata either do not contain authigenic illite or contain very little, making K–Ar and Ar–Ar dating unsuitable, and the other is that calcite is highly susceptible to recrystallization, leading to strong ambiguity in U–Pb ages.

Hydrothermal fluids rich in silica could lead to the formation of various siliceous minerals such as quartz and chert as they migrate through faults and interact with the host rock in tectonically active environments [35]. Because uranium (U) and lead (Pb) are not easily incorporated into the crystal lattice of siliceous minerals, their contents in these minerals originate primarily from hydrothermal fluids present during their formation and are relatively resistant to alteration by other fluids during later diagenesis, compared to calcite, which has higher U and Pb contents with similar ionic radii to calcium, allowing them to be easily incorporated into the calcite crystal lattice and making them more susceptible to alteration by various fluids during later diagenesis. This makes siliceous minerals more resistant to diagenetic alteration, preserving the original chemical and isotopic compositions of the hydrothermal fluids from which they formed. As a result, U–Pb dating of siliceous minerals can provide reliable insights into tectonic hydrothermal events and the evolution of strike-slip faults.

The Fuman oilfield in the Tarim Basin is characterized by the development of strike-slip faults, with a high rate of siliciclastic rocks occurring within the faults. Currently, among the wells drilled along the strike-slip faults, more than 20 wells have encountered siliceous minerals in the Ordovician Yijianfang Formation and Yingshan Formation, such as the GL3-H well in the F_I5 fault, the YM703 well in the F_I8 fault, the MS5 well in the F_I17 fault, the MS711 well in the F_I19 fault, and the MS802 well in the F_I20 fault. These siliceous rocks exhibit diverse forms and colors but are consistently associated with fault zones and show significant evidence of hydrothermal diagenesis.

In this study, we conducted a petrological analysis of siliceous minerals encountered during drilling in strike-slip faults and performed laser in situ U–Pb dating to obtain absolute geological ages. This allowed us to classify the activity of strike-slip faults in the Fuman oilfield into distinct stages. By integrating these findings with the regional tectonic background and seismic interpretation, we reconstructed the architecture of ultra-deep strike-slip faults at different stages and synthesized their overall evolutionary history.

2. Geological Setting

The Fuman oilfield is situated on the northwestern margin of the Tarim Basin, bordered by the Awati Depression, the Mangjiaer Depression, the Tazhong Uplift, and the Tabei Uplift. It serves as a transitional zone between the northern slope of the Tazhong Uplift and the southern slope of the Tabei Uplift, as well as between the Awati Depression and the Mangjiaer Depression. Structurally, it forms a “saddle-shaped” low ridge trending NS (Figure 1) [2,36,37,38]. The Fuman region has experienced multiple phases of tectonic evolutions [7,36]. During the Early Caledonian, the Tarim Basin was characterized by an extensional environment, leading to subsidence in this region. From the Middle–Late Caledonian to the Early Hercynian, the area was influenced by a compressional regime within the Tarim Basin. During the Middle–Late Hercynian, the Fuman region was uplifted due to the subduction and closure of the Southern Tianshan Ocean. It subsequently underwent overall subsidence and stabilized during the Himalayan movement [39,40]. These tectonic events resulted in the extensive development of strike-slip faults in the Fuman oilfield. To date, 21 first-order strike-slip faults have been identified, labeled F_I1 to F_I21 from west to east, with predominant NE and NW trends [36,41,42]. Additionally, a few second-order strike-slip faults with NNW and SN trends are also present (Figure 1a). The Ordovician strata in the Fuman oilfield are well preserved, with a gentle structure dip and an average burial depth exceeding 7000 m. The primary exploration targets are the Middle Ordovician Yijianfang Formation and Yingshan Formation, which have a combined thickness of approximately 800–970 m. Their lithology is predominantly composed of granular limestone, deposited in open-platform grain shoals and micritic limestone, and formed in an intertidal marine environment (Figure 1d).

3. Samples and Methods

3.1. Collection of Samples

Siliceous minerals are extensively developed in the Yijianfang Formation and Yingshan Formation of the Fuman oilfield. Core and outcrop observations reveal that the siliceous minerals exhibit three colors: dark gray, grayish white, and transparent colorless. Their textures include bedding-parallel, nodular, speckled, and linear forms. Within fractures, the siliceous minerals primarily appear as nodular, speckled, and linear forms, with all three colors represented.

For this study, various hydrothermal siliceous minerals distributed along fractures in the core samples were selected for U–Pb dating and geochemical analyses (Table 1, Figure 2). To validate the reliability of the U–Pb dating results for the siliceous minerals, a siliceous sample located 20 cm away from a diabase intrusion in the Penglaiba Formation of the Yong’anba outcrop in the southwest depression were also analyzed (Figure 1b).

