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Shale gas (SG) wells in the Wufeng-Longmaxi Formation in northern Guizhou differ considerably in their production capacities. Preservation conditions are a crucial factor affecting the formation of SG reservoirs. In this study, the formation water (FW) in four wells in northern Guizhou was analyzed to determine the type; the Cl/Mg, Ca/Mg, and Na/Cl coefficients ( C  Cl/Mg , C  Ca/Mg , and C  Na/Cl , respectively), and the coefficient of desulfurization ( C  d ); the SG in these wells was tested to identify its composition and the sources of its components. The results show the following: wells TX1 and CY1 contain two types (CaCl 2 and NaHCO 3 ) of FW, each with low C  Na/Cl , low C  d , high C  Ca/Mg , and high C  Cl/Mg , suggesting a high level of FW retention. The FW in wells TM1 and CD1 is of NaHCO 3 type and is characterized by high C  Na/Cl , high C  d , low C  Ca/Mg , and low C  Cl/Mg , indicating a high level of connectivity between the FW and surface water. Nonhydrocarbon gases account for a high proportion of SG in composition; with a 15 N value between -8.7 and -4.2‰, N 2 accounts for approximately 20% of SG in wells TX1 and CY1, respectively. This result indicates the ammonification of organic matter (OM) during pyrolysis and hydrocarbon generation as sources of N 2 . In contrast, with a 15 N value between -3.6 and 0‰, N 2 accounts for over 70% of SG in wells TM1 and CD1, respectively, indicating the atmosphere, the ammonification of OM, and the presence of poor preservation conditions due to a high level of connectivity between the shale bed and the atmosphere acted as the sources of N 2 . The metamorphism coefficients of FW in conjunction with the SG composition and the sources of nonhydrocarbon gases can serve as auxiliary indicators of the quality of SG preservation conditions.

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Composition of Formation Water and Natural Gas as an Indicator of Shale Gas Preservation Conditions: A Case Study of the Marine Shale of the Niutitang Formation in the Cengong Block in Northern Guizhou, China