A quantitative seismic prediction technique for the brittleness index of shale in the Jiaoshiba Block, Fuling shale gas field in the Sichuan Basin

In this paper, a quantitative seismic prediction technique of multi-parameter shale brittleness index suitable for the environment with complex structural stress was developed in order to confirm highly brittle intervals and favorable fracturing zones in the shale reservoirs in the Jiaoshiba Block, Fuling shale gas field, Sichuan Basin. Firstly, the effect of structural compression stress on the brittleness characteristics of rocks were figured out by analyzing structures, mineral composition, development degree of fractures in cores, well logging and seismic data comprehensively. Secondly, according to the Rickman formula, a new brittleness index prediction model based on Young's modulus, Poisson's ratio and shear modulus × density was established by introducing the shear modulus which reflects lateral shear stress–strain. Finally, the quantitative prediction technique of multi-parameter shale brittleness index suitable for the environment with complex structural stress was developed by virtue of the superiority of an elastic rock brittleness index method based on mineral composition to accurately calculate the brittleness index of a full hole. Field application shows that this technique is reliable, since its prediction results coincide with the calculated brittleness index of exploratory wells which are not used for modeling, with a relative error margin below 4%; and that the brittleness index of good shale of Upper Ordovician Wufeng Fm–Lower Silurian Longmaxi Fm in the Jiaoshiba Block increases from the top to the bottom and is stably distributed laterally. Particularly, the Wufeng–Longmaxi 11 is the highest in brittleness index, so it is the most favorable interval for penetrating and fracturing of horizontal wells. Keywords: Sichuan Basin, Fuling shale gas field, Late Ordovician, Early Silurian, Stress environment, Shear modulus, Brittleness index, Quantitative seismic prediction, Favorable interva