3.2. Experimental Methods

Two duplicate samples containing silica ranging in size from 1 cm × 1 cm to 2.5 cm × 3 cm were collected from each sample. One duplicate was prepared into an 80 μm thick section for electron backscatter (BSE) image analysis and wavelength dispersive spectrometry (WDS) chemical composition analysis by an electron probe microanalyzer (EPMA). The other duplicate was made into a 100 μm thick section and examined under an optical microscope (OM) to study the micro-texture characteristics of the sample. Subsequently, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to analyze the trace element contents of different textures within the thin section. The siliceous samples with greater U content than Pb content were cut into 1.2 cm × 1.2 cm pieces and placed on the sample stage of a laser ablation multi-collector inductively coupled plasma mass spectrometer (LA-MC-ICP-MS) for laser ablation experiments to determine the U–Pb isotopic ages of the siliceous materials.

The silica from the drilled samples was crushed, and the silicon isotopes (δ³⁰Si V-NBS28) were analyzed using a MAT-253 gas isotope mass spectrometer from Thermo Fisher, Waltham, MA, USA. The electron probe instrument used in the experiments was the EMPA-1720 from Shimadzu, Kyoto, Japan. The laser ablation system for the LA-ICP-MS was the ReSOlution LR-S155 from Applied Spectra Inc., Fremont, CA, USA, with a wavelength of 193 nm and a beam diameter of 160 μm. The mass spectrometer used was the iCAP-TQ from Thermo Fisher, Waltham, MA, USA. The U–Pb dating instrument consisted of a multi-collector inductively coupled plasma mass spectrometer (Neptune Plus, Thermo Fisher, Waltham, MA, USA) combined with a laser ablation system (GeoLasHD, Coherent, Sachsenburg, PA, USA). A laser wavelength of 193 nm, beam diameter of 160 μm, frequency of 10 Hz, energy density of approximately 8 J/cm², single-point acquisition time of 20 s, test temperature of 20 °C, and relative humidity of 50% were used. The reference standard used was siliceous rock from the top of the Maokou Formation in the Sichuan Basin, with an age of 250 ± 4 Ma. Before sample testing, NIST614 was used to correct for instrumental signal drift and the ²⁰⁷Pb/²⁰⁶Pb ratio, followed by calibration of the ²³⁸U/²⁰⁶Pb ratio using the reference siliceous standard. During sample testing, a sequence of 4 standards was first ablated, followed by the ablation of 6 samples. Each sample was measured at 100 spots.

To validate the siliceous dating results and ensure the accuracy of stage division and evolution of strike-slip faults, laser in situ U–Pb dating of calcite in some fractured siliceous samples was also conducted (Table 1). The experimental procedures and parameters of calcite laser in situ U–Pb dating followed the methods described by Pan et al. (2020) and Qiao et al. (2023) [30,31].

4. Results

4.1. Petrological Characteristics

Except for sample YAB in dolomite strata, the host rocks of other samples were granular limestone (Figure 2 and Figure 3). The siliceous minerals in samples filled in fractures (Figure 2) and exhibited variations in color and texture. The siliceous minerals were primarily dark gray (Figure 2a–e), grayish white (Figure 2f–i), and transparent colorless in the central parts of the siliceous minerals (Figure 2c,d). In some cases, the siliceous minerals, as the main component, completely filled fractures in blocky or breccia materials (Figure 2a–e,h,i). In other instances, the siliceous minerals, as secondary components, filled fractures as calcareous breccia (Figure 2f,g).

There are three types of siliceous minerals including cryptocrystalline silica, crystalline silica, and radial siliceous aggregates (abbreviated as radial silica, Figure 3), which were revealed using optical microscopy. They are commonly observed in grainstone texture and are the products of the metasomatism of lime grainstone by silica. The metasomatic grains primarily consist of algal grains and bioclasts from the protolith. Cryptocrystalline silica is the most prevalent form of siliceous minerals and is present in all samples. Crystalline silica, on the other hand, is SiO₂ with a crystal structure visible to the naked eye. It is mainly micro-crystalline, with some instances reaching fine-crystalline sizes. It predominantly occurs as intergranular siliceous cement (Figure 3a,b,j–l) and as siliceous fillings in fractures and vugs (Figure 3c–e,g,i–l). Radial silica consists of aggregates of cryptocrystalline and microcrystalline silica, typically filling fractures and vugs. It is often located at the center of cryptocrystalline silica and is a characteristic feature of chalcedony (as seen in the transparent colorless siliceous mineral in Figure 2d).

Based on these petrographic characteristics, we interpret that the siliceous minerals in the samples are primarily composed of SiO₂, ranging from amorphous to micro-crystalline forms, with residual calcite as an auxiliary component.

