Paleo-heat flow evolution of the Tabei Uplift in Tarim Basin, northwest China

Journal of Asian Earth Sciences 37 (2010) 52–66

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Paleo-heat flow evolution of the Tabei Uplift in Tarim Basin, northwest China Meijun Li a,*, Tieguan Wang a, Jianfa Chen a, Faqi He b, Lu Yun c, Sadik Akbar c, Weibiao Zhang d a

State Key Laboratory of Petroleum Resources and Prospecting, Faculty of Natural Resources and Information Technology, China University of Petroleum, Beijing 102249, PR China Exploration and Production Department, SINOPEC, Beijing 100029, PR China c Northwest Company, SINOPEC, Urumqi 830011, PR China d Exploration Research Center, Exploration and Production Research Institute, SINOPEC, Beijing 100083, PR China b

a r t i c l e

i n f o

Article history: Received 8 December 2008 Received in revised form 5 July 2009 Accepted 10 July 2009

Keywords: Paleo-heat flow Thermal regime Vitrinite reflectance Basin modeling Tarim Basin

a b s t r a c t The paleo-heat flow evolution of the Tabei Uplift in the Tarim Basin is investigated based on burial and thermal history reconstruction of 14 wells and using basin modeling. Numerous geological parameters, such as, temperature data and missing sediment thickness by erosion were used in the modeling. The basin model was calibrated using 460 measured vitrinite reflectance (%VRo) and vitrinite-like maceral reflectance (%VLMRo) values to constrain the validity of the maturity model. The heat flow history of the Tabei Uplift, Tarim Basin shows the following characteristics: (1) the highest paleo-heat flow was predicted to have occurred in the Early Ordovician as 65 ± 5 mW/m2, and gradually decreased to 55 ± 5 mW/ m2 during the Late Carboniferous; (2) a thermal kick was modeled to have occurred in the Permian as suggested by an abrupt rise in the heat flow; (3) the heat flow gradually decreased since the Triassic; (4) the present day heat flow was predicted to be as low as 38 mW/m2. This heat flow history honors the geologic and tectonic evolution history of the Tabei Uplift and is suggested as the best case heat flow model. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction The Tarim Basin is one of the most petroliferous basins of China. The Tahe Oilfield is situated in the southern slope of the Ackule High within the North Yarim Uplift (Fig. 1) (Kang and Kang, 1996; Zhang, 1999, 2003; Zhou et al., 2001; Li and Xu, 2004; Wang et al., 2008). The Tarim Basin is considered to be a typical ‘‘cool” basin (Wang et al., 1995a, 2000). Zhang and Liu (1992) studied the thermal gradient and heat flow of the basin based on geo-temperatures and spectral logs and suggested an average gradient of 2.0 °C/100 m. Wei (1992), Wang et al. (1995b,c, 1999) and Liu et al. (2004) studied the heat flow of the Tabei Uplift (the northern uplift of the Tarim Basin), Tazhong areas and other main structure units on the basis of geothermal gradient and measured thermal conductivities of rock samples. Xie and Zhou (2002) calculated the Cambrian–Ordovician paleo-temperature using pyrolysis kinetics simulation experiments on samples from the Tacan1 well. Qiu and Wang (1998), Qiu et al. (2006) estimated that the geothermal gradient in TZ12 well ranges from 2.4 to 3.0 °C/100 m by using free radical concentrations of kerogen determined by Electron Spin Resonance (ESR) spectrometry. However, the previous studies were mainly based on temperature data from measurements of

rock thermal conductivities of limited samples. These studies were primarily focused on present day thermal regime, gradients and heat flow, mainly in the Tazhong, Luntai and Yarim areas of the Tabei Uplift. However, systematic research on the heat flow history of the Tabei Uplift, especially in the Tahe Oilfield area, has not been conducted. The link between the evolution history and the paleo-heat flow of the basin was established from the Sinian to the present day by some authors, who identified four thermal episodes in the Tarim Basin (Pan et al., 1996; Li et al., 2000, 2005). In this study, the evolution of the heat flow history in the Tabei Uplift was evaluated, using 1-D basin modeling of 14 wells. The primary input data include the paleo-surface temperature, paleo-sea-level and depth, unconformities, missing sediment thickness during the erosion periods, and geological properties of sediments. A total of 460 vitrinite reflectance (%VR) and vitrinite-like maceral reflectance (%VLMRo) values measured from 14 wells were used as maturity indicators to calibrate the model. The study provides a thermal approach to the tectonic behavior of the Tabei Uplift and helps to establish a link between the tectonic events and the heat flow history. 2. Regional geological setting

* Corresponding author. Address: Organic Geochemistry Lab, China University of Petroleum, Beijing 102249, PR China. Tel.: +86 10 89731709; fax: +86 10 89731109. E-mail addresses: [email protected], [email protected] (M. Li). 1367-9120/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2009.07.007

