First Direct Dating of Alteration of Paleo-Oil Pools Using Rubidium-Strontium Pyrite Geochronology

Direct dating of petroleum systems by hydrocarbon or associated authigenic minerals is crucial for petroleum system analysis and hydrocarbon exploration. The precipitation of authigenic pyrite in petroliferous basins is commonly genetically associated with hydrocarbon generation, migration, accumulation, or destruction. Pyrite rubidium-strontium (Rb-Sr) isotope dilution thermal ionization mass spectrometry (ID-TIMS) is a well-developed technique, and its successful application for high-temperature ore systems suggests that this dating method has the potential to directly date key processes in the low-temperature petroleum systems. Rb-Sr data for pyrites in two Ordovician carbonate rock specimens collected from ~4952 m in the YD-2 well in the Yudong region, northern Tarim Basin (NW China), yield two identical isochron ages within analytical uncertainties: 206 ± 13 (2σ) and 224 ± 28 Ma (2σ). SEM investigations demonstrate that Rb and Sr atoms mainly reside in the crystal lattice of the pyrites due to the absence of fluid and mineral inclusions. The rigorous Rb-Sr isochron relations documented in the samples indicate that such residency can result in sufficient Rb/Sr fractionation and initial Sr isotopic homogenization for geochronology. In addition, the closure temperature (227–320 ◦C) for the Rb-Sr isotope system in pyrite is higher than the precipitation temperature for pyrite in petroleum-related environments (below 200 ◦C), suggesting that the Rb-Sr age of pyrite was not overprinted by post-precipitation alteration. Integrating the lead-strontium-sulfur isotopes of the pyrites with burial history analysis, the ages are interpreted as the timing of alteration of the paleo-oil pool by a hydrothermally-triggered thermochemical sulfate reduction process. This study, for the first time, demonstrates that Rb-Sr pyrite geochronology, combined with radiogenic and stable isotopic analyses, can be a useful tool to evaluate the temporal evolution of oil pools. This approach bears great potential for dating of petroleum systems anywhere else in the world.


Introduction
Establishing the absolute timing for key events in petroleum systems (e.g., petroleum generation, charge and alteration) is crucial for petroleum system analysis and hydrocarbon exploration, but is still poorly constrained. Recent studies have demonstrated that isotope chronometers in hydrocarbons or associated minerals can be used for direct dating of petroleum generation and post-generation events [1,2]. For example, rhenium-osmium (Re-Os) isotopic analysis of petroleum samples can yield isochron ages approximating the timing of petroleum generation; however, the ages often have large errors [1]. Authigenic minerals, such as clays, which are precipitated and associated with petroleum may also be datable targets because radioactive atoms (e.g., potassium (K), rubidium (Rb)) can be incorporated into the minerals during their growth [2]. In addition, authigenic minerals usually have a genesis not as complex as petroleum, which is a hydrocarbon mixture generated by multiple source rocks or multi-stage generation processes. The genesis of authigenic minerals can be determined by radiogenic or stable isotopic analysis, enabling the unambiguous interpretation of the age data [3]. Therefore, more geochronological investigations on petroleum-related authigenic minerals are required in order to obtain high-precision age data for petroleum system analysis and exploration targeting.
Authigenic pyrite is a ubiquitous mineral in petroliferous basins and is commonly associated with the deposition of the source rock [4], hydrocarbon generation/expulsion [5], secondary migration/charge [6], and post-accumulation alteration [7]. Trace element systematics of pyrites are directly related to the depositional environment, such as water body, hydrothermal system and organic-inorganic fluid [8][9][10]. Therefore, the preserved metallic radiometric isotopic systems, such as rubidium-strontium (Rb-Sr) and lead (Pb), in authigenic pyrites, have the potential to serve as a chronometer and source-indicator, similar to that in well documented high-temperature ore deposits [3]. The sensitive isotope-dilution thermal ionization mass spectrometry (ID-TIMS) and low blank Rb-Sr separation/purification techniques developed in our laboratory permit high-precision Rb-Sr isotopic analysis of very small sample sizes [11,12]. In contrast to the large amount of sample used in conventional techniques (>100 mg for each analysis) [3], only 2-6 mg (average 3 mg) of pyrite was used in this study. This allows us to address sample heterogeneity by working directly on small sub-samples, and avoid the mixing problem present when utilizing the conventional ID-TIMS method that uses a much larger amounts of material, thus masking any fine-scale heterogeneity and hampering generating a meaningful isochron age. Therefore, theoretically, Rb-Sr pyrite geochronology should have the potential to date key processes in petroleum systems.
The Yudong region is a significant petroleum exploration province in the Kuqa Depression, northern Tarim Basin ( Figure 1). Here, Ordovician carbonate rocks are reservoirs for hydrocarbons generated from Cambrian-Ordovician marine source rocks [21]. YD-2 is the only well so far drilled into Ordovician-hosted hydrocarbons in the Yudong region, but limited heavy oil of uncommercial value (~0.2 m 3 /d) has been recovered from the interval between 4945 m and 4983.5 m during the formation test of the Ordovician strata [22]. Pyrite is commonly disseminated in the Ordovician oil-bearing interval in the drillhole YD-2 ( Figure 2). This, therefore, provides a valuable opportunity to test the applicability of Rb-Sr pyrite geochronology to petroleum systems.
Minerals 2020, 10, x FOR PEER REVIEW 3 of 16 value (~0.2 m 3 /d) has been recovered from the interval between 4945 m and 4983.5 m during the formation test of the Ordovician strata [22]. Pyrite is commonly disseminated in the Ordovician oilbearing interval in the drillhole YD-2 ( Figure 2). This, therefore, provides a valuable opportunity to test the applicability of Rb-Sr pyrite geochronology to petroleum systems.     [22]. Pyrite is commonly disseminated in the Ordovician oilbearing interval in the drillhole YD-2 ( Figure 2). This, therefore, provides a valuable opportunity to test the applicability of Rb-Sr pyrite geochronology to petroleum systems.