4.2. Major Elements

The chemical compositions of the three types of siliceous minerals also vary significantly, in addition to their texture differences. Backscattered electron images from the electron probe micro-analyzer (EPMA) show that cryptocrystalline silica contains calcite of various sizes (Table 2, Figure 4a), including calcite crystals smaller than 100 μm and calcite patches ranging from hundreds of micrometers to millimeters. In contrast, the compositions of crystalline silica and radial silica are homogeneous, with SiO₂ contents generally exceeding 90% (Figure 4b,c, Table 2, point 4, point 14). Notably, the maximum SiO₂ content in radial silica reaches up to 97% (Figure 4e). The edges of calcite crystals or the transitional zones where cryptocrystalline silica patches are in contact with the limestone often exhibit a bay-shaped morphology with silica inclusions within the crystals. This provides clear evidence of limestone dissolution and subsequent replacement by silica. At the interface between siliceous and calcareous components, transitional products of calcite to wollastonite are observed, as indicated by points 6, 7, 9, 10, 13, 15, and 16 in Figure 5 and Table 2. Quantitative analysis of these points does not yield the chemical stoichiometric percentages of pure calcite (CaCO₃) or silica (SiO₂). Instead, the data reflect varying proportions of Ca and Si. For instance, the data corresponding to points 5 and 9 closely match the chemical stoichiometric composition of wollastonite (Ca₃Si₃CO₉), suggesting that calcite has undergone a phase transformation under the influence of high-temperature silicon-rich hydrothermal fluids (Figure 4f).

4.3. Silicon Isotopes

The δ³⁰Si values of the Ordovician samples from the Fuman oilfield range from 0.7‰ to 2.8‰, falling within the highly positive range (Table 1 and Figure 5), similar to those of metamorphic and hydrothermal sedimentary siliceous rocks such as chert, siliceous dolomite, siliceous stromatolite, and siliceous rocks formed from hydrothermal deposits [43]. Among the samples, the δ³⁰Si values of the metasomatic cryptocrystalline silica are relatively low, at less than 1‰, while those of crystalline silica and radial silica are higher, exceeding 1‰. The YAB sample exhibits the highest δ³⁰Si value, reaching 2.8‰.

4.4. U–Pb Dating of Fracture-Fillng Silica

According to the methodology outlined in Section 3.2, laser in situ U–Pb dating was conducted on the silica and calcite in the experimental samples. The obtained 12 sets of siliceous U–Pb ages range from 458 ± 78 Ma to 174 ± 35 Ma (Table 1, Figure 6), spanning from the Middle Caledonian to the Yanshanian.

5. Discussion

5.1. Origin of Siliceous Minerals

Research has confirmed that silicon isotope fractionation is limited in high-temperature geological processes [44,45], whereas it is more pronounced in low-temperature biogeochemical processes [46,47]. In high-temperature fluid–rock interactions, precipitated minerals with stronger Si-O bonds may become enriched in heavier silicon isotopes (³⁰Si), while the fluids may be enriched in lighter silicon isotopes (²⁸Si). In contrast, in low-temperature environments, processes such as biological uptake, chemical weathering, and adsorption tend to favor lighter silicon isotopes, resulting in negative δ³⁰Si values. Unlike biological uptake mechanisms, the silicon metabolism mechanism of siliceous radiolarians tends to favor a mineralization process. During this process, the siliceous skeletons of radiolarians preferentially incorporate heavier silicon isotopes (³⁰Si), while lighter silicon isotopes (²⁸Si) are more likely to remain in the environment. For example, siliceous radiolarians absorb heavier silicon isotopes from seawater, leading to a positive δ³⁰Si value. Silicate reservoirs, such as ferromagnesian volcanic rocks and meteorites, typically exhibit slightly negative δ³⁰Si values, ranging from 0‰ to −1‰ (Figure 5).

The relatively low δ³⁰Si value of the metasomatic silica may be attributed to the preservation of properties inherited from the original limestone. This includes the incorporation of detrital silica and biogenic silica in algal grains and bioclasts, which could lower silicon isotopic values (Figure 3b,h,j).

