Abstract:

The Labudalin Basin in Northeastern Inner Mongolia is an important land-based area for the potential formation of high-latitude permafrost-associated gas hydrates in China. It is expected to become an important complement to the high-altitude permafrost-associated gas hydrates in the Qilian Mountain, offering significant research value. Currently, investigations and studies on gas hydrates in this area are relatively limited. In 2023, the Geological Survey and Research Institute of Inner Mongolia deployed two exploration wells for gas hydrates in this region for the first time. This study uses the ST-1 well, which has more comprehensive data, as an case study to conduct an thorough analysis and regional comparison. The aim is to investigate anomalies related to gas hydrates, identify favorable conditions for their formation, and establish a symbiotic model for the shallow-gas and gas hydrate system in the study area. The study results indicate that: (1) During drilling, phenomena such as bubbling on the core surface, well-fluid eruptions, high anomalies in hydrocarbon gas measurements, elevated desorbed gas content from the core samples, and the occurrence of authigenic calcite were observed, all of which may be directly or indirectly related to gas hydrates. (2) The composition of the desorbed gas from the core samples is primarily CH₄, followed by CO₂, with a small amount of C₂H₆. This gas is mainly sourced from coal-type gas and may also contains a small portion of microbial-derived gas and mixed pyrolytic gas. (3) The organic carbon content of the Lower Cretaceous Damoguaihe Formation (K₁d), encountered during drilling, ranges from 3.2% to 31.2%, with an average of 12.5%. The hydrocarbon generation potential varies from 1.50 to 113.78 mg/g, with an average of 45.50 mg/g. The organic maceral components are primarily vitrinite and sapropelinite, indicative of II₂-III organic matter. The reflectance of kerogen vitrinite (R_o) ranges from 0.63% to 0.80%, indicating a stage from low maturity to maturity, which provides favorable conditions for hydrocarbon generation. This formation may be the primary potential source rock for gas hydrate formation. Compared to the five source rock sets in the region, the Upper Jurassic Manketouebo Formation (J₃m) is also expected to be a significant potential source rock for gas hydrate exploration. (4) The logging temperature at 30, 54, and 78 h after drilling was unstable at depths shallower than 200 m, particularly between 43.5 and 70.6 m, where it fluctuated significantly. This may be related to local thermal melting zone caused by stratigraphic fragmentation or fracture development at certain depths, suggesting that the logging temperature may not accurately represent the geothermal conditions of the strata. However, the study area still presents favorable conditions and significant potentials for gas hydrate formation.(5) During drilling, the highest gas values reached 12.0%-16.7% across multiple depth intervals, with desorbed gas contents ranging from 0.95 to 3.19 m³/t and an average of 1.84 m³/t. This indicates that the shallow-gas source is relatively well-developed in the study area. The drilling data revealed that the development features of thin coal layers, mudstone layers, sandstone layers are not favorable for the preservation and accumulation of shallow-gas, based on their physical properties and fault or fissure characteristics. In summary, the development of permafrost in the study area creates conditions suitable for a gas hydrate stable zone (GHSZ). The permafrost, combined with the GHSZ, provides effective sealing and preservation conditions for shallow gas, resulting in a symbiotic shallow-gas and gas hydrate system. This work provides important geological insights for subsequent investigations and research on gas hydrate or shallow-gas in the study area or similar regions.

Key words: Northeastern Inner Mongolia, Labudalin basin, permafrost, gas hydrate, drilling, geological indicator