China Shale Gas & Liquids - Songliao and Junggar Basins
By Zhi Yang, Research Institute of Petroleum Exploration and Development (RIPED), PetroChina, China
Both tight and shale oil occur in the shale strata of the Permian Junggar Basin. The early-middle Permian formations in the Junggar Basin are in sedimentary environments of saline lake basins formed after a section of sea was closed off. Lacustrine-facies dark mudstone and dolomite interbedded deposits are primarily found in these basins. Organic-rich shales are mainly distributed in the Lower Permian Fengcheng Formation in the Mahu Sag, the Middle Permian Lucaogou Formation in the Jimsar Sag, and the Middle Permian Pingdiquan Formation in the Kelameili Front, in layers of large thickness and wide plane distribution; these are the oldest continental shale series in China and the most important oil source rocks in the Junggar Basin. China's oldest continental shale is in the Permian strata of the Junggar Basin, which are composed of fine sediments in an offshore saline lake basin[1][2]. The Middle Permian Lucaogou Formation in the eastern Junggar Basin contains globally rare organic-rich shale, and acts as both source and reservoir of shale oil. It is characterized by superimposed thin layers, large thickness, overall oiliness, and continuous distribution[3]. The Jimsar Sag is a representative sag containing oil-rich shale. Recently, significant progress was made in the development of petroleum resources in the Lucaogou Formation shale in the Jimsar Sag. SRV fracturing was used in horizontal wells and two continuously distributed “sweet sections” were discovered[1][3][4][5]. Using abundant experimental data from the Lucaogou Formation shales in the Jimsar Sag and by analyzing the source rocks, reservoirs, and oil-bearing layers, lithofacies distribution and lithology division were studied, with the aim to arrive at new bases for selecting “sweet sections”.
The Jimsar Sag in the eastern Junggar Basin covers about 1200 km2. It is a dustpan-like sag faulting in the east and overlapping in the west over a Middle Carboniferous fold basement[1][3][4][5]. The Jimsar Sag experienced the Hercynian, Indo-China, Yanshan, and Himalayan movements, among others, and developed thick Upper Paleozoic-Cenozoic strata up to 5000m (Fig. 1-C). The Permian and Cretaceous strata on the eastern slope margin are thicker in the west and thin toward east until ultimately pinching out (Fig. 1-C). The strata in the Jimsar Sag are Carboniferous, Permian (Jiangjunmiao Formation, Lucaogou Formation, and Wutonggou Formation), Triassic, Jurassic, Cretaceous, Tertiary, and Quaternary from the bottom to the top[1]. The Middle Permian Lucaogou Formation consists of semi-deep to deep lacustrine shale with high organic matter abundance and hydrocarbon generation potential; in this formation, favorable reservoirs are developed, such as dolomite, dolomitic siltstone, and argillaceous siltstone. The Lucaogou Formation is 1000–4500m deep and 200–300m thick. It is an oil-rich assemblage with source rocks and reservoirs. Since 2010, over 20 exploration wells drilled in the Lucaogou Formation have found oil/gas, and over 10 wells have commercial oil flow. The horizontal well Ji 172-H, drilled in the “upper sweet section” of this formation in 2012, initially produced 77.8 t/d[1]. Then, significant progress was made using new methods in two horizontal and four vertical wells over the past two years[3][4][5]. This promoted the discovery of large shale oil reserves and demonstrated good prospect for oil extraction from the Lucaogou Formation shales. There are two sets of “sweet sections” in the Lucaogou Formation in the Jimsar Sag and they are characterized by the coexisted combination of tight oil and shale oil (Fig. 2)[3][4][5].
References
- ↑ 1.0 1.1 1.2 1.3 1.4 Kuang, L., Y. Tang, D. Lei, Q. Chang, M. Ouyang, L. Hou, and D. Liu, 2012, Formation conditions and exploration potential of tight oil in the Permian saline lacustrine dolomitic rock, Junggar Basin, NW China: Petroleum Exploration and Development Online, vol. 39, no. 6, p. 657–667.
- ↑ Zhang, Y., X. Qi, X. Cheng, and Z. Luo, 2007, Approach to sedimentary environment of late carboniferous-Permian in Junggar Basin: Xinjiang Petroleum Geology, vol. 28, no. 6, p. 673–675.
- ↑ 3.0 3.1 3.2 3.3 3.4 Yang, Z., L. Hou, S. Lin, X. Luo, L. Zhang, S. Wu, and J. Cui, 2018, Geologic characteristics and exploration potential of tight oil and shale oil in Lucaogou Formation in Jimsar sag: China Petroleum Exploration, vol. 23, no. 4, p. 76–85.
- ↑ 4.0 4.1 4.2 4.3 Yang, Z., C. Zou, S. Wu, S. Lin, S. Pan, X. Niu, G. Men, et. al, 2019a, Formation, distribution and resource potential of the “sweet areas (sections)” of continental shale oil in China: Marine and Petroleum Geology, vol. 102, p. 48–60.
- ↑ 5.0 5.1 5.2 5.3 Yang, Z., C. Zou, L. Hou, S. Wu, S. Lin, X. Luo, Z. Zhao, J. Cui, and S. Pan, 2019b. Division of fine-grained rocks and selection of “sweet sections” in the oldest continental shale in China: Taking the coexisting combination of tight and shale oil in the Permian Junggar Basin: Marine and Petroleum Geology, vol. 109, p. 339–348.