5.2. Geochronology of Siliceous Minerals

The U–Pb ages of the three types of silica vary significantly. Metasomatic cryptocrystalline silica exhibits the oldest age (Table 1), ranging from 458 ± 78 Ma to 376 ± 19 Ma. The oldest U–Pb age is from sample GL1, where the siliceous minerals primarily consist of algal grains replaced by silica (Figure 3a). Crystalline silica has the second oldest age, with sample YAB dating to 285.2 ± 7 Ma (Figure 7). Radial silica shows the youngest age, with samples GL3-H and YM5 dating to 252 ± 54 Ma and 174 ± 35 Ma, respectively (Figure 6e,f). Although the laser ablation target for sample GL3 was radial silica, the area where the radial silica developed is relatively small, and the laser also ablated portions of the metasomatic cryptocrystalline silica and calcite (Figure 4b). This may explain why its U–Pb age is higher than the other two radial silica ages, resembling the U–Pb age of crystalline silica. Additionally, the metasomatic cryptocrystalline silica around the bioclasts of sample MS5 and in sample MS711 contains some cemented crystalline silica and filled radial silica, which may affect the final U–Pb age results. These findings suggest that siliceous hydrothermal fluid continuously altered the host limestone along the fractures, with the alteration mechanism evolving from metasomatism to cementation and then filling. This progression corresponds to a change in crystal size from cryptocrystalline to crystalline and then radial, indicating a gradual increase in diagenetic intensity and an improvement in crystallinity. In other words, the same fracture or strike-slip fault remained active and was altered by geological fluids during different tectonic movements.

The five sets of calcitic U–Pb ages obtained (457 ± 13 Ma to 284 ± 60 Ma, Figure 6l–p) fall within the range of the siliceous U–Pb ages. However, except for the rhombic calcite age in MS711, which aligns with the siliceous age data, the other four calcite ages do not match their siliceous ages. For example, in sample GL3-H, there are two siliceous ages and one calcite age in the fractures (Figure 2d and Figure 3c–f): the cryptocrystalline silica has an Early Hercynian age, the radial silica has a Late Hercynian age, and the sparry calcite has a Middle to Late Hercynian age. This also confirms that the fractures or faults were continuously active during their development and evolution, constantly affected by flow events from different periods.

It is worth noting that U–Pb dating of metasomatic cryptocrystalline silica tends to have larger errors, likely due to the influence of U and Pb content in the residual calcite of the protolith. This is because the ionic radii of U and Pb are closer to that of Ca for calcite, making it easier for U and Pb ions to enter calcite crystals through isomorphism compared to siliceous minerals. In addition, the higher the residual calcite content in the protolith, the greater the potential error (Figure 4). In contrast, crystalline silica and radial silica with high silica content and homogeneous structures yield more reliable U–Pb ages, such as sample YAB (Figure 7).

The color of the siliceous rock where sample YAB is located gradually changes with distance from the dolomitic host strata. The part adjacent to the host rock is dark gray, while the color shifts to grayish white and transparent colorless closer to the diabase (Figure 7a). Energy spectrum analysis reveals that the SiO₂ content of the grayish white–transparent colorless minerals (Figure 7b) is 98.9%, with a δ³⁰Si value of 2.8‰, indicating metamorphism influenced by magmatic-hydrothermal fluids [47,48]. During the interaction between the volcanic hydrothermal fluids and dolomite, heavy silicon isotopes (³⁰Si) tend to enrich in the solid dolomite, while light silicon isotopes (²⁸Si) remain in the fluid phase. This process results in the silicon isotopic composition of sample YAB being significantly higher δ³⁰Si than that of other samples.

Optical microscopy observations reveal that the siliceous crystals are primarily cryptocrystalline and micro-fine crystalline. Cryptocrystalline silica retains the microstructural characteristics of dolomite and is a product of metasomatism. The micritic fine crystalline silica fills dissolution pores (Figure 7c), representing direct precipitation from volcanic silicate hydrothermal fluids. The laser ablation U–Pb age of filling silica is 285.2 ± 7.3 Ma (Figure 7d), consistent with the zircon U–Pb ages of 272–291 Ma of the Permian diabase in this area [49,50].

5.3. Stages and Evolution of Strike-Slip Faults

It is widely accepted that development of strike-slip faults in the Ordovician strata of the Fuman oilfield began during the Middle Ordovician (Middle Caledonian) and was subsequently influenced by multiple tectonic events, including the Late Caledonian–Early Hercynian, Late Hercynian, Indosinian–Yanshanian, and Himalayan movements, due to the periodic extensional–compressional cycles of the Tarim Basin [7,31,51]. For instance, Wu et al. [51] and Qiao et al. [31] reported the U–Pb ages of calcite in fractures and vugs from the central and northern Tarim Basin as 460 ± 12 Ma, 462.6 ± 6.8 Ma, and 460.8 ± 3.4 Ma, respectively. Combined with seismic interpretation, they proposed that the development of strike-slip faults in the central and northern Tarim Basin started in the Late Middle Ordovician, consistent with the ages of 458 ± 78 Ma and 454 ± 50 Ma obtained from the metasomatic silica in samples GL1 and YM6 in this study, as well as the 457 ± 13 Ma age measured from calcite in fractures of sample MS5.

