Ultra-deep (buried depth larger than 6000 m) marine carbonate strata have great potential for oil and gas resources and are important replacement area for oil and gas exploration and development ^[1-2]. A series of major achievements have been made in the exploration and development of ultra-deep marine carbonate strata in the Tarim Basin. In the early stage, Lunnan-Tahe weathering crust large oilfield and central Tarim reef beach-weathering crust condensate gas field were discovered in the Ordovician carbonate strata of the northern and central Tarim uplifts ^[3-4]. They are the largest marine carbonate oilfield and condensate gasfield, respectively, in China. In recent years, the exploration and development of fault- controlled oil and gas reservoirs in the northern depression has been carried out ^[5]. High-yield oil flows have been obtained from the Ordovician carbonate reservoirs in the strike-slip fault zones, and the oil exploration and development has reached the 7000 m “deep death line”. As a result, super-large ultra-deep fault-controlled oilfields were discovered with geological reserves of 10×10⁸ t, such as Fuman oilfield which has been efficiently built with a production capacity of over 200×10⁴ t/a. A new field has been opened up ^[5⇓⇓-8], and become a model for the exploration and development of ultra-deep fault-controlled oil and gas fields in the Tarim Basin.

Recent studies have shown that ultra-deep strike-slip faults in the Tarim Basin not only control the distribution of the Ordovician carbonate fracture-cavity “sweet spots”, but also control the migration, accumulation and enrichment of oil and gas ^[9-10]. However, strike-slip faults affect the formation and adjustment of oil and gas in multiple phases, which results in complex oil and gas distribution, and restricts the deployment of oil and gas exploration and development. In the early stage, a small amount of pore-filled fluid inclusions in calcite have been studied on the accumulation period mainly based on temperature measurement. The research on the Shunbei area of the Aman transition zone believes that the accumulation period of the strike-slip fault zone was the end of the Middle Ordovician and the Silurian ^[11]. Some studies speculate that the Late Caledonian accumulation was destroyed in the Early Hercynian period, and the main charging was in the Late Hercynian-Indosinian period ^[12], some studies also believe that there was a hydrocarbon filling process in the Late Caledonian, Late Hercynian and Himalayan periods ^[13]. Studies on the adjacent Halahatang area suggest that there are three accumulation periods (402-412 Ma, 9-11 Ma and 6-8 Ma) ^[14], or two periods (410-419 Ma and 8-16 Ma) ^[15], or three accumulation periods (Early Permian ^[16], Silurian-Permian and period after Triassic) ^[17]. Studies on Tahe oilfield suggest that there are three stages of accumulation in Middle and Late Caledonian, Late Hercynian and Himalayan periods^[18]. Studies on the Yingmaili area suggest that there are three periods of accumulation in the Late Caledonian-Early Hercynian, Late Hercynian and Himalayan periods ^[19]. All the studies focused on fluid inclusions, but there are obvious differences in understanding the accumulation periods in the study area, which may be caused by the following three main reasons for the analysis: (1) The Tarim Basin has experienced complex geothermal field evolution, and the early fluid inclusions were thermally reformed and rebalanced due to the influence of later deep burial warming or tectonic-thermal events, resulting in varying degrees of increase in the homogenization temperature of the inclusions. The temperature peak difference is large and cannot indicate the accumulation periods. (2) Inaccurate selection of fluid inclusions, not brine inclusions symbiotic with oil inclusions captured at the same time. (3) The burial history-thermal evolution history curves used to calibrate the homogenization temperature of the inclusions are quite different. It can be seen that fluid inclusion is still the most important target for the identification of reservoir forming period. However, for the analysis of ultra-deep multi-stage hydrocarbon accumulation and adjustment, as well as the evolution of multi-stage fluid temperature field, the accuracy of fluid inclusion sampling, lithofacies analysis and data analysis is very important ^[20⇓-22], and it is the key to whether the research results conform to the actual geological conditions. In addition, fluid inclusion association (FIA) is a group of inclusions formed by the same inclusion capture event and identified by petrographic methods ^[20-21]. Taking FIA as the research object is an important basis for the validity of homogenization temperature data of ultra-deep fluid inclusions ^[11].

This paper focuses on the strike-slip fault zone of the Halahatang-Fuman oilfield in the northern Tarim Basin, and takes into account the strike-slip fault zone in eastern Lunnan. Taking the calcite veins filled in fractures in Ordovician carbonate rock cores as the research object, and strictly taking the FIA as the basic unit, through petrographic observation, fluorescence observation, fluorescence spectrum analysis and microscopic temperature measurement (by cycling to obtain a bracketed Th), and obtained effective temperature measurement data of fluid inclusions in the cores of Ordovician carbonate reservoirs in 12 wells. On this basis, combined with the analysis of source rock and hydrocarbon generation history, fault dating, reservoir and trap forming periods, the key accumulation period of ultra-deep fault-controlled reservoirs in northern Tarim Basin and its enlightenment to oil and gas enrichment and exploration and development are discussed.

