Thermal evolution and maturation of lower Paleozoic source rocks in ...

Thermal evolution and maturation of lower Paleozoic source rocks in the Tarim Basin, northwest China Nansheng Qiu, Jian Chang, Yinhui Zuo, Jiyang Wang, and Huili Li

ABSTRACT The Tarim Basin is one of the richest basins in oil and gas resources in China. The Cambrian and Middle–Upper Ordovician strata are the most important source rocks. Previous early Paleozoic thermal histories have led to varied hypotheses on the evolution of the lower Paleozoic source rocks, causing a significant impact on petroleum exploration in the basin. A new Paleozoic thermal history of the Tarim Basin was reconstructed in this article using the integrated thermal indicators of apatite and zircon (uranium-thorium)/helium ages, apatite fission tracks, and equivalent vitrinite reflectance data. The modeled results indicate that different parts of the basin experienced widely differing early Paleozoic thermal gradient evolution. The eastern and central regions of the basin experienced a decreasing thermal gradient evolution from 37 to 39°C/km during the Cambrian and Ordovician to 35 to 36°C/km in the Silurian, whereas the northwestern region of the basin had an increasing early Paleozoic thermal gradient evolution from 28 to 32°C/km in the Cambrian to 30 to 34°C/km in the Ordovician and Silurian. The Lower Cambrian thermal gradient resulted from the higher thermal conductivity of the 800- to 1000-m (2625- to 3280-ft) thickness of gypsum and salt in the Cambrian strata. The basin experienced an intracratonic phase during the late Paleozoic and a foreland basin phase during the Mesozoic and Cenozoic, with the thermal gradient decreasing to the present-day value of 20 to 25°C/km. The sensitivity of thermal modeling by the best-fit method is less than ±5% in our study, and the

Copyright ©2012. The American Association of Petroleum Geologists. All rights reserved. Manuscript received February 17, 2011; provisional acceptance June 16, 2011; revised manuscript received July 31, 2011; final acceptance September 7, 2011. DOI:10.1306/09071111029

AAPG Bulletin, v. 96, no. 5 (May 2012), pp. 789–821

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AUTHORS Nansheng Qiu State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing 102249, China; present address: Research Center for Basin and Reservoir, China University of Petroleum, Beijing 102249, China; [email protected] Nansheng Qiu received his Ph.D. in structural geology from the Institute of Geology, Chinese Academy of Sciences, in 1994. He is now a professor of geology at the China University of Petroleum, Beijing. He has conducted tectonothermal history and hydrocarbon generation history research in several Chinese basins. His current research interests include tectonothermal evolution of sedimentary basins and hydrocarbon accumulation history. Jian Chang State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing 102249, China; [email protected] Jian Chang received his bachelor’s degree in mineral survey and exploration from the China University of Geosciences and is now a candidate for Ph.D. at the China University of Petroleum, Beijing. His current interests include tectonothermal evolution of sedimentary basin and hydrocarbon accumulation history analysis. Yinhui Zuo State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing 102249, China; [email protected] Yinhui Zuo received his master’s degree in mineral survey and exploration from Chengdu University of Technology and is now a Ph.D. candidate at the China University of Petroleum, Beijing. His current interests include tectonothermal evolution of sedimentary basin and hydrocarbon accumulation history analysis. Jiyang Wang Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; [email protected] Jiyang Wang is an academician in the Chinese Academy of Sciences. He received his Ph.D. from the Moscow Geology Institute in 1962. He has worked on geothermics in Chinese basins since 1978. His current research work focuses on the thermal history of sedimentary basins and heat flow in the continental area of China.

Huili Li Institute of Petroleum Exploration and Development, Northwest Oil Company, China Petroleum and Chemical Corporation (SINOPEC), Urumqi 830011, China; [email protected] Huili Li received her Ph.D. from the China University of Petroleum, Beijing, in 2005. She is now a senior geologist at Northwest Oil Company, China Petroleum and Chemical Corporation (SINOPEC). Her interest is in the thermal history reconstruction of basins and petroleum accumulations.

differences of the early Paleozoic thermal gradient evolution in different regions of the basin may result in differential maturation of lower Paleozoic source rocks. The maturity histories of the source rocks, modeled based on the new thermal histories, indicate that the lower Paleozoic source rocks in most areas of the basin matured rapidly and reached the late mature to dry-gas stage during the Paleozoic but experienced slower maturation during the Mesozoic and Cenozoic. These new data on the Paleozoic thermal history and lower Paleozoic source rock maturity histories of the Tarim Basin provide new insights to guide oil and gas exploration of the basin.

ACKNOWLEDGEMENTS This study was supported by the National Natural Science Foundation of China (no. 41072103), the Key Project, Ministry of Education of China (no. 308005), and the State Key Laboratory of Petroleum Resource and Prospecting (no. PRPJC200801). Grateful acknowledgments are made to the Northwest Oil Company (China Petroleum and Chemical Corporation [SINOPEC]) and the Tarim Oil Company (China National Petroleum Corporation [CNPC]) that contributed cores and geologic data for this work. We thank Peter Reiners from the University of Arizona and Nicolescu Stefan from Yale University, who provided assistance for our sample analyses and manuscript improvement. Yun Lu, Qian Yixiong, and Chen Yue gave much help during sample collection. Platte River Associates, Inc., provided the software for burial history study. Richard A. Ketcham provided the HeFTy software and Shengbiao Hu provided the Thermodel software to model thermal histories. Our heartfelt gratitude also goes to Keyu Liu and Zhuoheng Chen for their helpful comments and linguistic editing. We thank AAPG reviewers Barry J. Katz, Colin P. North, Jennifer D. Shosa, Stephen E. Laubach, Paul F. Green, and Nicholas B. Harris for their thorough and critical reviews and suggestions to improve the manuscript. The AAPG Editor thanks the following reviewers for their work on this paper: Barry J. Katz and Colin P. North.

