Abstract

With an aim to increase the understanding about sedimentary environment and isotopic and chemical characteristics of fillings in fracture cavities with multiple compositions, we conducted scanning electron microscope (SEM), fluid inclusion testing (FIT), common and trace element chemistry, full analysis testing, isotopic compositions (δ13C, δ18O, 87Sr/86Sr), and apatite fission track testing to study the formation environment of Aksu area, Tarim Basin. According to outfield and microscope observations, combined with SEM results, three textural and compositional type fractures and cavities were distinguished. Through fine analysis of geochemistry characteristics on fractures, cavities, multiple filling periods, and environments were interpreted. Constrained by rare earth element (REE) pattern diagram, relationships between carbon and oxygen isotopes, strontium isotope, the compositional patterns, and generation environment of the fracture and cavities were determined. The results show that (1) cavity, fracture filling, and wall rock primarily consist of calcite, with a proportion of 56.85%, 80.48%, and 81.00%, respectively. (2) Four fracture sets have been distinguished in the Ordovician limestone of the karst cave, Middle-Late Caledonian (Set 1), Early Hercynian (Set 2), Indo-Yanshanian (Set 3), and Himalayan orogeny (Set 4). Two stages of cave filling deposition are distinguished. Stage I was coeval with the Middle-Late Caledonian Set 1 fractures and is attributable to the circulation of freshwater fluid. Stage II was coeval with the Early Hercynian Set 2 fractures and is attributable to deep hydrothermal fluid circulation. (3) Cavity, fracture filling, and wall rock in Ordovician strata are slightly influenced by diagenesis alteration and territorial supply. Three significant filling stages were distinguished, freshwater fluid with strong oxidizing environment (Middle-Late Caledonian), hydrothermal fluid with authigenic abnormal enrichment (indicating obvious hypoxic sedimentary water, Early Hercynian), and high-temperature hydrothermal fluid from deep earth (primarily influenced by magmatism, Indo-Yanshanian, and Himalayan).

1. Introduction

Fillings in fractures, veins, and cavities have been highly used to obtain paleohydrogeological information, especially in volcanic and sedimentary rocks, and ore deposits, and related with past and active geothermal systems [15]. Typically, signatures of major and trace element can potentially provide critical characteristics to improve the understanding of genetic conditions of different types of mineral formation if these were well interpreted and characterized in their petrogenetic compositions [69]. The preservation of significant trace element distribution patterns could be used as important evidence to interpret multistage genetic development of minerals and reservoirs [10, 11]. For deep sedimentary and crystalline rocks, isotopic compositions (δ13C, δ18O, 87Sr/86Sr) have been used to study inflow and mixing of different fluid types and distinguish calcite formed from modern groundwater or numerous older calcite generations [3, 12]. The changing of δ13C in seawater was closely related to tectonic activity, sea level changing, climate changing, and other factors [13, 14]. δ13C was not fractionated significantly during precipitation process of carbonate, and it could basically represent the δ13C value of seawater associated with sedimentary period [15, 16].

The Tarim Basin, located in NW China, has undergone multiple stages of tectonic activities and complex deposition processes ([1720]., [21]). Major and trace element testing, associated with X-ray fluorescence (XRF), has been used to study magmatic source and evolution of basalt in Keping area, Tarim Basin [22]. Numerous scholars analyzed tectonic activities and evolution, formation environment of source rock, and fluid sources of reservoirs in NW Tarim, by geochemistry methods of trace element testing, REE, and U-Pb chronology testing [15, 2325]. However, there are few researches concentrated on fractures and cavity fillings in Ordovician formation in Aksu area, Tarim Basin, particularly fluid source, syntagmatic patterns, and sedimentary environment. It is urgent to make a delicate study on fractures and cavity fillings, including characterization of geochemistry testing, comprehensive analysis of fluid sources, tectono-genetic evolution, and sedimentary environment.

The present study targeted a better tectonic, geo-chemical, and mineralogical understanding of fracture and cavity fillings with the objective of providing further constraints of fracture cavity reservoirs. Based on observations of typical outfield, incorporating geochemistry testing of microscope, scanning electron, porosity, and permeability testing, mineral components of fracture and cavity fillings were interpreted. An understanding of common, trace element geochemistry and full analysis testing of associated calcite and wall rock is necessary to determine fluid sources and sedimentary environment. Multiple Sr isotope and C and O isotope compositions, including δ13C, δ18O, 87Sr, and 86Sr, of fracture and cavity fillings provide key information to constrain the possible sources and environment during the time of deposition. Additionally, combined with apatite fission track (AFT), acoustic emission testing (AET), and fluid inclusion testing (FIT), significant tectonic activities were deduced. Subsequently, filling sequence pattern among fractures and cavities was identified and summarized. Detailed trace element compositions and characteristic metallic distributions were integrated for a better understanding on the relative timing and mechanism of fracture and cavity filling formation, chemical evolution of fluids, and the source of fluids. The research could provide a further guidance for studying on fracture and cavity reservoirs of Ordovician strata in Tarim Basin.