The Tarim Basin, located in the southern Xinjiang Uygur Autonomous Region, northwest China, is one of the world’s largest

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Fig. 1. Map showing the locations of the modeled wells in the Tabei Uplift, Tarim Basin, and major tectonic terrains.

frontier basins, with an area of 560,000 km2 (Fig. 1). Detailed geological characteristics of the entire Tarim Basin have been summarized by many authors (e.g. Li et al., 1996; Jia and Wei, 2002; Zhang and Huang, 2005). It is a Palaeozoic cratonic basin, overlain in the south and north by Mesozoic–Cenozoic foreland depressions (Li et al., 1996). Fluctuating crustal activity has resulted in multiple unconformities (Jia and Wei, 2002; Zhang and Huang, 2005), causing the basin to form several tectono-stratigraphic entities (Fig. 1). The Tabei Uplift is located in the northern part of the Tarim Basin (Fig. 1). The Tahe Oilfield lies in the southern part of the Tabei Uplift and is bounded by the Manjiaer and Awati depressions to the south (Fig. 1). A total of 6500–9500 m thick sedimentary sequence, comprising Cambrian to Tertiary strata, rests on the Archaean and Proterozoic crystalline basement (Zhang, 1999; Jin et al., 2008; Wang et al., 2008). The Cambrian stratigraphic sequence consists mainly of tidal, platform and platform-margin marls, mudstones and carbonates/ evaporites. The overlying Lower Ordovician section is primarily composed of platform dolomite and argillaceous limestone,

whereas the Middle–Upper Ordovician interval is composed of platform and marginal slope-shelf carbonate sediments (Kang and Kang, 1996; Jin et al., 2008). The Middle–Upper Ordovician sequence can be subdivided into four formations, including the Yijianfang (O2yj), Qiaerbake (O3q), Lianglitage (O3l) and Sangtamu (O3s) formations from bottom to top. The O2yj formation consists mainly of wackestone, bioclast packstone and intraclast packstone. The O3q and O3l formations are composed of micrite limestone with thin clay interlayers. The O3s formation is comprised of siliciclastic mudstone and sandstone. The black/dark-gray Ordovician mudstones and mud-rich limestones, particularly in the Manjiar Sag, are the major source rocks for the Ordovician reservoirs (Graham et al., 1990; Zhang and Huang, 2005; Wang et al., 2008). The Sinian to Quaternary sequence is well preserved in the cratonic region, reaching up to 14 km in thickness in the Manjiaer Depression (Huang et al., 1999). The stratigraphy (Fig. 2) of the Tarim Basin consists of several marine, continental and transitional deposits. The Palaeozoic strata were deposited almost entirely in

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Fig. 2. Generalized stratigraphic column of the Tabei Uplift, Tarim Basin showing the penetrated thicknesses in each modeled well, chronostratigraphy, and missing sediment thickness for each erosion event. (F: formation of model layer; D: stratigraphic intervals that have been completely eroded away; E: erosional event; H: hiatus).

marine settings (Li et al., 1996; Jia and Wei, 2002). The Cambrian– Lower Ordovician section, with approximately 3 km of proximate thickness, is comprised of shallow marine to lagoonal carbonates,

while the Middle–Upper Ordovician sequence is associated with a marine transgression event. The Upper Paleozoic marine and continental transitional sediments were accumulated following the

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deposition of the Silurian and Devonian fine-grained red beds and sandstones. Since the Late Permian, a maximum of 6 km fluviolacustrine sediments have been accumulated in the Mesozoic– Cenozoic.

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3. Data used in the modeling

et al., 2003; personal communication). Lithologies, assigned to the model layers, are based on the well log interpretation (e.g. composite well logs) and comprise a mixture of six major lithologies (sandstone, siltstone, shale, limestone, dolomite and igneous). The lithologies for the eroded sections were estimated using analogies from the preserved sections.

3.1. Data inventory

3.3. Vitrinite reflectance and vitrinite-like maceral reflectance data

Fourteen wells from various exploration blocks of the Tahe Oilfield were selected for 1-D thermal modeling using BasinMod 1-D version 5.4 (Platte River Associates Inc., 2003). Stratigraphic data (e.g. well markers) were obtained from the well completion reports of the Exploration and Production Research Institute of the Northwest Oilfield Company, SINOPEC. The bottom hole temperature (BHT) and temperatures obtained during well tests (DST) were used for calibration. Default parameters in the BasinMod 1-D were used for the initial porosity, matrix density, thermal conductivity and heat capacity. Mechanic compaction, coupled with the Falvey and Middleton reciprocal porosity-depth relationship was used to model the burial history.

