Good material basis and unique diagenetic evolution process are the keys to the formation of high-quality shale gas reservoirsJul. 26,2022
Shale gas is generated and stored in the micro- to nano-scale pores and throats in organic-rich shale (fine-grained sedimentary rocks with particle diameters less than 0.003 9 mm) and interlayers. It is an unconventional natural gas with adsorption and free state as its main occurrence modes. With the sudden emergence of shale oil and gas in the field of global oil and gas exploration and development, shale oil and gas have an increasing proportion in the world’s energy structure, and have become a hot spot in global oil and gas exploration and development.
As an unconventional oil and gas reservoir integrating source rock and reservoir properties, the formation and evolution of shale gas reservoirs have undergone the combined modification of inorganic and organic diagenesis, mainly including compaction, cementation, transformation of clay minerals, unstable mineral dissolution, hydrocarbon generation, etc. The diagenetic evolution process of shale is of great significance to the formation of high-quality shale reservoirs, the evaluation and optimization of shale gas sweet spots, and the enrichment and improvement of unconventional oil and gas geological theories.
In the article named “Diagenetic evolution of key minerals and its controls on reservoir quality of Upper Ordovician Wufeng-Lower Silurian Longmaxi shale of Sichuan Basin” published in Petroleum Geology & Experiment by WANG Ruyue, HU Zongquan, BAO Hanyong, WU Jing, DU Wei, WANG Pengwei, PENG Zeyang and LU Ting, the authors have conducted studies on the diagenetic evolution progress and its controls on reservoir quality of Upper Ordovician Wufeng-Lower Silurian Longmaxi shale of Sichuan Basin based on macro- to microscopic observations, X-ray diffraction analysis, energy spectrum analysis, and carbon and oxygen isotopes of carbonate rocks.
Results indicated that good material basis (rich in organic matter and bio-quartz) and a unique diagenetic evolution process were the keys to the formation of high-quality shale gas reservoirs. (1) The supporting framework composed of pyrite, microbial dolomite and biological quartz formed in the early diagenetic stage is critical for the formation of high-quality shale reservoir. (2) In the middle and late diagenetic stages, the synchronicity of clay mineral transformation, dissolution pore generation, hydrocarbon generation and pressurization promoted the development of organic pores and micro-fractures, which is beneficial to the formation of high-quality shale reservoirs and the enrichment and high yield of shale gas.
Relevant research results were published in Petroleum Geology & Experiment, Volume 43, Issue 6, 2021 (doi: 10.11781/sysydz202106996), and a bilingual version is now online (http://jtp.cnki.net/bilingual/detail/html/SYSD202106010).
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