China Shale Gas & Liquids - Bohai Bay Basin
By Yuanjia Han, China University of Petroleum, Wuhan, China
After the first discovery of Hua8 well in 1961 (Song, 2018), petroleum production has been ongoing in the Bohai Bay Basin. The Bohai Bay Basin consists of seven subbasins, including the Linqing, Jizhong, Huanghua, Jiyang, Bozhong, Liaodong Bay, and Liaohe subbasins (Figure 1). A total of more than 2.2 billion tons of oil and nearly 240 billion cubic meter of natural gas have been produced to date (Jiang, 2014), much of that from the well-known Shengli Oilfield, Jiyang subbasin. Nearly 33% of China’s domestic oil production comes from the Bohai Bay Basin (Hao et al., 2011), which makes it one of the most petroliferous basins in China.
Geological Setting
The Bohai Bay Basin is a Cenozoic rifted basin formed on the basement of the North China Craton (Tian et al., 2017). As a consequence, the Bohai Bay Basin has a relatively high thermal gradient (2.4-5.5°C/100m) and a relatively high pressure gradient (1.2-1.9 MPa/100m). A series of syndepositional normal faults were induced by the intense extension during Paleogene. Meanwhile, the synrift sequence, i.e., the Kongdian, Shahejie and Dongying Formations (Figure 2), were restricted to the grabens and half grabens and deposited in fluvial-lacustrine environment. The postrift sediments include the Neogene Guantao, Minhuazhen and Pinyuan Formations were deposited primarily in fluvial environment (Ma et al., 2017). The main petroleum source rock, Shahejie Formation, overlies the Paleocene Kongdian Formation and underlies the Oligocene Dongying Formation, in which the Second member of Kongdian Formation (Ek2), the Shahejie Formation (Es1-4) and the Third member of Dongying Formation (Ed3) are the primary targets for shale oil exploration (Xue et al., 2020).
Shale Oil Exploration
In 2013, Well Guandong-6X1 tapped the first commercial shale oil flow in the Huanghua Subbasin (Pu, 2019). As of 2019, two horizontal wells, the GD1701H and GD1702H wells have produced oil from Ek2 shale, with a stable daily oil production of 145 barrels and 220 barrels (Figure 3), respectively. The laminated Ek2 shale mainly consists of interbedded layers of felsic shale, calcareous shale and carbonates. The Ek2 shale is in overall good-very good source rock containing Type I-II kerogen, in average of 3.6% TOC, and at the oil-mature stage with 0.68% to 1.16% Ro (Zhao et al., 2018). The shale matrix has a porosity of less than 10% and permeability of less than 1×10-3 μm2. Intergranular pores in analcite, intercrystalline pores in dolomite and interlayer micro-fractures make tight carbonate, calcareous shale and felsic shale effective reservoirs, with brittle mineral content of more than 70% (Zhao et al., 2018), in which a significant amount of analcite up to 59 % is identified (Figure 4) (Zhang et al., 2015). Analcite of volcanic origin (Han et al., 2021) is believed one of the most important factors for the successful development of Ek2 shale oil play. The commercial shale oil production in the Dagang Oilfield marks the start of lacustrine shale oil play in the Bohai Bay Basin.
The exploration of shale oil in Jiyang subbasin begins from 2008 (Song et al., 2019). The Shahejie Formation consists of interbedded sandstone and carbonate layers, with laminate shale and musdstone accounting about 31% (Song et al., 2015). The lacustrine Shahejie shale contains 1.5-19.6% TOC in general of Type I-II organic matter origin and mainly at the immature to high mature stage up to 1.3% Ro (Zhang et al., 2012). At the very beginning, the Shahejie Formation specialized horizontal wells are generally low in productivity, with the highest daily oil production of 2.3-9.5 t/d, while the concurrent vertical wells are higher in productivity, with daily oil production of 5.8-154.0 t/d (Song et al., 2020). There is a observable trend of systematic increase in GOR from 13 to 385 m3/t with depth increase to about 3500m (Song et al., 2015). Oil mobility in the liquid-rich Shahejie shale is believed especially crucial for economic success of resource development (Li et al., 2019). Although the averaged shale porosity is in range of 4.5% to 13.1% (Song et al., 2015), oil in-place within the Shahejie shale is characteristic of high sulphur content (~2.3%), high resins and asphaltenes (in average 39.7% and 11.2%, respectively), and therefore, high density and viscosity (~57.7 mPa·s) (Figure 5), e.g., poor mobility. In addition, the terrestrial shale in the Jiyang subbasin is characterized by high clay content, weak diagenesis, and strong heterogeneity, adding difficult to fracturing and exploiting. At present, pilot tests have been carried out back to old wells, industrial oil flows varying from 6.3 to 44.0 t/d have been obtained after fracturing (Song et al., 2020).
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