Vitrinite reflectance (%VRo) is the most widely used maturity indicator in geohistory modeling calibration, despite uncertainties arising from the sample quality and measurement method. In this study, a total of 340 core and cutting samples were collected from 14 wells for vitrinite reflectance measurement and were used to construct a geologically sound heat flow history. The %VRo values were measured on polished rock blocks using a Leica Model MPV-SP microscopic photometer (ASTM, 1994). Due to the absence of true vitrinite originating from higher plants and the relatively high level of maturity of the organic matters of lower Palaeozoic sediments in the Tarim Basin, many conventional maturity parameters are not suitable for calibration. Buchardt and Lewan (1990) reported the occurrence of vitrinitelike macerals in Cambrian–Ordovician Alum Shale, Southern Scandinavia, and used the reflectance of vitrinite-like macerals as a thermal maturity index. Vitrinite-like macerals are also of widespread occurrence in the Lower Palaeozoic rocks of the Tarim Basin (Wang et al., 1995a; Cheng and Fang, 1997; Xiao et al., 2000). Although the origin of vitrinite-like macerals in the Tarim Basin is still a matter for further investigation, the relationship between vitrinite-like maceral reflectance (%VLMRo) and vitrinite reflectance (% VRo) has been established to assess the thermal maturity of

3.2. Chronostratigraphy Formation thickness was obtained from well completion reports and deposition ages were obtained from the SINOPEC (Zhang

Fig. 3. Temperature profiles from well-logging in the Tabei Uplift, Tarim Basin.

Fig. 4. Temperature data from oil-testing in the Tabei Uplift, Tarim Basin.

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Fig. 5. %Ro (including measured %VRo and estimated %VRo from measured %VLMRo) profiles in S47, T204, T401, S60, S109 and TP7 wells.

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Table 1 Present day heat flow, estimated by the previous workers in the Tabei Uplift of the Tarim Basin. Tabei Uplift

Tazhong West and Awati Manjiaer Average References southwest depression depression Tarim

44.14

34.42

43.93 38–52

60–50

40–45

65–72.3

39.19

<40

>50

30–40

44.05

Zhang and Liu (1992) Wei (1992) Wang et al. (1995c) Wang et al. (1995b)

collected from 12 wells for vitrinite-like maceral reflectance (%VLMRo). Two of three linear regression equations proposed by Xiao et al. (2000) were used to calculate the %VRo through %VLMRo:%VRo = 0.28  %VLMRo + 1.03 (for VLMRo = 0.75–1.5%); and %VRo = 0.81  %VLMRo + 0.18 (for VLMRo > 1.5%). The measured vitrinite reflectance (%VRo) and estimated vitrinite reflectance through %VLMRo are collectively expressed as (%Ro) in this paper.

3.4. Maturity models for %Ro calculation Fig. 6. Paleo-surface temperature used in the 1-D models.

Lower Palaeozoic sediments (e.g. Wang et al., 1996; Xiao et al., 2000). In this study, a total of 120 core and cutting samples were

Three models including the Lopatin temperature and time indices (TTI, Waples, 1980), Simple-Ro method (Suzuki et al., 1993), and the standard LLNL (Easy %Ro, Sweeney et al., 1987; Sweeney and Burnham, 1990) model in BasinMod can be used to calculate thermal maturity parameters. In this study, we used the Easy

Fig. 7. (a) Thermal maturity model showing the measured vitrinite reflectance data %Ro and modeled maturity profiles, and (b) paleo-heat flow history plots from different models for the S47 well. Profile 1: modeled maturity profile from the ‘‘best case” heat flow model; profile 2 and profile 3: calculated maturities using a constant heat flow of 38.0 mW/m2 and 45 mW/m2 through time. %VRo and %VLMRo values also in Figs. 8–13 were measured by the State Key Laboratory of Coal Resources and Mine Safety at China University of Mining and Technology, Beijing (Fm: Formation).

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%Ro, which was widely recognized as an industrial standard to calculate Ro profiles (Waples et al., 1992a, 1992b; He and Middleton, 2002). 3.5. Temperature data The most reliable temperature data, used to characterize the geothermal field of a basin, are those from systematic steady-state temperature measurements and/or well tests. Wireline logging of old holes that have cooled for a year or more can be a useful way of getting valid temperature profiles. However, this is rarely practical, since wells are often cemented before being abandoned (Barker, 1996). A great number of oil-testing temperature data (Fig. 3) and 8 well-logging temperature curves (Fig. 4) have been collected in this study. The oil-testing temperature data are more reliable than others since they were tested directly from oil reservoirs, after several hours’ circulation. Meteorologic data from 19 observation stations show that the present day mean annual land surface temperature in the Tarim Basin ranges from 12 to 16 °C (Fu et al., 2003). Extrapolation of well-logging temperature data from 8 wells (Fig. 4) shows that the land surface temperature of the Tahe Oilfield changes from 12 to 32 °C. Extrapolation of 500 oil-testing data indicates that the surface temperature is 14 °C (Fig. 3). Here we took the present day mean annual land surface temperature at the locations of wells for thermal modeling to be 15 °C. Surface temperatures in the past were estimated by considering paleo-latitude and paleo-climate. A great number of paleo-magnetism measurements indicate that the Tarim block was at a southern latitude in the Sinian and continually drifted northward to a

