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Ahead of Print Abstract
DOI:10.1306/10212524114
Organic pore preservation in marine organic-rich shales of the southern China: Insights from pore fluid pressure evolution
Tao Luo12 , Xiaowen Guo1 , Zhiliang He1 , Zhihui Xiao1 , Jian-xin Zhao2 , Tian Dong1 , Rui Yang1 , and Ze Tao3
1 Key Laboratory of Tectonics and Petroleum Resources (China University of Geosciences), Ministry of Education, Wuhan 430074, China
2 Radiogenic Isotope Facility (RIF), School of the Environment, The University of Queensland, Brisbane, QLD 4072, Australia
3 Key Laboratory of Exploration Technologies for Oil and Gas Resources, Ministry of Education, Yangtze University, Wuhan 430100, China
Ahead of Print Abstract
Marine organic-rich shales in the southern China maintained overpressure conditions caused by hydrocarbon thermal cracking during the deep-burial stage and gas leakage during the period of tectonic uplift, accompanied by organic pore development and preservation. Presently, there is no comprehensive framework exists for revealing the preservation of organic pores in shale reservoirs. In this study, multiple approaches including CO2 and N2 adsorption, scanning electron microscopy, fluid inclusion microthermometry, Laser Raman spectroscopy, and thermodynamic modeling, were used to quantitatively evaluate organic pores and investigate pore fluid pressure evolution in the Wufeng Formation and Longmaxi Formation shale reservoir in the eastern Sichuan Basin. Measured results show that organic micropores in shale samples from three wells at different structural units have similar values of volume and specific surface area. Organic mesopore volume accounts for 40% to 70% of the organic pore volume. The degree of organic mesopores development in wells JYA, JYB, and JYC is proportional to the pressure coefficients with shale reservoirs. Pore fluid pressure evolution within shale reservoirs can be divided into two stages, 160 to 85 Ma and 85 Ma to present day. Overpressure was generated in shale pores during the first stage, with pressure coefficients increasing from 1.57 to 2.2, and the overpressure protected most of the organic pores from compaction. The second stage was characterized as the reduction and dissipation of overpressure during the Yanshanian and Himalayan Orogenies. By comparing the closed-system modeling of overpressure and pore fluid evolution across different structural zones, this study demonstrates that possible increase in overpressure and decrease in organic pore volume during the Yanshanian Orogeny. In intensely tectonically active zones, the development of faults and fractures can lead to overpressure dissipation within shale reservoirs, resulting in poor organic mesopore preservation and low hydraulic fracturing productivity during the Himalayan Orogeny. Conversely, in weakly tectonically active zones, the shale reservoir maintained an overpressured state during the Himalayan uplift, with large-diameter and high-roundness organic mesopores preservation, indicating that overpressure during uplift can provide important preservation for organic mesopores. This contribution provides greater insight into organic pore preservation and shale gas enrichment in other tectonic settings around the world.
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