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INTRODUCTION The Tarim Basin is located in the Uygur Autonomous Region of Xinjiang, northwest China (Figure 1). It is the largest basin in China, having an area of 56 × 104 km2 (22 × 3861 mi2). There have been oil and gas exploration activities in this basin for 40 yr, and several large oil and gas fields, with reserves of more than 1 × 108 tons (7 × 108 bbl), have been found since the 1990s, including the TZ4 field, the Tahe field (the largest marine oil field in China), and the Kela-2 field. Oil and gas exploration has rapidly developed during the past 10 yr, and more than 6924 × 104 tons of oil and 584.07 × 108 m3 of natural gas had been produced in the Tarim Basin by the end of 2007 (Song and Jiang, 2008). All these discoveries indicate that the Tarim is a region rich in oil and gas resources. The thermal history of source rocks controls the timing of hydrocarbon generation and expulsion (Carminati et al., 2010; Hudson and Hanson, 2010). The hydrocarbon source rocks in the Tarim Basin are mainly found in the Cambrian and Ordovician strata, and they are deeply buried and mostly thermally overmature (Zhao et al., 1999, 2008; S. C. Zhang et al., 2000, 2001; B. M. Zhang et al., 2005; Mu, 2009). The early Paleozoic thermal history was an important factor in the evolution of the lower Paleozoic source rocks because Paleozoic strata may achieve thicknesses of up to 7000 m (22,966 ft). Previous studies have reported opposing Paleozoic thermal histories, among which most researchers have agreed that the basin was warm and that the thermal gradients were relatively high, up to 40°C/km in the early Paleozoic (Pan et al., 1996; Qiu et al., 1997, 2002; Xie and Zhou, 2002; Wang et al., 2003; Li et al., 2004, 2005, 2010; Xiao et al., 2008), whereas other studies have indicated a cool basin with a thermal gradient of only 20°C/km during the early Paleozoic (Tu, 1994; Jin, 1997). This controversy concerning whether the early Paleozoic

Thermal Evolution and Maturation of Lower Paleozoic Source Rocks, Tarim Basin, China

Figure 1. (A) Isopach map of Cambrian to Quaternary (–C-Q) succession and location of study wells. Isolines of thickness of C– -Q succession, structural units, and the north-south cross section of the Tarim Basin are from Jia et al. (1995). I = Kuqa depression; II = Northern uplift; III = Northern depression; IV = Central uplift; V = Southwest depression; VI = Southern uplift; VII = Southeast depression. (B) North-south cross section of the Tarim Basin. The symbols are the abbreviations of the strata and are shown in Figure 2. AnZ = Pre-Sinian.

thermal state of the Tarim Basin was warm or cool has led to two opposing hydrocarbon generation scenarios for the lower Paleozoic source rocks, implying very different perspectives on the oil and gas exploration potential of the Tarim Basin. In particular, uncertainty about lower Paleozoic source rock maturation has restricted further oil and gas exploration in the basin.

Vitrinite reflectance (Ro) and apatite fissiontrack (AFT) data are two commonly used indicators in the thermal history reconstruction of sedimentary basins. However, Ro cannot be used in the study of the early Paleozoic thermal history because of a lack of vitrinite macerals in the subDevonian succession (Jin, 1997; Hanson et al., 2007). Previous studies on the early Paleozoic Qiu et al.

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Figure 2. The stratigraphic columnar section of the Tarim Basin. Thermal conductivities (K) and radiogenic heat generation data (A) are from Qiu (2002) and Wang et al. (1999).

thermal history of the Tarim Basin have been based mainly on equivalent Ro (Requ) data converted from bitumen or vitrinitelike maceral reflectance 792

(Chen et al., 1995; Xiao et al., 2000; Wang et al., 2003). The AFT analysis is particularly important to basin analysis and hydrocarbon exploration


because annealing temperatures, which generally have been considered to be between 65 and 120°C, approximately corresponding to liquid hydrocarbon generation temperatures in nature (Green et al., 1989; Mora et al., 2010). The process of fissiontrack annealing in apatite has been calibrated in the laboratory (Laslett et al., 1982; Duddy et al., 1988; Green et al., 1989; Crowley, 1991), and several equations have been proposed to describe the annealing behavior of apatite (Laslett et al., 1987; Corrigan, 1991; Crowley, 1991). The (uraniumthorium [U-Th])/helium (He) thermochronometry of apatite and zircon is an emerging method based on the production of He by the nuclear decay of U and Th in some radioactive minerals (e.g., apatite and zircon). Systematic diffusion studies indicate that apatite He ages provide thermochronological information for temperatures between approximately 40 and 75°C (Wolf et al., 1996; Farley, 2000), which is the apatite He partial retention zone (PRZ). However, zircon has a higher He PRZ of approximately 170 to 190°C (Reiners et al., 2002). Because the above thermal indicators span different temperature ranges, the potential for an enhanced ability to constrain the thermal histories of basins by integrating interpretations of different thermal indicators exists. This article contributes to the understanding of the thermal history of the Tarim Basin with a new interpretation of its thermal evolution by applying the apatite and zircon (U-Th)/He ages, AFT, and Requ data as thermal indicators. More specifically, this work modeled the maturity histories of lower Paleozoic source rock based on the new thermal history and investigated the effects of different thermal scenarios on the maturity histories of the main source rocks in the Tarim Basin.

