Submarine fan deposition in the Miocene abyssal plain of the Ulleung Basin, Korea: insights into deep-sea depositional environments and hydrocarbon exploration in a back-arc basin

This study presents the first integrated core- and well log-based analysis of deep-sea fan deposits from the abyssal plain of the Ulleung Basin, providing direct constraints on depositional processes in a back-arc setting where previous interpretations have relied primarily on seismic data. Sedimentological analysis of conventional cores identifies seven sedimentary facies and five facies associations, interpreted to have been deposited by sediment gravity flows such as hybrid flows and high-density turbidity currents. Petrophysical analysis of wireline log data enables quantitative evaluation of shale volume and porosity. Integration of sedimentological and petrophysical datasets reveals eight sandier-upward cycles organized into a systematic stacking pattern of submarine-lobe sandstone over outer lobe mudstone, showing a transition from progradation to retrogradation.With newly-revised chronostratigraphic constraints based on zircon U-Pb dating, this study reconstructs the evolution of a Middle Miocene deep-marine fan in the Ulleung Basin. The stacking pattern and facies distribution indicate that lobe development was primarily controlled by third-order eustatic sea-level fluctuations. Repeated sea-level falls promoted sediment bypass and lobe progradation within the submarine fan, whereas sea-level rise enhanced thick mud deposition with fan abandonment. These depositional processes likely contributed to the development of sand-rich, connected reservoir bodies. Consequently, this study demonstrates that eustatic forcing can exert a dominant control on deep-water sedimentation, providing important implications for predicting reservoir distribution as well as for future deep-water hydrocarbon exploration and CO₂ storage potential in the Ulleung Basin.