Geological Background
The Kuqa Depression is situated in the northern part of the Tarim Basin, northwest (NW) China, and consists of seven structural units (Northern Monolinal Belt, Kelasu Thrust Belt, Yiqikelike Thrust Belt, Baicheng Sag, Qiulitage Thrust Belt, Yangxia Sag and Southern Slope Belt, Figure 1). The Yudong region lies in the Southern Slope Belt of the Kuqa Depression ( Figure 1) and consists of Paleozoic-Cenozoic sediments [23]. Three major tectonic events have affected the Kuqa Depression: (1) a peripheral foreland basin stage (late Permian to Middle Triassic), (2) a rift basin stage (Late Triassic to Middle Jurassic), and (3) a rejuvenated foreland basin stage (early Neogene to the present-day) [23]. In YD-2, the Lower Ordovician is directly overlain by Jurassic strata (Figure 2A), indicating the Middle Ordovician to Triassic rocks have been eroded during the multi-stage tectonic movements that affected this region [24].
Ordovician hydrocarbon reservoirs in the Tarim Basin are significantly influenced by magmatism and associated hydrothermal activity [18][19][20]. Hydrothermal activity resulted in the precipitation of minerals such as calcite, and it created a high-temperature environment that was favorable for the occurrence of TSR in the reservoirs [13][14][15][16][17][18][19]. TSR is an important process for the formation of pyrite in the Ordovician reservoirs, and pyrites of TSR genesis are characterized by enriched sulfur isotope compositions [13,15,18,19]. In addition to TSR-derived pyrites, there are also pyrites precipitated under low-temperature conditions [18,19]. These pyrites have depleted sulfur isotope compositions and were formed by bacterial sulfate reduction [14,[16][17][18][19].