northern latitude (e.g. McFadden et al., 1988; Sharps et al., 1989; Chen et al., 1992; Fang et al., 1998; Fang and Shen, 2001; Jia et al., 2007). After the formation of the crystalline basement of the Tarim Plate in the Mesoproterozoic, the Paleo-plate was at a latitude of 20°S (Jia et al., 2007). It drifted northward to a latitude 20°N in the Late Carboniferous–Permian (Fang and Shen, 2001; Jia et al., 2007). It moved to the 30°N latitude in the Jurassic and then returned to lower north latitude in the Cretaceous and finally migrated to the present position. This study assumes a surface temperature of 20 °C for the early Ordovician, a maximum of 25 °C in the Late Ordovician, and then continuously decreasing to the present day surface temperature of 15 °C, with the exception of a kick in the Cretaceous (Fig. 5). 3.6. Erosional events Multiple stratigraphic unconformities initiated in all stratigraphic sections of the Tarim Basin, implying multiple periods of uplift and erosion. Uplifting occurred in the Late Silurian by the early stage of Hercynian Tectonic Event, followed by a major uplift and erosion during the Late Permian. All other tectonic events are relatively minor periodic uplifting (Tang, 1997). The Ro% that changes irreversibly with increasing temperature is commonly used in reconstructing sections with unconformities (Dow, 1977; Katz et al., 1988; Barker, 1996; Hu et al., 1999). As illustrated in Fig. 6, Ro% profiles of all sections in the northern uplift of the Tarim Basin show abrupt increase in reflectivity across major unconformities (e.g. Carboniferous/Silurian and Carboniferous/Ordovician). The amount of missing sections by erosion can be roughly estimated by extrapolating the Ro-depth trend below this

Fig. 8. (a) Thermal maturity model showing the measured maturity data %Ro and modeled maturity profiles, and (b) paleo-heat flow history plots from different models for the T401 well. Profile 1: modeled maturity profile from the ‘‘best case” heat flow model; profiles 2 and 3: calculated maturities using a constant heat flow of 38.0 mW/m2 and 45 mW/m2 over time, respectively.

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unconformity to a typical surface value of 0.2%Ro (Barker, 1996). The thickness of missing sections by erosion varies from 3200 to 4500 m across the study area. The amount of offset in %Ro is a function of the thermal history, maximum burial depth prior to erosion and re-burial depth afterwards, the duration of erosion and intensity of the uplift (Katz et al., 1988). The thermal maturity model (e.g. Easy Ro%), which integrates complete thermal and burial histories, can be used to quantitatively assess the amount of uplift and estimate missing thickness during the erosion, using the %Ro profiles. The values that were used in the reconstruction of the burial and missing sections in the Tabei Uplift, were also adjusted to provide the best fit between the measured and calculated %Ro profiles. Zhang et al. (2000), Qi and Liu (1999) used ‘‘wave process” analysis, and estimated that there was approximately 1600 m of Ordovician, Silurian and Devonian sections missing in the Tabei Uplift. Based on the fluid inclusion homogenization temperature, Zhao et al. (2005) estimated that the missing section in the Lunnan 1 well of the Tabei Uplift is about 2595 m. Li et al. (2007) predicted 500–600 m of missing section for the Upper Ordovician and 500– 900 m for the Silurian in the Ackule area of the Tabei Uplift based on regional geology and seismic data. In wells S76, T903, S67, T904, S47, T204, T401, S60, the Silurian and Devonian strata are completely missing, where the Lower Carboniferous rocks overly the Upper Ordovician strata directly. Some authors suggested that the stratigraphic unconformity between the Lower Silurian (S1k) and the Upper Ordovician (O1s) strata represents a major uplift (Qi and Liu, 1999). As illustrated in Fig. 6e and f, the Ro profile along the S1 and O periods are continuous, and the only offset corre-

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sponds to the D3–S1 or C1–S1 boundaries. Thus, the offset of Ro across the unconformity of C1–O ought to be the result of the erosion of the Silurian rather than the Ordovician rocks. The Lower Silurian deposits in the Tarim Basin consist mainly of fine clastics deposited in a shallow littoral and tidal flat environment. The maximum thickness of the Silurian strata is about 3600 m in the Majaer Depression (Zhu et al., 2001). The Silurian stratum is missing in most of the sections in the Tabei Uplift, except for some wells in the south and southwest slope (e.g. S112, S109, TP7 wells). The Triassic/Permian or Triassic/Carboniferous boundaries marked other major tectonic events (Qi and Liu, 1999; Zhang et al., 2000). The Upper Permian in the Tabei Uplift is mostly eroded. Thus, the Lower Triassic directly overlies on the Lower Permian, even the Lower Carboniferous at some regions (e.g. in the T204, S47, T401, S60 wells). According to Qi and Liu (1999), Zhang et al. (2000), the missing thickness from erosion is over 2000 m in the Lunan1 well, ranging from 1000 to 2000 m in the study area. The modeled Ro profiles in this study also show abrupt lateral variations in the eroded thickness corresponding to the Triassic–Permian or Triassic–Carboniferous unconformities. But the Ro offsets in all wells are apparently less than those of the Carboniferous–Silurian (Ordovician) or Devonian–Silurian, indicating a relatively lower magnitude of erosion. In our models, approximately 2000 m of missing thickness was assigned to this erosion event, which provides the best agreement between the calculated and the measured Ro% values. The Late Triassic Indian–Sinia Orogeny resulted in a regional uplift and erosion and the formation of the base Jurassic unconformity (Qi and Liu, 1999; Zhang et al., 2000; Hu, 2004). According to