The Tarim Basin covers about 56×10⁴ km², which is a composite basin composed of peripheral Cenozoic foreland basin and Paleozoic-Mesozoic craton basin, with a broad and gentle structural pattern of craton inner uplifts and depressions ^[22]. The thick Nanhua-Quaternary sedimentary rock series developed on the metamorphic basement of the Pre-Nanhua System, in which the Cambrian-Ordovician marine carbonate strata are the main target for oil and gas exploration and development in the basin area. Recently, 70 large-scale strike-slip fault zones of grades I and II have been identified in northern uplift-northern depression-central uplift, with a total length of 4000 km. The circum-Aman strike-slip fault system with a distribution range of 9×10⁴ km² has been identified (Fig. 1)^[5,23].

Studies have shown ^[5,23] that the circum-Aman strike-slip fault system is divided into two areas with F_I5 strike-slip fault zone as dividing line in east-west direction, and forming northern Tarim, Aman and central Tarim zones in north-south direction (Fig. 1). The strike-slip fault zone of grade I is more than 80 km long (F_I5 is up to 290 km), and the strike-slip fault zone of grade II is 30-80 km long. But the displacement of the strike-slip fault is small. For most faults, the horizontal displacement is less than 500 m, while the vertical displacement is less than 100 m in the Ordovician carbonate rocks. Compression-torsion strike-slip faults are developed in the Lower Paleozoic carbonate rocks. Local strike-slip faults in the central uplift extend upward to the Carboniferous-Permian, while the northern uplift may inherit and develop to the Mesozoic-Paleogene, mainly with tension-torsion faults. The section is dominated by high and steep upright faults, with multiple layers of positive flower-like and negative flower-like structures superimposed. The large strike-slip fault zone is divided into several sections along the strike. A variety of strike-slip structures are developed, such as linear structure, echelon structure, flower structure, horsetail structure, “X” type shear structure, pull-apart structure and braided structure, and forming a variety of fault combinations^{[5⇓⇓⇓⇓-10]}. The “X” type conjugate strike-slip fault system is developed in Halahatang area ^[24]. And the strike-slip faults in the east-west direction gradually transit to the northeast and northwest strike-slip faults. The south is roughly bounded by the northern ancient uplift, and dominated by the northeast strike-slip faults in Fuman oilfield (Fig. 1). Studies have shown ^[24⇓-26] that there exist various non-Anderson fault patterns dominated by connection and growth in the small displacement long strike-slip fault zone. The conjugate strike-slip fault in northern Tarim Basin regulates the deformation of the cross-cutting parts through successive sliding mechanism, and adjusts the main displacement and deformation through strong action of the overlapping area. These mechanisms result in the differences in the geometry and kinematics characteristics of the strike-slip fault zones in different regions.

The Fuman-Halahatang oilfield is rich in oil resources with multiple sets of reservoir-cap assemblages and oil-bearing intervals dominated by Ordovician limestone^{[5⇓⇓⇓⇓-10]}. The discovered oil and gas are mainly distributed in the Middle Ordovician at the top of the Yijianfang Formation-Yingshan Formation and at 6000-8000 m. The reservoir space is dominated by three types of pores including secondary dissolution pores, caves and fractures^{[5⇓⇓⇓⇓-10]}. The porosity of reservoir matrix is mostly less than 3%, and the permeability is generally less than 1×10^-3 μm². However, the maximum porosity of the large-scale fractured-vuggy carbonate reservoir interval drilled locally can reach 50%, and the permeability is more than 5×10^-3 μm², which is mainly distributed along the strike-slip fault zone and is the main drilling target. Oil and gas distribution in Fuman oilfield is mainly controlled by the strike-slip fault damage zone ^{[5⇓⇓⇓⇓-10]}, and fractured-vuggy reservoirs are distributed within 300 m from the main fault. The Ordovician fractured-vuggy reservoir in Halahatang area is controlled by interlayer karst and strike-slip fault, which is mainly located within 800 m from the strike-slip fault zone. The distribution of oil and gas in carbonate reservoir is very uneven, and the output is very unstable. However, the crude oil production of strike slip fault zone in Fuman oilfield of depression area is stable, and there are many wells with high productivity, which have the characteristics of differential enrichment along the damage zone “sweet spots” ^[5].

Fluid inclusions can directly capture hydrocarbon fluids in the process of hydrocarbon charging to form oil inclusions, which have important implications for the history of hydrocarbon charging in sedimentary basin^{[11⇓⇓⇓⇓⇓⇓⇓-19]}. Fluid inclusion association (FIA) is a group of inclusions associated with lithofacies and generally isochronous. The temperature measurement of fluid inclusion under FIA constraint can effectively indicate the accumulation period ^[20⇓-22].