The division of later strike-slip fault stages, beginning in the Late Middle Ordovician, varies slightly due to differences in strike-slip faulting, the length of the fault, and measurement precision. Jia et al. [7] pointed out, based on the basin-level tectonic dynamic cycles, that the Middle Caledonian was the initial stage of the strike-slip faulting in the northern and central Tarim Uplifts. This was followed by a tectonic stabilization period during the Late Caledonian–Early Hercynian and Late Hercynian–Indosinian, characterized by compressional stress. The Yanshanian–Himalayan periods were considered unrelated to the significant strike-slip faults due to relatively weak tectonic activity. Qiao et al. [31] reported U–Pb ages of calcite fillings in fractures and vugs from Ordovician strata in the Halahatang Oilfield in the northern Tarim, obtaining ages of 460.8 ± 3.4 Ma, 448.6 ± 5.3 Ma, 364 ± 53 Ma, 335 ± 19 Ma, 324 ± 23 Ma, 307.5 ± 9.3 Ma, 282.9 ± 5.4 Ma, 244 ± 13 Ma, and 158 ± 17 Ma (Figure 8). Based on these data, they concluded that the northern Tarim Basin experienced tectonic activities throughout the Hercynian and Indosinian–Yanshanian. Although the U–Pb ages of silica in this study differ slightly from those of Qiao et al. [31] due to the regional and sample variations, they are generally consistent when considering the calcite ages (Figure 6l–p).

Based on this, it can be inferred that the strike-slip faulting in the Fuman oilfield began in the Middle Caledonian (458–454 Ma), followed by six reactivation events in the Late Caledonian (440–403 Ma), Early Hercynian (399–371 Ma), Middle Hercynian (322 Ma), Late Hercynian (292–252 Ma), and Yanshanian (174 Ma) (Figure 8). Each stage of strike-slip faulting developed successively on the previous fault architecture and was subjected to hydrothermal alteration by silicon-rich fluids simultaneously.

This multi-stage evolution has resulted in the diverse and complex characteristics of strike-slip faults in the Fuman oilfield [52,53]. During the Middle Ordovician, the Tarim Basin transitioned from extension to compression due to the subduction and the closure of the Paleo-Kunlun Ocean at the southern margin of the Tarim Basin [7,54]. This compressional stress led to the development of ultra-deep-burial compressional–torsional strike-slip faults in the Fuman oilfield (Figure 9), characterized by high-angle linear architectures vertically and predominantly NE-trending horizontally. From the Late Caledonian to the Early Hercynian, the strike-slip faults were reactivated due to the subduction and the closure of the Paleo-Alkin Ocean at the southeastern margin of the Tarim Basin. This resulted in extended fault connectivity from the main fault to the branch fault, particularly during the Early Hercynian when the stress direction shifted from the NE to the NE–SW, resulting in the development of NW, NNW, and NNE branch faults. Consequently, X-conjugate fault systems formed, such as the F_I5–F_I9 faults in the northern Fuman oilfield (Figure 1a).

The strike-slip faults were relatively inactive during the Middle Hercynian due to weak tectonic activity [7,51]. However, during the Late Hercynian, the Fuman oilfield experienced intense compressional stress due to the closure of the northern Tianshan Ocean, leading to the development of shallow-to-middle depth tensional strike-slip faults superimposed on pre-existing faults (Figure 9c). These faults exhibited more complex semi-flower and flower-like architectures [52]. During the Indosinian and Yanshanian, the uplift and compression of the Tianshan, Kunlun, and Altun mountains around the Tarim Basin developed shallow echelon faults in the clastic formation above the Ordovician (Figure 10), but the ultra-deep strike-slip faults of the Ordovician were less affected in a deep burial stage at that time [31].

Figure 10 illustrates the seismic profile, fault interpretation, and fault evolution process of the F_I17 fault in the Fuman oilfield. As shown, the strike-slip faults originated as high-angle linear architectures during the Middle Cambrian (Figure 10b) and penetrated the salt cap by the end of the Middle Caledonian into the Lower–Middle Ordovician (Figure 10c). Late Ordovician tectonic activity was blocked by the plastic mudstone of the Upper Ordovician and extended to the top of the Yijianfang Formation (Figure 10c). The linear faults below the Ordovician extended upward (Figure 10d), and the upper strata began to develop semi-flower and flower-like architectures, with the Late Caledonian movement active during the Middle Silurian. During the Late Hercynian, tectonic activity intensified again, and the flower-like architectures extended upward (Figure 10e). Subsequently, due to deep burial and weakened tectonic activity, the strike-slip faults in the Fuman oilfield basically became fixed, with only shallow echelon and normal faults developing under the Yanshanian compression (Figure 10f). Although some studies suggest that strike-slip faults continued to develop during the Himalayan period in the Fuman oilfield, the impact of the Himalayan tectonic movement on the Ordovician strike-slip faults was minimal due to their ultra-deep burial stage.

