Thermal evolution characteristics and petroleum geological significance of Paleogene Yacheng Formation source rocks on northern slope of Lingshui Sag, Qiongdongnan Basin

The source rocks of the Paleogene Yacheng Formation in the Lingshui Sag of the Qiongdongnan Basin are the most important source rocks for the "Deep Sea No. 1" gas field. However, oil and gas exploration results on the northern and southern slopes of the Lingshui Sag differ significantly, and the thermal evolution characteristics and hydrocarbon generation history of the Yacheng Formation source rocks remain unclear, which constrains further exploration breakthroughs in this area. Based on detailed interpretation of 3D seismic data combined with basin modeling technology, the burial history, thermal history, and maturity evolution history of the Yacheng Formation source rocks in this area were simulated. The study clarified the thermal evolution characteristics and regional differences of the source rocks and explored their significance for hydrocarbon accumulation. The results showed that: (1) The source rocks of the Yacheng Formation on the northern slope of the Lingshui Sag were widely distributed, with burial depth gradually increasing from the Lingshui low uplift to both sides. The maximum burial depth in the Lingshui subsag reached 10 800 m. (2) The northern slope underwent a three-stage evolution process of early rapid burial, mid-stage stable subsidence, and late accelerated subsidence. (3) The thermal evolution of the Yacheng Formation source rocks on the northern slope exhibited a "higher in the south and lower in the north" pattern. Specifically, in the Lingshui subsag, the source rocks reached the hydrocarbon generation threshold in the early Late Oligocene (Ro=0.5%), reached the mature stage in the mid-Late Oligocene (Ro=0.7%), evolved to the high-maturity stage by the end of the Late Oligocene (Ro=1.3%), entered the overmature stage in the mid-Miocene (Ro=2.0%), and are currently in the stage of gas depletion (Ro=4.5%). In contrast, maturation evolution in the low uplift and gentle slope areas was relatively delayed, with the hydrocarbon generation threshold reached in the mid-Late Oligocene, the mature stage in the Early Miocene, high-maturity stage in the mid-Miocene, and currently the overmature stage (Ro=2.75%), dominated by gas generation.