Mechanism of Coal Accumulation in Fault-Depressed Basins and Exploration Insights: A Case Study of Member 2 of the Nantun Formation in the Huhehu Depression, Hailar Basin

Enrichment of coalbed methane is fundamentally governed by the formation and distribution of coal seams. In continental fault-depression basins, the mechanisms controlling peat accumulation are more complex than those in stable basins, extending beyond traditional sea-level control models. This study examines the synergistic control of provenance, climate, tectonics, and sedimentation on coal accumulation in such settings, focusing on Member 2 of the Lower Cretaceous Nantun Formation (K₁n²) in the Huhehu Depression, Hailar Basin—a typical continental fault-depression basin and its major source rock interval.By integrating multiple analytical techniques, including organic geochemistry (e.g., biomarkers, carbon isotopes), elemental geochemistry (major, trace, and rare earth elements), petrographic analysis, and seismic-log based sedimentary facies interpretation, this research systematically reconstructs the paleoenvironment during peat accumulation, delineates the spatial distribution of coal seams in detail, and quantitatively to semi-quantitatively evaluates the contributions of various controlling factors.
Key findings are as follows:
(1) Paleoenvironmental Reconstruction and Provenance Characteristics: Comprehensive geochemical indicators reveal that during the peat-forming period of K₁n², the study area experienced warm and humid paleoclimatic conditions with a freshwater environment. The provenance was dominated by intermediate-acidic igneous rocks, exhibiting a weakly neutral to acidic geochemical signature. This combination effectively suppressed large-scale terrigenous clastic input, providing a critical geochemical foundation for the development of low-ash, high-organic-content coal.
(2) Sedimentary-Tectonic Synergistic Control Mechanism and 3D Distribution Patterns: This study clearly identifies the littoral-shallow lacustrine facies as the only dominant peat-accumulating facies within this fault-depression lacustrine setting. Its spatial distribution is strictly constrained by the basin’s "eastern faulting, western overlapping" half-graben structural framework. Coal seams mainly developed in the transitional zone from the eastern steep slope belt to the central depression (sag), where the rate of accommodation space creation remained in long-term balance with the rate of peat accumulation. Seismic attribute analysis and isopach mapping clearly demonstrate an asymmetric distribution of cumulative coal seam thickness—thicker in the east and thinner in the west—trending along the fault zone and thickening significantly toward the east (downthrown block), with thicknesses ranging from 30.84 to 151.24 meters. This distribution forms the material basis for coalbed methane enrichment.
Based on these findings, this study innovatively establishes a comprehensive sedimentary model applicable to continental fault-depression basins: "Weakly Neutral-Acidic Provenance Supply – Warm-Humid Freshwater Environment – Littoral-Shallow Lacustrine Facies Dominance – Peat Accumulation in Steep Slope Belt to Depression Zone." This model systematically elucidates the dynamic coupling among provenance characteristics, paleoclimate, syn-sedimentary tectonic activity, and lake-level fluctuations. It represents an important supplement to and advancement of the classical sea-level-controlled coal accumulation paradigm, forming a novel theoretical framework for peat accumulation in continental fault-depression basins.This research deepens the understanding of the intrinsic mechanisms controlling coal measure formation in fault-depression basins.