In view of the fact that it is difficult to form and preserve the isochronous brine inclusions symbiotic with oil inclusions in the open system of porous calcite cements, the sampling selects the fracture calcite completely filled in the same generation. And the fractures should heal quickly, so it is conducive to the formation of a closed system which can preserve early inclusions and inhibit the formation of the later brine inclusions. In addition, through petrographic analysis, isochronous brine FIAs that are symbiotic with oil inclusions should be strictly selected to avoid the influence of different periods of brine inclusions and brine inclusions not symbiotic with oil inclusions. In this study, 40 samples were selected from the fracture calcites in the cores of 12 wells of Ordovician carbonate strata in the strike-slip fault zone in the northern Tarim Basin, and 24 groups of effective FIAs were obtained in 10 wells (the sampling position is shown in Fig. 1a). The fluid inclusion thick sections are thinner than 200 μm and polished on both sides. Petrologic observation was completed in the Carbonate Rock Laboratory, School of Geosciences and Technology, Southwest Petroleum University. Cathode luminescence and inclusion microscopic temperature measurement were completed in the State Key Laboratory of Reservoir Geology and Development Engineering, Southwest Petroleum University. Cathode luminescence that distinguishes calcite cements at different diagenetic temperatures, was observed using a CL8200 MK5 cathode luminescence microscope operating at 7-10 kV and 400-500 mA. Microscopic temperature measurement of inclusions was performed using a British THMSG600 hot and cold stage, with a temperature measurement range of -196 °C to 600°C, a temperature accuracy of 0.1 °C, and a heating/freezing rate of 0.1-150 °C/min. In order to identify oil inclusions with different maturity levels, the quantitative analysis of fluorescence spectrum was carried out on the basis of FIA identification and fluorescence thin section observation, and the test was completed in the Microscopic Test Laboratory of Reservoir Forming Dynamics of the College of Geosciences, Yangtze University.

This study of fluid inclusions mainly constrains the data by the FIA. The cycling to obtain a bracketed Th method was used to measure the homogenization temperature of fluid inclusions ^[20]. The specific procedures are as follows: Select an observation temperature that is less than the homogenization temperature. Raise the temperature of the fluid inclusions to above the observation temperature, and bubbles can be observed obviously. Cool the fluid inclusions after reaching the homogenization temperature. When the temperature decreases below the observation temperature, no bubbles can be seen because of the metastable nucleation of bubbles. If the inclusions are homogenized, bubbles will not appear soon when cooling. If not, bubbles will appear quickly when cooling. Although the cycling to obtain a bracketed Th method is time-consuming, the data obtained are more reliable ^[20].

By observing the thick sections of inclusions under fluorescence microscope, the fluorescent inclusions are oil inclusions, while the non-fluorescent inclusions are brine or gas inclusions ^[20]. The fluorescence color of oil inclusions has certain indicative significance for its maturity, oil source and charging period, but the fluorescence color observed by naked eye is only qualitative description, and there may be errors. Therefore, it is necessary to quantitatively characterize the fluorescence color, namely, fluorescence spectrum analysis ^[20]. Fluorescence spectrum analysis mainly focuses on the main peak wavelength of oil inclusion fluorescence. Different main peak wavelengths represent different fluorescence colors.

In addition, in-situ LA-ICP-MS dating of calcite in the same fracturing period was carried out in the Radioisotope Laboratory of University of Queensland, Australia; and in-situ trace element analysis of the same sample was carried out in the Carbonate Laboratory of Southwest Petroleum University ^[26-27]. Based on the analysis of source rocks, referring to the results of previous studies^{[11⇓⇓⇓⇓⇓-17,28]} and the newly drilled well Luntan 1, the data of denudation amount are added to simulate the hydrocarbon generation history of single well by PetroMod simulation software.

The Ordovician limestone has developed fractures in the strike-slip fault zone in the study area, with various fracture types and characteristics ^[29]. There are generally 1-3 stages of fractures, and the Fuman oilfield in the south is dominated by high angle fractures. The fracture fillings are mainly calcite, but mud fillings are more in the northern buried mountain area. The calcite has 1-4 generations, but most of them are cemented filling of coarse-grained calcite (Fig. 2a-2e). Local fractures contain oil, and show the characteristics of multi-stage fracture activity. According to the thin-section cathode luminescence analysis (Fig. 2f-2h), the cement is mainly calcite, with a small amount of ferro-calcite, and locally fluorite and other hydrothermal minerals. The cathode luminescence characteristics of carbonate minerals have a certain indication for the formation sequence and diagenetic stage. The non-luminescence-dark luminescence usually indicates the diagenetic environment of seabed, supratidal, tidal bottom, mixed water, atmospheric freshwater, etc. The resulting carbonate mineral is the product of syndiagenetic stage, and its paleogeotemperature is usually close to the surface normal temperature (about 25 °C). Bright luminescence usually indicates a shallow diagenetic environment. The resulting carbonate mineral is the product of early diagenetic stage, and its paleogeotemperature is usually 25-80 °C ^[20]. Therefore, on the basis of the relative stages of fracture development, typical broad, single-composition and single-structure calcite cements of the same generation were selected, considering with calcites of different types and characteristics.

The identification of fluid inclusion type is one of the contents of petrographic observation of fluid inclusions. In this study, two types of fluid inclusions were identified, namely brine inclusions and oil inclusions, which can be further divided into four classes according to the different phase states, including liquid-rich gas-liquid two- phase brine inclusions, liquid-rich gas-liquid two-phase oil inclusions, all-liquid brine inclusions and all-liquid oil inclusions (Fig. 3). Among the FIAs in which brine inclusions and oil inclusions coexist, various combinations have been identified (Fig. 3). The combination types of fluid inclusions in the same FIA are mainly yellow-green fluorescent liquid-rich gas-liquid two-phase oil inclusions and liquid-rich gas-liquid two-phase brine inclusions, which coexist in the same healed crack, and yellow-fluorescent liquid-rich gas-liquid two-phase oil inclusions, liquid-rich gas-liquid two-phase brine inclusions, and single-liquid-phase brine inclusions coexisting in the same healed crack (Fig. 3).

