Longmen Shan fold-thrust AUTHORS Dong Jia Department of Earth Sciences, belt and its relation to the Nanjing University, Nanjing 210093, People’s Republic of China; [email protected] western Sichuan Basin in Dong Jia is a professor in the Department of Earth Sciences of Nanjing University in China. central China: New insights He received his Ph.D. in structural geology from Nanjing University in 1988. His current from hydrocarbon exploration research projects are focused on structural geometries and timing of deformation in the sedimentary basins of China. Dong Jia, Guoqi Wei, Zhuxin Chen, Benliang Li, Qing Zeng, and Guang Yang Guoqi Wei Research Institute of Petro- leum Exploration and Development, Langfang Branch, PetroChina Company Limited, Lang- fang 065007, People’s Republic of China; ABSTRACT [email protected] The Longmen Shan fold-thrust belt is one of the key regions of de- Guoqi Wei is a chief geologist in the Langfang monstrable Mesozoic–Cenozoic tectonic evolution in China, and the Branch of the Research Institute of Petroleum Sichuan Basin was the first natural-gas-producing area in China. In Exploration and Development, PetroChina this article, the structural features of the Longmen Shan belt are Company Limited. He received his Ph.D. from Nanjing University in 2000. His current re- presented, using both seismic profiles and field data. The complex searches are focused on sedimentary basins in structures of the northeast-trending Longmen Shan fold-thrust belt western China. and its foreland in the western Sichuan Basin are formed by southeast-directed thrusting. Several eastward-verging, rootless Zhuxin Chen Department of Earth Sci- thrust sheets and imbricates of Cambrian–Triassic rocks have been ences, Nanjing University, Nanjing 210093, recognized in the northern Longmen Shan belt. Evidence suggests People’s Republic of China; that the northern Longmen Shan belt experienced at least two major [email protected] periods of deformation in the Late Triassic and Cenozoic. However, Zhuxin Chen received his B.A. degree in the southern Longmen Shan belt is represented by the basement- petroleum geology from the Chinese University involved thrust structures and klippen, and its major periods of of Petroleum in 2001. He is currently finish- deformation were in the latest Cretaceous–early Cenozoic. Sedi- ing his Ph.D. (geology) at Nanjing University, mentary features in the western Sichuan Basin reflect a two-phase where he specializes in seismic interpretation and balancing cross sections. flexural-loading history and illustrate that the Late Triassic fore- land basin extends along the foredeep of the entire length of the Benliang Li Research Institute of Petro- Longmen Shan belt, but the uppermost Cretaceous–Paleogene re- leum Exploration and Development, Langfang juvenated foreland basin is restricted in the southern part of the Branch, PetroChina Company Limited, Lang- western Sichuan Basin. fang 065007, People’s Republic of China; Structural geometries suggest that prospective traps are mainly [email protected] developed in the frontal zone of the Longmen Shan fold-thrust belt Benliang Li is a senior geologist in PetroChina and in the southern part of the western Sichuan Basin. One of the Company Limited. He earned a B.A. degree major contributions of this article is finding preexisting Paleozoic in geology from the Chinese Southwestern rift basins under the Cenozoic thin-skinned thrust belts, which Petroleum Institute in 1995 and obtained a represent a new potential hydrocarbon play. Ph.D. in geology from Nanjing University in 2000. His current research focuses on the Mesozoic–Cenozoic basin in western China. Copyright #2006. The American Association of Petroleum Geologists. All rights reserved. Manuscript received May 10, 2005; provisional acceptance December 1, 2005; revised manuscript received January 18, 2006; final acceptance March 23, 2006. DOI:10.1306/03230605076 AAPG Bulletin, v. 90, no. 9 (September 2006), pp. 1425–1447 1425 Qing Zeng Southwest Oil & Gasfield INTRODUCTION Company, PetroChina Company Limited, Chengdu 610051, People’s Republic of China; The northeast-trending Longmen Shan (Shan = mountains) lies along [email protected] the eastern boundary of the Tibetan Plateau and is one of the key Qing Zeng is a senior research scientist in regions of demonstrable Mesozoic–Cenozoic tectonic evolution of PetroChina Company Limited. He obtained his the China continent (Sengo¨r and Hsu¨ , 1984; Mattauer et al., 1985; B.A. degree in structural geology from Nanjing Huang and Chen, 1987; Xu et al., 1992). This area is trapped among University in 1986. Presently, he is working on the North China, South China, and Qiangtang blocks in the tec- several projects on structural reconstruction tonic collage of China (Figure 1). The recent phase of exploration and trap analysis in the Sichuan Basin. has provided new evidence for structural geometries, deformation Guang Yang Southwest Oil & Gasfield timing, and potential hydrocarbon plays in the western Sichuan Company, PetroChina Company Limited, Basin. Chengdu 610051, People’s