Discovery of shale gas from Wufeng-Longmaxi Formation in Xianfeng area, Hubei Province, China

1.   Objective

The Xianfeng area in Hubei Province is located in the central anticlinorium of the fold belt in western Hunan-Hubei, with a stable tectonic structure. It was in a deep-water shelf sedimentary environment from Wufeng period of Late Ordovician to Longmaxi period of Early Silurian, contributing to the formation of organic-rich shale in this area (Li B et al., 2016; Zhang H et al., 2019; Wu X et al., 2019). The well E Feng Di 1 (Well EFD1) was drilled in Xianfeng area in the south of the central anticlinorium by preliminary geological survey and favorable shale gas areas optimization. During the drilling, shale gas was discovered from Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation (Wufeng-Longmaxi Formation). The geological conditions for shale gas enrichment were further defined through the analysis of the data obtained from the well drilling, well logging, and sample testing. The discovery will provide an important basis for further exploration and development of shale gas in this area.

2.   Methods

Well EFD1 was drilled with the ZJ30 drilling rig, and gas logging and comprehensive logging were carried out while drilling. Continuous coring was conducted for the shale of Wufeng-Longmaxi Formation, which was the target strata. Seventy-three shale samples were systematically collected (sampling density: One sample every meter). The gas content of the samples was measured with KD-II desorption analyzers, with an overall desorption duration of over 40 hours. The content of total organic carbon (TOC) and sulfur content of the samples were measured with the CS230 carbon-sulfur analyzer. Other relevant data of Wufeng-Longmaxi Formation shale were obtained through logging, including natural gamma, natural potential, resistivity, and the content of radioactive elements of U, Th, and K.

3.   Results

Organic-rich shale from Wufeng-Longmaxi Formation is revealed by the drilling of Well EFD1 in Xianfeng area, and the gas-bearing shale is 75.74 m in thickness (Fig. 1). It mainly consists of black carbonaceous shale and carbonaceous siliceous shale. The horizontal bedding is widely developed, and the high-dip micro-fractures are filled with calcite locally. The pyrite, fossils of graptolites and radiolarians are rich in the shale, which indicates the sedimentary environment is a deep-water shelf. The carbonaceous shale of Longmaxi Formation was initially discovered at a well depth of 1460.00 m. After that, the anomalous values of total hydrocarbon content and methane content continued to rise according to gas logging. They rise from 0.32% to 3.41% and from 0.22% to 2.99% respectively during the drilling of the first-barrel core at a depth of 1463.20–1474.12 m. Furthermore, they surged to 9.62% and 7.59% respectively at a depth of 1526.32–1528.47 m from the top of Wufeng Formation. Eight barrels of cores were obtained by continuous coring from Wufeng-Longmaxi Formation shale, with drilling footage of 74.50 m and a total core length of 72.00 m, leaving the core recovery rate at 96.60%. Core flooding experiments were conducted on-site. Large numbers of moniliform bubbles were visible and emerged continuously during the experiments, indicating the richness of shale gas. The desorbed gas content (excluding residual gas) of the shale cores is 0.20–2.53 m³/t, with an average of 1.67 m³/t.

Figure 1.  Histogram of the comprehensive evaluation of shale gas from Wufeng-Longmaxi Formation as revealed by Well EFD1 (1:500).

DownLoad: Full-Size Img PowerPoint

The TOC and sulfur content of the samples of Wufeng-Longmaxi Formation shale were measured. The TOC is 0.81%–4.60%, with an average of 1.74%. The sulfur content is 1.03%–18.43%, with an average of 3.16%. The shale is especially favorable at a depth of 1523.47–1535.34 m from the bottom of Longmaxi Formation to the top of Wufeng Formation, with a thickness of nearly 12 m. In this shale section, TOC is 1.74%–4.60%, with an average of up to 3.57%. The respective content of U, Th, and K of Wufeng-Longmaxi Formation shale are shown to be 11.36×10^–6–52.40×10^–6 (19.39×10^–6 on average), 9.52×10^–6–74.78×10^–6 (18.56×10^–6 on average), and 0.48%–6.14% (1.35% on average) by gamma-ray spectroscopy logging results. The shale at a well depth of 1525.24–1529.24 m (the thickness is 4 m) features a high natural gamma, showing abnormally high content of radioactive elements of U, Th, and K.

According to analytical results, the Wufeng-Longmaxi Formation shale in Xianfeng area of Hubei Province has a great thickness, high TOC and rich gas. It is especially favorable from the bottom of Longmaxi Formation to the top of Wufeng Formation. The content of radioactive elements of U, Th and K is revealed to be closely correlated with TOC and gas logging indication by gamma-ray spectroscopy logging results. The shale section with high natural gamma indicates a strongly reductive sedimentary environment, which is conducive to the formation and the enrichment of shale gas.

4.   Conclusion

Shale gas from Wufeng-Longmaxi Formation was discovered in Xianfeng area, Hubei Province by drilling of Well EFD1, and survey data of the shale gas was obtained. These have deepened the overall understanding of the shale gas in this area, including development characteristics of organic-rich shale and conditions for the formation and enrichment of shale gas. The Wufeng-Longmaxi Formation in Xianfeng area lies in the favorable sedimentary facies zone. As a result, the gas-bearing shale of Wufeng-Longmaxi Formation in this area has a great thickness (75.74 m), high TOC, and rich gas, thus implying a good material foundation and preservation conditions of shale gas as well as the considerable potential of shale gas resources. Horizontal well drilling and staged fracturing tests are recommended to be further carried out to obtain data on shale gas productivity. A new breakthrough is expected to be achieved based on the discovery of shale gas in Xianfeng area.

Acknowledgment

This study is funded by the project “Shale Gas Geological Survey in Key Areas in South China on a Scale of 1:50000” (DD20190561) initiated by China Geological Survey.