2. Geological Setting

The research area is located at the northern Tarim Basin and southern edge of the northwest of the Tianshan Mountains in China (Figure 1(a)) [26, 27]. The Tarim Basin, one of the largest sedimentary basins, has undergone multitectonic movements [18, 19, 21]. Three typical outcrops primarily representing fractures and cavities were selected, namely, Yijianfang (YJF), Liuhuanggou (LHG), and Xinnengshipianchang (XNSPC), respectively, in the Keping Tectonic Belt, north Tarim Basin (Figures 1(b) and 1(c)). Keping, Bachu, and Kepingtage thrust zones consist of main tectonic structures. Through 314 National road to 218 Provincial road (11 km), from Keping to Tumushuke (more than 50 km), YJF fracture cavity zone was developed. LHG profile, located at 1135 km of 314 National road in Bachu country, primarily developed a significant fault-controlled cavity characterized with multiple mineral fillings. XNSPC profile, located at 10 km from Keping town, was a typical location which has undergone critical tectonic activities. The Ordovician (siliceous rocks and red, grey-white carbonate stones) and Devonian (red thick sandstone, grey mud stone, and green sandstone) strata were the main exposed formations in the typical outfields. The Silurian Formation is composed, from top to bottom, of sandstone (ca.800 m thick), interlayered sandstone, grey mudstone (ca.200 m), and yellow conglomerate (ca.100 m). The Ordovician Formation is mainly composed of subgroups of seven and six in Keping and Bachu areas, respectively. The total exposed thickness could reach 500 m, primarily carbonate. Emphatically, Penglaiba group, Yingshan group, Yijianfang group, and Dawangou group were the significant formations [2729]. Respectively, Penglaiba group is mainly off-white fine-meso crystalline dolomite, Yingshan group consists of gray and light gray microcrystalline limestone. The Yijianfang group is mainly light gray, dark gray microcrystalline bioclastic limestone, and sandstone limestone. Dawangou group consists of gray, dark gray nodular bioclastic limestone [30].

Aksu area has yielded multiple tectonic activities of the Caledonian, Hercynian, Indo-Yanshanian, and Himalayan periods [18, 21]. In the Early Caledonian period, Aksu region overall declined; huge thick carbonate formations were deposited, especially Yingshan group and Yijianfang group. In Middle-Late Caledonian, affected by the northward subduction of the South Tianshan oceanic crust plate, three uplifts occurred in the Aksu outcrop area, associated with I, II, and III episodes, respectively. As the Yijianfang and Yingshan Formations were exposed, then karstification occurred. The ancient karst cave system is generally developed and covered by the Ordovician and Silurian clastic rock strata. During the Indo-Yanshanian period, the subduction collision of the Indosinian plate towards the Eurasian continent produced a strong far-source collision effect in the northern Tarim region, which caused a strong horizontal compression and tectonic uplift in the Aksu area. Current north-south thrust nappe structure was finally formed [29]. In the Himalayan period, the stratum at the southern edge of the Kepingtag structural belt has undergone strong uplift and bending deformation, and suffered some degree of erosion. However, the karst cave system (KCS) developed at the southern edge of the Kepingtag structural belt was a KCS formed during the Middle Caledonian movement. In conclusion, the Caledonian and Hercynian were the most significant periods with greatest impact on the study area [30].

Primarily, four set fractures are generated in the study area, Caledonian horizontal fractures (F-I), tensile-shear fractures generated in the Hercynian period (F-II), low-angle shear fractures associated with the Indo-Yanshanian period (F-III), and high-angle fractures developed in the Himalayan period (F-IV) (Figures 2(a) and 2(b)). Two typical fracture cavities are developed, cavity associated with underground river and fault-controlled river, characterized by horizontal layering and vertical zoning of filling, respectively (Figures 2(c) and 2(d)). The fractures are primarily filled by micrite gray breccia, and calcite (Figures 2(e), 2(f), 2(i), and 2(j)). The cavities are significantly filled by calcite and fluorite, quartz, albite, dolomite, illite, and semectite (Figures 2(g), 2(h), 2(k), and 2(l)). However, due to fluorite is a valuable mineral, most of the fluorite was mined and almost invisible. Dissolution pores could be observed among the grains in cavity filling (Figures 2(g) and 2(h)). Through observations of fluid inclusions under microscope, the fracture and cavity filling primarily consisted of two phases: (gas-liquid) inclusion and gaseous inclusion (Figures 2(m)–2(p)). Most of the intergranular voids show yellow fluorescence, and a lot of gas-hydrocarbon inclusions in the sample are scattered in the fissures.