Samples and Analytical Method
Pyrite samples analyzed in this study were separated from two pyrite-rich core specimens (A and B) taken at~4952 m in the YD-2 well (Figure 2A). The samples were prepared as thin sections. Stereo binocular microscopy and scanning electron microscope-energy-dispersive X-ray spectroscopy (SEM-EDS, 200 kV, Zeiss Ultra plus Field Emission SEM at CSIRO) were employed to investigate the petrographic features of the samples.
Pyrite was separated by crushing the specimens in a porcelain mortar, followed by gravity sorting, electromagnetic separation and hand-picking under a binocular microscope. The obtained pyrite separates were cleaned with Milli-Q water and analytical-grade ethanol to further improve their purity and remove any contamination introduced by the mineral separation process. The two separate pyrite samples were prepared and dried completely on a hotplate prior to undertaking the chemistry.
Pyrite sub-samples were randomly picked from each sample so that they would potentially have a more homogeneous initial 87 Sr/ 86 Sr ratio (Sr i ). Each sub-sample was weighed (2-6 mg) using an AG104 Mettler Toledo analytical balance for Rb-Sr-Pb isotopic analyses and dissolved along with 87 Rb-84 Sr isotopic tracers in 14 N HNO 3 (~0.1 mL) in screw-top PFA Savillex vials. Rubidium, strontium, and lead fractions of each sub-sample were separated and purified using a mini-column containing~30 µL of Sr Spec resin [12]. Isotope ratios of Rb and Sr were determined using ID-TIMS on a multi-collector Triton plus instrument at the State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS) in Beijing [12,25]. Lead isotope ratios were also measured by TIMS using the same instrument. 87 Sr/ 86 Sr ratios were normalized to 88 Sr/ 86 Sr = 8.375209 [12,25] using the exponential law. Approximately 2 ng of the standard NBS 987 was loaded onto the filament and analyzed during the course of study, yielding a mean 87 Sr/ 86 Sr value of 0.710244 ± 0.000012 (2σ, n = 4) in good agreement with the reported values (0.710241-0.710255) [12,25,26]. Analytical uncertainties for 87 Rb/ 86 Sr ratios were less than 1%. The blank during the analytical session was less than 3 pg for Rb, 6 pg for Sr and 100 pg for Pb. Therefore, the contribution of total blank on the Rb-Sr and Pb isotopes can be ignored. In addition, based on the highly sensitive TaF5 emitter, we obtained strong signal intensity for Sr (~3000 mV of 88Sr), even for 2 ng sample size during TIMS measurements. This technique yielded a good precision (≤0.000039) for 87 Sr/ 86 Sr in this study. Rb-Sr isochron ages of pyrites were calculated using the ISOPLOT 3.0 software [27], applying a decay constant (λ 87 Rb) of 1.42 × 10 −11 yr −1 [28]. For Pb isotopes, in-run precision of individual analyses was better than 0.02% (2 RSE). Acids used during the Rb-Sr-Pb chemistry were all analytical reagent (AR) grade and were purified utilizing a SavillexTM DST-1000 sub-boiling distillation system. Ultrapure water with resistivity of 18.2 MΩ.cm −1 obtained from a Milli-Q Element system was used throughout this work.
Sulfur isotopic ratios of pyrites (25-50 mg for each) were measured by a Finnigan MAT 252 mass spectrometer using the dual inlet method at IGGCAS. Sulfur has four stable isotopes ( 32 S, 33 S, 34 S and 35 S), and the fractionation of 34 S/ 32 S ratio can be observed in low-temperature settings.

Petrographic Observations
Pyrite crystals were characterized by using both stereo binocular microscope photomicrographs ( Figure 3A,B) and scanning electron microscope-energy-dispersive X-ray spectroscopy (SEM-EDS) images ( Figure 4A-I). SEM images illustrate that pyrite, calcite and quartz are intergrown and show embayment structures ( Figure 4A), suggesting that these minerals were precipitated contemporaneously from hydrothermal fluids which may have migrated along faults near the YD-2 well ( Figure 2B). The coexistence of pyrite and bitumen was also observed in this study ( Figure 3B), which indicates that the formation of pyrite in the sampled reservoir rock may be the result of the interaction between hydrocarbon and hydrothermal fluid.