6. Conclusions

A variety of siliceous minerals precipitated from silicon-rich hydrothermal fluids during strike-slip faulting. Due to the stable properties of siliceous minerals, which are less susceptible to later diagenetic changes, silica is suitable for U–Pb dating to determine the timing and evolution of strike-slip faults. Laser micro U–Pb dating based on the petrological analysis of various siliceous minerals in the strike-slip faults in Fuman allowed the stages of the Ordovician strike-slip faulting to be determined. Combined with macroscopic seismic interpretation, the evolutionary process of the strike-slip faulting was discussed as follows:

Three types of siliceous minerals fill the strike-slip faults in the Fuman oilfield: cryptocrystalline silica, crystalline silica, and radial silica. Cryptocrystalline silica is the most common, formed by the metasomatism of siliceous fluids on algal grains and bioclasts in limestone. Protolith texture and residual calcite are preserved in the siliceous minerals, with residual calcite indicating that limestone was dissolved and replaced by silicon-rich hydrothermal fluids. Crystalline silica exists as intergranular cement and intergranular vug fillings, with microcrystalline crystals being the most common. Radial silica, in the form of chalcedony, fills fractures and vugs and is often located at the core of siliceous minerals. The SiO₂ content of cryptocrystalline silica is the lowest relatively; crystalline silica and radial silica exhibit homogeneous compositions with high SiO₂ contents.

Laser in situ U–Pb dating of 12 sets of silica in fractures yielded ages ranging from 458 ± 78 Ma to 174 ± 35 Ma. Among these, the metasomatic cryptocrystalline silica has the oldest ages, followed by crystalline silica, while radial silica has the youngest ages. This indicates that the siliceous hydrothermal fluids continuously altered limestone along the faults. The diagenetic processes evolved from metasomatism to cementation and then filling, with crystal forms transitioning from cryptocrystalline to crystalline and then radial. This progression reflects a gradual increase in diagenetic intensity and improvement in siliceous crystallinity. In other words, the same fault was persistently active during different tectonic movements. Combined with the five groups of calcitic U–Pb ages, it was concluded that the strike-slip faults in the Fuman oilfield could be divided into six tectonic evolutionary stages: the Middle–Caledonian (458–454 Ma), Late Caledonian (440–403 Ma), Early Hercynian (399–371 Ma), Middle Hercynian (322 Ma), Late Hercynian (292–252 Ma), and Yanshanian (174 Ma).

Multi-stage evolution has led to the diverse and complex characteristics of the strike-slip faults in the Fuman oilfield. Under compressive stress, high-angle linear strike-slip faults developed in the Ordovician during the Middle–Late Caledonian. During the Early Hercynian, the strike-slip faults remained active, evolving from main faults to branch faults, with semi-flower and flower-shaped strike-slip faults developing. During the Late Hercynian, middle, shallow, and branch faults developed. Finally, during the Yanshanian, echelon faults developed in the shallow clastic strata.

Author Contributions

Funding acquisition, C.Y. and X.L.; data curation, Z.Z. and J.C.; methodology and experiment, S.C. and B.L.; review and editing, Z.Q. and T.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundations of China (No. 42372169, No. U23B20154), the Petrochina Science and Technology Major Project (Research on Hydrocarbon Accumulation and Preservation Mechanisms of the Middle and Lower Assemblages of Superimposed Basins in China, No. 2023ZZ02), and the Tarim Oilfield Research Project (Study on the effect of volcanic hydrothermal activity on reservoir evolution in Fuman Region II, No. 113-42412).

Data Availability Statement

Detailed information describing the experimental data is in the main text.

Acknowledgments

We acknowledge the precious advice of the editors and reviewers.

Conflicts of Interest

All authors are employees of Petrochina. Author C.Y. and Author X.L. have received a research grant from Tarim Oilfield Research Project for No. 113-42412. Author Z.Q. has received research grants from the National Natural Science Foundations of China for No. 42372169 and No. U23B20154 and the Petrochina Science and Technology Major Project for No. 2023ZZ02. The funder was not involved in the study design, collection, analysis, and interpretation of data, writing of this article, or decision to submit it for publication.

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Figure 1. Distribution of the Ordovician strike-slip faults and drilled samples (a), locations of the Fuman oilfield and YAB outcrop (b), location of the Tarim basin (c), and lithological column (d). F_I and F_II mentioned in Figure 1a indicate first-order faults and second-order faults, respectively.

Figure 2. Photo of experimental core samples: (a) GL1, (b) GL2, (c) GL3, (d) GL3-H, (e) YM5, (f) YM6, (g) YM703, (h) MS5, and (i) MS711. Siliceous minerals occurred in fractures with three colors including dark gray (a–e), grayish white (f–i), and colorless (c–e).