GEOLOGIC AND THERMAL SETTINGS The Tarim Basin is one of the most petroliferous basins in China. 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 (Figure 1). Numerous publications on the re-

gional geologic setting of the Tarim Basin exist (Kang and Kang, 1996; Li et al., 1996; Jia and Wei, 2002; Dai et al., 2009; Yang et al., 2009). The basin has been divided into several structural zones, including four depressions and three uplift regions, that is, the Kuqa depression, the Northern depression, the Southwest depression, the Southeastern depression, the Northern uplift, the Central uplift, and the Southern uplift (Figure 1). The Tarim Basin, which developed on an Archean and Proterozoic metamorphosed basement, experienced a long geologic history from the Sinian to the Quaternary. The complex and superimposed geologic configuration of the basin controls the basic characteristics of hydrocarbon accumulation and distribution (Zhang, 2000; He et al., 2005). The tectonic evolution of the basin has been divided into six stages (Jia et al., 1995) (Figure 2): (1) between the cratonic peripheral aulacogen from the Sinian to the Ordovician (Z∼O), (2) between the intracratonic depression from the Silurian to the Mississippian (S∼C1), (3) between the intracratonic rift from the Pennsylvanian to the Permian (C2∼P), (4) between the foreland basin stage in the Triassic (Tr), (5) between the intracontinental depression from the Jurassic to the Paleogene (J–E), and (6) the recombined foreland basin from the Neogene to the Quaternary. Several unconformities developed during the tectonic evolution, among which five important regional unconformities are (Figure 2) (Jia et al., 1995; Jia and Wei, 2002; He et al., 2005) (1) the Silurian and the Ordovician or older strata during the Caledonian movement stage II, (2) the Carboniferous and the Devonian or older strata during the Hercynian movement stage I, (3) the Upper Permian–Triassic and the underlying strata during the Hercynian movement stage II, (4) the Jurassic and the Triassic or older strata during the Indosinian movement, and (5) the Tertiary and the Cretaceous or older strata during the Yanshan and Himalayan movement. The above multiple episodes of uplift and subsidence resulted in contrasts in the tectonic and thermal evolution of individual structural zones. The history of the basin includes deposition of marine to nonmarine facies during the Sinian to the Early Permian and nonmarine facies during the Qiu et al.

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Figure 3. Thickness of gypsum and salt in the Cambrian stratum (modified from Fan, 2003) and migration of the depocenter of the Tarim Basin (modified from Gu, 1994).

Late Permian to the Quaternary (Li et al., 1996; Jia and Wei, 2002) (Figure 2). A total thickness of up to 15,000 m (49,213 ft) of sediments was deposited in the Northern depression (Huang et al., 1999). The Cambrian succession consists mainly of tidal, platform and platform-margin marls, mudstones, and carbonates and evaporites. Up to 1000 m (3281 ft) of gypsum and salt were deposited in the central and northwestern parts of the basin (Fan, 2003) (Figure 3). These sediments have higher thermal conductivities than sandstone and mudstone and result in the relatively lower thermal gradients in these successions. The overlying Ordovician section is primarily composed of platform dolomite and marginal slope-shelf carbonate sediments (Kang and Kang, 1996; Jin et al., 2008). The upper Paleozoic marine and continental transitional sediments were accumulated after deposition of the Silurian and Devonian fine-grained red 794

beds and sandstones. A set of fluviolacustrine sediments of approximately 6000 m (∼19,685 ft) thickness accumulated during the Mesozoic and the Cenozoic. The depositional center of the basin shifted as the basin evolved (Gu, 1994). It was located at the eastern part of the basin in the early Paleozoic and developed three depocenters in the eastern, western, and southwestern regions of the basin during the late Paleozoic. The depocenter moved to the northern part of the basin and the Southwest and Southern depressions during the Mesozoic. Several depocenters developed in front of the Tianshan and Kunlun mountains during the Cenozoic (Figure 3). Several hydrocarbon source rocks are found in the basin, including four sets of marine source – ], Lower Ordovician [O1], rocks (Cambrian [C Middle–Upper Ordovician [O2+3], and Carboniferous [C]) and two terrestrial potential source


Figure 4. Burial histories of studied wells and sampling locations. The unfilled and filled circles on the right of the diagrams are apatite and zircon sampling depths, respectively.

rocks (Triassic–Jurassic [Tr-J] and Cretaceous– Paleogene [K-E]). The Tr-J and K-E source rocks are mostly immature or distributed in limited areas of the basin (S. Liu et al., 2006; Y. K. Lui et al., 2007). The effective hydrocarbon source rocks in the Tarim Basin are mainly the Cambrian carbonate successions and the Middle–Upper Or-

dovician strata (B. M. Zhang et al., 2000; S. C. Zhang et al., 2000, 2001; Zhao et al., 2008; Mu, 2009). They are currently buried to depths of more than 5000 m (16,404 ft) in most areas of the basin, and as a result, these lower Paleozoic source rocks are at a high-mature or overmature stage (Xiao et al., 2000; Wang et al., 2003). The black Qiu et al.

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796 Table 1. Measured (U-Th)/He Ages of Apatite Samples from the Tarim Basin* Thermal Evolution and Maturation of Lower Paleozoic Source Rocks, Tarim Basin, China

Sample Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Sample ID

Strat.**

Depth (m)

Mass (mg)

Sh1-1a Sh1-3a Sh1-4a Sh1-9a Sh1-10a Sh1-12a Sh1-14a Z2-1a Z2-2a Z2-3a Z2-6a Z2-8a Z2-9a Z2-10a Z2-12a Z11-4a Z11-5a Z11-7a Z11-8a Z11-10a S14-7a S14-10a S14-11a S14-12a S14-13a S14-14a T1-1a T1-2a T1-3a T1-4a T1-5a KQ1-1a

E Tr Tr C S S S K Tr Tr P D S S O C D S S S N K K K J Tr S S S S S S

1825 2964 3311 4563.5 4583.5 4961 5331.1 2085 2553 2963 3972 4804 4962 5138 5462 4181.56 4351.5 4701.2 4922 5091 2748 3680 4015 4274 4550 4785 1567.8 1694.3 1791.5 1901.5 2095 2402.1