ults etrographic Observations
yrite crystals were characterized by using both stereo binocular microscope photomicrogr re 3A,B) and scanning electron microscope-energy-dispersive X-ray spectroscopy (SEMs ( Figure 4A-I). SEM images illustrate that pyrite, calcite and quartz are intergrown and s yment structures ( Figure 4A), suggesting that these minerals were precipi mporaneously from hydrothermal fluids which may have migrated along faults near the Figure 2B). The coexistence of pyrite and bitumen was also observed in this study ( Figure   he high-resolution SEM images show that pyrites in this study contain no mineral or ions ( Figure 4B-I). Furthermore, potassium elemental mapping of pyrite indicates that t nt is below the detection limit of the instrument ( Figure 5A-H). The lack of K also indicate ce of K-bearing inclusions that might also accommodate Rb. The high-resolution SEM images show that pyrites in this study contain no mineral or fluid inclusions ( Figure 4B-I). Furthermore, potassium elemental mapping of pyrite indicates that the K content is below the detection limit of the instrument ( Figure 5A-H). The lack of K also indicates the absence of K-bearing inclusions that might also accommodate Rb.   The mapping results show that the K content of the pyrites is below the detection limit of the instrument, supporting the absence of K-rich inclusion in the pyrites.

Rb-Sr-Pb-S Isotope Data and Isochron Ages
Pyrite sub-samples from specimen A contain 0.1353-0.1655 ppm Rb and 0.1358-0.1958 ppm Sr and have 87 Rb/ 86 Sr and 87 Sr/ 86 Sr ratios of 2.355-3.379 and 0.718340-0.721307, respectively (Table 1), which yield an isochron age of 206 ± 13 Ma (2σ), with an initial 87 Sr/ 86 Sr ratio (Sri) of 0.71138 ± 0.00048 (2σ) ( Figure 6A). Pyrite sub-samples from specimen B contain 0.1891-0.2611 ppm Rb and 0.2154-0.2511 ppm Sr (Table 1). An isochron age of 224 ± 28 Ma (2σ) with a Sri of 0.7100 ± 0.0011 (2σ) ( Figure  6B) is regressed from their 87 Rb/ 86 Sr and 87 Sr/ 86 Sr ratios, which are 2.543-3.124 and 0.718049-0.719882, respectively ( Table 1). The two Rb-Sr isochrons result in identical ages and Sri, within analytical uncertainties, indicating that these ages are not mixing isochron ages, and that the analyzed pyrites were precipitated synchronously.  Figure 4B-I (A-H). The mapping results show that the K content of the pyrites is below the detection limit of the instrument, supporting the absence of K-rich inclusion in the pyrites.  (Table 1), which yield an isochron age of 206 ± 13 Ma (2σ), with an initial 87 Sr/ 86 Sr ratio (Sr i ) of 0.71138 ± 0.00048 (2σ) ( Figure 6A). Pyrite sub-samples from specimen B contain 0.1891-0.2611 ppm Rb and 0.2154-0.2511 ppm Sr (Table 1). An isochron age of 224 ± 28 Ma (2σ) with a Sr i of 0.7100 ± 0.0011 (2σ) ( Figure 6B) is regressed from their 87 Rb/ 86 Sr and 87 Sr/ 86 Sr ratios, which are 2.543-3.124 and 0.718049-0.719882, respectively ( Table 1). The two Rb-Sr isochrons result in identical ages and Sr i , within analytical uncertainties, indicating that these ages are not mixing isochron ages, and that the analyzed pyrites were precipitated synchronously.

Pyrite Rb-Sr Geochronology: A Robust Dating Technique for Petroleum Systems
In contrast to the Rb-Sr budget in ore-associated pyrite, which is mainly attributed to K-rich inclusions [3,29], Rb and Sr atoms mainly reside in the crystal lattice as inclusions are absent in the studied pyrite (Figure 4). The Rb-Sr data also support the absence of K-rich inclusions: (1) Pyrites in this study have Rb and Sr content generally lower than ore-related pyrites. A compiled Rb-Sr dataset suggests that most (~70%) ore-related pyrite have Rb and Sr content higher than the pyrite samples in this study ( Figure 7); (2) randomly-selected sub-samples of each specimen have similar Rb-Sr concentrations (Table 1). This also implies that the Rb and Sr budget in this case cannot be mainly attributed to randomly distributed K-rich inclusions. In addition, the identical ages within analytical uncertainties yielded by Rb-Sr data from the two specimens at the same sampling site also exclude the dominant residence of Rb and Sr atoms in K-rich inclusions.