Figure 3. Optical microscope images under plane-polarized light of samples, except for c with cross-polarized light. (a) GL1, grain limestone with grains replaced by cryptocrystalline silica and cemented by microcrystalline silica. (b) GL2, same description as (a). (c) GL3, same description as (a) except for the radial silica filling in the center of siliceous component. (d–f) GL3-H, two stages of siliceous minerals with cryptocrystalline silica on the outside and radial silica at the core. (g) YM5, cryptocrystalline silica replacing grains and becoming cloudy, and chemically homogeneous radial silica. (h) YM6, silica replacing grains along fracture. (i) YM703, cryptocrystalline silica replacing grains and crystalline silica filling in the fractures. Silica cut by late fractures filled with sparry calcite. (j–k) MS5, bioclastic grain and matrix replaced by cryptocrystalline silica, intergranular and dissolved pores filled by crystalline silica, and residual calcite located inside and at the edges of the bioclastic grain. (l) MS711, same description as (a). Red dots in the figure represent siliceous U–Pb dating targets, while blue dots represent calcareous U–Pb dating targets.

Figure 4. BSE images and chemical analysis points of the siliceous minerals for GL3 (a,b), GL3-H (c–e), and YM5 (f). Red dots correspond to the points in Table 2. Black represents fracture or pores, dark gray represents silica, grayish white represents calcite, and white represents pyrite. The picture in the lower-left corners of (a,c,d,f) is the energy spectrum. Square points in (b) are the laser ablation points for U–Pb dating.

Figure 5. Comparison of δ³⁰Si of experimental samples with different geological reservoirs (Revised from Wang et al. [43]; Zhang et al. [44]; Deng et al. [45]; and Savage et al. [46,47]).

Figure 6. U–Pb dating results of silica (red) and calcite (blue) in sampled fractures by LA-MC-ICP-MS. (a–k) Silica U–Pb dating of GL1, GL2, GL3, GL3-H (cryptocrystalline silica), GL3-H (radial silica), YM5, YM6, YM703, MS5 (cryptocrystalline silica of bioclast), MS5 (cryptocrystalline silica of matrix), and MS711, respectively; (l–p) Calcite U–Pb dating of GL2, GL3-H, YM703, MS5, and MS711; more details shown in Table 1 and Figure 3.

Figure 7. The petrological characteristics and U–Pb age of siliceous rocks in the siliceous streak, 20 cm away from diabase intrusion of the Penglaiba Formation in the Yong’anba outcrop. (a) Outcrop photo; (b) sample photo of YAB; (c) optical microscope photo of YAB; (d) U–Pb dating result of YAB.

Figure 8. The developments of strike-slip faults in the Fuman oilfield based on U–Pb dating [31,51].

Figure 9. Seismic profiles and strike-slip fault interpretation of GL3-H well in F_I5 (a), YM5 well in F_I7 (b), and MS5 well in F_I17 (c).

Figure 10. Seismic profile (a) and evolution process at different periods (b–f) of F_I17.

Table 1. Characteristics of samples.

Sample	Host Rock	Depth/m	Strata	Siliceous Mineral		Siliceous U–Pb Dating		Calcite U–Pb Dating
Sample	Host Rock	Depth/m	Strata	Diagenesis	δ³⁰Si_V-NBS28 ‰	Target	Age Ma	Target	Age Ma
YAB	Fine-powder dolomite	Outcrop	O₁p	Cryptocrystalline silica replacing dolomite, Crystalline and radial silica filling in vug	2.8	Crystalline silica	285.2 ± 7	/	/
GL1	Grain limestone	7318.18	O₂y	Cryptocrystalline silica replacing calcareous grain, crystalline silica cementing intergranular boundaries	0.8	Cryptocrystalline silica	458 ± 78	/	/
GL2		7389.10	O₂y	Cryptocrystalline silica replacing calcareous grain, crystalline silica cementing intergranular boundaries	1.1	Cryptocrystalline silica	440 ± 67	Rhombic calcite at the edge of silica	284 ± 60
GL3		7938.42	O_1-2y	Cryptocrystalline silica replacing calcareous grain, radial silica filling in vug	1.4	Crystalline silica filling in the vug	292 ± 47	/	/
GL3-H		7789.23	O₂y	Cryptocrystalline silica replacing matrix	0.9	Cryptocrystalline silica	403 ± 41	Rhombic calcite at the siliceous edge	322 ± 28
GL3-H		7789.23	O₂y	Radial silica filling in fracture and vug	1.5	Radial silica filling in fracture and vug	252 ± 56	Rhombic calcite at the siliceous edge	322 ± 28
YM5		7276.42	O₂y	Cryptocrystalline silica replacing matrix, crystalline silica cementing intergranular and radial silica filling in fracture and vug	0.9	Radial silica filling in vug	174 ± 35	/	/
YM6		7303.92	O₂y	Cryptocrystalline silica replacing matrix	0.7	Metasomatic cryptocrystalline silica	454 ± 50	/	/
YM703		7292.78	O₂y	Cryptocrystalline silica replacing matrix, crystalline silica filling in fracture and vug	1.2	Cryptocrystalline siliceous bioclast	399 ± 32	Calcite in the siliceous vug	290 ± 19
MS5		7609.38	O₂y	Cryptocrystalline silica replacing matrix	1.7	Cryptocrystalline siliceous bioclast	376 ± 19	Calcite at the siliceous edge	457 ± 13
MS5		7609.38	O₂y	Cryptocrystalline silica replacing matrix, crystalline silica filling in fracture and vug	1.7	Cryptocrystalline silica	371 ± 19	Calcite at the siliceous edge	457 ± 13
MS711		7712.1	O_1-2y	Cryptocrystalline silica replacing matrix, crystalline silica filling in fracture and vug	1.1	Metasomatic cryptocrystalline silica	376 ± 30	Rhombic calcite	383 ± 64