3.49 1.03 2.15 1.37 2.16 1.03 6.98 4.82 2.36 4.2 1.39 9.93 11.01 4.01 3.67 1.75 6.8 0.96 1.3 1.97 0.8 4.35 2.94 1.13 2.04 3.19 0.91 0.9 3.24 2.4 2.46 11.85

He (nmol/g)

Th/U (atomic)

U (ppm)

Th (ppm)

Sm (ppm)

Ft

Corrected Age (Ma)

±d (Ma)

eU (ppm)

T (°C)

0.31 0.85 0.32 0.0006 0.06 0.0078 0.0044 0.1179 8.854 1.29 0.15 0.043 0.026 0.01 0.005 0.007 0.0016 0.0043 0.0018 0.0026 0.0346 0.79 0.064 0.002 0.05 0.0014 0.011 0.1276 0.03 0.01 0.335 3.609

0.31 1.89 4.15 5.63 1.62 3.86 6.32 0.54 2.41 3.05 1.12 3.48 7.57 1.02 20.45 4.26 0.92 1.24 5.1 18.35 2.43 1.06 0.65 0.64 0.28 3.03 15.73 1.58 0.67 1.77 17.67 4.84

28.88 84.38 22.62 0.52 24.01 7.29 5.11 11.23 117.1 15.25 249.44 7.43 2.23 12.77 9.03 14.21 26.03 62.47 20.99 14.08 2.54 32.95 18.91 30.37 12.16 3.66 1.84 43.79 16.01 6.05 18.84 25.93

8.83 155.35 91.45 2.85 37.83 27.39 31.51 5.87 275.02 45.27 272.51 25.19 16.43 12.67 180.09 59.06 23.45 75.36 104.34 251.84 6.02 34.05 11.99 18.97 3.34 10.82 28.14 67.46 10.49 10.43 324.57 122.47

117.22 387.59 185.74 0.27 106.73 154.64 182.48 149.82 343.39 223.76 354.03 28.7 22.71 328.88 253.33 153.43 77.76 1006.12 154.06 439.02 34.28 32.59 115.27 89.25 159.22 72.53 352.62 196.15 68.68 52.88 135.32 160.42

0.73 0.6 0.66 0.61 0.68 0.59 0.77 0.75 0.67 0.73 0.64 0.79 0.79 0.73 0.71 0.65 0.78 0.59 0.6 0.63 0.601 0.73 0.708 0.64 0.683 0.698 0.6 0.58 0.72 0.68 0.67 0.81

32.32 92.98 42.11 5.33 10.83 7.49 0.53 21.08 251.12 132.34 4.52 3.35 3.94 1.76 0.33 1.8 0.08 0.77 0.41 0.23 147.21 50 11.67 0.63 22.22 0.85 18.55 33.92 5.68 5.82 17.5 56.11

0.62 1.4 0.62 3.4 0.26 1.02 0.06 0.42 6.95 2.05 0.09 0.12 0.18 0.2 0.07 0.13 0.03 0.11 0.25 0.1 8.76 0.87 0.31 0.23 0.61 0.28 1.07 0.64 0.17 0.32 0.28 0.81

30.96 120.89 44.11 1.19 32.9 13.72 12.52 12.63 181.73 25.89 313.48 13.35 6.09 15.75 51.36 28.09 31.54 80.18 45.51 73.26 3.96 40.95 21.73 34.83 12.94 6.2 8.45 59.64 18.47 8.5 95.12 54.71

46 68 75 99 100 107 114 54 64 72 93 110 112 115 120 94 97 104 108 112 64 80 86 91 96 100

4

*Ft = alpha correction factor (Farley et al., 1996); eU = effective uranium (U) concentration, a parameter that weighs the decay of the two parents for their alpha productivity, calculated from the contents of U and thorium (Th), eU = U + 0.235Th (Reiners et al., 2005). Two replicate analyses per sample were tested for most samples, but several samples had three replicate analyses. The error of corrected age (d) was calculated with the two replicate or three replicate analyses. Wells Sh1, Z2, Z11, and S14 have systematic sampling depth and obtained apatite helium (He) ages systematically. Temperature data are calculated from the thermal gradient of the sampling wells and depth for each sample. The thermal gradient in each well is based on the regression curve between the DST (drill-stem test) and BHT (bottom-hole temperature) data and their depth (e.g., 19.6°C/km in well Sh1, 20°C/km in wells Z11 and S14, and 21°C/km in well Z2. The surface temperature is 10°C). Strat. = stratum. **Abbreviations in this column are defined in Figure 2.

8.27 18.88 4.17 0.62 0.73 0.54 23.74 46.57 13.84 0.69 0.75 0.74 108.86 274.19 44.51 23.03 37.96 4.06 2.86 9.96 3.22 8.26 3.91 1.3 0.0567 0.432 0.0232 3.4 5.29 4.41 2593.8 3201.5 4554.5 S S O KQ1-2a KQ1-5a KQ1-7a 33 34 35