Fig. 9. (a) Thermal maturity model showing the measured maturity data %Ro and modeled maturity profiles, and (b) paleo-heat flow history plots from different models for the T204 well. Profile 1: modeled maturity profile from the ‘‘best case” heat flow model; profiles 2 and 3: calculated maturities using a constant heat flow of 38.0 mW/m2 and 45 mW/m2 over time, respectively.

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Zhang et al. (2000), the missing rock thickness during that erosional event at this unconformity is about 400–500 m in the study area. The Late Cretaceous Yanshanian Orogeny played a minor role in the evolution of the Tarim Basin. It led to regional uplift with small scale erosion (Tang, 1997; Qi and Liu, 1999). The missing thickness by this erosional event is estimated to be around 60–150 m in the Tabei Uplift (Qi and Liu, 1999). In this study, a missing thickness of 100–150 m was applied in the modeled wells.

Du et al. (1997) constructed a regional sea-level curve for the Sinian–Devonian period, based on sequence stratigraphy work from the outcrops. Ding et al. (2000), Chen et al. (2000) proposed four cycles of sea-level changes in the Carboniferous. Xue et al. (1998) studied the sea-level fluctuations from the late Cretaceous to Eogene, associated with the deposition of marine carbonate sequence of the southwest Tarim Basin.

4. Results and discussions 3.7. Palaeobathymetry and sea-level variations 4.1. Previously proposed present day heat flows The palaeobathymetry and sea-level variations are based on depositional environment interpretations, tectonic evolution and sequence stratigraphy studies. For example, the Early and Middle Ordovician deposits of the Tabei Uplift are typical platform carbonates (Fan and Wu, 2004). By considering the abundance of calcareous algae and calcified cyanobacteria, which are often associated with the shallow marine environment, Fan and Wu (2004) suggested a paleo-water depth of 20 m or less during the Early and Middle Ordovician. A value of 20 m of sea depth for the Early–Middle Ordovician was used in the model. The palaeo sea-level variations are determined by the studies of Yu (1996), Ding et al. (2000), Feng et al. (2000), Jiang et al. (2001) and Huang et al., 2006. Jiang et al. (2001) suggested a relatively low sea-level in the Early Ordovician, which rises rapidly from the Early to Mid-late Ordovician and reaches to a stable high sea-level in the Mid-late Ordovician. This interpretation is based on the variations of the carbon and strontium isotopes of the Ordovician carbonates.

Many authors have studied the heat flow distribution in the Tarim Basin (e.g. Zhang and Liu, 1992; Wang et al., 1995b,c; Li et al., 2004, 2005). These studies predicted a relatively lower heat flow for the Tabei Uplift, compared to the Tazhong Uplift and the Majiaer Depression, ranging from 40 to 45 mW/m2 (Table 1). The present day heat flow of the Tabei Uplift of Tarim Basin ranges from 38 to 45 mW/m2, similar to the shields of Canada and Australia. A large mismatch between the measured and calculated maturity profiles was observed when 38 and 45 mW/m2 of the heat flow values were used in the model (Figs. 7–12). 4.2. Heat flow history A constant heat flow value over time cannot honor the evolution history of the Tabei Uplift due to complexities in the tectonic history of the Tarim Basin. A relatively higher heat flow

Fig. 10. (a) Thermal maturity model showing the measured maturity data %Ro and modeled maturity profiles, and (b) paleo-heat flow history plots from different models for the S60 well. Profile 1: modeled maturity profile from the ‘‘best case” heat flow model; profiles 2 and 3: calculated maturities using a constant heat flow of 38.0 mW/m2 and 45 mW/m2 over time, respectively.

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from Sinian to the Ordivician, a decline in heat flux from the Silurian to Palaeozoic with a Permian thermal kick and a gradually decreasing thermal stress from the Mesozoic to the Present Day better honors the tectonic history of the Tabei Uplift. A ‘‘best case” heat flow model was constructed for the 14 modeled wells.

