Introduction

Due to the improvements in exploratory drilling techniques and promotions in the exploration activities, oil and gas in deep and ultra-deep strata have been a hot topic in both research and exploration and it will be the most important field for raising reserves in the future (Dyman et al. 2002; Bai and Cao 2014; Liu et al. 2018; Yao et al. 2018; Liu et al. 2020a). Significant breakthroughs and discovery of oil and gas resources have been made recently in the Ordovician formations characterized by a burial depth of more than 6300m of Tarim Basin, providing a new resource target for deep and ultra-deep oil and gas exploration of Western China (Zhu et al. 2018; Zhou et al. 2019; Qi 2016; Wang et al. 2014; Liu et al. 2020b; Yang et al. 2020).

The thermal history of source rocks is of vital importance to the evaluation of source rocks, which controls the timing of hydrocarbon generation and expulsion (Pang et al. 2012; Carminati et al. 2010; Hudson and Hanson 2010; Kosakowski and Krzywiec 2013). The hydrocarbon source rocks in the Tarim Basin are mainly found in the lower Cambrian and lower Ordovician strata characterized by a burial depth of more than 6500m (Li et al. 2015; Cai et al. 2015; Chang et al. 2014a, b; Zhang et al. 2005; Zhang et al. 2014; Wang et al. 2010). They have the characteristics of high abundance of organic matter (the total organic carbon (TOC) content is 0.5–3.36 wt%, average TOC content = 2.84 wt%; Sun et al. 2003) with the kerogen type of I–II, and the degree of thermal evolution is currently in the high-over mature stage (Zhu et al. 2016; Zheng et al. 2018; Xiong et al. 2015; Qi 2016; Yan et al. 2020; Wang et al. 2006). The black and dark-gray Cambrian-Ordovician mudstones are both relatively high-quality source rocks and they are the major source rocks for the Ordovician reservoirs (Mu 2009; Zhang et al. 2005), whereas there is no consensus on the contribution of these two sets of source rocks to the hydrocarbon in Ordovician reservoirs. The main reason for the dispute of the consistency of thermal history in the Tarim Basin has been difficult to reach. Some scholars believed that the geothermal gradient of Tarim Basin had shown a gradual decline since the early Paleozoic, with relatively higher values in the central and eastern regions (Qiu et al. 2011, 2012; Liu et al. 2019; Chang et al. 2014a, b), while others argued that due to the influence of magmatic activity, the peak geothermal gradient appeared in the Carboniferous-Permian of Tabei and Tazhong uplift (Li et al. 2010; Li et al. 2016; Yu et al. 2010; Xiang et al. 2013). Uncertainty about the thermal history and the maturity evolution of lower Paleozoic has restricted further oil and gas exploration in the Tarim Basin.

In this study, we combined a large number of new and previously published equivalent vitrinite reflectance (Requ), (U–Th)/He ages, and fission tracks to analyze the tectono-thermal evolution of different tectonic units in Tarim Basin, and modeled the thermal evolution of the Cambrian-Ordovician source rocks. In addition, based on the occurrence state of Ordovician oil and gas reservoirs, the main source rocks are defined from the perspective of thermal evolution of source rocks. This study has an important guiding significance for hydrocarbon exploration in the Tarim Basin.

Geological setting

The Tarim Basin is the largest hydrocarbon-bearing basin in the northwest of China (Fig. 1a) and has an area of 560×103 km2. The basin is bounded by the Tian Shan Mountains to the north, the Kulugetake Mountains to the northeast, the Kunlun Mountains to the southwest, and the Altun Mountains to the southeast (Jia 1997; Qiu et al. 2012; Tang et al. 2004) (Fig. 1b). The basin can be subdivided, based on basement morphology, major faults, structural deformation, and sediment distribution, into several structural zones, including five depressions (i.e., the Kuqa depression, the Awati sag, the Manjiaer sag, the Tanggu depression, the Southwestern depression belt), five uplift regions (the Tabei uplift belt, the Shuntuoguole low rise, the Tazhong uplift belt, the Bachu uplift belt, and Tadong uplift), and two slopes (the Kongquehe slope and the Maigati slope) (Ma et al. 2015; Jia and Wei 2002) (Fig. 1b).

