Gas-Bearing Characteristics and Origin Analysis of Shale Gas in Longtan Formation, Permian, Shaoyang Sag, Central Hunan

In order to evaluate shale gas resources potential in the Permian Longtan Formation, Shaoyang sag, Central Hunan, a shale gas survey well-2015H-D3 was done in this study. The gas-bearing properties and its influencing factors were quantitatively analyzed by the well on-site desorption with other related tests, combined with gas stable carbon and hydrogen isotopes, preliminary discussion on the origin of hydrocarbon gas is made. The gas logging values become abnormal from the depth of 150 m, and the gas content of desorption firstly increases gradually and then decreases with the increase of depth. 300-425 m is the highest gas-bearing zone, with a cumulative thickness of 125 m. The gas content of desorption is all over 0.5 m³/t, the highest value is 2.35 m³/t, with an average value of 1 m³/t, indicating that the Longtan Formation has a good potential for shale gas exploration. The gas content of desorption is controlled by organic carbon content while the depth is more than 300 m, and the desorption gas content is not simply constrained by organic carbon content but rather restricted by the preservation conditions while the buried depth is less than 300 m. The gas carbon isotope test shows that δ¹³C₁ is between -29.87‰ and -36.82‰ with the average value of -34.52‰, δ¹³C₂ is between -29.45‰ and -31.02‰ with the average value of -30.09‰, and δD₁ is between -131.2‰ and -178.4‰ with the average value of -167.4‰. The gas origin analysis reveals that the shale gas in Longtan Formation belongs to the type of oil source gas in thermal genesis. It can be determined that the Longtan Formation is the transitional environment through sedimentary phenomena, which is consistent with the results of hydrogen isotope, but it is still unable to determine the specific judgment interval.