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

[Image: see text] 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.