4.2.1. Heat flow in the S47 well The S47 well is located on the northern part of the Tahe Oilfield in the Tabei Uplift. The boundary between the Lower Carboniferous and the Lower Ordovician is a major unconformity. The Middle– Upper Ordovician, the Silurian to the Devonian sections are missing at the well location. We estimated a 4100 m of eroded section during the Silurian and the Devonian, based on the Ro data and previous geological studies. The boundary between the Lower Triassic and Lower Carboniferous is another important conformity. The Permian and Upper Carboniferous sediments are completely missing at this well. Approximately 2100 m of rock section were eroded, based on the Ro data and other studies (e.g. Zhang et al., 2000). The thermal models using a constant heat flow through time do no provide a good match between the measured and calculated %Ro profiles (Fig. 7a). Profiles 2 and 3 are %Ro profiles calculated by using a minimum 38 mW/m2 and maximum 45 mW/m2 constant heat flow through time in this area. It appears that profile 2 has a good agreement with measured %Ro profile for the Triassic and younger sediments, and profile 3 for the Carboniferous and Ordovician sections. However, neither of these models can simulate a

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%Ro offset at the major unconformities despite the assigned missing thickness to the modeled erosional events. A good fit to the maturity data, was achieved by applying the ‘‘best case” model, where he heat flow is as high as 61 mW/m2 during the Ordovician and decreases gradually to 50 mW/m2 during the Late Carboniferous, followed by a Permian thermal kick for simulating the igneous activity (Fig. 7, profile 1). The modeled maximum heat flow applied in this model is about 61 mW/m2, 35% higher than the predicted maximum present day heat flow. 4.2.2. Heat flow in the T401 well T401 well is located northwest of S47. The stratigraphic sequence is similar to that of well S47. The Lower Carboniferous sequence overlies directly on the lower Ordovician Yingshan Formation. We predicted missing sections of 3900 m and 2500 m corresponding to the base Carboniferous and base Triassic unconformities, respectively, on the basis of measured %Ro profile and previous geological studies. The %Ro profiles of 2 and 3, in Fig. 8a, were calculated using a present heat flow, ranging from 38 to 45 mW/m2. The thermal models using a constant heat flow through time could poorly calibrate to the measured %Ro data (Fig. 8a profiles 2 and 3). It appears that Curve 2 has a good fit to the measured %Ro data for the Triassic and younger sediments, and profile 3 for the Carboniferous and Ordovician sections. Moreover, the constant heat flow models cannot simulate a maturity jump at major unconformities, despite the assigned missing thickness to the associated erosional events. A good fit to the maturity data was provided using the ‘‘best case” model (Fig. 8, profile 3). The heat flow is as high as 65 mW/

Fig. 11. (a) Thermal maturity model showing the measured maturity data %Ro and modeled maturity profiles, and (b) paleo-heat flow history plots from different models for the T903 well. Profile 1: modeled maturity profile from the ‘‘best case” heat flow model; profiles 2 and 3: calculated maturities using a constant heat flow of 38.0 mW/m2 and 45 mW/m2 through time.

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m2 during the Ordovician, decreasing gradually to 50 in the Late Carboniferous with a Permian thermal kick. The maximum heat flow of about 65 mW/m2 is 44% higher than the maximum present day heat flow. The paleo-heat flow during the Jurassic to the present day is lower than 45 mW/m2. 4.2.3. Heat flow in the T204 well Well T204 is located to the southeast of S47. Unlike the S47 and T401 wells, this well contains an Upper Ordovician sequence. Thus, the Lower Carboniferous Bachu Formation overlies the Upper Ordovician Lianglitage Formation. The missing section corresponding to this unconformity is estimated to be around 4000 m by ‘‘best case” model. This model also predicts a total of 2300 m of eroded sediment thickness for the Lower Triassic–Lower Carboniferous unconformity. The thermal models using a single heat flow through time could not calibrate to the measured %Ro data (profiles 2 and 3 of Fig. 9a and b. However, the ‘‘best case” matches the measured maturity indicators (profile 1 of Fig. 9a). In the ‘‘best case” model, the heat flow is relatively high in the Early Ordovician and gradually decreases from the Late Ordovician to Late Carboniferous. A thermal kick was simulated by elevating the heat flow in Permian, followed by a gradually decreasing heat flow through the present day. The highest applied heat flow in the model was about 65 mW/m2, similar to that of the T401 well. 4.2.4. Heat flow in the S60 well Well S60 is in the northeast of Tahe Oilfield. It has a similar stratigraphic sequence and paleo-heat flow history as Well T204 (Fig. 10a). The ‘‘best case” model matches the measured %Ro data.