It is worth noting that many single liquid brine inclusions have been observed in the petrographic study of fluid inclusions. Due to the lack of bubbles in the cavity, the homogenization temperature cannot be measured, so the single liquid brine inclusions are not used as the research object to judge the hydrocarbon accumulation period, and often easily ignored. All-liquid brine inclusions in diagenetic minerals in sedimentary basins have three origins: entrapment at low temperatures (less than 50 °C), significant metastability and necking down ^[20-21]. The all-liquid brine inclusions of low temperature origin do not have bubbles after freezing and cooling. The all-liquid brine inclusions of significant metastability origin have bubbles after freezing and cooling. The all-liquid brine inclusions of necking down origin often show petrographic evidence ^[20]. Therefore, the all-liquid brine inclusions have important indicative significance, which can indicate the fluid inclusion caused by one-stage entrapment at low temperatures.

According to the observation and description of fluorescence characteristics, oil inclusions with yellow fluorescence and yellow-green fluorescence are mainly developed (Fig. 3). After analyzing the fluorescence spectra of these two kinds of fluorescence, it is found that the main fluorescence peaks were mainly distributed in two intervals, 450-500 nm and 500-550 nm (Fig. 4), indicating that the oil inclusions in this batch of samples do have two types of fluorescence. This is consistent with the results of thin section observation. According to the analysis of the corresponding relationship between the fluorescence color and the degree of thermal evolution, there are differences in the maturity of crude oil, and it is speculated that two stages of crude oil charging may have occurred.

The homogenization temperature of oil inclusions is influenced by extremely complex factors, so it cannot be directly used to represent the temperature of oil and gas charging period. The homogenization temperature of brine inclusions coexisting with oil inclusions in the same FIA can be measured to represent the forming temperature of oil inclusions, and the temperature obtained is reliable ^[20-21]. Since the same FIA is usually planar in three- dimensional space, the thickness of the inclusion thick sections is within 200 μm, and the size of the fluid inclusions observed under a microscope is 2-20 μm. Therefore, by adjusting the focal length of the microscope (upper and lower), the fluid inclusions in different positions of the same FIA in three-dimensional space can be observed (Fig. 5). Combined with the cycling to obtain a bracketed Th method, the homogenization temperature of fluid inclusions in different FIAs can be measured more accurately to reflect the lowest temperature (lower temperature limit) when the fluid inclusions are captured. Since the fluorescence colors of oil inclusions in the samples are mainly yellow and yellow-green, the accumulation can be divided into two stages. Since different fluorescence colors of oil inclusions represent an accumulation stage, the homogenization temperature of each FIA is analyzed according to different fluorescence colors of oil inclusions.

The statistical analysis of the homogenization temperature of brine inclusions symbiotic with yellow fluorescent oil inclusions (Fig. 6) shows that low temperature range accounts for a large proportion, and most FIAs have low temperature range and relatively high temperature range data. FIA3 only has relatively high temperature range, while FIA5, FIA6, FIA10, FIA13 and FIA16 only have low temperature data. There may be two explanations for the above temperature differences: (1) The oil inclusions with yellow fluorescence represent a low-temperature charging stage (all-liquid brine inclusions of low-temperature origin are of great significance, with the homogeneous temperature generally less than 50 °C), and the oil and gas charging time is relatively early. The high temperature information represents different degrees of thermal transformation of low-temperature brine inclusions during continuous burial. Most of FIAs have undergone partial thermal reequilibration. Among them, FIA3 was completely reequilibrated by thermal transformation, while FIA5, FIA6, FIA10, FIA13 and FIA16 were not reequilibrated by thermal transformation, so the high temperature data have no meaning of accumulation temperature. (2) The yellow fluorescent oil inclusions undergone a stage of low-temperature charging and a stage of high-temperature charging. However, due to different degrees of thermal reequilibration, the data in the high-temperature interval are relatively dispersed. Among them, FIA5, FIA6, FIA10, FIA13 and FIA16 lack high temperature data, which may be due to the lack of effective data points in the samples and no high temperature inclusions were detected.

The statistical analysis of the homogenization temperature of brine inclusions symbiotic with yellow-green fluorescent oil inclusions (Fig. 7) shows that this type of FIAs does not contain low-temperature range data. Among them, the homogenization temperature data intervals of FIA1, FIA2, FIA3 and FIA4 are relatively consistent. The homogenization temperature distribution range of FIA1 and FIA3 is 90-130 °C, and the temperature of 90-110 °C is dominant. The homogenization temperature distribution range of FIA2 is 70-150 °C, of which the temperature of 110-130 °C is dominant. The homogenization temperature distribution range of FIA4 is 50-130°C, of which the temperature of 90-110 °C is dominant. FIA5, FIA6, FIA7 and FIA8 show high temperature data greater than 150 °C, which may be related to tectonic thermal events (magmatic activity). To sum up, this study believes that the yellow-green fluorescent oil inclusions represent a relatively high temperature charging period, with the charging temperature of 90-110°C. At the same time, the temperature range is also recorded in the brine inclusions symbiotic with yellow fluorescent oil inclusions. It is worth noting that such FIAs are distributed in uplift areas at higher locations.