and dark-gray Ordovician mudstones and mudrich limestones are the major source rocks for the Ordovician reservoirs (S. C. Zhang et al., 2000, 2001; B. M. Zhang et al., 2005; Mu, 2009). Many of the previous studies have focused on the depositional environments, distribution characteristics, organic geochemistry, macerals, and hydrocarbon generation of these source rocks (Zhao et al., 1999, 2008; B. M. Zhang et al., 2000, 2005; S. C. Zhang et al., 2000, 2001; Wang et al., 2003; Chen et al., 2006). The total organic carbon (TOC) of the Middle–Upper Ordovician (O2+3) mudstone source rock ranges from 0.67 to approximately 5.4%, and the organic matter of the O2+3 is mainly type II and type III kerogen based on elementary analytical diagrams (Zhang et al., 2005; Mu, 2009). Most Middle–Upper Ordovician source rocks are mature, with Requ of 0.8 to approximately 1.6%. However, the TOC of the Cambrian carbonate source rock ranges from 0.48 to 4.0%, with a higher TOC of 1.84 to approximately 5.52% in mudstone (S. C. Zhang et al., 2000, 2001; Mu, 2009). The organic matter of the Cambrian carbonate is mainly type I kerogen and some type II1 kerogen based on elementary analytical diagrams. Most Cambrian source rocks are highly mature or overmature, with a Requ of 1.6 to approximately 4.0% (Wang et al., 2003; Zhang et al., 2005). Studies of the thermal regime of the Tarim Basin have been conducted since the 1990s and include measurements of the present-day thermal gradients and heat-flow distribution characteristics (J. Wang et al., 1995; L. S. Wang et al., 1995, 1996, 2003, 2005) as well as the lithospheric thermorheological structure (Wang et al., 2000; S. W. Liu et al., 2006). The basin was believed to have had the coolest thermal regime in China, with an average present-day thermal gradient of only 20°C/km (J. Wang et al., 1995) and an average heat flow of 44 mW/m2 (L. S. Wang et al., 1995). Some previous thermal history studies have indicated a cooling trend to the thermal history of the basin with a thermal gradient decreasing from approximately 40°C/km during the early Paleozoic to 20 to 25°C/km in the present day (Pan et al., 1996; Xie and Zhou, 2002; Li et al., 2004, 2005, 2010; Xiao et al., 2008), whereas other studies have Qiu et al.

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indicated a cool basin with thermal gradients of approximately 20°C/km during the early Paleozoic, which is almost the same as present-day conditions (Tu, 1994; Jin, 1997). This lower gradient during the early Paleozoic was obtained based on the marine geologic background and some thermal indicators of free radical density of organic matters (Tu, 1994; Jin, 1997). However, the cool basin hypothesis has now already been rejected by others.

SAMPLES AND EXPERIMENTS Samples To reconstruct the Paleozoic thermal history, obtaining paleothermal indicators that can reliably record Paleozoic thermal information is important. Burial history reconstruction forms the basis for sample collection and thermal history reconstruction. Thousands of meters of Mesozoic and Cenozoic strata exist in most areas of the Tarim Basin. Most of the Paleozoic strata have been deeply buried, resulting in the resetting of He and AFT ages and preventing the application of these indicators in the reconstruction of the Paleozoic thermal history. Although the PRZ and partial annealing zone of apatite could be obtained in some wells in depressions (e.g., wells Sh1 and Sh8), ideally, we would prefer to select Paleozoic thermal indicators from wells where significant erosion occurred at the end of the Paleozoic or the Mesozoic because of tectonic uplift (e.g., wells KQ1 and T1). In these cases, the thermal indicators may retain their recorded thermal information and reveal the Paleozoic thermal history because they were not as deeply buried. The burial histories of wells sampled in this article were reconstructed based on the backstripping method using the BasinMod software (Figure 4). Compaction corrections will have significant impact on thermal history, which in turn affects the timing of source rock maturity, petroleum generation, and expulsion. In our modeling, Sclater and Christie’s (1980) compaction and porosity exponential reducing factors were used. Because AFT and apatite and zircon He ages are sensitive to erosion, these erosion data directly 798

affect how these age data are interpreted. The main regional unconformities that developed in the basin would result in different amounts of erosion. Erosion of less than 200 m ( 2.0%; Liu and Shi, 1994); Requ = 1.26Rv + 0.21 (Rv < 0.75%; Xiao et al., 2000); Requ = 0.28Rv + 1.03 (0.75% < Rv < 1.50%; Xiao et al., 2000); Requ = 0.81Rv + 0.18 (Rv > 1.50%; Xiao et al., 2000).

In addition, an absence of vitrinite originating from higher plants is evident in the lower Paleozoic; equivalent Ro (Requ) data, which are converted from bitumen reflectance (Rb) or vitrinitelike maceral reflectance (Rv), have been used to study the maturity of lower Paleozoic source rocks (Pan et al., 1996; Wang et al., 2003; Li et al., 2004, 2005, 2010). As to different maturity stages, there have been several formulae for converting Requ from bitumen reflectance (Feng and Chen, 1988; Jacob, 1989; Liu and Shi, 1994; Wang et al., 2003)

and vitrinitelike maceral reflectance (Chen et al., 1995; Xiao et al., 2000). Here, we collected Rb and Rv data from the study wells, which were provided by the Tarim Oil Company (China National Petroleum Corporation [CNPC]), and then converted them into Requ values (Table 3). The Ro data in the upper Paleozoic and Mesozoic are all referred to be Requ in our study for consistency. Furthermore, AFT length and ages from 11 samples in the Sinian to Paleogene in wells H4, T1, and HT1 were tested in our study (Table 4). Some AFT data sampled systematically by depth from two wells were also collected (e.g., wells Sh1 and KQ1); these data were provided by the Northwest Oil Company (China Petroleum and Chemical Corporation [SINOPEC]). Analytical Results of Thermochronological Indicators An apatite (U-Th)/He age profile in the Tarim Basin was obtained based on data from four wells with systematic sampling depth (e.g., wells Sh1, Sh8, S14, and Z2) (Figure 5A). These wells are located in different structural zones, all of which were deeply buried during the Mesozoic and Cenozoic (Figure 4). We believe that the presentday temperature is the highest temperature these samples have ever experienced independent of their thermal histories. In this way, a natural apatite He age-temperature profile was developed by combining data from these wells. In this profile, although some scatter in ages with regard to temperature exists, in general, He age decreases with increasing temperature and approaches zero at approximately 90°C. Here, temperature data are calculated from the thermal gradient of the sampling wells and depth for each sample. The thermal gradient in each well is obtained based on the regression curve between the drill-stem test (DST) and bottom-hole temperature (BHT) data and their depth (e.g., 19.6°C/km in Sh1 well, 20°C/km in Z11 and S14 wells, and 21°C/km in Z2 well). Zircon He ages with systematic sampling depth in well Sh1 indicate that the zircon He ages for the samples at shallow depths are slightly older than the stratigraphic ages in well Sh1 but become Qiu et al.