4.2.5. Heat flow in the T903 well Well T903 was drilled in the northeastern part of the Tahe Oilfield. The well has a relatively thick (115 m) Upper Ordovician Sangtamu Formation. The boundary between the Lower Carboniferous Bachu Formation and the Sangtamu Formation is unconformable. The missing sediment thickness associated erosion event is about 2900 m, which is much thinner than that of the S47, T401, T402 or S60 wells. The eroded thickness, corresponding to the Lower Triassic/Carboniferous unconformity is minor, resulting in a minor jump on the %Ro profile (Fig. 11). The %Ro profile of the ‘‘best case” model calibrates well with the %Ro data, while the constant heat flow models provide a large misfit to the measured %Ro data (Fig. 11). The maximum heat flow in the Ordovician was modeled as 68 mW/m2, which gradually decreased to 55 mW/m2 in the Late Carboniferous, with a Permian thermal kick.

4.2.6. Heat flow in the S109 well The S109 well was drilled in the western part of the southern margin of the Tabei Uplift (Fig. 1). This well penetrated the entire Ordovician strata including the Upper Ordovician Sangtamu Formation (227 m). The Lower Silurian and the Upper Devonian sequences are also present in this well (Fig. 6c). The boundary between the Upper Devonian Donghetang Formation and the Lower Silurian sequence is unconformable. The eroded sediments during the associated erosion event is estimated to be around 3300 m, which is approximately 600–800 m less than that of the other wells (e.g. T204 and T401). This well also contains a thin Permian f succession of 54 m. The eroded sediment thickness, associated

Fig. 12. (a) Thermal maturity model showing the measured maturity data %Ro and modeled maturity profiles, and (b) paleo-heat flow history by different models for the S109 well. Profile 1: modeled maturity profile from the ‘‘best case” heat flow model; profiles 2 and 3: calculated maturities using a constant heat flow of 38.0 mW/m2 and 45 mW/ m2 over time, respectively.

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Fig. 13. Sensitivity of the heat flow on the modeled %Ro profiles, calculated from ‘‘best case” model, for the T401 well. The sensitivity test is performed by increasing or decreasing the applied heat flow by 5% and 10%.

to the base Devonian unconformity is estimated to be 1800 m, which is also thinner than that the T401 and T204 wells.

Despite the similarities between the applied heat flow trends in this well and the others, the S109 well model requires a slightly

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lower heat flow in order to calibrate against the observed thermal indicators. The model estimates a heat flow value of 55 mW/m2 for the Ordovician period (Fig. 12). The AD4, TP7, S76, S67, S112 and S69 wells were modeled using similar heat flow history to the rest of the modeled wells.

sured and calculated %Ro profiles can also be established. But there is no enough evidence to validate the removal of more than 7 km of sediment during the erosion event.

5. Conclusions 4.3. Summary of modeled paleo-heat flow history in the Tabei Uplift The geothermal models constructed on six wells show that the calculated maturity profiles from the ‘‘best case” model have a good agreement with the measured %Ro profiles. However, the models using a constant present day heat flow with respect to time fails to calibrate to the measured maturity data. In summary, the heat flow is about 60 ± 5 W/m2 in the Early Ordovician and gradually decreases to 55 ± 5 W/m2 during the Late Carboniferous. It had an abrupt increasing during Permian and then decreased to the current heat flow value (38–45 W/m2). 4.4. The uncertainties and reliability of the models The lack of measured %Ro data from the Cretaceous and Mesozoic sediments due to no samples available may reduce the reliability of the ‘‘best case” model to a certain degree. The thermal maturity profiles were also calculated assuming an increase or a decrease of heat flow values by 5% or 10% obtained by the ‘‘best case” model (e.g. T401 in Fig. 13). When the heat flow values were increased or decreased by 10%, the %Ro profile appears to be inconsistent with the measured Ro data, especially in the Ordovician section. When the heat flow values were increased or decreased by 5%, the %Ro profile in the Ordovician remains inconsistent with the measured %Ro data. However, it is largely consistent with the measured %Ro data in the Mesozoic strata. The %Ro data plays a key role in the construction of the thermal model. The mean values of %Ro were used in this study. However, the measurement errors may reduce the reliability of the %Ro data. The errors of the data used in our model are generally ±10%. The error bar data in Fig. 13 indicates the range of the %Ro values. The calculated %Ro profile by increasing or decreasing 10% apparently falls outside the range of the %Ro data. When the heat flow values were increased or decreased by 5%, the %Ro profile approximately fall within the range of %Ro data. Thus, the accuracy of the heat flow values by our model is approximately estimated as ±5%. Sensitivity analysis is critical to test geological scenarios, since there are inevitably many uncertainties in the input data (e.g. Deming and Chapman, 1989; Waples et al., 1992a,b; Barker, 1996). In the ‘‘best case” model, the missing sediments during the erosion are sensitive parameters. Thus variable erosion intensities were tested to determine the sensitivity of this parameter in the models. In the ‘‘best case” model, we assumed a 3000–4000 m of eroded thickness for the base Carboniferous erosional event (Erosion 8). Previous workers (e.g. Zhang et al., 2000, 2007) estimated an average of 2000 m missing thickness for this event in the northern Tahe Oilfield (e.g. T401 well). We assumed a 2000 m of rock loss thickness in Erosion 8. The model, however, fails to simulate a jump in the %Ro profile at this unconformity using the applied model. Only by applying a 120 mW/m2 of heat flow in the Ordovician a reasonable fit to the measured %Ro profile could be achieved and at the same time, the maturity jumps at the unconformities could be simulated. But the heat flow value of 120 mW/m2 is apparently not reasonable considering the regional geological settings and geothermal conditions. We could also simulate a %Ro jump across the unconformity by using a uniform heat flow through time. For example, in the T401 well, by using a 38 mW/m2 of present heat flow through time and eroded thickness of 7300 m for the Erosion 8, a good fit of mea-