To sum up, there are at least two stages of oil and gas charging in the oil inclusions in the sample. The first stage is the early low-temperature charging below 50 °C. The other stage is the late high-temperature charging at 90-110 °C. Yellow and yellow-green fluorescent oil inclusions observed by naked eyes generally have relatively high thermal maturity ^[30]. Low-temperature oil inclusions have higher thermal maturity, which may be due to the increase of burial temperature in the late stage.

The Ordovician carbonate strata in the Tarim Basin have experienced multiple stages of complex accumulation and evolution ^{[11⇓⇓⇓⇓⇓⇓⇓-19]}. A variety of methods are adopted below to comprehensively determine the accumulation period.

Although the temperature measurement method of inclusions has been widely used in the study of the accumulation period in the Tarim Basin, the reliability of the data is low because the inclusions are easily affected by late thermal effects ^[20-21]. Therefore, it is necessary to select effective inclusion data, and exclude the data influenced by factors such as late thermal action (including volcanic activity), in order to more accurately determine the accumulation period.

Previous work in northern Tarim Basin usually ignored the all-liquid brine inclusions, and did not detect and analyze the all-liquid brine inclusions below 50 °C, while the brine inclusions at 70-80 °C were used to calibrate the Caledonian accumulation ^{[13⇓-15,18 -19]}. However, this study shows that FIAs to the south of the Ordovician weathering crust in the northern Tarim uplift have all-liquid brine inclusions coexisting with oil inclusions below 50 °C (Fig. 6). In particular, FIA5, FIA6, FIA10, FIA13 and FIA16 only have brine inclusions below 50 °C, corresponding to the mudstone deposition period of the Upper Ordovician Sangtamu Formation with a thickness of 1000 m (about 440-450 Ma). A large amount of high-temperature data are also recorded in the FIA with low-temperature all-liquid brine inclusions (Fig. 6), indicating that the FIA has experienced late thermal effect and undergone thermal reequilibration ^[20-21]. Due to the differences in closure of different inclusions, temperature rose differently, resulting in a wide temperature range, which may correspond to the thermal effect of Hercynian-Yanshan period, but cannot be used to indicate the accumulation stage.

It is worth noting that no inclusions from late accumulation period were detected in the southern inner area far away from the weathering crust of the paleo-uplift. This may be caused by the following reasons: (1) Late Ordovician accumulation was accompanied by multiple strong charging events, but late oil and gas charging was weak. (2) No inclusions appeared in Late Hercynian oil and gas charging, which may because the crude oil was saturated or the fractures in calcite were completely closed. (3) No inclusions were detected because of too few samples. All-liquid brine inclusions entrapment at low temperatures below 50 °C were detected in the Shunbei area of the Aman transitional zone ^[11], indicating that Caledonian accumulation was universal. However, the temperature range of the high-temperature brine inclusions detected in the Shunbei area varies greatly ^[12-13], and no all-liquid brine inclusions below 50 °C were analyzed, which is consistent with the temperature of 70-130 °C detected in this study. This is most likely caused by the influence of late thermal effects.

In the northern region of Halahatang, all-liquid brine inclusions associated with oil inclusions entrapment at low temperatures below 50 °C were also detected in wells X101 and H801 (Fig. 6), indicating that there was oil charging in the late Caledonian period. However, higher temperature inclusions were detected in northern Tarim buried hills ^{[14⇓⇓⇓-18]}. In this study, low-temperature inclusions were not detected in wells X8H and H6-1, but a large number of inclusions with temperature greater than 150 °C were detected in Well X8H (Fig. 7). According to the analysis of the weathering crust adjacent to the paleo-uplift in this area, the early paleo-reservoirs have been damaged. In addition, a large number of asphalt sandstones were found in the Silurian, which is generally considered to be the product of the destruction of paleo-reservoirs in Caledonian period ^[14-15]. However, the inclusions at the temperature of 70-130 °C may undergo late thermal heating, so it is not suitable to use the peak value of homogenization temperature to represent the accumulation period. But the lowest temperature of the FIA may represent the temperature of the accumulation period. The homogenization temperature of the brine inclusions symbiotic with yellow-green fluorescent oil inclusions represents the accumulation period with higher maturity, in which the lower temperature in the range of 70-90 °C in the same FIA can indicate the accumulation period (Fig. 7), corresponding to the Late Hercynian accumulation period ^{[13⇓⇓⇓⇓⇓-19]}. Based on the relevant data, a large number of inclusions at 70-90 °C have been detected in Lunnan-Tahe-Halahatang-Yingmaili areas ^{[13⇓⇓⇓⇓⇓-19]}, indicating an important accumulation period, and revealing that the paleo-uplift area in northern Tarim was controlled by Late Hercynian accumulation. The high temperature is caused by late thermal effect, and the abnormally high temperature detected in Well X8H may be affected by Permian volcanic activity. A wide range of homogenization temperature data of inclusions were also detected in Lunnan-Tahe areas. Within it, a large number of Late Hercynian temperature data may represent oil and gas adjustment period, which cannot be determined to be multi-stage accumulation period.