Pyrite Rb-Sr Geochronology: A Robust Dating Technique for Petroleum Systems
In contrast to the Rb-Sr budget in ore-associated pyrite, which is mainly attributed to K-rich inclusions [3,29], Rb and Sr atoms mainly reside in the crystal lattice as inclusions are absent in the studied pyrite (Figure 4). The Rb-Sr data also support the absence of K-rich inclusions: (1) Pyrites in this study have Rb and Sr content generally lower than ore-related pyrites. A compiled Rb-Sr dataset suggests that most (~70%) ore-related pyrite have Rb and Sr content higher than the pyrite samples in this study ( Figure 7); (2) randomly-selected sub-samples of each specimen have similar Rb-Sr concentrations ( Table 1). This also implies that the Rb and Sr budget in this case cannot be mainly attributed to randomly distributed K-rich inclusions. In addition, the identical ages within analytical uncertainties yielded by Rb-Sr data from the two specimens at the same sampling site also exclude the dominant residence of Rb and Sr atoms in K-rich inclusions. The absence of K-rich inclusions in pyrite suggests that initial Sr isotopic homogenization can be easily attained during the precipitation of pyrite. The obtained Rb-Sr ages for pyrite, thus, should be true isochron ages rather than mixing isochron ages. In addition, closure temperature for the Rb-Sr isotope system in pyrite is estimated between 227 °C and 320 °C [3], which is higher than the temperature for petroleum system events, e.g., 60-100 °C for oil generation [7], below 100 °C for hydrocarbon charge [6], and 100-180 °C for destruction of hydrocarbon reservoirs [7]. This implies that the Rb-Sr geochronometer in pyrite is insensitive to post-precipitation events and difficult to overprint in petroleum systems. Therefore, the Rb-Sr isotopic system in pyrite documents the time of its crystallization. The age data can be further used to constrain petroleum system events by integrating radiogenic (e.g., Pb, Sr) and stable isotopic (e.g., S) analyses of the dated pyrite.

Source of Metals in Hydrothermal Fluid
Ordovician carbonate rocks intersected by YD-2 were deposited in a marine environment, which is characterized by the general lack of terrigenous input [44]. Therefore, pyrite in the Ordovician reservoir is unlikely to be of detrital origin, but was most likely formed through in-situ precipitation [45]. Hydrothermal fluid is an important carrier of ore metals (e.g., Fe, Pb) [46] and can provide a potential source of iron for the formation of pyrite in carbonate rocks [47,48]. The coexistence of pyrite with other hydrothermal minerals (e.g., calcite, quartz) and the embayment structure between these minerals shown in the photomicrographs ( Figure 4A) demonstrate a hydrothermal origin for the pyrites. The hydrothermal origin of these pyrites is also supported by their coeval precipitation ages. The Rb-Sr isochron age of the pyrites, ca. 224-206 Ma, is younger than the depositional age of their Figure 7. Rb-Sr concentration data are compared between pyrite in this study and pyrite samples in high-temperature ore systems (log-log scale). Rb-Sr data sources: [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43].
The absence of K-rich inclusions in pyrite suggests that initial Sr isotopic homogenization can be easily attained during the precipitation of pyrite. The obtained Rb-Sr ages for pyrite, thus, should be true isochron ages rather than mixing isochron ages. In addition, closure temperature for the Rb-Sr isotope system in pyrite is estimated between 227 • C and 320 • C [3], which is higher than the temperature for petroleum system events, e.g., 60-100 • C for oil generation [7], below 100 • C for hydrocarbon charge [6], and 100-180 • C for destruction of hydrocarbon reservoirs [7]. This implies that the Rb-Sr geochronometer in pyrite is insensitive to post-precipitation events and difficult to overprint in petroleum systems. Therefore, the Rb-Sr isotopic system in pyrite documents the time of its crystallization. The age data can be further used to constrain petroleum system events by integrating radiogenic (e.g., Pb, Sr) and stable isotopic (e.g., S) analyses of the dated pyrite.

