Shale gas storage is a dominant factor to economically evaluate the shale play. A series of Lower Silurian Longmaxi marine shale samples in southeast Chongqing, China, were collected to investigate the reservoir characteristics, and a suit of methane adsorption isotherms were fitted using a supercritical Ono-Kondo model to better understand the adsorption capacity of Longmaxi shale. The saturated adsorption of a monolayer presents a greatly positive relationship with the total organic carbon (TOC) content. A negative relationship with clay was observed due to the predominant influence of organic matter on the methane adsorption. Methane adsorption also increases with increasing pressure and decreases with increasing temperature. On the basis of the relationships, one new estimation algorithm related to TOC content, pressure, and temperature was established to calculate the methane adsorption capacity on the basis of the Ono-Kondo model. Furthermore, with higher TOC content, the adsorption capacity of shales correspondingly increases and the maximum of the adsorption capacity tends to a deeper depth. Considering geological characteristics of Longmaxi shale, one new gas-in-place (GIP) model was proposed to apply, considering the controlling factors, TOC, porosity, gas saturation, pressure, and temperature. The relationships of GIP, adsorbed gas, and free gas with increasing depth shows that (1) free gas increases rapidly and equally; (2) adsorbed gas initially increases rapidly at less than 1000 m, and then decreases with depth increases; and (3) GIP rapidly increases at shallow depths, and then gently increases more than 1000 m.