Abstract
Numerous large gas fields have been discovered in the Qiongdongnan basin of the South China Sea, demonstrating its huge exploration potential. However, extensive exploration targeting the Yacheng formation source rocks in the northern depression zone has yielded suboptimal results. To accelerate the development of the "Trillion-Cubic-Meter Gas Province" in the South China Sea, it is imperative to investigate the characteristics of source rocks in the Yacheng Formation. This study focuses on the Yacheng formation mudstones from Well ST31X in the southern gentle slope of the Songxi Sag. Element analyses were conducted to evaluate provenance, sedimentation rate, depositional environment, redox conditions, and paleoproductivity conditions. For the first time, the geochemical characteristics of trace and REEs were utilized to assess the source rock features of the Yacheng formation in Songxi Sag, identifying favorable areas for future exploration. The REE distribution patterns of Yacheng formation mudstone exhibit light rare earth element (LREE) enrichment and heavy rare earth element (HREE) depletion, with significantly steeper LREE curves compared to HREE curves. A distinct negative Eu anomaly is observed, while no notable Ce anomaly is detected. The provenance is dominated by inputs from Hainan Island, indicating rapid near-source sedimentation. The depositional environment is characterized by terrestrial freshwater conditions without significant marine transgression. The oxidizing to weakly reducing water conditions are unfavorable for organic matter preservation. The western sub-sag of the Songxi Sag, influenced by Fault No. 5, primarily developed alluvial fan-fan delta systems with limited source rock accumulation. The eastern sub-sag, adjacent to the tidal flat and gentle slope of the Songtao uplift, represents a favorable zone for source rock development in the Yacheng Formation, which is conducive to natural gas accumulation and reservoir formation.