Combining Isotopic Geochemical Data and Logging Data to Predict the Range of the Total Gas Content in Shale: A Case Study from the Wufeng and Longmaxi Shales in the Middle Yangtze Area, South China

The marine shales of the lower Silurian Longmaxi formation (S₁l) and the upper Ordovician Wufeng formation (O₃w) in the middle Yangtze area of southern China have been recognized as effective gas shales with an enormous potential. To estimate the total gas content in the shales, shale gas samples were collected at 20 °C, 48 °C/65 °C, and 90 °C in the gas release process of each core sample during desorption experiments. Based on the isotope fractionation data during the desorption of gas in shale, an adsorbed gas content₍ₘᵢₓₜᵤᵣₑ gₐₛ₎/free gas content₍ₘᵢₓₜᵤᵣₑ gₐₛ₎ to δ¹³C₁₍ₘᵢₓₜᵤᵣₑ gₐₛ₎ model was established to estimate the proportions of free and adsorbed gas in shale by using the δ¹³C₁ data of shale gas. The proportion of adsorbed gas₍ₘᵢₓₜᵤᵣₑ gₐₛ₎ in the Wufeng shales is higher than that in the Longmaxi gas shales. The adsorption capacity of the Wufeng shales with relatively high total organic carbon (TOC) is greater than that of the Longmaxi shales with a relatively low TOC. Subsequently, the free gas content and the adsorbed gas content could be calculated during the desorption experiments. Combining the free gas and adsorbed gas estimated from the logging data, we calculated the minimum and maximum total gas contents of each core sample from well LY1. The total gas content in the Wufeng shales, ranging from 1.654 to 3.371 m³/t, is higher than that of the Longmaxi shales, ranging from 0.398 to 1.769 m³/t. Compared with the minimum and maximum total gas contents predicted by the isotopic geochemical data and logging data, the value of the total gas content estimated by using the United States Bureau of Mines method is not always located within the range of the total gas content, which is influenced by the relationship between free gas₍ₗₒggᵢₙg dₐₜₐ₎ (m³/t) and free gas₍ₘₑₐₛᵤᵣₑd gₐₛ₎ (m³/t).