3. Testing and Analytical Methods

3.1. Sampling

More than 250 specimens were drilled from typical outfields, YJF, LHG, and XNSPC profile, characterized with highly developed fracture and cavity system. All samples were selected and drilled from facture, cavity filling, and wall rock. Among these specimens, 46 samples were selected to be used for casting thin sections and scanning electron microscopy. 34 specimens were used to conduct FIT, and 80 samples were conducted to whole-rock mineral testing in order to analyze mineral compositions. 54 specimens were conducted to carbon and oxygen isotope testing, and 20 were used for strontium isotope testing to interpret sedimentary environment of facture and cavity filling. 20 specimens were selected to be used for trace element testing, aiming to discuss fluid source of fracture and cavity filling. 20 samples were used to make a full analysis of fracture filling, cavity filling, and wall rock.

3.2. Microscopic Observation Method

The cast thin section (CTS) is a rock flake, prepared by injecting and subsequently solidifying colored liquid glue into the pore spaces of the rock sample under vacuum pressure [31, 32]. Interpretation of cast thin sections is one of the conventional methods to analyze rock minerals and oil-gas reservoirs. Based on electron microscope, two-dimensional morphological characteristics of pores inside rocks could be seen visually. Additionally, development characteristics of fracture filling, cavity filling, and wall rock could be described. The thin sections of the specimens in the study area were made by a high-pressure casting machine (ZT-2) in the Petroleum Geology Testing Center, Exploration and Development Research Institute, Shengli Oilfield Branch, China Petroleum and Chemical Corporation (CPCC). Electron microscopy imaging (EMI) of the samples was performed using a Quanta200 scanning electron microscope and EDAX spectrometer online equipment. The chemical compositions of fracture filling, cavity filling, and wall rock were analyzed. The testing of EMI was also conducted in petroleum geology testing center. Combined with polarizing electron microscope (PEM), combined with D/max-2500PC diffractometer, mineral analyses of whole rock in the testing samples were conducted.

3.3. Carbon, Oxygen, and Strontium Isotope Testing

Using a MAT253 gas isotope mass spectrometer, complying with the standard of “Determination of carbon and oxygen isotopes in carbonate minerals or rocks,” carbon and oxygen isotope testing (COIT) was conducted in the Petroleum Geology Testing Center, Exploration and Development Research Institute, Shengli Oilfield Branch, CPCC. Due to stability characteristics of C13 and O18 isotopes, COIT was applied to analyze source of fracture filling, cavity filling, and the sedimentary environment. Based on standard of “Determination of Lead, Antimony, and Strontium Ions in Rocks,” the strontium isotope testing (SIT) was carried out in the Analysis and Testing Center of the Beijing Institute of Geology, Nuclear Industry. Using a Phoenix hot surface ionization mass spectrometer, strontium isotopes were determined, with a maximum accuracy of 3%, a temperature of 20°C, and 40% humidity. The value of 87Sr/86Sr keeps stable after deposition and could be used to well characterize paleosedimentary environment of carbonate rocks. Associated with testing results of carbon and oxygen isotopes, the strontium isotopes can be used to discriminate fracture filling periods, cave filling periods, and the paleoenvironment.

3.4. Full Analysis Testing, Trace Element Testing, and Common Element Testing

Using a plasma mass spectrometer (NexION300D, No. 10742), combined with the standard of “Carbonate rock chemical analysis method: Determination of 44 elements,” trace element testing was conducted in the Analysis and Testing Center of the Beijing Institute of Geology, Nuclear Industry. The temperature was set to 22°C, and relative humidity was set to 21%. Samples primarily drilled from fracture filling, cavity filling, and wall rock. According to the standard of “Methods for chemical analysis of silicate rocks: determination of ferrous oxide and Determination of 16 major and minor components”, AB104L, AxiosmAX, and X-ray fluorescence spectrometer were used to detect content of major, minor components and individual trace elements. The temperature was set to 23°C, and relative humidity was set to 31%. The full analysis of fracture filling, cavity filling, and wall rock were also tested in the Analysis and Testing Center of the Beijing Institute of Geology, Nuclear Industry. Based on ICP-AES analyzer, common element testing was conducted in the Petroleum Geology Testing Center, Exploration and Development Research Institute, Shengli Oilfield Branch, CPCC.

3.5. Apatite Fission Track (AFT) and Fluid Inclusion Testing

Based on the requirement of experimental analysis, apatite fission track testing was carried out in Zekang’en Science and Technology Limited Liability Company, Beijing. All the samples were preprocessed, composing of crushing, screening, washing, magnetic separation, heavy liquid separation, and binocular selection. In order to calculate fission track density and perimeter track length and obtain the Dpar value per particle, automatic scan system was adopted. The apatite fission track ages were also calculated by zeta (value of 265±7.98). By using AFT length, age, kinetic parameter (Dpar), and geological setting, the kinetic simulation was conducted based on annealing activities of AFT. The software of HeFTy v1.9.1 was conducted to do tectonic thermal simulations on the six samples. In each simulation, 10,000 paths were tested, K-S test was selected to evaluate the fitting, and only the best-fit curve was accepted. In order to study tectonic movements that rock has undergone, fluid inclusion test (FIT) was used. Using Renishaw RM2000 laser Raman probe, the composition of samples’ fluid inclusions could be identified. The FIT of these samples was tested in the Petroleum Geology Testing Center, Exploration and Development Research Institute, Shengli Oilfield Branch, CPCC. According to uniform temperature of FIT, the tectonic movements that rock fillings yielded could be deduced [33]. Results of FIT could be used to analyze and verify tectonic movements in Aksu area.