Due to lack of data, there has been a long-term dispute over the main source rocks of the Middle-Lower Cambrian and the Middle-Upper Ordovician in the platform area of the Tarim Basin ^[31]. In recent years, the Middle-Upper Ordovician reservoir has been drilled in different zones of the Central Tarim-Aman-Northern Tarim areas, but no effective source rocks of the Middle-Upper Ordovician were found. Recently, high-abundance dark mudstone source rocks of the Lower Cambrian Yuertusi Formation with a thickness of 10-30 m have been found both in outcrops and underground wells ^[31⇓-33]. The average TOC value is greater than 2%, the kerogen is of types I-II, and the vitrinite reflectance is 1.3%-1.8%, indicating the high-quality source rock in the highly mature to over mature stage. By tracing on seismic profiles, it is found that the Lower Cambrian reservoir in the Aman transitional zone is obviously thick, which may be an effective hydrocarbon generation center ^[5]. Through oil-source correlation, it is found that the oil and gas was mainly come from the Lower Cambrian, which is the primary source rock in the platform basin area ^[33-34].

Through the analysis of burial history and thermal evolution history, during the filling and subsidence period of the Sangtamu Formation, the burial depth of the Lower Cambrian source rock was 3000-5000 m, entering the mass hydrocarbon generation and expulsion stage. Previous studies showed that, in addition to the influence of the volcanic activity in the Early Permian, the Phanerozoic geothermal field was a process of gradual annealing, and the paleogeothermal gradient gradually decreased from 35 °C/km to 20 °C/km in the Tarim Basin ^[28]. Combined with previous research results ^{[11⇓⇓⇓⇓⇓⇓⇓-19]}, the burial history and thermal evolution history curves of typical wells in the Aman transitional zone and the Halahatang slope area were compiled under the constraint of R_o and erosion calibration from Well Luntan 1 (Fig. 8). The results show that the Paleozoic Halahatang area has a slow subsidence process under the action of multi-stage uplifting and subsiding. There may be two periods of crude oil accumulation corresponding to the Late Ordovician-Silurian (Middle-Late Caledonian) and the Permian (Late Hercynian), respectively. The Middle Cenozoic entered the dry gas stage (Fig. 8a). The hydrocarbon generation center of the Aman transitional zone experienced rapid sedimentation in the Early Paleozoic, decelerated sedimentation in the Late Paleozoic, slow sedimentation from the Mesozoic to Paleogene and rapid sedimentation since the Neogene. Due to the continuous decrease of the paleogeothermal gradient, the source rock at the bottom of the Lower Cambrian entered the peak period of oil generation in the Middle and Late Caledonian period (Fig. 8b). The Late Hercynian period entered the mature gas generation stage, and then hydrocarbon generation stagnated after the Mesozoic.

It is worth noting that Well LT1 (recently completed) encountered high-abundance source rocks of the Yuertusi Formation at the bottom of the Lower Cambrian at 8600 m. The maturity of the source rock is 1.5%-1.7% estimated by asphalt reflectance, Raman spectroscopy and organic geochemical parameters of rock extracts ^[33], which is far lower than the maturity predicted by early studies. It is revealed that the source rock of the Lower Cambrian in the uplift area might generate oil in the Late Hercynian. It can be concluded that the circum-Aman area in the Tarim Basin has effective source rocks for multi-stage oil generation. At the same time, the study found that the maturity of crude oil in the northern Tarim-Aman transition zone is mainly 0.7%-1.2%, and partially over 1.2% (Fig. 9), indicating that the discovered oil reservoirs have crude oil mainly generated in the early stage, corresponding to the Middle and Late Caledonian periods.

The calcite dating and seismic data analysis show that the circum-Aman strike-slip fault system was formed at the end of the Middle Ordovician ^[26], and with interlayer karstification occurred before the deposition of the Lianglitage Formation in the Late Ordovician, forming the high-quality fracture-cavity reservoir at the top of Yijianfang Formation-Yingshan Formation along the strike-slip fault zone. The exact U-Pb age of cement in fractures was also obtained in this study. The age of calcite precipitation of the top fractures in the Yifang Formation in Well R4 are (462.6±6.8) Ma and (468±16) Ma. The precipitation age of calcite in the top fractures of the carbonate rocks of the Yingshan Formation in Well Q1 is (459±28) Ma, and the precipitation age of calcite in the top fractures of the carbonate rocks of the Yijianfang Formation in Well H6-1 is (449.8±7.3) Ma. These data indicate that a large number of calcite deposits with an age of 460-450 Ma were filled in the fractures along the strike-slip fault ^[26], revealing that fracture-cavity reservoirs in the strike-slip fault zone had been developed before the deposition of the Lianglitage Formation in the Late Ordovician. In addition, in-situ rare earth element analysis shows that the calcite has a positive Y anomaly, which may represent a shallow low-temperature environment. And the Y/Ho (chemical weathering intensity, the content ratio of yttrium to lanthanide) value is greater than 27, indicating a shallow low-temperature environment ^[35]. In addition, the negative Eu anomaly may exclude magmatic hydrothermal fluids and deep high-temperature environments, while REE+Y indicates obvious depletion features without the participation of atmospheric freshwater ^[27]. Comprehensive analysis shows that the calcite samples were originated from shallow dissolution and precipitation or filling in the burial period after freshwater dissolution, corresponding to the fracture filling in early burial period of 460-450 Ma. With the rapid subsidence of the Sangtamu Formation in the Late Ordovician, Lower Paleozoic source-reservoir-cap assemblages were formed, and fault-controlled paleo-reservoirs were formed through vertical connection of strike-slip faults. During the Middle-Late Ordovician, not only the Lower Cambrian source rock started to generate mass hydrocarbon, but also forming traps and migration conditions for large-scale oil reservoirs. On the contrary, if no hydrocarbon charged in that period, the pores in the Ordovician carbonate rocks may be completely cemented during the deep burial process ^[17,36]. It can be concluded that early hydrocarbon charging also played an important role in preserving reservoir pores.