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Table 4. Apatite Fission-Track Data in the Study Wells* Well

Depth (m)

Strat.**

n

H4 H4 H4 T1 T1 T1 T1 T1 HT1 HT1 HT1 Sh1 Sh1 Sh1 Sh1 Sh1 Sh1 Sh1 Sh1 Sh1 Sh1 Sh1 Sh1 Sh1 Sh1 Sh1 Sh1 KQ1 KQ1 KQ1 KQ1 KQ1 KQ1 KQ1 KQ1

1760 1862 2150 1567.8 1694.3 1791.5 2095 4809 3443.4 3806 4061.5 2342 2528 2724 2940 3148 3460.9 4564.3 4592.59 4638.64 4773 4833.41 4961.13 5047.26 5234.24 5319.51 5478.35 1818.3 2401.35 2596.78 2799.89 2956.93 3046.58 3803.36 4371.05

D S S E S S S Z D S O K K Tr Tr Tr P D D D D D S S S S O S S S S S S O O

12 10 5 28 24 28 29 30 28 29 31 20 18 21 19 20 21 22 27 21 24 23 20 22 22 22 20 20 21 20 20 21 21 21 20

rs (105/cm2) (Ns) 6.29 4.44 4.24 5.919 4.294 3.597 3.090 0.633 0.984 1.773 1.537 3.917 3.406 4.108 3.555 4.913 4.092 1.336 1.723 3.174 3.197 1.540 4.036 1.966 3.403 0.742 0.926 4.795 6.260 7.745 6.397 4.650 5.586 4.875 3.122

ri (105/cm2) (Ni)

(39) (326) (228) (647) (225) (594) (609) (131) (234) (326) (324) (370) (375) (361) (361) (369) (436) (154) (198) (287) (488) (144) (507) (287) (373) (85) (95) (486) (880) (615) (593) (607) (358) (311) (340)

30.65 17.81 19.89 12.460 11.471 7.781 7.047 1.933 9.013 7.899 12.287 11.157 6.485 9.638 9.060 14.767 11.778 9.332 9.277 18.896 10.730 9.050 10.284 10.933 12.673 18.154 10.855 5.831 10.763 11.911 9.234 8.250 9.627 10.298 9.209

(190) (1309) (1070) (1362) (601) (1285) (1389) (400) (2144) (1452) (2675) (1054) (714) (847) (920) (1109) (1255) (1076) (1066) (1717) (1638) (846) (1292) (1596) (1389) (2079) (1114) (591) (1513) (980) (856) (1077) (617) (657) (1003)

P (x2) (%) >50 2.0% The maturation modeling results indicated that by the end of the Ordovician, the bottom of the Cambrian entered the generation threshold (0.5% Ro) (Figure 18A). Mainly early mature source rock existed in the Southwest depression, the western part of Central uplift, the Northern uplift, and the Southeast uplift. The middle and late mature source rocks were distributed in the Northern uplift, the western part of the Northern depression and Central uplift, and the Southwest depression. The source rock reached the main gas stage in the eastern region of the basin, and it was at the drygas stage in the eastern part of the Northern depression, Central uplift, and Southwest depression (Figure 18A). By the end of the Permian, the source rock intervals which had reached the main gas stages covered most of the basin, except the northern part of the Northern uplift, the Southern uplift, the western end of Central uplift, and the Qiu et al.

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Figure 19. Maturation level of the base of the Middle and Upper Ordovician source rocks. The contour interval is 0.5%. Requ = equivalent vitrinite reflectance.

southern part of the Southwest depression, where the source rocks were still within the oil window (Requ < 1.3%) (Figure 18B). During the entire Mesozoic, the maturity mostly maintained the characteristics of the end of the Permian (Figure 18C). Currently, they are at the dry-gas stage (Requ > 2.0%), with the exception of the western part of the Central uplift (e.g., around well T1) and the Southern uplift, where the source rock is still at the oil window (Requ < 1.3%) (Figure 18D). The other set of source rock, the Middle–Upper Ordovician (O2+3), is absent in the western part of the Southwest depression, the western part of the Central uplift, and the northeastern part of the Northern uplift. Because of its thickness of more than 6000 m (>19,685 ft) in the eastern region of the basin, the O2+3 evolved into the main gas or drygas stage by the end of the Ordovician (Figure 19A). With the late Paleozoic and Mesozoic succession deposition over it, the O2+3 evolved into late and overmature stages over the basin (Figure 19B, C). During the Cenozoic, the maturity advanced to the 814

dry-gas stage in the northwestern part of the basin but only advanced slightly in the other parts of the basin (Figure 19D).