Fig. 1
figure 1

(a) A simplified view of the outline of China, (b) Simplified distribution map of the tectonic units in Tarim Basin, (c) Paleozoic stratigraphy of the Tarim Basin (derived from exploration and production research institute of Northwest Oilfield Branch Company, SINOPEC)

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The Tarim Basin has experienced five stages of tectonic evolution: basement formation stage, the Cryogenian-Ediacaran strong extension-compression stage, Cambrian-Ordovician weak extension-strong compression stage, Silurian-Cretaceous fluctuation and migration stage, and Cenozoic weak extension-strong compression stage. The Tarim Basin was initially formed in the middle and late Ordovician, and ancient Kunlun Ocean was subducted beneath the Tarim block. The strong squeezing force from the SW direction resulted in the formation of the Tazhong uplift (Li et al. 2013; Ren et al. 2011, 2012). From Silurian to Devonian, after the Kunlun Caledonian collision orogeny was ended, the Paleo-Tethys Ocean was developed (Yang et al. 2005). Synchronously, intense folding orogeny occurred in the Arkin tectonic domain (Liu et al. 2007), and some parts of the top of the Silurian and Devonian units in the northern of the Tarim Basin were eroded and truncated, which resulted in the transformation of the tectonic framework from the west part being higher than the east part to the east part being higher than the west part (Ding et al. 2009).

Cambrian and Ordovician strata constitute a complete source-reservoir-caprock assemblage in the Tarim Basin (from bottom to top: Yuertusi Formation, Penglaiba Formation, Yingshan Formation, Yifangfang Formation, and Sangtamu Formation) (Fig. 1c). Yuertusi Formation of the Lower Cambrian is widely developed in Tazhong uplift, which is mainly deposited within black mudrock. The Upper Cambrian-Lower Ordovician source rock is only distributed in the Mandong sag. The source rocks of the lower Cambrian are sapropelic, approximately 150–350m in thickness, and with total organic carbon (TOC) content of more than 1% (Wu et al. 2016; Chen et al. 2018), and the Upper Cambrian-Lower Ordovician source rock is also sapropelic with a TOC of 2.84% and an average thickness of 110m (Bao et al. 2018; Yu and Zhou 2005). Yijianfang and Yingshan Formations are mainly composed of mudstone and limestone, and the cracks and dissolved pores are relatively developed, which are good reservoirs. The thick mudstone layers of the Upper Ordovician Santamu Formation are stably distributed in the area and are good area caps.

Methods and samples

Methods

To enhance the accuracy of thermal history, the equivalent vitrinite reflectance (Requ), (U–Th)/He ages of apatite and zircon, and fission tracks of apatite and zircon were combined to simulate the thermal history of the Tarim Basin. Different models were used for different paleo-thermal indicators, the Easy %Ro model for Requ (Sweeney and Burnham 1990), the multi-component dynamic model for AFT ages (Ketcham et al. 2007), the parallel curvilinear model for ZFT ages (Yamada et al. 2007), the Farley model for AHe (Farley 2000), and the Reiners model for ZHe (Reiners et al. 2004). Our database contains 258 sets of vitrinite reflectance (Ro), 66 sets of (U–Th)/He ages for apatite and zircon (Table 1), and 52 sets of apatite fission tracks (Table 2), and these samples are distributed in Tabei uplift, Tazhong uplift, Shuntuoguole low uplift, Tanggu depression, Kongquehe slope, Tadong uplift, Bachu uplift, Awati sag, which have well representative and relatively scattered locations covering most area of the Tarim Basin.