Based on the analysis of relevant data, it is believed that the Paleozoic source rocks in Aman transition zone-northern Tarim uplift are dominated by the Lower Cambrian. In the Middle-Late Caledonian period, it has entered the peak oil generation period, which is the key accumulation period in this area and laid the foundation of oil and gas resources. A large number of Middle-Late Caledonian paleo-reservoirs have been preserved in the Aman transitional zone, and the paleo-reservoirs did not reach the cracking level during thermal evolution, so that the characteristics of liquid-rich reservoirs can remain. As a result, homogenization temperature data of a lot of inclusions below 50 °C were detected. However, the Middle-Late Caledonian paleo-reservoirs in the northern uplift area were destroyed, and in-situ oil generation rarely happened in the late stage. The paleo-reservoirs in depression area are the results of adjustment and re-accumulation during the Late Hercynian. Thus, inclusion homogenization temperatures of 70-90 °C were detected.

Based on comprehensive analysis, the fault-controlled oil and gas system of the strike-slip fault around Aman area was developed in the Middle and Late Caledonian period, and experienced multiple adjustments and reformations (Fig. 10).

At the end of the Middle Ordovician, the strike-slip fault system in circum-Aman area was formed ^[26], and with the fault-controlled karst occurred at the same time, forming the Middle Ordovician fault-controlled carbonate fracture-cavity reservoir. Under the rapid filling of very thick mudstone of the Sangtamu Formation in the Late Ordovician, not only a fault-controlled fracture-cavity trap was formed and well preserved, but also the Lower Cambrian source rock entered the peak hydrocarbon generation, with superior source-reservoir-cap-transport configuration.

Through the strike-slip faults, multiple sets of oil and gas reservoirs have been developed, such as the Lower Cambrian, the Upper Cambrian, the Lower Ordovician Penglaiba Formation and Yingshan Formation, the Middle Ordovician Yijianfang Formation and the Upper Ordovician Lianglitage Formation. A fault-controlled reservoir model related to strike-slip faults of “vertical migration and accumulation and compound accumulation” is formed (Fig. 10a). Since the hydrocarbon generation center is located in the Aman transition zone in the south, the Caledonian period has entered the peak period of oil generation, and the Late Hercynian period has entered the mature gas generation stage. Therefore, the continuous subsidence area in the southern depression only has the oil generation period in the Middle and Late Caledonian, and the northern paleouplift may also have the Late Hercynian oil generation period ^[33]. Based on basin simulation, it is found that a large amount of oil generated in the Middle and Late Caledonian may fully charge the fracture-cavity traps in the strike-slip fault zone in depression area.

From the Late Caledonian to the Early Hercynian, the Tarim Basin experienced strong tectonic transformation ^[22]. In the process of uplifting and erosion of paleo-uplift and strike-slip fault inheritance development, the oil and gas in the high part of the paleo-uplift was destroyed, and the paleo-reservoirs in the northern part of Halahatang- Lunnan areas were destroyed, but left asphalt. Obvious adjustment and reformation took place in the slope area between the uplift and the depression, and the fluid inclusions detected in this period represent reservoir adjustment. In the depression area, there is Upper Ordovician mudstone caprock with thickness of more than 1000 m, with gentle structure and possible rich oil resources (Fig. 10b).

In the Late Hercynian, with the overall subsidence of the Carboniferous-Permian, the Lower Cambrian source rocks in depression area entered the over-mature stage, while the slope area may continue to generate oil, which has a certain complement to the paleo-reservoir. The calcite precipitation in the fractures of the carbonate rocks in Well Q1 in northern Halahatang was dated at (288.6±8.8) Ma, indicating the existence of faulting and fluid activities in this period, which may induce fractures and hydrocarbon migration and accumulation. Previously, a large number of inclusion homogenization temperature data were used for statistical analysis, but due to the thermal effect of the burial period, the brine inclusion homogenization temperature data in a wide range of 80-120 °C were generated. Therefore, it was difficult to distinguish the fluid activities before or after the formation of the Permian igneous rock. This study found the homogenization temperature is 110-116 °C, which almost falls into the maximum subsidence period of the Permian (Fig. 8a), corresponding to the rapid subsidence period after the occurrence of the Early Permian igneous rock. The accumulation stage is well defined.