4. Geochemical Testing Results

4.1. Main Mineral Compositions

According to the testing results of CTS, EMI (Figure 2) and mineral analyses of whole rock (Table 1), main mineral compositions of fracture filling, cavity filling, and wall rock were finely studied. From the view of compared bar chart (Figure 3(a)), cavity filling primarily consisted of calcite (73.16%), followed by quartz (11.11%), barite (9.63%), clay (3%), stone salt (2.05%), plagioclase (0.63%), and potash feldspar (0.37%), in the Yijianfang profile. Fracture filling is mainly composed of calcite (90.72%), followed by dolomite (3.12%), clay (2.76%), stone salt (1.96%), quartz (1.04%), and some minerals (<1%), such as potash feldspar, plagioclase, and gypsum. Wall rock primarily included calcite (75.50%), followed by stone salt (12.75%), quartz (8.00%), clay (1.50%), potash feldspar (1.25%), and other minerals (dolomite and gypsum, less than 1%). In Liuhuanggou profile (Figure 3(b)), cavity filling primarily consisted of gypsum (42.13%) and calcite (36.20%), followed by clay (6.80%), plagioclase (6.80%), stone salt (4.20%), quartz (1.93%), siderite (1.00%), dolomite (0.60%), and potash feldspar (0.07%). Fracture filling was mainly composed of calcite (65.41%), followed by gypsum (19.35), clay (6.41%), quartz (5.24%), plagioclase (2.35%), siderite (0.94%), and dolomite (0.53%). Wall rock primarily included calcite (92.0%), followed by clay (6.00%), quartz (1.00%), and plagioclase (1.00%).

Integrating the testing data of Yijianfang profile and Liuhuanggou profile, the overall mineral compositions of Aksu area were interpreted (Figure 3(c)). Calcite was the primary mineral in the cavity filling, with a proportion of 56.85%. Content percentage of gypsum, quartz, barite, clay, plagioclase, and stone salt was 18.59%, 7.06%, 5.38%, 4.68%, 3.35%, and 3.00%, respectively. Proportions of potash feldspar, dolomite, and siderite were all less than 1.00%. Fracture filling primarily consisted of calcite (80.48%), followed by gypsum (7.86%), clay (4.24%), quartz (2.74%), dolomite (2.07%), stone salt (1.17%), and plagioclase (1.00%). The content percentage of potash feldspar and siderite was less than 1.00%. Wall rock was mainly composed of calcite (81.00%), followed by stone salt (8.50%), quartz (5.67%), and clay (3.00%). Proportions of potash feldspar, plagioclase, dolomite, and gypsum were all less than 1.00%.

4.2. Carbon, Oxygen, and Strontium Isotope Testing

Through the cutting relationship, combination pattern, extension type, and filling characteristics, different period fractures were distinguished in the outcrops. Constrained by the tectonic evolution background, primary five period fractures were identified: Middle-Late Caledonian, Early and Late Hercynian, Indo-Yanshanian, and Himalayan orogenies. With our goal to analyze the relationship among the fracture fillings, cavity fillings, and wall rock, the fracture fillings in different periods were samples and marked. Then, these specimens were conducted the COIT and SIT testing. According to geochemical testing results of COIT and SIT (Figure 4, Table 2), three primary categories were divided (Figure 4(a)). In type I, the value of δCPDB was between -1.9‰ and -0.5‰, and the value of δOPDB ranged from -10‰ to -12‰. The filling in type I primarily consisted of cavity filling (stage I), Caledonian fracture filling, and early Hercynian fracture filling. For type II, the value of δCPDB was between -0.8‰ and 1.0‰, and the value of δOPDB ranged from -6.5‰ to -10‰. The filling in type II were mainly composed of cavity filling (stage II), late Hercynian fracture filling, and wall rock. In type III, the filling primarily consisted of Indo-Yanshanian fracture filling and Himalayan fracture filling. The value of δCPDB was between -3.9‰ and -6.5‰, and the value of δOPDB ranged from -10.8‰ to -15.5‰. These fillings belonged to the later stage which occurred after formation period of cavity. Sedimentary environment of cavity filling (stage I) was significantly related with that of Caledonian fracture filling and early Hercynian fracture filling. Sedimentary environment of cavity filling (stage II) was significantly related with that of late Hercynian fracture filling and wall rock (Ordovician carbonate formation). Based on results of SIT (Figure 4(b)), the sedimentary environment of cavity filling can be divided into two stages. The one stage was primarily associated with the sedimentary environment of wall rock and late Hercynian fracture filling. The value of 87Sr/86Sr was less than 0.7095. The other stage of cavity filling was highly related with Caledonian fracture filling and early Hercynian fracture filling, with 87Sr/86Sr value of 0.7095-0.7110. The sedimentary environment of Indo-Yanshanian fracture filling and Himalayan fracture filling was not associated with cavity filling. It belonged to a third sedimentary environment, with 87Sr/86Sr value of 0.7100-0.7140.

