Shale gas preservation conditions of Lower Paleozoic Niutitang formation in southeastern Chongqing, China: Evidence from formation water and shale gas components

To quantitatively characterize the preservation conditions of shale gas in different wells, the formation water and gas composition characteristics of five wells (YC1, YY6, YY1, YK1, and QQ1) in southeastern Chongqing were analyzed. The formation water type of QQ1 and YK1 is CaCl2, and CY1 includes CaCl2 and NaHCO3 types. The C-Na/Cl and C-d are low, while C-Ca/Mg and C-Cl/Mg are high, indicating that the retention degree of formation water is high. The formation water of YY6 and YY1 is NaHCO3 type, the C-Na/Cl and C-d are high, while the C-Ca/Mg and C-Cl/Mg are low, indicating the strong connectivity of formation water. The N-2 content of wells QQ1 and YK1 is below 32%, and N-15 is between -7.33 parts per thousand and -1.98 parts per thousand, the source of nitrogen is ammoniated in the process of organic matter pyrolysis and hydrocarbon generation, and the preservation conditions are relatively good. Therefore, the preservation conditions combined with the metamorphic degree of formation water and gas composition can be judged.