Biogenic Gas Systems in Eastern Qaidam Basin
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ARTICLE IN PRESS Marine and Petroleum Geology 25 (2008) 344–356 www.elsevier.com/locate/marpetgeo Biogenic gas systems in eastern Qaidam Basin Yuqi Danga, Wenzhi Zhaob, Aiguo Sub, Shuichang Zhangb,Ã, Maowen Lic, Ziqiang Guana, Dade Maa, Xinling Chena, Yanhua Shuaib, Huitong Wangb, Yanhu Tana, Ziyuan Xua aPetroChina Qinghai Oilfield Company, Dunhuang, Gansu 736202, China bPetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China cGeological Survey of Canada, 3303-33 Street NW Calgary, Alberta, Canada T2L 2A7 Received 18 November 2006; received in revised form 2 April 2007; accepted 3 May 2007 Abstract Several giant biogenic gas fields (with proven gas reserves greater than 25 billion cubic meters) have been discovered in recent years in the Sanhu area of eastern Qaidam Basin. This area has an average surface altitude of around 2800 m, and forms the northern segment of the Qinghai-Tibet Plateau. The biogenic gas fields occur mostly within or adjacent to the depocenter of approximately 3400 m of Quaternary sediments. The gas reservoirs, with burial depth generally less than 1900 m, are unconsolidated sandstones with approximately 24–40% porosity, and are interbedded with mudstones containing on average 0.3% TOC. The occurrence of methanogens in the shallow Quaternary sediments appears to depend on both the sedimentary facies and burial depths, thus most of the biogenic gases in the Sanhu area appears to have derived from the source kitchens in the central sag above a biogenic gas floor at the depth around 1800 m. The key gas system elements for the formation of the giant biogenic gas accumulations include (1) secular low surface temperatures and lake water hypersalinity favor the preservation of suitable organic substrates for biogenic methane generation, (2) well-developed sand and mud interbeds, (3) sufficient cumulative thickness of water-saturated mudstones as caprocks, (4) presence of syndepositional anticlines of Pleistocene and later age, (5) a regional hydrogeological system favoring northeastward gas migration, and (6) ongoing dynamic gas migration and accumulation with abundant gas supply. r 2008 Elsevier Ltd. All rights reserved. Keywords: Biogenic gas; Qaidam basin; Natural gas in Chinese basins; Quaternary 1. Introduction northern continental shelf of South China Sea (He et al., 2002; Huang and Xiao, 2002; Pan et al., 2002), Bohai Bay Giant Quaternary biogenic gas fields (i.e. fields with Basin (Shi, 2002), Subei Basin (Zheng, 1998), Junggar more than 25 billion cubic meters of proven gas reserves) Basin (Liao et al., 2001; J. Wang et al., 2003; Li et al., are seldom reported worldwide. In recent years, commer- 2004), Songliao Basin (Zhang et al., 2004) and Hetao Basin cial biogenic gas discoveries have been made in many in Inner Mongolia (Yang, 2004). However, the Sanhu area sedimentary basins in China, including the Sanhu region of of the eastern Qaidam Basin is the only region in China the eastern Qaidam Basin (e.g. Pang et al., 2005), Luliang where giant biogenic gas accumulations have been dis- and Baoshan basins in Yunnan (Xu et al., 1999; Dai et al., covered (Pang et al., 2005). 1999; Song et al., 1999; Luo, 1999; Wang and Luo, 2000), There is still a significant gap in our understanding of the Baise Basin in Guangxi (Luo et al., 2003), alluvial plain shallow biogenic gas systems in the Sanhu area. This area areas along the southeastern coast (Lin et al., 1997, 1999, has an average ground surface altitude of approximately 2004; Jiang et al., 1997; Lu, 1998; Yan et al., 1998; Chen 2800 m above sea level. The high altitude, low surface et al., 2003), Yinggehai and Qiongdongnan basins in the temperature, high water salinity and high sedimentary rates are considered among the important geographic and ÃCorresponding author. Tel.: +86 13701229055. geological attributes that are responsible for the accumula- E-mail address: [email protected] (S. Zhang). tion and preservation of gas-source organic matter 0264-8172/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2007.05.013 ARTICLE IN PRESS Y. Dang et al. / Marine and Petroleum Geology 25 (2008) 344–356 345 (Gu, 1993; Zhou et al., 1994; Qi et al., 1997; Zhang et al., exploration strategies for shallow biogenic gases with an 2004a, b), and resultant biogenic gases (Guan et al., 1997, early generation system. 