4.3. Fluid Inclusion Testing and Apatite Fission Track Testing

According to Figure 5, the stratigraphic burial history and primary tectonic activities that Aksu area has yielded were interpreted. Caledonian, Early Hercynian, Late Hercynian, Indo-Yanshanian, and Himalayan were five significant tectonic movement periods. The primarily horizontal fractures developed in Caledonian orogeny (near NS compression stress). Low-angle shear-tensile fractures with calcite filling mainly generated in Hercynian orogeny (NW-SE, NS compression stress). Low-angle conjugate shear fractures primarily formed in Indo-Yanshanian orogeny (NE-SW compression stress). High-angle fractures mainly developed in Himalayan orogeny (NNW-SSE compression stress). Based on observations of fluid inclusion in fracture filling, cavity filling, and wall rock, the fluid inclusions were characterized by single-phase (Single-P), two-phase (Two-P), particle fissures, and irregular pattern. The mineral compositions of fluid inclusions were mostly calcite. The fluid inclusion types primarily consisted of gas hydrocarbon, asphalt, and oil-gas. Due to later tectonic evolution, some oil fluid inclusion could change to asphalt. This stage of fluid inclusion was mainly caused by the tectonic activity of Middle-Late Caledonian and Hercynian orogeny.

Combining with tectonic thermal evolution results (Table 3 and Figure 6), significant tectonic activities that Aksu area has yielded could be analyzed. The AFT results indicated that Aksu area has undergone four critical tectonic movements since Ordovician formation period. During the Late Hercynian period (ca.250-ca.200 Ma), Aksu area has yielded a rapid uplift with a cooling rate of 1.1 °C/Ma. In the Indo-Yanshanian period (ca.200-ca.70 Ma), Aksu area has undergone overall rapid uplift with a cooling rate of 1.3 °C/Ma, performing a slowly uplifting in the middle period (ca.120-ca.100 Ma). During the Himalayan period (ca.70 Ma-present), Aksu area performed a rapid uplifting with a cooling rate of 0.57°C/Ma. The results performed a good consistency with fluid inclusion testing results.

4.4. Trace Element Testing and Major Element Testing

According to trace and major element testing results (Tables 4 and 5), various intersection diagrams between different elements were drawn (Figures 79). The CaO content of carbonate samples in the study area was distributed in 8.01-55.64% (average, 46.12%), which indicated that the mineral primarily consists of calcite. The Mg content was very low (0.112-0.538%), indicating weak dolomitization. The SiO2 and Al2O3 content was 0.412-84.87% and 0.0245-0.2805%, respectively, which showed less influence by land source supply. Trace element testing results showed low content of redox-sensitive trace elements. The content of V, U, and Mo was 0.012-3.95 μg/g (average, 1.133), 0.252-1.95 μg/g (mean, 0.823), and 0.024-2.13 μg/g (average, 0.311), respectively.

Compared with clastic, carbonate rocks have low abundance of rare earth elements. Rare earth elements (REE) (Table 5) were standardized based on PASS [34, 35]. The ∑REE ranged from 1.539 to 19.697 μg/g, with average of 9.129 μg/g. LREE/HREE was between 4.880 and 10.311, and the average was 7.427. The high LREE/HREE primarily distributed in cavity filling. The research area is dominated by light rare earth elements (LREE). The Th/U primarily distributed in 0.077-0.854, and averaged was 0.443. The ΔCe was distributed in 0.743-1.041, and the average was 0.873. The ΔEu primarily ranged from 0.886 to 29.642, with an average of 6.262. The high ΔEu mainly distributed in cavity filling (Figures 7(a)–7(c)).

According to distribution pattern diagram (Figure 7), cavity, fracture filling, and wall rock have similar curve characteristics. The standardized curve is relatively flat, indicating low fractionation degree among rare earth elements. Eu showed a strong “V” sharp and Ce performed a weak “V” sharp (Figures 7(a)–7(c)). The Ba/La and Th/Nb value was 5.07-50446.3, 1.72-26.875, respectively (Figure 7(d)). The Zr/Nb and U/Pb values were 4.41-22.26 and 7.42-97.5 (Figures 7(e) and 7(f)). The La/Nb value ranged from 4.51 to 107.5 (Figure 7(g)). The Ba/Y and Nb/Y values were 1.97-27048.68 and 0.0014-0.234, respectively (Figure 7(h)). The Rb/Y value primarily distributed in 0.0295-2.585 (Figure 7(i)). Two significant categories were interpreted based on the testing results.