It is worth noting that previous studies have also detected fluid inclusions in the Permian, but they may not represent the accumulation period. Due to the finalization of the Lunnan paleouplift in the Late Hercynian, many reservoirs have been reformed and destroyed, and the detected inclusions may be the result of oil/gas adjustment and recharge ^[18], rather than the crude oil formed by the secondary hydrocarbon generation of source rocks. The Yuertusi Formation in the slope area of the northern paleo-uplift is thinned and pinched from about 30 m in thickness to the north. The potential of secondary oil generation is limited, but gas generation is strong. It is difficult to generate a huge amount of oil resource with geological reserve more than 10×10⁸ t in the Tahe-Lunnan area. On the other hand, due to the Late Hercynian tectonic movement and the revival of strike- slip faults, hydrocarbon may migrate upward through the faults from depression area and the deep paleo-reservoirs, thus forming Tahe Oilfield and Halahatang Oilfield (Fig. 10c). Therefore, the Late Hercynian inclusions detected in the northern Tarim uplift may be a response to the upward migration and re-accumulation of paleo-reservoirs from the depression and deep parts. At the end of the Late Hercynian and Indosinian-Yanshanian, multiple tectonic movements took place in northern Tarim uplift. The tectonic subsidence was slow and the hydrocarbon generation of source rocks was basically stagnant, but adjustment and reconstruction in the paleo-reservoirs never stopped ^{[14⇓⇓⇓-18]}.

The Late Himalayas entered the charging period of crude oil cracking gas and kerogen cracking gas, which has reached a consensus ^{[18,33 -34]}. It is worth noting that the gas charging in the missing part of the salt-gypsum layer of the Middle Cambrian in eastern Lungu-eastern Fuman Oilfield was strong, forming the condensate gas reservoir (Fig. 1a). However, the strike-slip faults in the western region have ceased activity, and the crude oil cracking gas may still be trapped under the salt-gypsum layer of the Middle Cambrian, and preserved to be a large number of paleo-reservoirs.

The study shows that the multi-stage structural-sedimentary evolution of the oil and gas system in circum- Aman area has formed multiple sets of reservoir-caprock assemblages and various trap types. The accumulation of traps connected by faults formed reservoirs in the Cambrian, Ordovician, Silurian, Devonian-Carboniferous, Triassic and Jurassic. The faulted interval controls the vertical migration of oil and gas (Fig. 10). In the depression area, vertical migration and accumulation was dominant, while in the paleo-uplift area, lateral migration and accumulation was strong along the faults or unconformity, forming the differential migration and accumulation mode of fault-controlled. It is a typical multi-stage hydrocarbon accumulation and adjustment system related to strike-slip faults, and causes the complexity of the oil and gas phase state. On this basis, the fault-controlled reservoir model of “early accumulation, vertical migration and accumulation, and sectional enrichment” in the depression area and the fault-related reservoir model of “multi-stage adjustment, multi-element control and local enrichment” in the uplift area are formed. A large number of oil and gas resources have been destroyed in the paleo-uplift area due to multi-stage accumulation and transformation, while the preservation conditions in the depression area are superior and with rich oil and gas, which has been confirmed by the exploration and development of Fuman oilfield and Shunbei oilfield ^[5,10].

To sum up, the source rocks in the Aman transition zone in the northern depression of the Tarim Basin are thick, early in accumulation, and well preserved, so the strike-slip fault zone in the depression is richer in oil. Following the exploration idea of “the deeper the depth, the richer the oil, and deep into the depression and faults”, more oil resources may be found in the deeper part of the depression area.

Fluid inclusions of paleo-reservoirs in northern Tarim Basin have undergone complex heating transformation, showing different homogenization temperature data. It is necessary to define the reservoir forming period by the lowest homogenization temperature of the brine inclusions in the oil-bearing FIAs. The FIAs in the calcite in the Ordovician carbonate reservoir of the strike-slip fault zone have two types of oil inclusions with different maturity degrees. One shows yellow fluorescence and the other shows yellow-green fluorescence. The homogenization temperature of their symbiotic brine inclusions is below 50 °C and 70-90 °C, respectively, which reveal the two oil charging stages (the Middle-Late Caledonian and the Late Hercynian). The Middle-Late Caledonian period is the key formation period of strike-slip faults, Ordovician carbonate fracture-cavity reservoirs and traps, and the main source rocks of the Lower Cambrian entered its peak oil generation period, which is the key accumulation period and lays the foundation of Paleozoic oil and gas resources in the northern Tarim Basin. The reservoirs in the Aman transition zone were mainly formed in the Late Ordovician, and Fuman oilfield has preserved the earliest original paleo-reservoirs. The northern uplift area is dominated by early Permian accumulation, and has secondary reservoirs migrating from the south in the Late Hercynian. There are great differences in the reservoirs in the depression area from the paleo-uplift area, and the preservation condition is the key to oil enrichment. The depression area has thick source rocks and excellent preservation conditions, so the related strike-slip fault-controlled oil reservoirs have a greater exploration and development potential.