Table 1 The tested (U–Th)/He ages of apatite and zircon samples in Tarim Basin
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Table 2 Geological information and results of apatite fission track from the Tarim Basin
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(U–Th)/He ages

The (U–Th)/He analyses of apatite and zircon were conducted at the School of Earth Science in University of Melbourne. The (U–Th)/He analyses were conducted following the procedures described by Gleadow et al. (2015). The extraction and measurement of helium (He) from a single crystal was followed by the measurement of U and Th by isotope dilution, Parr bomb dissolution, and high-resolution inductive-coupled plasma mass spectrometry (HR-ICPMS). The helium extraction involves laser heating of crystals inside Nb foil packets to a temperature of approximately 1300 °C for two to four 10-min extraction. Zircon He ages were calculated and corrected for α-emission following the approach of Farley et al. (1996). A total of nine zircon samples were tested to obtain (U–Th)/He ages. Three replicate analyses per sample were tested for all samples. The other (U–Th)/He ages for apatite and zircon samples were obtained from previous studies (Qiu et al. 2011, 2012).

Vitrinite reflectance

We compiled 258 vitrinite reflectance (Ro) or Requ values from 20 wells in the Tarim Basin. Vitrinite reflectance (Ro), bitumen reflectance (Rb), and vitrinitelike reflectance (Rv) of eight wells in Shuntuoguole low uplift were measured from dark mudstone at the China University of Petroleum, Beijing. Due to the lack of vitrinite originating from higher plants in the Lower Paleozoic, Requ data are converted from Rb or Rv, and Requ was used to study the maturity of lower Paleozoic. There are several formulae for obtaining Requ from Rb, which corresponds to different maturity stages (Jacbo 1989; Liu and Shi 1994; Petersen et al. 2009; Qiu et al. 2012; Xiao et al. 2000). The formulae for converting Requ from bitumen reflectance and vitrinite-like maceral reflectance in this study are used by Xiao et al. (2000). The other Requ data for Tabei uplift, Tazhong uplift, Tang-gu depression, Bazhu uplft, and Tadong uplift were obtained from previous studies (Wang et al. 2010; Liu et al. 2016; Li et al. 2016; Qiu et al. 2012).

Results

Analytical results of paleo-thermal indicators

Equivalent vitrinite reflectance (Requ)

The Requ value is controlled by the heating temperature and heating time of the organic matter, and is mainly affected by temperature (Buchardt and Lewan 1990; Burnham and Sweeney 1989). In the tectonically stable state, Ro and depth show a linear relationship (Barker et al. 1986; Barker and Goldstein 1990; Sweeney and Burnham 1990), whereas in the case of strata erosion and unsteady high heat source intrusion, it will lead to the offset of the Requ-depth profile. In the Tarim Basin, the equivalent vitrinite reflectance profiles (%Requ) from 20 wells all show increasing trends with depth (Fig. 2). The Requ values of Tabei uplift (Fig. 2a–c), Shuntuoguole low uplift (Fig. 2d–i), Bachu uplift (Fig. 2p–q), and Tanggu depression (Fig. 2s–t) are discontinuous in the Permian and Pre-Permian, and there are many sporadic extremely high Ro anomalies near the unconformity surface, recording obvious tectonic heat. The event revealed that the end of the Carboniferous strata experienced a higher paleotemperature (Li et al. 2010; Zhu et al. 2016a; Zhu et al. 2016b).

Fig. 2
figure 2

Requ versus depth of typical wells in Tarim Basin. (a)–(c): The wells of SL1, LN2, and T401 represent the Tabei uplift, and these data are from Liu et al. (2016); (d)–(k): The wells of SHB5, SHB2, SHB1, ST1, SX1, SN3, SN4, and SN501 represent the Shuntuoguole low uplift, and these data are from this study; (l)–(n): The wells of Z1, Z13, and TZ1 represent the Tazhong uplift, and these data are from this study and Wang et al. (2010); (o): The well of TD1 represent the Tadong uplift, and these data are from Qiu et al. (2012); (p)–(q):The wells of H4 and HT1 represent the Bachu uplift, and these data are from Li et al. (2016); (s)–(t): The wells of TC1 and TB2 represent the Tanggu depression, and these data are from Li et al. (2016)

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(U–Th)/He ages for apatite and zircon

