Different desorption types and geological influencing factors of Silurian shale gas in southeastern Sichuan Basin

The southeastern Sichuan Basin is a key area for the exploration and development of Silurian shale gas in China. In recent years, a number of shale gas wells have achieved good exploration results in this area, providing important support for the development of unconventional natural gas resources in China. However, as the exploration extends to the deep to ultra-deep complex structural areas within the basin and the residual syncline outside the basin, some shale gas wells have recently experienced complex situations such as "low gas content" and "high gas content but low yield", which seriously restricts the increase of reserves and production. To accurately evaluate the shale gas content and productivity, key field desorption experimental data from 19 shale gas wells under different geological conditions in the southeastern Sichuan Basin—including total desorption gas content, the proportion of first-order desorption gas, and initial desorption rate—were systematically analyzed. Combined with the regional geological background and experimental principles, the desorption characteristics of shale gas were studied in depth to reveal the classification characteristics of shale gas desorption behavior and its geological influencing factors. The results showed that: (1) according to desorption characteristics, the Silurian shale gas in southeastern Sichuan Basin could be classified into five types: long-term rapid-release type, full-temperature stable-release type, high-temperature burst-release type, low-temperature weak-release type, and full-temperature weak-release type. The desorption characteristics of different types of shale gas were significantly different. (2) Suitable maturity and good preservation conditions ensured the high total gas content and good productivity of long-term rapid-release-type and full-temperature stable-release-type shale gas, while differences in uplift amplitude were the main reason for the different reservoir pressure states and free gas proportions between the two types. High water saturation, strong tectonic movement, and excessively high maturity were the key factors limiting the productivity of high-temperature burst-release-type, low-temperature weak-release-type, and full-temperature weak-release-type shale gas. (3) Currently, the long-term rapid-release-type and full-temperature stable-release-type shale gas were the most favorable targets for achieving high production and large-scale extraction. The high-temperature burst-release-type shale gas had high gas content, but it could not be produced effectively under the current geological and engineering conditions, making it a potential target for future technical research. The low-temperature weak-release-type and full-temperature weak-release-type shale gas had relatively low exploration and development value.