Mesozoic horizontal stress in the East Sichuan Fold-and-thrust Belt, South China: Insights for Lower Paleozoic shale gas retention

Abstract

The Lower Paleozoic organic-rich shales are the primary targets of shale gas exploration in the East Sichuan Fold-and-thrust Belt (ESFB), South China. Horizontal compression during the Mesozoic orogeny caused the decollement of these shales and further influenced the retention of shale gas. In this study, seven sets of rock samples were recovered from outcrop sites in the ESFB, and the measured occurrences of the exposed strata in the ESFB were extracted from regional geological survey works. Acoustic emission tests under uniaxial compression were conducted on these outcrop samples. Magnitudes of the two principal horizontal effective stresses, SH-eff and Sh-eff, at the seven sampling sites, were acquired based on the Kaiser effect; magnitudes of the Mesozoic SH-eff experienced by the Lower Paleozoic shales in the different tectonic belts were then determined based on previous and newly measured paleo stress magnitudes, as well as their correlation with the measured dip angles of strata. Finally, the maximum depths of decollement (HD-max) of the shales were computed using the inferred paleo stress magnitudes. Measured SH-eff magnitudes of the outcrop samples ranged from 74.7 MPa to 117.0 MPa, and the ratios of SH-eff magnitudes to Sh-eff magnitudes fell between 1.10 and 1.55. The magnitudes of the paleo stress experienced by the Lower Cambrian shale were higher than those of the Lower Silurian shale due to a greater burial depth. The inferred maximum Mesozoic SH-eff magnitudes decreased from 180–200 MPa at the margin of the orogenic belt to 60–80 MPa at the margin of the basin, and the computed HD-max values decreased from more than 10 km to less than 5 km correspondingly. A lower magnitude of the Mesozoic horizontal stress and a shallower decollement contributed to better retention of the Lower Paleozoic shale gas inside the Sichuan Basin.