Zircon and apatite (U–Th)/He results are shown in Table 1. The ZHe ages of nine zircon grains of well SHB5, ST1, and AD1 are from this study, and ZHe ages range from 71.1±4.4 to 717.4±44.5 Ma, but some samples had a large variation among the single-grain ZHe ages. Previous studies believed that the main factors affecting the variation of He ages in the intra-sample included the crystal size (Reiners and Farley 2001), effective Uranium (Farley et al. 1996), and radiation damage (Shuster et al. 2006; Flowers et al. 2007) (Fig. 3). The grain size directly affects the diffusion loss of 4He. The larger the zircon grain size, the more 4He concentration will be preserved. However, some grains with higher He ages have smaller grain radius (such as AD1-4c and SHB5-4c), and these abnormal particles should be excluded for thermal history recovery. In addition, excess He is another key factor in the variation of He ages, in grains ST1-6a and SHB5-4c, and the 4He value is extremely high whereas the eU is relatively low, suggesting there was significant “parentless” 4He in these grains, and thus they also should be excluded. After removing the outliers, suitable samples were selected for analysis. The age of zircon He gradually decreases with the increase of stratum age except for SHB5-2a, and the zircon He ages are slightly smaller than the age of stratum (Fig. 4a–c). These findings suggest that these zircon samples are partially reset, and the highest temperature experienced entering a partial retention zone of zircon He age. In the well of KQ1, the apatite He ages are between 13.84 and 56.11 Ma, and these He ages are much younger than their stratigraphic ages, which implies that these apatite samples of Silurian and Ordovician strata are almost entirely reset. The zircon (U–Th)/He ages decrease with the depth of samples, and the Zhe ages of the Ordovician samples are slightly younger than their depositional ages, which implies that the highest temperature experienced by the Ordovician strata enters a partial retention zone of zircon He age (Fig. 4d). In the wells of T1, HT1, Z2, and Z11, the apatite and zircon He ages are also younger than the stratigraphic ages, indicating heating to high temperatures (Fig. 4e–f). Therefore, the maximum temperature experienced by the sample can be effectively constrained in the thermal history simulation.

Fig. 3
figure 3

(a) Relationship between zircon He age and the grain radius; (b) Relationship between zircon He age and eU concentration; (c) Relationship between zircon He age and the 4He concentration

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Fig. 4
figure 4

Relationship between (U–Th)/He ages of apatite and zircon and burial depth for typical wells in the Tarim Basin (dashed line indicates the stratigraphic age, the data of SHB5, ST1, and AD1 wells are from this study, and the data of KQ1, T1, HT1, Z2, and Z11 wells are from Qiu et al. (2012))

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Fission tracks for apatite and zircon

Fifty-two fission-track (FT) ages have been previously determined from the Kongquehe slope, Bachu Uplift, Tanggu depression, and Tazhong uplift (Table 2; Fig. 5). In general, the AFT central ages in the Tarim Basin are variable, ranging from 19.0 ± 2.0 Ma (HT1) and 25.0 ± 2.0 Ma (TZ6) to 232.7 ± 16.0 Ma (TZ45). Except for these three samples (T1, TZ45, TC1), the other AFT ages or age pattern are considerably younger than depositional or stratigraphic ages, indicating that the apatites in the detrital samples have undergone significant post depositional annealing and resetting. In Kongquehe slope, the AFT data sets all show that the AFTs have partially annealed. The AFT ages decrease from 164 Ma for the Upper Silurian sample to 55 Ma for the Ordovician sample. In the Bachu uplift, Tanggu depression, and Tazhong uplift, the AFT ages also decrease with increasing depth, and their AFT ages are younger than the corresponding stratigraphic ages of Paleozoic–Mesozoic strata, which suggest that these samples had partially annealed.