Source of Metals in Hydrothermal Fluid
Ordovician carbonate rocks intersected by YD-2 were deposited in a marine environment, which is characterized by the general lack of terrigenous input [44]. Therefore, pyrite in the Ordovician reservoir is unlikely to be of detrital origin, but was most likely formed through in-situ precipitation [45]. Hydrothermal fluid is an important carrier of ore metals (e.g., Fe, Pb) [46] and can provide a potential source of iron for the formation of pyrite in carbonate rocks [47,48]. The coexistence of pyrite with other hydrothermal minerals (e.g., calcite, quartz) and the embayment structure between these minerals shown in the photomicrographs ( Figure 4A) demonstrate a hydrothermal origin for the pyrites. The hydrothermal origin of these pyrites is also supported by their coeval precipitation ages. The Rb-Sr isochron age of the pyrites, ca. 224-206 Ma, is younger than the depositional age of their host rock (i.e., Ordovician) but approaches the onset of rifting in the Kuqa Depression (Late Triassic), when hydrothermal activity commonly occurred [23].
Metals in hydrothermal fluids from which the pyrites precipitated could have originated from three main repositories, i.e., the mantle, crust, and/or metamorphic basement of the sedimentary basin [49][50][51]. Lead-strontium isotope compositions of pyrites can provide important constraints on the sources of metal in fluid systems [33]. A compiled Pb-Sr isotope dataset for these three repositories within or adjacent to the Tarim Basin was employed to constrain the possible metal sources for the hydrothermal fluid (Figure 8).

Source of Sulfur
Sulfur in pyrites from carbonate reservoirs can be directly inherited from the thermal degradation of organosulfur compounds in crude oil or obtained from H 2 S, which can be generated either through TSR or BSR [47].
Sulfur isotopic composition of pyrites can provide an important constraint on the origin of their sulfur. As shown in Figure 9D, δ 34 S values of the pyrites are compared with sulfur isotopic ratios of potential sulfur providers (e.g., kerogen, oil, sulfate, formation water) and products of TSR and BSR (e.g., pyrites, H 2 S). The large discrepancy between δ 34 S values of pyrites in this study (+31.032% to +31.799% ) and kerogen or oil unaltered by TSR (−15.3% to +19.4% and +11.9% to +20.5% , respectively) precludes the possibility of direct inheritance of sulfur from hydrocarbon precursors ( Figure 9D). The pyrites are also unlikely to have been precipitated via BSR processes because their δ 34 S values are more enriched than those of BSR-derived pyrites, which have depleted δ 34 S values ranging from −25.7% to −11.6% ( Figure 9D). Instead, the similarity in δ 34 S values between the studied pyrites and pyrites (+11.200% to +29.700% ) and H 2 S (+14.210% to +33.000% ) that are produced by TSR processes in the Tarim Basin indicates that the pyrites from YD-2 most likely are by-products of TSR. Furthermore, the TSR origin is also supported by the insignificant sulfur isotopic fractionation between pyrites and sulfates (δ 34 S sulfate = +14.840% − 42.100% ), especially those in Ordovician strata (δ 34 S sulfate = +24.460% − 42.100% ) ( Figure 9D), because TSR results in less sulfur isotopic fractionation between sulfates and pyrites when compared with those in BSR [7].
TSR requires the presence of both sulfates (either as minerals or dissolved in formation water) and hydrocarbons, as well as a relative high temperature (100-180 • C) [7]. Several lines of evidence support that TSR might have taken place in the Ordovician reservoir intersected by the YD-2 well: (1) Both the industrial discovery (i.e., heavy oil flows) and evidence from photomicrographs of the samples (e.g., solid bitumen in Figure 3B) imply that there should have been sufficient reactive hydrocarbons for sulfate reduction to occur; (2) Anhydrite was identified in the Ordovician strata at~4950.1 m in the YD-2 well (i.e.,~1.9 m above the sampling depth) (Figure 2A) [22], which may have been the SO 4 2− source for the TSR; and (3) A case study of fluid inclusions hosted by hydrothermal minerals (e.g., calcite) from the Ordovician carbonate reservoir in the northern Tarim Basin suggests that the temperature of hydrothermal fluids in this region was higher than 154 • C, which can facilitate TSR [20]. In addition, petroleum recovered from the studied area has more enriched carbon isotopic compositions (δ 13 C = −25% − −28% , [59]) compared to ordinary Ordovician-hosted petroleum (δ 13 C = −31% − −33% , [21]), indicating that petroleum has likely been oxidized by TSR because the increase of temperature results in the enrichment in carbon isotopic compositions of petroleum [21].

