Abstract
Junggar Basin is one of the largest sedimentary basins in Northwest China. Carboniferous oil and gas fields have been found in different areas in the eastern part of the Junggar Basin on a large scale, indicating that the Carboniferous rocks of the Junggar Basin have a huge potential for oil and gas exploration. This study focuses on the Batamayineishan Formation in the eastern part of the Junggar Basin, which contains volcanic rocks and pyroclastic rocks, aiming to investigate the reservoir characteristics and to identify the formation mechanism of the rocks of this formation. The majority of the existent reservoir space in the volcanic rocks of the Batamayineishan Formation is dominated by secondary pores and fractures. Using the methods of petrography, pressure-controlled mercury injection (PMI), and electron probe microanalysis (EPMA), the reservoir characteristics and diagenetic history of the volcanic rocks of the Batamayineishan Formation in the Shuangjingzi area were studied. A theoretical framework is established to provide favorable guidance for exploring Carboniferous volcanic rocks in the Junggar Basin. The results of mercury injection indicate that the average pore throat radius and porosity of the volcanic rocks are 0.068 µm and 6.62%, respectively. Permeability remains stable and does not show a significant change with an increase in porosity. Despite the high porosity, the permeability is relatively low, reflecting isolated and non-connected primary pores. The average value of permeability is relatively low (0.424×10-3 µm2), which typically suggests narrow micro-throats. Primary gas pores fill and develop amygdales on a large scale. In addition, the dissolution pores developed by dissolution and alteration also compensated for the decrease in the original gas pore volume.
Highlights
Volcanic reservoirs have been found in many locations worldwide (Nakata, 1981; Petford and McCaffrey, 2003; Sruoga and Rubinstein et al, 2004; Sruoga and Rubinstein, 2007; Lenhardt and Götz, 2011; Barreto and de Lima et al, 2017; Navelot and Géraud et al, 2018)
This study focuses on the lithology, physical properties, reservoir space type, pore structure type and pore infilling of the Carboniferous Batamayineishan Formation in the Shuangjingzi area of the Junggar Basin, eastern China
The lithological characteristics of the volcanic rocks of the Batamayineishan Formation were determined through outcrop sample description and thin-section observations
Summary
Volcanic reservoirs have been found in many locations worldwide (Nakata, 1981; Petford and McCaffrey, 2003; Sruoga and Rubinstein et al, 2004; Sruoga and Rubinstein, 2007; Lenhardt and Götz, 2011; Barreto and de Lima et al, 2017; Navelot and Géraud et al, 2018). Since the discovery of the first volcanic reservoir in the northwestern margin of the Junggar Basin, volcanic oil and gas reservoirs have gained significant attention and have been reported in many petroliferous basins in China, such as the Junggar, Sichuan, Bohai Bay, Santanghu, Songliao, and Hailer basins (Yang and Hou et al, 2017; Sun and Zhong et al., 2018; Sun and Cao et al, 2018; Zheng and Sun et al, 2018). Primary pores formed by welding, volatile outgassing, deuteric recrystallization, and autoclastic brecciation may enhance the porosity and permeability (references). Secondary processes, such as compaction and alteration, could destroy primary pores; several secondary processes may contribute positively to enhancing reservoir quality by generating secondary pore systems, such as dissolution during the burial period, weathering, and leaching. Fractures and cracks coupled with pores play a key role in developing oil and gas reservoirs on a large scale
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