ABSTRACT The Junggar basin is one of the largest oil- producing areas in China, and contains Upper Permian lacustrine oil shales with some of the greatest hydrocarbon potental in the world. In this study, we present the diagenetic characteris- tics of Permian–Triassic sandstones from the southern Junggar basin and evaluate their reser- voir potential. The uppermost Permian and Lower Triassic Cangfanggou Group in the southern Junggar basin is characterized by alternating fluvial and lacustrine deposits, whereas the Middle–Upper Triassic Xiaoquangou Group was deposited predominantly in a lacustrine environment; fluvial and deltaic sedi- mentation was subordinate. The sandstones of the Cangfanggou and Xiaoquangou groups are volcanic litharenites. Their detrital modes and textures of vol- canic fragments suggest a primarily andesitic/ basaltic volcanic-arc provenance. Early diagenesis of the sandstones is characterized by nonferroan calcite cementation, grain-coating, pore-lining clay minerals, and the initial dissolution of detrital grains. Authigenic quartz; pore-filling phyllosilicates; pore-filling, grain-replacive zeolites; albitized detrital plagioclase; authigenic K-feldspar; illite; and late calcite dominate burial diagenesis. The formation of iron oxides and dissolution of cal- cite cement resulted from tectonic uplift during the Tertiary. Albitization and zeolite formation during burial are among the most pronounced diagenetic processes that affected these sandstones. Pore-filling clay minerals, calcite, and zeolites have substantially reduced sandstone porosity. However, appreciable primary porosity has been preserved by the formation of early clay coats and pore linings, which retarded further cementation. Secondary porosity is present to varying degrees in the sandstones and is the result of dissolution of unstable framework grains. In the Santai and adja- cent areas of the southern Junggar basin, the Cangfanggou Group sandstones can be moderate to good quality reservoirs (measured core porosity ranges from 12.9 to 23.7%, averaging 18.2%), whereas the Xiaoquangou Group sandstones in general have relatively poor reservoir quality. Based on burial and thermal histories and diagenetic pat- terns, we predict that equivalent Permian–Triassic sandstone intervals in the enormous western part of the southern Junggar basin have potential as oil and (particularly) gas reservoirs. INTRODUCTION The Junggar (also transliterated as “Zhungeer”) basin is one of the largest and most important oil- producing areas in China. The basin is located in far northwestern China, and is bounded by the sur- rounding mountain ranges: the Tian Shan (“Shan” means “mountain” in Chinese) to the south, the West Junggar Shan to the northwest, and the Kelameili Shan to the east (Figure 1). The basin occupies an area of about 140,000 km 2 and is cov- ered by a sedimentary succession of Upper Permian through Cenozoic nonmarine strata that can be more than 14 km thick (Wu, 1986) (Figure 1). The Upper Permian lacustrine oil shales in the Junggar basin rank among the thickest and richest petroleum source rock intervals in the world (Graham et al., 1990; Lawrence, 1990; Carroll et al., 1843 ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved. 1 Manuscript received April 15, 1996; revised manuscript received November 4, 1996; final acceptance May 2, 1997. 2 Department of Earth Sciences, University of Western Ontario, London, Ontario, N6A 5B7, Canada. 3 School of Geosciences, Queen’s University of Belfast, Belfast BT7 1NN, United Kingdom. This work was done at the Queen’s University of Belfast with funding from Exxon, BHP, Mobil, and ITOTAL, which we gratefully acknowledge. The Karamay Institute of Petroleum Exploration and Development generously provided borehole samples. Geng Ansong at the Institute of Geochemistry (Guangzhou), Academia Sinica, and Zhou Yisheng at the Xinjiang Institute of Petroleum are thanked for assistance with field work and for valuable discussions. Special thanks go to the staff of the Electron Microscopy Unit at the Queen’s University of Belfast. We also thank A. H. Ruffell and S. R. Lawrence for their constructive comments on an earlier version of the manuscript. Stable isotope analysis and ongoing research concerning the Junggar basin at the University of Western Ontario have been supported by the Natural Science and Engineering Research Council of Canada. The manuscript benefited substantially from critical commentary by Kevin T. Biddle, Suzanne Kairo, W. R. Kaiser, and an anonymous reviewer. Diagenesis and Reservoir Potential of Permian–Triassic Fluvial/Lacustrine Sandstones in the Southern Junggar Basin, Northwestern China 1 Zhaohui Tang, 2 John Parnell, 3 and Fred J. Longstaffe 2 AAPG Bulletin, V. 81, No. 11 (November 1997), P. 1843–1865.