The burial and thermal histories of 14 wells in the Tabei Uplift, Tarim basin were modeled to evaluate and predict the heat flow history from the Ordovician to the present day by using 460 measured %Ro, data as calibration parameters. The paleo-heat flow was predicted to be about 60 ± 5 mW/m2 in the Early Ordovician, which gradually decreases to 55 ± 5 mW/m2 in the Late Carboniferous. It subsequently gradually decreases to the present day value of 38– 45 mW/m2 after a thermal kick in the Permian. It has demonstrated that heat flow scheme, not only enables a good calibration with the measured maturity data but also honors the paleo-tectonic evolution of the Tabei Uplift. It suggests that the ‘‘best case” model may be one of the most reliable ones. Acknowledgements We thank Northwest Oilfield Company of SINOPEC for providing samples and data, and for permission to publish this work. We gratefully thank Dr. Ozkan Huvaz, Professor M. Faure, and another anonymous reviewer for their constructive comments, suggestions, which have greatly improved the manuscript. We wish to extend our thanks to Dr. Haiping Huang of the University of Calgary and Dr. Keyu Liu of the CSIRO of Australia for their editing and Prof. Feiyu Wang for his assistance in modeling using the BasinMod 1-D program. This research is primarily funded by a Grant (No. 2006CB202307) from the China State 973 Scientific Program. The work is also financially supported by the foundation of the State Key Laboratory of Petroleum Resources and Prospecting (Grant No. PRPDX2008-01) and CNPC Innovation Fund (Grant No. 07E001). References American Society for Testing and Materials (ASTM), 1994. Standard test method for microscopical determination of the reflectance of vitrinite in a polished specimen of coal: Annual Book of ASTM Standards: Gaseous Fuels; Coal and Coke, sec. 5, v. 5.05, D 2798-91, pp. 280–283. Barker, C., 1996. Thermal modeling of petroleum generation: theory and application. Elsevier, Development in Petroleum Science 45, 1–503. Buchardt, R., Lewan, M.D., 1990. Reflectance of vitrinite-like macerals as a thermal maturity index for Cambrian–Ordovician Alum Shale, Southern Scandinavia. American Association of Petroleum Geologists Bulletin 74, 394–406. Chen, Y., Cogné, J.P., Courtillot, V., 1992. New Cretaceous paleomagnetic poles from the Tarim Basin, North western China. Earth and Planetary Science Letters 114, 17–38. Chen, G.J., Wang, Z.Y., Xue, L.H., Zhang, X.B., 2000. Sedimentary sequence and the sea-level changing of carboniferous in Tarim Basin (in Chinese with English abstract). Xinjiang Geology 18 (2), 141–147. Cheng, D.S., Fang, J.F., 1997. Genesis and thermal evolution of vitrinite-like macerals in hydrocarbon source rocks of Lower Palaeozoic (in Chinese with English abstract). Petroleum Exploration and Development 24 (1), 11–13. Deming, D., Chapman, D.S., 1989. Thermal history and hydrocarbon generation: example from Utah-Wyming thrust belt. American Association of Petroleum Geologists Bulletin 73 (12), 1455–1471. Ding, X.Z., Liu, X., Fu, D.R., Yao, J.X., Wang, R., Wu, S.Z., Yan, Y., 2000. Sequence stratigraphy and sea-level changes of Carboniferous in the north western margin of Tarim Plate, NW China (in Chinese with English abstract). Regional Geology of China 19 (1), 58–65. Dow, W.G., 1977. Kerogen studies and geological interpretation. Journal of Geochemical exploration 7, 79–99. Du, X.D., Wang, P.J., Kuang, L.C., Wang, D.P., 1997. The Reconstruction and origin of sea-level changes of Sinian Period to Devonian period in Tarim Basin (in Chinese with English abstract). Acta Sedimentologica Sinica 15 (3), 14–18. Fan, J.S., Wu, Y.S., 2004. Palaeoenvironmental analysis of Ordovician rocks in the Northern Uplift of Tarim Basin in terms of calcareous algae and cyanobacteria (in Chinese with English abstract). Acta Micropalaeotologica Sinica 21 (3), 251– 266.

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