Based on identification diagrams of diagenetic transformation (Figure 8), there were no obvious correlations among ΔEu, DyN/SmN, LaN/SmN, ∑REE, and ΔCe. The DyN/SmN value ranged from 0.562 to 1.163, with an average of 0.957. The LaN/SmN value primarily distributed in 0.209-1.217, and the average was 0.703. Combined with ΔEu (primarily less than 1), the rare earth elements were less influenced by diagenetic evolution. According to intersection diagrams (Figures 9(a)–9(d)), there were no obvious correlations among redox-sensitive trace elements (V, U, Mo, and Zn) and Al2O3. It indicated rocks primarily self-generated and weak influence of terrigenous clastic. Associated with relationships among ΔCe, ∑REE, and Al2O3 (Figures 9(e) and 9(f)), there were no apparent correlations, which revealed less influence by terrigenous clastic. There is a strong positive correlation (R2=0.819) between K2O and Al2O3 concentrations in samples from cavity, fracture filling, and wall rock (Figure 9(g)). The SiO2 concentrations have no direct correlation (R2=0.538) with Al2O3 concentrations in the study area (Figure 9(h)). The relationship between Al2O3 and 87Sr/86Sr concentrations have no visible mathematical correlation (Figure 9(i)). The Al2O3 and 87Sr/86Sr are primarily distributed in 0.05-0.2% and 0.709-0.711 g, respectively.

5. Discussions

5.1. Characteristics and Significance of REE

The partition curve of carbonate rock is relatively flat (Figures 7(a)–7(c)), characterizing by enriching LREE and depleting HREE, with an average LREE/HREE of 7.43. The ∑REE is primarily less than 184 μg/g, indicating pristine seawater, open sea, and relatively stable structural depositional environment, where ΔCe<1, namely, negative exception, indicates reduced sedimentary environment and where ΔCe>1, namely, positive exception, reveals oxidized sedimentary environment [36]. Mostly, ΔCe were less than 1, performing reduced sedimentary environment (Figure 8). ΔCe in few cavity fillings was larger than 1, indicating oxidized sedimentary environment. Two critical sedimentary stage was interpreted.

Under reduced environment, Eu3+ is reduced to Eu2+, enriching in sediments. The hydrothermal fluid from the midocean ridge shows obvious positive anomalies (Eu) [37]. Combined with ΔEu distribution characteristics (Figures 7(a)–7(c)), two stages of cavity deposition were distinguished. Some REE distribution in cavity and fracture filling is light-REE enriched and exhibits strong positive ΔEu (largest, 29.64), indicating intrinsically linked to high-temperature hydrothermal fluids (Nos. 6, 7, 8, 10, 11, 4, 5, and 20). Stage I was coeval with the Middle-Late Caledonian and early Hercynian fractures, and it is attributable to circulation of freshwater fluid. REE distribution in cavity, fracture filling, and wall rock is also light-REE enriched and slight negative ΔEu (Nos. 2, 3, 13, 15, 19, 9, 16, 17, 1, 12, 14, and 18), revealing typical sediments associated with seawater. Stage II was coeval with the late Hercynian fractures and wall rocks, and it is attributable to the deep hydrothermal fluid circulation. Combined with correlation judgement diagrams of fluid source (Figures 7(d)–7(i)), two significant fluid sources were distinguished. Type I was significantly influenced by melt-related effects and high-temperature fluid activity, indicating hydrothermal fluid source. Type II was highly fluid-related rocks, primarily freshwater fluid. It showed a good agreement with REE distribution.

5.2. Characteristics and Significance of Major Elements

According to major elements testing results (Table 4), average Mg/Ca value is 0.0066 in cavity, fracture filling, and wall rock in study area, indicating primarily calcite. Relatively high SiO2 content was distributed in fracture filling and wall rock and low content in fracture filling. High Al2O3 and TiO2 content primarily was distributed in cavity and fracture filling and low content in wall rock. The Al2O3 and TiO2 content was low overall. Two significant sedimentary stages were distinguished. Stage I indicates shallow water body during deposition period, resulting in small amount of terrigenous detrital material. Stage II reveals deep water body during sedimentary period, resulting in low Al2O3 content, belonging to platform marginal sedimentary environment.