Timing of Alteration of the Paleo-Oil Pool
The wide distribution of hydrothermal minerals in Ordovician hydrocarbon reservoirs indicates the common occurrence of hydrothermal activities in the Tarim Basin [48]. The hydrothermal mineral assemblage of pyrite + calcite + quartz is also present in samples in this study ( Figure 4A). Previous studies have indicated that migrated hydrocarbons can be altered by hydrothermal fluid in three aspects: (1) modifying the molecular composition of hydrocarbons, e.g., generating typical biomarkers of combustion; (2) the preferential loss of components depleted in carbon isotopic compositions; and (3) facilitating the TSR by providing a high temperature environment [21].
Although hydrothermal activities and their influence on hydrocarbons have been well documented, their absolute timing is poorly constrained. This study obtained Rb-Sr isochron ages of ca. 224-206 Ma, indicating that the pyrites precipitated from hydrothermal fluid associated with Late Triassic rifting in the Kuqa Depression, and not early Permian magmatism, as previously proposed [20]. The pyrite Rb-Sr geochronological evidence is consistent with the lack of early Permian magmatic records in the region, since few Permian magmatic rocks have been identified in the Yudong area, suggesting that any thermal influence of early Permian magmatism was insignificant [49].
As discussed in the preceding sections, the studied pyrites are also indicators of the TSR process. Therefore, Rb-Sr isochron ages of pyrite may also record the timing of TSR alteration of the paleo-oil pool if hydrothermal activity and TSR occur contemporaneously. Burial history analysis indicates that there was regional uplift during the late Permian to Middle Triassic in the Kuqa Depression [23]. Hydrogen sulfide generated prior to the uplifting event could have potentially leaked from the Ordovician reservoir due to erosion of the caprocks. Therefore, hydrogen sulfide precursors of the pyrites may have been generated after the Middle Triassic. Considering the Late Triassic precipitation age of the pyrites, it is reasonable to deduce that both the hydrothermal activity and TSR occurred at ca. 224-206 Ma. Hydrothermal fluid flow can result in a high-temperature environment, which is essential for the TSR. Thus, we propose that hydrothermal activity was a key factor in triggering the TSR in the Ordovician reservoirs of the Yudong region.

Conclusions
This is the first study to integrate the low-blank and high-sensitive Rb-Sr ID-TIMS dating technique with Pb-S isotope analysis for direct dating and characterizing key events in petroleum systems using small pyrite samples (2-6 mg). Through applying this method to samples taken from an Ordovician carbonate reservoir in the Yudong region of the Kuqa Depression, northern Tarim Basin, the timing of hydrothermally-triggered TSR alteration of a paleo-oil pool is established at ca. 224-206 Ma. SEM investigations demonstrate that Rb and Sr atoms mainly reside in the crystal lattice of the pyrites because neither fluid nor mineral inclusions are found in the pyrites. The rigorous Rb-Sr isochron relations documented by the samples indicate that such residency can result in sufficient Rb/Sr fractionation and initial Sr isotopic homogenization for constructing an isochron. In addition, the closure temperature (227-320 • C) for the Rb-Sr isotope system in pyrite is estimated to be higher than the precipitation temperature for pyrite in the petroleum-related environment (below 200 • C), suggesting that the Rb-Sr age for pyrite was not overprinted by post-precipitation alterations. This study, therefore, confirms that Rb-Sr pyrite geochronology is potentially an important tool for constraining petroleum system events. Besides obtaining reliable isotopic ages, the determination of stable (e.g., S) and radiogenic (e.g., Sr and Pb) isotopic ratios of pyrites can also lead to a better understanding of petroleum systems. The applicability of this geochronometer to petroleum system analysis could be enhanced by in-situ analyses of Fe-S isotopes and trace elements (e.g., transition metals, such as molybdenum and zinc) of pyrite, which can provide additional constraints on its origin and precipitation environment.