Fig. 5
figure 5

Relationships between AFT ages, lengths, and strata of different tectonic units in the Tarim Basin. (These data are from Qiu et al. (2010); Xiang et al. (2013); Chang et al. (2014a, b); Qiu et al. (2012) and Xiao et al. (2011))

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The mean track lengths of these samples vary from 8.8 μm (TZ45) to 12.6μm (KQ1) with corresponding standard deviation of 0.9 to 2.9 μm. Specifically, the values of mean track lengths in the Tazhong uplift (from 9.4 μm to 12.0 μm) are all less than 12.0 μm, shorter than those of the Kongquehe slope (from 10.0 μm to 12.6 μm) and Bachu uplift (from 10.0 μm to 12.1 μm). These track lengths are evidently smaller than the standard of unannealed mean track lengths of 16.3 ± 0.9 μm (Gleadow et al. 1986). The track length distribution (Fig. 5) of most samples exhibit the signature of a typical “mixed” distribution pattern (Gleadow et al. 1986). From the “mixed” type of length distribution illustrated above, we can conclude that samples were probably affected by a multistage thermal history, in which some tracks have been thermally shortened while others have been generated by subsequent cooling (Gleadow et al. 1986).

Model results

Burial history reconstruction

Burial-history reconstruction is the basis for the reconstruction of thermal history. To construct burial histories, thickness, depths, and average lithology data in the wells referred to the well completion reports provided by previous studies. Erosion events and the according eroded thickness are site-specific, and were thus considered separately (Chen et al. 2014; Sun 2017; Wang et al. 2003, 2010; Qiu et al. 2010, 2011, 2012; Li et al. 2016). The burial histories show that in the Tabei and Tazhong uplift, these areas are almost continuously subsiding since the Cambrian period, and the current stratum depth is the largest depth. However, the Tadong uplift and Bachu uplift underwent a large scale of uplift. The Hercynian and Indosinian orogenies were dominant factors in the tectonic transformation of the eastern Tarim Basin. The Yanshanian and Himalayan orogenies resulted in rapid uplift and sedimentation in the Tarim Basin during the Mesozoic–Cenozoic (Fig. 6).

Fig. 6
figure 6

Burial history of different tectonic units in the Tarim Basin. (a)–(b): The wells of LN2 and T401 are modified after Chen et al. (2014) and Sun (2017); (c)–(h): The wells of SHB5, SHB2, ST1, SN3, and SN4 are from this study; (i)–(k): The wells of Z2, Z11, and TZ1 are modified after Wang et al. (2010); (l): The well of TD1 is modified after Wang et al. (2003); (m): The well of KQ1 is modified after Qiu et al. (2012); (n)–(r): The wells of MC1, F1, T1, HT1, and H4 are modified after Qiu et al. (2011) and Qiu et al. (2010); (s)–(t): The wells of TC1 and TB2 are modified after Li et al. (2016)

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Thermal history modeling

In order to better restore the thermal history of the Tarim Basin, based on the analysis of the characteristics of the paleo-thermal indicators, (U–Th)/He ages, FT ages, and equivalent vitrinite reflectance (Requ) were combined and used to reconstruct the thermal history for Tarim Basin. The software HeFTy (version 1.9.1) was applied to model the thermal histories. In our modeling, 5000–10,000 thermal paths were developed using the Monte Carlo inverse-modeling method, and the best-fit time-Temperature (t-T) path model indicated the thermal history of the sample. The thermal history of six typical drilling samples was modeled from different tectonic units (Fig. 7). In Shuntuoguole low uplift, the modeling results of the samples SHB5-4 and ST1-7 show that this area gradually warmed up during the Late Sinian to Devonian phase reaching approximately 80–90 °C, then the temperature began to rise rapidly and the peak paleo-temperature appeared in the Early Permian, reaching 160–170°C. The formation temperature then rapidly cooled and returned to normal thermal conditions during the Middle Permian. After the Early Permian, the formation temperature slowly increased and the current temperature has reached 120–140 °C (Fig. 7a). In Bachu uplift and Kongquehe slope, the thermal models show that these samples underwent first an overall heating stage after their deposition until the Devonian period, then followed by two major cooling episodes which could be recognized from the t-T paths. These two cooling events are closely related to the tectonic activities of Hercynian and Yanshanian in the Tarim Basin (Fig. 7b and Fig. 7c). In Awati sag, the thermal models showed a very similar thermal history with Shuntuoguole area. During the Late Permian, the samples reached a maximum paleotemperature of approximately 140–160 °C. Then it began to cool down rapidly and returned to normal thermal state during the middle-late Permian (Fig. 7d).