DISCUSSION Generally, thermal gradient evolution is controlled by the tectonic setting. The Tarim Basin developed initially as a cratonic peripheral aulacogen resulting from the opening of the paleo–Asian Ocean in the Cambrian and the Early Ordovician (Jia et al., 1995). The stretching and thinning of the crust resulted in high geothermal gradients at that time. In our study, the thermal gradient of the Cambrian and Early Ordovician in the eastern and central basin is 37 to approximately 39°C/km. The cratonic periphery aulacogen disappeared when the paleo–Asian Ocean was closed in the Late Ordovician. Subsequently, the basin entered an intracratonic phase in the Silurian, and the thermal gradient decreased gradually from the Silurian


to the Permian. Since the Triassic, the basin experienced the formation of a foreland basin during the Mesozoic and the Cenozoic, and the geothermal gradient decreased continuously to its present-day value of 20 to 25°C/km. In addition, the geothermal gradient was affected by the thermal conductivity of the strata. Generally, mudstone has the lowest thermal conductivity value, sandstone and carbonate have higher thermal conductivity values, and gypsum and salt have the highest thermal conductivity values in the sedimentary rocks. The carbonate successions of the Cambrian and the Ordovician and gypsum and salt in the Cambrian strata have higher thermal conductivity values (Figure 2). Those strata with higher thermal conductivity values have lower thermal gradients. In the central and northwestern parts of Tarim Basin, approximately 800 to 1000 m (∼2625–3280 ft) of gypsum and salt exist in the Cambrian strata, which may have resulted in lower geothermal gradients during the Cambrian and the Ordovician (e.g., wells T1 and H4) because of their higher thermal conductivity values (4.75 ± 0.5 W/m.K). Effective Paleozoic Thermal Indicators The Paleozoic thermal history controls the maturation of the lower Paleozoic source rocks. In previous studies, only Requ data have been used to model the thermal history of the Cambrian to the Early Ordovician, and a gradient of 29.5 to 30°C/km was determined (Xie and Zhou, 2002; Wang et al., 2003). The current study integrated the different temperature ranges of apatite and zircon (U-Th)/He ages, AFT, and Requ data to reveal the thermal history of the Tarim Basin for the first time. Our study shows that a more complete reconstruction of thermal history of the well section was possible with the integration of multiple thermochronometers than by application of any one technique alone. The (U-Th)/He thermochronometry has been widely used to study the thermal evolution in orogenic belts. However, the use of (U-Th)/He thermochronometry for the study of thermal histories of sedimentary basins is less common (House et al., 1999, 2002; Crowhurst

et al., 2002; Lorencak et al., 2004). The (U-Th)/He ages can be used as a new kind of thermal indicator for sedimentary basins. The apatite He age– temperature profile (Figure 5A) indicates that the closure temperature of apatite He in the Tarim Basin is higher than that reported in previous works (e.g., Wolf et al., 1998; Warnock et al., 1997; Farley, 2000). This closure temperature from the geologic samples of the Tarim Basin can reveal the natural evolution of He closure within the apatite crystal during the geologic time and temperature, and the higher closure temperature can be used to model thermal history with a higher temperature. More recent studies showed that radiation damage associated with U and Th decay altered the apatite He diffusion parameters and thus closure temperature to evolve through time (Shuster et al., 2006; Shuster and Farley, 2009; Flowers et al., 2009). Shuster et al. (2006) found that radiation damage at typical levels may impede the mobility of He, and an apatite with higher eU will accumulate more damage “traps.” The damage trap is a kind of isolated site and He will be confined within these traps (Shuster et al., 2006). An apatite with higher eU will develop a higher He closure temperature than an apatite with lower eU that accumulates fewer damage traps. If this is true, eU values in most samples in our study are higher than the typical eU value of 28 ppm, based on Flowers et al. (2009). These higher eU values result to the higher He closure temperature of apatite in the Tarim Basin (Figure 5B). In addition, most of our study samples are collected from the Paleozoic and Mesozoic strata, and these long periods will accumulate more eU, which in return will result in a higher He closure temperature of apatite. Thermal indicators of the Paleozoic samples in deeply buried parts of the basin did not retain the recorded Paleozoic thermal information because they were reset by the deep burial and high temperatures caused by the overburden of the Mesozoic and Cenozoic successions. Because the Paleozoic thermal history was overprinted in the Mesozoic and the Cenozoic, some wells on highs were selected to model the Paleozoic thermal histories. These highs had been uplifted beginning with the Hercynian movement stage I, where Qiu et al.

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the strata of the Devonian, the Carboniferous, the Permian, and the Triassic were eroded or not deposited. The Paleozoic strata had a relatively shallow burial depth in this area, thus, the thermal indicators were preserved and the Paleozoic thermal information can be extracted and applied to model the Paleozoic thermal history (e.g., wells KQ1 and T1). The Tarim Basin was a cratonic peripheral aulacogen during the Sinian to the Ordovician (Jia et al., 1995); these highs did not exist at these times and almost no structural differentiation existed throughout the basin. These highs were uplifted by the Caledonian tectonic movement or Hercynian tectonic movement during the late Paleozoic (Jia et al., 1995; Jia and Wei, 2002; He et al., 2005). Subsequently, structural differentiation existed in the basin. Therefore, a Paleozoic thermal history from the highs can be extrapolated to the other regions within the basin. Based on this, wells KQ1, T1, H4, TD1, and TZ1 were selected to study the Paleozoic thermal histories. The presence of some interbedded marine clastic sediments in Middle–Upper Ordovician strata (e.g., well KQ1) and the basal Cambrian in some wells (e.g., well T1) provided the opportunity to investigate the apatite and zircon samples to analyze the fission-track and (U-Th)/He ages. Samples from the Precambrian and the Cambrian strata of well T1 and samples from the Ordovician strata in well KQ1 contained effective thermal information for the early Paleozoic thermal history reconstruction. In addition, the uncertainties of the thermal modeling results are important to the interpretation of thermal maturity profiles. In our study, no measured Requ data were available for the Mesozoic and the Cenozoic strata because of the absence of samples; this lack of measurements may reduce the reliability of the modeling results to some extent. Thermal maturity profiles were calculated by assuming that the uncertainty of the thermal gradient values was ±5 or ±10% of the best-case result (e.g., well H4 in Figure 20). When the thermal gradient values were changed by ±10%, the Requ profile appeared to be inconsistent with the measured values, especially in the lower Paleozoic section. When the thermal gra816