“/” as untested.

Table 2. Mass fraction (wt %) of major elements for GL3, GL3-H, and YM5 using EPMA from the points in Figure 4.

Point	MgO	CaO	Na₂O	K₂O	SiO₂	SO₃	MnO	FeO	Sum
1	0.00	0.16	0.17	0.10	94.09	0.00	0.04	0.00	94.56
2	0.00	0.33	0.10	0.03	76.26	0.00	0.00	0.05	76.77
3	0.51	54.69	0.00	0.00	1.08	0.00	0.00	0.00	56.28
4	0.00	0.03	0.66	0.13	93.66	0.11	0.00	0.09	94.68
5	0.29	34.30	0.53	0.09	25.48	0.05	0.00	0.00	60.74
6	0.30	23.06	0.54	0.10	24.03	0.05	0.00	0.00	48.09
7	0.27	52.24	0.02	0.00	6.92	0.02	0.00	0.00	59.48
8	0.00	0.27	0.00	0.01	81.62	0.00	0.00	0.00	81.89
9	0.29	34.30	0.53	0.09	25.48	0.05	0.00	0.00	60.74
10	0.15	43.92	0.08	0.00	15.18	0.00	0.00	0.00	59.33
11	0.20	53.42	0.00	0.00	1.23	0.00	0.00	0.00	54.86
12	0.53	43.95	0.08	0.14	20.94	0.00	0.00	0.06	65.69
13	0.18	49.75	0.17	0.01	9.19	0.00	0.05	0.14	59.48
14	0.00	0.00	0.41	0.12	96.39	0.11	0.00	0.00	97.02
15	0.10	45.51	0.00	0.00	17.40	0.00	0.00	0.00	63.01
16	0.33	34.01	0.28	0.03	13.49	0.05	0.00	0.04	48.23

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Yao, C.; Qiao, Z.; Luo, X.; Zhang, T.; Li, B.; Chang, S.; Zhang, Z.; Chen, J. Stages and Evolution of Strike-Slip Faults of the Ultra-Deep-Burial Ordovician Strata in Fuman Oilfield, Tarim Basin: Evidence from U-Pb Geochronology of Siliceous Minerals. Minerals 2025, 15, 270. https://doi.org/10.3390/min15030270

AMA Style

Yao C, Qiao Z, Luo X, Zhang T, Li B, Chang S, Zhang Z, Chen J. Stages and Evolution of Strike-Slip Faults of the Ultra-Deep-Burial Ordovician Strata in Fuman Oilfield, Tarim Basin: Evidence from U-Pb Geochronology of Siliceous Minerals. Minerals. 2025; 15(3):270. https://doi.org/10.3390/min15030270

Chicago/Turabian Style

Yao, Chao, Zhanfeng Qiao, Xiao Luo, Tianfu Zhang, Bing Li, Shaoying Chang, Zhenyu Zhang, and Jiajun Chen. 2025. "Stages and Evolution of Strike-Slip Faults of the Ultra-Deep-Burial Ordovician Strata in Fuman Oilfield, Tarim Basin: Evidence from U-Pb Geochronology of Siliceous Minerals" Minerals 15, no. 3: 270. https://doi.org/10.3390/min15030270

APA Style

Yao, C., Qiao, Z., Luo, X., Zhang, T., Li, B., Chang, S., Zhang, Z., & Chen, J. (2025). Stages and Evolution of Strike-Slip Faults of the Ultra-Deep-Burial Ordovician Strata in Fuman Oilfield, Tarim Basin: Evidence from U-Pb Geochronology of Siliceous Minerals. Minerals, 15(3), 270. https://doi.org/10.3390/min15030270

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