Fig. 7
figure 7

Thermal history (t-T) histories of the Tarim Basin samples based on AFT ages and length data, Requ data, and (U–Th)/He ages using HeFty program. (a) Shuntuoguole low uplift, including SHB5, and ST1. (b) Bachu uplift, including T1 and H4. (c) Kongquehe slope, including KQ1 well. (d) Awati sag, including AD1 well. The results in the t-T curves show three reliability paths, that is, green = acceptable fit, magenta=good fit, and black = best fit

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Using the thermal conductivity (K) (Liu et al. 2020a), the general paleo heat flow (q) history of the deep boreholes in the Tarim Basin is shown (q=−K• Gt) in Fig. 8. The value of heat flow in the Tarim Basin has almost continuously decreased during the entire geologic history. While the heat flow evolved differently in the different structural zones. Higher heat flow exists in the Tadong and Tazhong uplift than in the Bachu uplift, Tabei uplift, and Shuntuoguole low uplift (Fig. 8). In general, there are three phases of heat flow evolution in the Tarim Basin: (1) The gradual cooling stage during Ordovician-Carboniferous, the heat flow is gradually decreasing during this period; (2) Rapidly raised stage in the early Permian period, the heat flow in Tabei and Tazhong uplift are raised rapidly during this period and the maximum is 51~62mW/m2; (3) The decline stage since Middle Permian. The tectonic activity is relatively stable and the geothermal heat flow is gradually reduced during this period.

Fig. 8
figure 8

Heat flow histories of different structural zones in the Tarim Basin

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The maturity of the source rocks

The maturation histories of the source rocks control the timing of hydrocarbon generation and expulsion (Pang et al. 2012; Carminati et al. 2010; Kosakowski and Krzywiec 2013; Hudson and Hanson 2010). The maturity histories of the Cambrian–Ordovician source rocks were calculated based on the newly obtained Requ data and ZHe data of Shuntuoguole low uplift, and the maturities of the source rock for the other wells were taken from Qiu et al. (2012). The division of mature evolution stages of source rocks is an indispensable reference premise for the research of maturity history, and the value of Requ was used to indicate the stage of the maturity history classified as follows: Immaturity stage (Requ < 0.5%); Early maturation stage, (0.5% < Requ < 0.7%); Middle maturation stage, (0.7% < Requ < 1%); Late maturation stage, (1% < Requ < 1.3%); Main-gas stage, (1.3% < Requ < 2%).

The modeling results show (Fig. 9) that the maturation of the source rocks in different areas underwent different maturation evolutions. In the well of SHB2, SHB5, and ST1, the Lower Cambrian and Lower Ordovician source rocks entered the Early maturation stage (0.5%<Ro<0.7%) and began to form hydrocarbons since the Caledonian period. At the end of the Early Permian, under the abnormal heat flow of magmatic activity, the maturity of the source rock was rapidly entered the main-gas stage (1.3% <Ro < 1.5%) (Fig.9a–b). In the well of SN3, SN501, and SN2, the maturity of the Lower Cambrian and Lower Ordovician source rocks reached the hydrocarbon generation threshold in the middle Caledonian period. During the Hercynian–Yanshan–Hishan period, the maturity of the Lower Cambrian and Lower Ordovician source rocks gradually increased. The Lower Cambrian source rocks reached the stage of dry gas generation (>2%), and the Lower Ordovician source rocks reached the high maturity stage (1.3% < Requ < 2%) (Fig. 9c–e). According to the occurrence state of hydrocarbons and the maturity evolution of Lower Ordovician and Lower Cambrian source rocks, it can be seen that the degree of the lower Cambrian source rocks is well matched with the distribution of oil and gas reservoir types in Shuntuogule low uplift. Therefore, only from the perspective of source rock evolution, the main source rock in the Shuntoguole area of Tarim Basin may be the Lower Cambrian source rock.

