Shale gas occurrence under burial conditions

Shale samples were collected from the Longmaxi Formation in southeastern Sichuan Basin to carry out quantitative studies of shale gas occurrence under burial conditions.The impacts of TOC content, temperature and moisture on adsorption capacity were studied using gravimetric isothermal adsorption, and some related empirical equations were established to calculate adsorbed gas content under burial conditions. In addition, free gas storage space was derived by deducting the volume of pore water and adsorbed gas from total space. The density of free gas was calculated using the Peng-Robinson equation. Therefore, a quantitative calculation method for shale gas occurrence was proposed. Under overpressure condition, adsorbed gas content decreases with the increase of burial depth, while free gas content increases and becomes dominant. Shale gas content of well Jiaoye1 was calculated using 22 samples mainly from 38 m interval. The adsorbed gas fraction is 27.1%-47.8% with an average of 34.3%, and the free gas fraction is 52.2%-72.9% with an average of 65.7%.