The main controlling factors on shale gas occurrence characteristics in deep and high-over mature shales: A case study of Silurian Longmaxi Formation in the Sichuan Basin, southern China

Deep shales refer to the shales with buried depth greater than 3500 m. Yields of shale gases depend on the form of their occurrence at the depth, but the major controlling factors on shale gas occurrence state in deep high-over mature shales are poorly understood. Using the Longmaxi Formation as an example, the shale gas occurrence characteristics of deep high-over mature shales were quantitatively characterized, and the effects of temperature, pressure, pore structure, and organic matter abundance on the occurrence characteristics of shale gas were studied by using the control variable method. The results show that the Longmaxi shale is mainly organic-rich argillaceous shale, with an average porosity of 7.54%. The pore volumes are mainly contributed by mesopores, whereas the total specific surface areas are primarily by micropores. The Longmaxi shale has high gas contents, with total gas contents of 3.82∼7.89 m3/t rock, in which the proportion of free gas varies from 71% to 83%, with an average of 77%. The high proportion of free gas depends on the particular geological conditions in the Longmaxi Formation. High temperature is not conducive to shale gas storage, and its negative effect on adsorbed gas is greater than that on free gas. High pressure is beneficial to shale gas storage, and its positive influence on free gas is greater than that on adsorbed gas. The large pore volume dominated by mesopores provides sufficient storage place for free gas. High organic matter content is beneficial for shale gas enrichment but reduce the proportion of free gas. In conclusion, high pressure and large pore volume are the major factors for the high proportion of free gas.