There were no obvious correlations among U, V, Mo, Zn, and Al2O3 concentrations (Figures 9(a)–9(c)), indicating primary self-generation. In order to analyze influence of terrigenous debris on REE composition, we studied relationships among ∑REE, ΔCe, and Al2O3. The ∑REE and ΔCe concentrations have no direct correlation (R2=0.0065, R2=0.22, respectively) with Al2O3 concentrations (Figures 9(e) and 9(f)). There are strong and weak positive correlations (R2=0.954, 0.805, 0.011, respectively) between K2O and Al2O3 concentrations in wall rock, cavity filling, and fracture filling, respectively (Figure 9(g)). There are strong and moderate positive correlations (R2=0.836, 0.726, 0.616,respectively) between SiO2 and Al2O3 concentrations in wall rock, fracture filling, and cavity filling, respectively (Figure 9(h)). There are strong and weak positive correlations (R2=0.591, 0.290, respectively) and weak negative correlations (R2=0.342) between 87Sr/86Sr and Al2O3 concentrations in fracture, cavity filling, and wall rock, respectively (Figure 9(i)). The least 87Sr/86Sr value in the study area was higher than 0.708, indicating rocks under Archaean seawater were characterized by a mantle-like isotope signature [38]. The increasing 87Sr/86Sr value are primarily caused by increasing hydrothermal flux, decreasing seafloor undergoing rates, and increasing undergoing weathering and sea-level fall [39, 40]. The relatively high Sr isotope ratios of cavity, fracture filling, and wall rock may have been caused by increasing hydrothermal fluids and sea-level fall in the study area. The highest 87Sr/86Sr value (0.7135) was found in a specimen with a relatively high component of clastic material (Al2O3=0.6%) from the cavity filling. This relationship supports isotopic exchange of carbonate with a cavity filling, fracture filling, and wall rock. One similar sedimentary period was interpreted between cavity and fracture filling. This showed a good consistency with C-O testing results and REE results.

5.3. Redox Analysis

Redox-sensitive trace elements (RSTE, U, V, Mo, and Zn) perform different chemical valence and occurrence form in water with different oxygen content. RSTE is reduced to a low valence state in an oxygen-deficient environment and easily adsorbed by organic matter to be enriched in sediments [41, 42]. Zn primarily exists in the form of dissolved Zn2+ or ZnCl+ in oxidizing water, while precipitating as a compound in an oxygen-poor or anaerobic environment [42].

We used PASS average Al content to standardize cavity, fracture filling, and wall rock in study area [42, 43]. Then, we calculated enrichment factor of common redox-sensitive trace element through Equation (1).
(1)EFelementX=X/AlSampleX/AlPASS,
where EFelementX<1, it indicates X element relatively deficient in the testing sample; where 1<EFelementX<10, it reveals X element relatively enriched; where EFelementX>10, X element shows significant authigenic abnormal enrichment, indicating obvious hypoxic sedimentary water.

Enrichment coefficient of U, V, Mo, and Zn in the cavity, fracture filling, and wall rock has a wide distribution range (Figures 10(a)–10(d)). Two significant categories were distinguished. EF(U), EF(Mo), and EF(Zn) values in stage I are larger than 10, with an average of 28.36, 17.74, and 42.12, respectively. This indicates cavity, fracture filling, and wall rock characterized by obvious spontaneous abnormal enrichment, revealing significant anoxic deposition environment. EF(U), EF(V), EF(Mo), and EF(Zn) values in stage II are less than 10, with an average of 6.49, 0.43, 5.12, and 3.53, respectively. It indicates cavity, fracture filling, and wall rock perform relatively high RSTE enrichment factor, revealing relatively significant oxygen fluctuations and associated with fluid activity. The RSTE enrichment factor in cavity filling is higher than that in facture filling, indicating more oxygen fluctuations during the cavity development period.

Generally, Th/U and V/Cr have a significant response to oxidation of sedimentary water, where Th/U>8 or V/Cr<2, rocks highly associated with strong oxidizing water, where 2<Th/U<8 or 2<V/Cr<4.25, belonging to weak oxidizing environment, where Th/U<2 or V/Cr>4.25, highly associated with reducing water [43, 44]. The Th/U in cavity, fracture filling, and wall rock is all less than 1, indicating reducing environment (Figure 10(f)). The V/Cr primarily distributes in 0-2, rarely greater than 10 (Figure 10(e)). This indicates two significant sedimentary environment, strong oxidizing and reducing water. It performs a good agreement with the RSTE enrichment factor results.

5.4. Comprehensive Analysis of Fracture-Cavity Filling Sequence

Redox environment and deep fluid activity are significant factors for fracture-cavity carbonate reservoir [45]. With our goal to analyze the combination patterns among the tectonic orogenies, filling period, and the fluid source, we integrated the testing results and the geological background analysis. According to comparison among main mineral compositions, carbon, oxygen, and strontium isotope, fluid inclusion and apatite fission track, trace element and major element testing, cavity, fracture-filling sequence pattern was analyzed (Figure 11). Through analyses on different period calcite filling source and various filling mineral sources, the combination relationship among the different filling minerals, tectonic movement background, and diagenesis history were summarized. Based on the above results, three significant filling stages were interpreted. First stage is freshwater fluid with strong oxidizing environment. The second stage is hydrothermal fluid with authigenic abnormal enrichment, indicating obvious hypoxic sedimentary water. The third stage is high temperature hydrothermal fluid from deep earth, primarily influenced by magmatism.