Republic of China Guang Yang is the project manager in Petro- China Company Limited. He received his B.A. Tectonic Setting degree in geology from Nanjing University in 1984, followed by his M.S. degree in 1989. The foreland fold and thrust belt of the Longmen Shan is about His interests include petroleum geology, basin 500 km (310 mi) long and 30–50 km (18–31 mi) wide and can be analysis, and entrapment assessment. traced from the Micang Shan of the Qinglin Mountains through the northern and southern Longmen Shan to the Kangdian paleo- high (Figure 1). To the west of the Longmen Shan is a vast triangu- ACKNOWLEDGEMENTS lar region underlain by strongly deformed Middle and Upper Triassic This research was supported by grants from deep-marine strata. These Triassic rocks comprise the Songpan- the National Science Foundation of China Garzeˆ basin, recognized as an ancient remnant ocean basin by Yin (Grant No. 40372091). Seismic profiles and and Nie (1993). To the southeast lies the western Sichuan Basin, drill data for this study, as well as consent to cored by the Yangtze craton (eastern part of the South China block). publish selected data, were generously pro- The Qiangtang block (North Tibet) and one to three accreted vol- vided by PetroChina Company Limited. We are especially grateful to Jia Chengzao, Dai Jinxing, canic arcs of late Paleozoic to early Mesozoic age tectonically bound Deng Qidong, Lu Huafu, Chen Hanlin, and the Songpan-Garzeˆ and Longmen Shan to the southwest (Sengo¨r Xiao Ancheng for their time, support, and geo- and Hsu¨ , 1984; Xu et al., 1992). logic insight. This article benefited from con- The Longmen Shan forms a northeast-trending chain whose structive and very helpful reviews by Jim Granath, internal structures parallel the trend of the chain. In contrast, the Peter Hennings, and Alexander A. Kitchka. folds and thrusts in the Songpan-Garzeˆ basin form a series of convex-to-the-south thrusts and folds (Figure 1) (Burchfiel et al., 1995; Harrowfield and Wilson, 2005). In this region, the Longmen Editor’s Note: Shan rises to more than 6 km (3.7 mi) above the Sichuan Basin, Color versions of figures may be seen in the forming one of the steepest mountain fronts along any margin of online version of this article. the Tibetan Plateau. Burchfiel (2004) provided the interpretation that the steep topographic front of the Longmen Shan results from resistance to eastward crustal flow because of a strong, less ductile crust underlying the adjacent Sichuan Basin (Lebedev and Nolet, 2003). However, the timing and geometry of the complex struc- tures of the northeast-trending Longmen Shan fold-thrust belt remain controversial (Lu et al., 1989; Luo, 1991; Xu et al., 1992; Burchfiel et al., 1995; Liu et al., 1995; Chen and Wilson, 1996; Jia et al., 2003; Wallis et al., 2003; Roger et al., 2004). It is gen- erally assumed that at least two major orogenic events occurred in the Longmen Shan belt since the Mesozoic: a Late Triassic compressional event (Indosinian orogeny) and a Cenozoic defor- mation related to the India–Asia collision (Dirks et al., 1994; Burch- fiel et al., 1995). Many geologists propose that the Longmen Shan 1426 Longmen Shan Fold-Thrust Belt Figure 1. Tectonic setting and generalized map of the Longmen Shan fold-thrust belt and western Sichuan Basin in central China. The locations of structural cross sections and seismic lines are indicated. QCF = Qinchuan fault zone; BCT = Beichuan thrust; MJT = Majiaoba thrust; WCT = Wenchuan thrust; WLT = Wulong thrust; LGT = Lingguan thrust. Jia et al. 1427 belt in the Late Triassic evolved as a peripheral fore- foredeep history of the Upper Triassic foreland and the land system during the closure of the Songpan-Garzeˆ uppermost Cretaceous–Paleogene rejuvenated fore- ocean basin (Lu et al., 1989; Luo, 1991; Liu et al., 1994, land in the western Sichuan Basin. 1995). Exploration History STRATIGRAPHY AND BASIN EVOLUTION The western Sichuan Basin covering approximately The stratigraphic relationships between the Longmen 66,000 km2 (25,482 mi2) was the first natural-gas– Shan and the western Sichuan Basin are complicated producing area of China. In historical records of ancient by multiple phases of tectonism in the Late Triassic China, several shallow wells drilled in AD 76–147 at and Cenozoic. The chronostratigraphy in Figure 2 has Chengdu and Qionglai produced small quantities of been compiled from published data and proprietary natural gas that were used locally for boiling salt. Ex- information from recently drilled wells. In general, the ploration in this basin began with the drilling of wells western Sichuan Basin consists of a Proterozoic base- near oil seeps and seismic surveys in the 1950s. The ment of the Yangtze craton covered by a thin, incom- first commercial gas field (Zhongba gas field) near plete succession of Sinian (uppermost Proterozoic) Jiangyou was discovered at the PetroChina, Chuan 19 to Middle Triassic shallow-marine rocks interrupted well in 1971, which has produced 1.2 Â 1010 m3 (4.23 Â by Permian basic magmatism in the south and Upper 1011 ft3) gas.