China Shale Gas & Liquids - Sichuan Basin
By Tian Dong, Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan, China
The Sichuan Basin, approximately covering 230000km2, is a large basin on the Yangtze Platform and contains up to 6-12 km of Pre-Cambrian Sinian to Cenozoic sediments. The Upper Ordovician to Lower Silurian Wufeng-Longmaxi Formation is widely distributed across the entire Yangtze region of South China, and the thickest organic-rich shales of Wufeng-Longmaxi Formation is located in the Sichuan Basin and adjacent areas (Fig. 1) (Zou et al., 2019). The shales display varying TOC content in the range of 0.2% to 6%, and are primarily composed of black siliceous mudstone, shelly limestone, gray silty-shaly interlaminated mudstone, gray argillaceous mudstone, and calcareous mudstone (Lu et al., 2020). The commercial gas production is typically from the lower part of Wufeng-Longmaxi Formation that is dominated by black siliceous mudstone (Fig. 2). The commercially producing shale interval has a thickness of 20-40m, an average TOC content of 2-4%, vitrinite reflectance (Ro) of 2-3.5%, porosity of 1.85-8.32%, and pressure coefficient of 1.3-2.1 (Zou et al., 2019; Guo et al., 2020).
The Wufeng-Longmaxi Shale play is estimated to have 17-81 trillion m3 of technically recoverable shale gas resources. In 2012, the first commercially producing well JY1 was drilled in the Jiaoshiba area with daily production over 200×103 m3, indicative of the shale gas breakthrough in China. In 2015, the annual shale gas production from Wufeng-Longmaxi Shale Play reached 7.5 billion m3, and in 2019, the annual shale gas production reached 15 billion m3. According to the new statistics, the annual gas production from Wufeng-Longmaxi Shale Play exceeds 20 billion m3 in 2020. In the Sichuan Basin, the favorable exploration area of shale gas reservoir with burial depth < 3500m is over 63000 km2, and the geological resource is approximately 2.6 trillion m3 (Ma et al., 2018). Until now, in the Sichuan Basin, China has built four giant shale gas fields in the Fuling, Weiyuan, Changning and Zhaotong area typically with burial depth<3500m. The exploration area of shale gas reservoir with burial depth >3500m is estimated to be 128000 km2, and the geological resource is twice as shallow burial shale gas resource (He et al., 2020). In the current stage, the next goal is to commercially produce gas from shale reservoirs with burial depth >3500m. Sinopec and PetroChina have drilled tens of wells in the deep shale gas reservoirs (>3500m), and achieved high initial shale gas production, for example, well DY4 drilled in the southeast part of Sichuan Basin has a daily production exceeding 200×103 m3. In order to achieve big breakthrough in the deep shale reservoir exploration, it is of great important to innovate drilling technologies and equipments.
The gas-producing shale interval, or called “high quality shale”, is characterized by high gamma ray (GR), high uranium (U), high acoustic (AC) and low density (DEN) (Fig. 3). Moreover, the gas-producing shale interval has high brittleness, dominantly organic matter-hosted pores, high pressure coefficient, and is rich in organic matter and quartz content. The abundance of organic pores is positively correlated with shale gas content, well production and recoverable reserves, indicative of the importance of organic pores to shale gas storage and accumulation (Ma et al., 2018). The dissolution and reprecipitation of siliceous skeletons, such as radiolarians, sponge spicules provides the major silica source for authigenic quartz formation that is favorable for both porosity preservation and brittleness enhancement (Dong et al., 2019). Abundant equant euhedral microcrystalline quartz are nucleated to form aggregates that is able to form a rigid framework (Fig. 4), resulting in enhanced brittleness. Migrated organic matter including both bitumen and pyrobitumen were commonly observed to fill in intercrystalline pores between euhedral quartz crystals, and generating organic pores during gas window stage. These organic pores can be preserved by rigid quartz framework during late tectonic movements (Guo et al., 2020). In summary, for the high quality shale interval, high porosity provides abundant pore space for shale gas storage, and high brittleness make the shale interval more likely to fracture under stress and to maintain open fractures.
After the maximum burial in the Jurassic-Cretaceous stage, the Wufeng-Longmaxi shale has experienced multiple tectonic uplifts, generating faults and fractures that cut through shale reservoirs. Excepting TOC content, thermal maturity, porosity, preservation condition is the most significant factor controlling shale gas enrichment and well production. High pressure coefficient is an effective indicator for good gas preservation conditions. In summary, the combination of high TOC content, high quartz content, high porosity and good gas preservation conditions are fundamental to gas enrichment and gas production for the Wufeng-Longmaxi Shale Play.
References
Dong, T., He, S., Chen, M., Hou, Y., Guo, X., Wei, C., Han, Y., Yang, R., 2019. Quartz types and origins in the Paleozoic Wufeng-Longmaxi Formations, Eastern Sichuan Basin, China: Implications for porosity preservation in shale reservoirs. Marine and Petroleum Geology 106, 62-73.
He, Z., Nie, H., Hu, D., Jiang, T., Wang, R., Zhang, Y., Zhang, G., Lu, Z., 2020. Geological problems in the effective development of deep shale gas: a case study of Upper Ordovician Wufeng-Lower Silurian Longmaxi formations in Sichuan Basin and its periphery. Acta Petrolei Sinica 41, 379-391.
Guo, X., Li, Y., Teng, G., Wang, Q., Yuan, T., Shen, B., Ma, Z., Wei, F., 2020. Hydrocarbon generation and storage mechanisms of deep-water shelf shales of Ordovician Wufeng Formation-Silurian Longmaxi Formation in Sichuan Basin, China. Petroleum Exploration and Development 47, 193-201.
Lu, Y., Hao, F., Lu, Y., Yan, D., Xu, S., Shu, Z., Wang, Y., Wu, L., 2020. Lithofacies and depositional mechanisms of the Ordovician-Silurian Wufeng-Longmaxi organic-rich shales in the Upper Yangtze area, south China. AAPG Bulletin 104, 97-129.
Ma, Y., Cai, X., Zhao, P., 2018. China’s shale gas exploration and development: Understanding and practice. Petroleum Exploration and Development 45, 589-603.
Zou, C., Zhu, R., Chen, Z., Ogg, J.G., Wu, S., Dong, D., Qiu, Z., Wang, Y., Wang, L., Lin, S., Cui, J., Su, L., Yang, Z., 2019. Organic-matter-rich shales of China. Earth-Science Reviews 189, 51-78.