Fracture is primarily developed during the Mid-Late Caledonian, Early and Late Hercynian, Indo-Yanshannian, and Himalayan period. Five diagenesis stages were distinguished, atmospheric freshwater, shallow buried, epigenetic, shallow buried, and shallow-deep buried diagenesis. And five filling types were identified in cavity and fracture, calcite, debris, fluorite, quartz, and siliceous. Three calcite formation stages were distinguished, Mid-Late Caledonian (I), Early Hercynian (II), and Indo-Yanshanian period (III). Debris and calcite (I), primarily formed during Mid-Late Caledonian period, are associated with strong oxidizing freshwater. Fluorite and calcite (II) mainly developed during the Hercynian period, highly associated with shallow hydrothermal fluid. Two quartz-forming stages and three siliceous-forming stages were interpreted in the study area, while they were mostly associated with high-temperature hydrothermal fluid. In conclusion, siliceous, quartz, and calcite (III), primarily formed during the Indo-Yanshanian and Himalayan period, are significantly influenced by deep hydrothermal fluid with high-temperature (larger than 135°C). A clear cavity and fracture filling provide a better guidance to study fracture-cavity reservoir in Ordovician strata, Tarim Basin. The Mid-Late Caledonian and Hercynian period are two significant stages for formation and preservation of Ordovician fracture-cavity reservoirs. The two stages can provide sufficient space and channels supporting oil and gas migration and storage. In other words, the dissolution and cavity development were primarily formed during Caledonian and Hercynian orogenies. Affected by the atmospheric freshwater, underground water, hydrothermal fluid, the cavity scale, and available storage space increased. Additionally, due to filling and dissolution differences in the formation with multiple fractures, the dissolution along the bedding surface and preexisting fractures was developed. Then, numerous sufficient channels and space for the oil and gas migration were generated.

6. Conclusions

  • (1)

    Cavity filling primarily consists of calcite, with a proportion of 56.85%. Content percentage of gypsum, quartz, barite, clay, plagioclase, and stone salt was 18.59%, 7.06%, 5.38%, 4.68%, 3.35%, and 3.00%, respectively. Fracture filling primarily consisted of calcite (80.48%), followed by gypsum (7.86%), clay (4.24%), quartz (2.74%), dolomite (2.07%), stone salt (1.17%), and plagioclase (1.00%). Wall rock primarily composed of calcite (81.00%), followed by stone salt (8.50%), quartz (5.67%), and clay (3.00%)

  • (2)

    Four fracture sets were distinguished in the karst cave Ordovician limestone. Set 1 includes subhorizontal (0-10°) partially filled fractures, 0.25-1 m-long, striking NNE 40° that are interpreted to formed during the Middle-Late Caledonian orogeny. Set 2 involves inclined (30-60°) tensile-shear fractures, 0.5-2.5 m-long, striking NNE330° and NEE70° that likely formed during the Early Hercynian orogeny. Set 3 includes variably oriented (0-50°), fully filled conjugate shear fractures, 0.25-1.25 m-long, striking NEE 70° that developed during the Indo-Yanshanian orogeny. Set 4 involves high-angle (dip=5090°) shear fractures, 0.3-3 m-long, striking NNE30° and NWW310° that formed during the Himalayan orogeny

  • (3)

    The cave is primarily filled by the debris and chemical deposits, including sulfur-rich, giant crystal calcite, travertine, sand mud, and gypsum. Two cave filling deposition stages were distinguished. Stage I was coeval with the Middle-Late Caledonian Set 1 fractures and is significantly influenced by the circulation of freshwater fluid. Stage II was coeval with the Early Hercynian Set 2 fractures and is attributable to deep hydrothermal fluid circulation

  • (4)

    Cavity, fracture filling, and wall rock in Ordovician strata slightly influenced by diagenesis alteration and territorial supply. Three significant filling stages were distinguished, freshwater fluid with strong oxidizing environment, hydrothermal fluid with authigenic abnormal enrichment (indicating obvious hypoxic sedimentary water), and high temperature hydrothermal fluid from deep earth (primarily influenced by magmatism)

  • (5)

    Three calcite formation stages were distinguished, Mid-Late Caledonian (I), Early Hercynian (II), and Indo-Yanshanian period (III). Debris and calcite (I), primarily formed during the Mid-Late Caledonian period, associated with strong oxidizing freshwater. Fluorite and calcite (II) are developed during the Hercynian period, highly associated with shallow hydrothermal fluid. Siliceous, quartz, and calcite (III), primarily formed during the Indo-Yanshanian and Himalayan period, significantly influenced by deep hydrothermal fluid with high-temperature (larger than 135°C)

Data Availability

All the underlying data could be found in the tables in the manuscript.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This study was funded by the Major Scientific and Technological Projects of CNPC under grant ZD2019-183-006, the National Science and Technology Major Project of China (2016ZX05014002-006, 2016ZX05047003-003, and 2017ZX05013006-003), the National Natural Science Foundation of China (42072234), and the Fundamental Research Funds for the Central Universities (17CX05010). The authors would like to appreciate all the people, which supported the data, testing, and analyses. Many thanks to the anonymous reviewers, whose comments will improve the quality of our manuscript. And our sincere gratitude to the China Scholarship Council, who provided QR with the opportunity and funds to learn in Canada.

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