Integrated characteristics of the Niudong gas reservoir in the Altun Mountains Front Belt, Qaidam Basin: implications for coal-derived gas accumulation and exploration

The distribution of hydrocarbons, along with the accumulation conditions and mechanisms, constitutes a critical challenge in oil and gas exploration. In this study, natural gas components and carbon isotopic compositions, organic geochemistry data, casting thin sections, scanning electron microscopy (SEM), seismic profiles, fluid inclusion analyses, and hydrocarbon generation history were integrated to comprehensively investigate the origin, genetic type, maturity, accumulation conditions and mechanisms of natural gas in the Niudong area, Altun Mountains Front Belt (AMFB). Key findings are summarized as follows: (1) The Natural gas in the Niudong area exhibited mixture characteristics of dry and wet gas, where wet gas constitutes the dominant fraction (80%). It is identified as typical coal-derived gas, developed from the same source rocks during two distinct thermal evolution stages (same source but different maturities). (2) Influenced by paleo-structures (paleo-uplifts and paleo-slopes), the Niudong nose-shaped uplift occupied a favorable position for sustained hydrocarbon migration and developed an efficient vertical transport system dominated by deep faults within the AMFB. (3) The Niudong area experienced two distinct hydrocarbon charging episodes. The first episode (late Oligocene to early Miocene, late E3–early N1) involved concurrent crude oil and natural gas migration. The second episode (late Miocene, late N1) was dominated by natural gas charging. (4) Two primary accumulation models were characterized in the Niudong area (i.e., faulted anticline structural gas reservoir and stratigraphic–lithologic gas reservoir), and deep faults might serve as the dominant control on gas accumulation. The Lower Jurassic (J1) strata in the Niudong area exhibited optimal reservoir conditions, representing key targets for future exploration. The central Kunteyi Sag and adjacent slope zones, positioned proximal to high-maturity source rocks, exhibit highly favorable conditions for stratigraphic–lithologic hydrocarbon accumulation. These areas should be prioritized as the primary exploration targets in future. This study delivers pivotal advancements in understanding coal-derived gas exploration within the AMFB, establishing a conceptual framework with threefold significance for unconventional gas exploration. As such, it provides transferable insights for hydrocarbon exploration in geologically analogous basins across Northwestern China and other global foreland systems with similar depositional environments.