Fig. 9
figure 9

Thermal evolution of source rocks for the typical drilling source rocks in the Tarim Basin

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According to the thermal evolution results of the source rocks recovered by single wells, the Kriging interpolation method was used to simulate the maturity evolution plane distribution of the bottom interface of the Lower Cambrian source rock. The maturation modeling results indicated that by the end of the Ordovician, the bottom of Cambrian entered the generation threshold (0.5% Ro). Mainly, early mature source rock existed in the Bachu uplift and Tabei uplift, and the middle and late mature source rocks were distributed in the Northern uplift, the Tanggu depression, Tazhong uplift, and the Kongquehe slope. The source rock reached the main gas stage in the eastern region of the basin, and it was at the dry gas stage in the Manjiaer depression (Fig. 10a). By the end of the Permian, the source rock intervals which had reached the main gas stages covered most of the basin, except the Tabei uplift, Bachu uplift, and the southern part of the Southwest depression, where the source rocks were still within the oil window (Requ < 1.3%) (Fig. 10b). By the end of the Mesozoic, the dry gas stage was mainly distributed in the south of Shuntuoguole low uplift, Manjiaer depression, Tadong uplift, and Kongquehe slope (Fig. 10c). Currently, the maturity mostly maintained the characteristics of the end of the Mesozoic (Fig. 10d).

Fig. 10
figure 10

Maturation level of the bottom interface of the Lower Cambrian source rock. The contour interval is 0.5%. Requ = equivalent vitrinite reflectance (modified after Qiu et al. 2012; Yang et al. 2020)

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Discussion

The limitations of the paleo-temperature indicators

Although the various paleo-temperature indicators have their own advantages, they also have some limitations in the restoration of thermal history. Vitrinite reflectance is the most commonly used paleo-temperature method for thermal history research, and it can be used to objectively characterize the organic matter maturity of source rocks since the Late Paleozoic. However, for the early Paleozoic marine strata, due to the lack of vitrinites from higher plants, some other organic matter maturity indicators (asphalt reflectance and vitrinite-like reflectance) are usually converted into equivalent vitrinite reflectance in order to recover the thermal history. However, the organic matter paleo-temperature indicators as the highest paleo-thermometer, which can only record the highest paleo-temperature experienced in the geological history period, the paleo-temperature information recorded before this period will be erased, and the subsequent lower temperature information cannot be recorded.

The apparent age of low-temperature thermochronology has no direct geological significance in many cases. Only when the sample is continuously cooled in a linear manner can its apparent age be interpreted as the time when the sample passes its sealing temperature. Therefore, when analyzing the low-temperature thermochronological samples, it is necessary to combine other information to constrain their ages. In addition, the pre-processing of low-temperature thermochronology samples is more complicated and cumbersome. Laser etching-ICPMS will become a mainstream method of apparent age testing and analysis. The development of other paleo-temperature scales, such as the concentration of organic matter free radicals and rock thermoacoustic emission lacks effective quantitative models to model the thermal histories of the sedimentary basins. Therefore, a single paleo-temperature scale method may lead to uncertainty and diversification of the thermal history recovery results. Multiple ancient temperature scale methods (organic matter paleo-temperature, low-temperature thermochronological ancient temperature, etc.) should be combined with each other to take advantage of each other’s advantages.

