Abstract: The latest studies show that multi-stage magmatic intrusion events occur in the Changchang Sag of the Qiongdongnan Basin, northern South China Sea; however, the influences of these magmatic intrusions on the thermal evolution of source rocks in the Changchang Sag are still unclear. With the latest seismic, geochemical and drilling data, and the seismic facies technology, we identified nine magmatic intrusions of four stages in the sag. We then conducted 3D basin modeling to quantitatively assess the impacts of these magmatic intrusions on the thermal evolution of source rocks in the Changchang Sag. The results indicate that
magmatic intrusions can accelerate the thermal maturity of source rocks, but the influence scope is limited. The size of intrusive
body and its influence scope have the following rules: when the diameter of the magmatic body is less than 2 km, and the radius of its influence scope is also less than 2 km; the diameter of magmatic body is larger than 2 km but less than 5 km, and the radius of its influence scope is less than 5 km; while the diameter of magmatic body is larger than 10 km but less than 20 km, and the radius of its influence scope is less than 16km. Therefore, magmatic intrusion events do not affect the thermal evolution of source rocks in the Changchang Sag, but they may bring non-hydrocarbon charging risks. Based on the analysis of the influence of magmatic intrusions on two drilling wells in the Changchang Sag, the hydrocarbon generation threshold of the Changchang Sag is determined to be 2300-2500 m below the seafloor. These findings have laid a foundation for next oil and gas exploration in this sag.

Key words: magmatic intrusion events, maturity, thermal simulation, Changchang Sag