Improved\nCalculation Method of Undervalued Free Gas\nin Shale: A Case Study of Well Z302 Rock in Wufeng-Longmaxi Formation,\nSichuan Basin

Shale gas content data always plays an important role\nin the process\nof shale resource evaluation. However, free gas is always underestimated\nin the evaluation of the gas content. As a result, there are always\ncontradictions between the on-site desorption data and actual production\ndata. This phenomenon will lead to the evaluation process of shale\ngas resources; some of the potential shale gas blocks are considered\nto be of no development value. Through the on-site shale desorption\nand organic geochemical experiment on shale of Ordovician Wufeng-Silurian\nLongmaxi Formation in Well Z302, Sichuan Basin, the following conclusions\nare drawn: (1) the desorbed gas release in a sequence of N<sub>2</sub>, followed by CO<sub>2</sub>, CH<sub>4</sub>, and C<sub>2</sub>H<sub>6</sub>; (2)the δ<sup>13</sup>C of methane and ethane showed\ndifferent fractionation characteristics during on-site desorption,\nand ethane showed “desorption hysteresis” during desorption;\n(3) the shale gas is formed by oil cracking under the condition of\nhigh overmaturity, and the parent material of the oil-generating gas\nis mainly type II; (4) in the late stage of gas desorption, carbon\nisotope rollover occurred due to the stronger diffusion ability of <sup>12</sup>C-rich methane and the reversal phenomenon caused by residual\nenrichment of <sup>13</sup>C methane isotopes; (5) the transition\nmodel of gas abundance during shale desorption is established. At\nthe same time, the separation method of free gas and adsorbed gas\nin the shale desorption process is determined. Finally, a new calculation\nmethod of the ratio of free gas and adsorption gas is proposed to\nsolve the problem that free gas is underestimated in the evaluation\nof shale gas resources.