Figure 20. Sensitivity of the thermal gradient on the modeled equivalent vitrinite reflectance (Requ) profiles for well H4. The sensitivity test is performed by the uncertainty of thermal gradient values by ±5 or ±10% of the best case. The crosses are measured Requ data. a = –10%; b = –5%; c = best case; d = +5%; e = +10%. C– = Cambrian; O = Ordovician; C = Carboniferous; P = Permian; S1 = Lower Silurian; D = Devonian; N = Neogene.

dients were adjusted by ±5%, the Requ profiles in the Cambrian–Ordovician remained inconsistent with the measured Requ data. This poor fit between models and data in the lower Paleozoic strata indicates that the sensitivity of thermal modeling by the best-fit method is less than ±5%. However, the gradient was mostly consistent with the measured Requ data in the upper Paleozoic strata. In our study, the thermal indicator data were also a key factor in the thermal history reconstruction. However, measurement errors may reduce the reliability of the modeled results. Errors in the (U-Th)/He ages used in our model were generally less than ±3%, and the errors in the Requ data were smaller than ±10%. We agreed with Li et al. (2010) that the accuracy of the thermal modeling results using Requ data is approximately ±5%. Paleozoic Thermal Scenarios for the Maturation of the Main Source Rocks The thermal evolution of source rocks in the Paleozoic stratigraphic sequences has long been an outstanding problem for petroleum exploration in


Figure 21. Effects of different thermal scenarios on the maturity history of Cambrian (C– ) source rocks in well TZ1. a = 20°C/km; b = 35° C/km; c = 45°C/km. Requ = equivalent vitrinite reflectance; O = Ordovician; O1 = Lower Ordovician; C = Carboniferous; P = Permian; Tr = Triassic; J = Jurassic; K = Cretaceous; E = Paleogene; S = Silurian; D = Devonian; E~Q = Cenozoic; N + E = Tertiary.

the Tarim Basin because the thermal history of the Paleozoic could not be reconstructed objectively as a result of the lack of effective thermal indicators in the early Paleozoic carbonate successions. However, it is the Paleozoic thermal history that controls the oil generation of the Cambrian and the Lower Ordovician source rocks. The maturity history of Cambrian source rock for different early Paleozoic thermal scenarios was modeled (Figure 21). The results show that a low Paleozoic thermal gradient (e.g., 20°C/km) was favorable to the maturation of the source rocks, and the source rock was still within the oil window, which may reflect a positive scenario for petroleum exploration. However, the predicted Requ data did not fit the measured values occurring during the Cambrian and the Ordovician strata. The early Paleozoic thermal gradient in our study resulted in the main gas generation stage in the Middle–Late Ordovician, which indicated that the oil accumulation from the Cambrian source rock was very early (e.g., O2+3, Jin and Wang, 2004). The maturity of the Cambrian source rock reached

the maximum value at the end of the Ordovician, and no post-Paleozoic increase in thermal maturity existed in the high within the basin (but the maturity may have increased in the deep depressions within the basin). However, a higher thermal gradient (e.g., 45°C/km) would result in higher Requ values of the Cambrian and the Ordovician strata than those of the measured values. The Paleozoic thermal gradient in this study fits well with the measured Requ data.

CONCLUSIONS Based on the data from core and cuttings samples in the Tarim Basin, a new model of apatite He ages with temperature was obtained. The higher He closure temperature of apatite in our study might have resulted from higher eU values accumulated during the long geologic period. The (U-Th)/He data provide a new kind of thermal indicator for thermal reconstruction of the Tarim Basin. By Qiu et al.

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integrating multiple thermal indicators, He ages, AFT, and Requ data, with thermal history reconstruction, this study provides better definition of thermal gradient evolution of the Tarim Basin during the Paleozoic. The modeled results showed that the Paleozoic thermal gradients differed greatly in different parts of the basin. The eastern region of the basin experienced a decreasing thermal gradient evolution from 37 to approximately 39°C/km during the Cambrian and the Ordovician to 35 to approximately 36°C/km in the Silurian (e.g., wells TD1, KQ1, and TZ1), and then it decreased to 29 to approximately 34°C/km in the late Paleozoic. However, the northwestern part of the basin initially experienced an increasing thermal gradient history from 28 to approximately 32°C/km in the Cambrian to 30 to approximately 34°C/km in the Ordovician and the Silurian (e.g., wells T1 and H4) and then began to decrease to 29 to approximately 33°C/km in the late Paleozoic. The lower early Paleozoic gradient resulted from the 800- to approximately 1000-m (2625- to ∼3280-ft) thickness of gypsum and salt in the Cambrian strata and their higher thermal conductivity values. The basin entered an intracratonic phase in the late Paleozoic and foreland basin phase in the Mesozoic, during which thermal gradients decreased continuously to the 20 to approximately 25°C/km gradients of the present day. The sensitivity of thermal modeling by the bestfit method is less than ±5% in our study, and the differences of the early Paleozoic thermal gradient evolution in different regions of the basin may result in differential maturation of lower Paleozoic source rocks. The maturity history of the lower Paleozoic source rocks was modeled based on the new thermal history. The modeled results showed that the lower Paleozoic source rocks evolved rapidly and reached the main gas generation stage in most areas of the basin during the Paleozoic, with relatively slow maturation during the Mesozoic and the Cenozoic. The Paleozoic thermal history and the maturity history of the lower Paleozoic hydrocarbon source rocks of the Tarim Basin may provide new evidence for oil and gas accumulation periods and the oil and gas exploration potential of the different parts of the basin. 818

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