Relationship between the thermal history and tectonic evolution

Generally, thermal evolution is controlled by the tectonic setting. The Tarim Basin was a stable craton basin during the Paleozoic (Jia and Wei 2002), and the stretching and thinning of the crust resulted in a higher geothermal gradient at that time. During the Permian, the heat flow began to increase and during the Middle-Late Permian, reached a peak value in Tabei and Tazhong uplift. Li et al. (2016) indicated that the heat flow peak in the Tarim Basin was mainly controlled by the magmatic activity which created a thermal pulse. We calculated the maximum geothermal gradients for each well at the time of peak burial temperature. The results show that the maximum geothermal gradients have a range of 26.75°C/km to 46.28°C/km and exhibit a southward decreasing trend. Such high calculated geothermal gradients clearly indicating a thermal pulse which probably was related to the magmatic activity as suggested by some researchers (Li et al. 2010; Li et al. 2016). The well log section of the Tabei and Tazhong area also shows that magmatic rocks are generally developed in the Permian, and the thickness of the magmatic rocks in the northern area is thicker than that in the southern area (Liu et al. 2020b). A mantle plume was assigned to be origin of Large Igneous Province in Tarim basin (Yang et al. 2013; Li et al. 2014; Xu et al. 2014), is generally characterized by a wide lateral expanse, can vertically affect the geothermal regime over a large area (Gouveia et al. 2018), and thus could be regarded as a plausible heating source. The nearby Sichuan basin, which has similar tectonic history with Tarim basin, is well-recognized for Permian mantle plume activity and is a good example for comparison. Geothermal studies in Sichuan basin have revealed a high thermal anomaly induced by mantle plume and generate geothermal gradients up to > 40°C/km, with gradients the highest in the center of mantle plume and decrease away from the center (Zhu et al. 2016a, b; Jiang et al. 2018). For the Tarim basin, the calculated geothermal gradients closely resemble those in Sichuan basin influenced by mantle plume. And the spatial pattern of peak burial temperature and geothermal gradients in this study corresponds to the distribution of Large Igneous Province (LIP). These pieces of evidence all support the hypothesis of a thermal pulse resulting from mantle plume activity in the Tarim basin. The spatial variation of calculated geothermal gradients would also illustrate that the center of the mantle plume to be near the Tabei area, consistent with previous research (Li et al. 2014), and the lateral extent of thermal influence exerted by mantle plume could be in the Tazhong uplift.

Through sedimentary stratigraphic studies, it is determined that the crust uplifted rapidly before the magmatic activity. The strong uplift occurred at ~300Ma, which caused the unconformity between the Carboniferous and Permian. Large-scale magmatic activity is between 300 and 288 Ma (Xu et al. 2014; Li et al. 2014). The heat flow began to rise rapidly, and the degree of thermal exhibits a southward decreasing trend. The degree of stratum denudation during this period also tended to gradually decrease from north to south. Therefore, the abnormally high paleo-temperature in the Permian period may be caused by magmatic activity and structural uplift. After 290 Ma, most of the heat generated by the mantle plume disappeared, and the geothermal gradient began to rapidly decrease and returned to a normal thermal state. The subsequent geothermal gradient was mainly controlled by the depth of the stratum (Fig. 11).

Fig. 11
figure 11

The evolution of mantle plume in the Tarim Basin from 310 to 290Ma and the geothermal gradient distribution during the Permian period

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Conclusions

  1. (1)

    The thermal history of the Tarim Basin was reconstructed integrating multiple thermal indicators as follows: He ages, Fission tracks, and Requ data. The modeled results showed that the Tarim Basin underwent a gradual cooling stage during Ordovician-Carboniferous. During the early Permian period, the heat flow rapidly increased and reached a peak value (51–62 mW/m2) in Tabei and Tazhong uplift, and then decreased since Middle Permian. After the Middle Permian, the tectonic activity was relatively stable and the geothermal heat flow was gradually reduced during this period.

  2. (2)

    The maturity histories of the source rocks, modeled based on the new thermal histories, indicate that the maturation of the source rocks in different areas underwent different maturation evolutions. The Cambrian source rocks experienced rapid burial and heating during the Caledonian period, entered the hydrocarbon generation stage during the end of Ordovician, and at the end of Mesozoic, the Cambrian source rocks reached the dry gas stage in the south of Shuntuoguole low uplift, Manjiaer depression, Tadong uplift, and Kongquehe slope. Currently, the maturity mostly maintained the characteristics of the end of the Mesozoic.