Shales in the Qiongzhusi and Wufeng–Longmaxi Formations: a rock-physics model and analysis of the effective pore aspect ratio |
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| Authors: | Zhi-Qiang Yang Tao He Chang-Chun Zou |
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| Institution: | 1.Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education (School of Earth and Space Sciences, Peking University),Beijing,China;2.Key Laboratory of Geodetection (China University of Geosciences), Ministry of Education,Beijing,China |
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| Abstract: | The shales of the Qiongzhusi Formation and Wufeng–Longmaxi Formations at Sichuan Basin and surrounding areas are presently the most important stratigraphic horizons for shale gas exploration and development in China. However, the regional characteristics of the seismic elastic properties need to be better determined. The ultrasonic velocities of shale samples were measured under dry conditions and the relations between elastic properties and petrology were systemically analyzed. The results suggest that 1) the effective porosity is positively correlated with clay content but negatively correlated with brittle minerals, 2) the dry shale matrix consists of clays, quartz, feldspars, and carbonates, and 3) organic matter and pyrite are in the pore spaces, weakly coupled with the shale matrix. Thus, by assuming that all connected pores are only present in the clay minerals and using the Gassmann substitution method to calculate the elastic effect of organic matter and pyrite in the pores, a relatively simple rock-physics model was constructed by combining the self-consistent approximation (SCA), the differential effective medium (DEM), and Gassmann’s equation. In addition, the effective pore aspect ratio was adopted from the sample averages or estimated from the carbonate content. The proposed model was used to predict the P-wave velocities and generally matched the ultrasonic measurements very well. |
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