2001; Dang et al., 2003, 2004; Zhu et al., 2003). The very recent dynamic accumulation is believed to be significant 2. Geological settings for the formation of giant biogenic gas fields in the eastern Qaidam Basin (Qi et al., 1997; Yang et al., 1999; Jin et al., The Qaidam Basin in northwestern China is a large 2002; Xu et al., 2002; Dai et al., 2003; M. Wang et al., 2003; Mesozoic–Cenozoic continental basin, bounded by three Ma et al., 2004; Xu and Xie, 2004). major maintain ranges (i.e. Kunlun, Qilian and Altun The purpose of this manuscript is to review the Chinese mountains, Fig. 1). Based on the basement structure and literature devoted to shallow biogenic gas systems, with a sedimentary covers, this basin can be divided into three first- particular focus on the Sanhu area. This review is intended order structural units: the northern margin fault-fold belt, to provide the first steps toward recognition of specific andthewesternandeasterndepressions. Granite, gneiss and Mountains Qilian Mountains Qaidam Basin Altun Lenghu China Youshashan West Taijinnaier East Taijinnaier Qaidam Basin Seli Dabuxun River+Lake Desert High mountains (>3400 m) Medium mountains (3000-3400 m) Low mountains (2600-3000 m) Kunlun Mountains Qilian Mountains A sion Nor thern Fault Block Taijinaier Dacaidan Western DepresWest Taijinaier East TuofengshanC’ C Taijinaier Tainan North Sebei 1 Slope Yanhu Sebei 2 Eastern DepressionSeli Dabuxun Central sag South A’ Slope Deformed boundary 1st-order structural unit Golmud 2nd-order structural unit Gasfield Section Location Anticline River Lake Town Kunlun Mountains Fig. 1. (a) Geographic location of the Qaidam Basin and (b) the division of structural units in the central and eastern Qaidam Basin. ARTICLE IN PRESS 346 Y. Dang et al. / Marine and Petroleum Geology 25 (2008) 344–356 green schist occur in the basement of the eastern and western the depocenter being shifted eastward, and the develop- depressions. Weakly metamorphosed Paleozoic carbonates ment of the large-scale lake basin then terminated (Zhai, are also present in most areas of the basin, with the exception 1990; Huang et al., 1996; Jin et al., 1999, 2002; Hanson of the eastern depression and the Kunlun piedmont zone that et al., 2001; Xia et al., 2001). contains igneous rocks (Zhai, 1990). With the eastward shift in depocenter, fluvial-lacustrine It is this difference in basement composition that sediments were deposited in the Sanhu area (Fig. 2). The resulted in the structural response in different parts of the thickness of the Quaternary sediments is up to 3400 m and basin to the north-south compression. The evolution of mostly in the range of 1500–2000 m, with the mean past the Qaidam Basin has experienced three major stages: the sedimentary rate around 0.6–0.7 mm per year. In the early piedmont fault depression stage in the Mesozoic, the Pleistocene fluvial-deltaic sediments were deposited with unified depression stage in the Tertiary, and the cyclic abundant terrestrial debris. The development of the lake folding in the center and west but subsiding depression in basin reached its climax by the middle Pleistocene, with the the east in the Quaternary. At the end of the Pliocene, the deposition of dominantly deepwater fine-grained argillac- intense northward subduction of the Indian Plate led to eous sediments. In the late Pleistocene, the Sanhu area was significant elevation of the central and western Qaidam continuously elevated due to the influence of the Quatern- Basin, while the Sanhu area in the east remained relatively ary neo-tectonic movement. As the climate became drier, stable and formed a large inland lake in the Quaternary. the lake shrank with waters becoming shallower and Since the Holocene, the eastern depression has uplifted as saltier. In the Holocene, the lake dried out as the whole part of the re-upheaval of the Qinghai-Tibet Plateau, with Qaidam Basin was elevated (Table 1). Dabuxun A lake A’ 3 Q N 2 5.5 Ma - Now 2 N 2 1 N2 40.0 - 5.5 Ma N1 E2 Mz-E 1+2 208.0 - 40.0 Ma A A’ Quanternary Neogene facies Deep lake Paleogene Shallow and shore lak facies Fig. 2. (a) Schematic cross section showing the shift in depocentres as a function of geological time; and (b) variation in sedimentary facies in eastern Qaidam Basin. See Fig. 1 for the location of the cross section. Table 1 Quaternary stratigraphy in the Sanhu area, Qaidam Basin Stratigraphic system Average thickness (m) Sedimentary facies Q Holocene Yanqiao Fm. (Q4) 20 Saline lake/kavir Pleistocene Upper Dabuxun Fm. (Q3) 80 Lacustrine bog/saline lake Middle Qigequan Fm. (Q1+2) 1700 Shallow to semi-deep water lacustrine Lower ARTICLE IN PRESS Y. Dang et al. / Marine and Petroleum Geology 25 (2008) 344–356 347 The Qigequan Formation (Q1+2) is the main Quaternary occasionally up to 19% in carbonaceous mudstones (Zhou stratum in the Sanhu area (Table 1), with a maximum et al., 1994; Xu and Xie, 2004). thickness of 3200 m and an average thickness of 1700 m. Based on seismic reflectance features, this forma- 3. Gas system description tion can be divided into 14 units (K0-K13, from top to base). All of the gas pays discovered in the Sanhu area are 3.1. Gas geochemistry and early gas generation within this formation, which are grouped into four payzones (Fig. 3). The critical moment of gas generation, migration and Spore-pollen assemblages and geomagnetic dating data accumulation is an important characteristic for unconven- indicate several alternating cold and dry–warm and humid tional biogenic gas systems.