Tectonic migration and its impact on the development of source rocks in the Paleogene Lishui Sag, East China Sea Basin

Tectonic migration is an important geological phenomenon in the episodic evolution process of faulted basins, which significantly influences the development and distribution of high-quality source rocks. Based on the most recent compilation of 3D seismic, drilling, and organic geochemical information, combined with tectonic-stratigraphic framework, the activity of major faults, as well as the spatial-temporal distribution of sedimentary-subsidence centers, we revealed for the first time the migration patterns of the Paleogene structures in the Lishui Sag of the East China Sea Basin. Furthermore, we investigated the genetic mechanisms governing tectonic migration and its implications on source rock formations and the potential for hydrocarbon generation. Results indicate that the depression and sedimentary center of the Lishui Sag underwent a westward migration during the Paleocene. The fault depression-deposition center of Lishui Sag migrated from east to west in the Paleocene. The regional sedimentation center was located in the Lishui East Sub-Sag in the early Paleocene, migrated to the Lishui West Sub-Sag in the middle Paleocene, and developed inheritedly in the late Paleocene. The phenomenon of tectonic migration occurs as the result of the westward movement of major active faults and sedimentary-subsidence centers, which is driven by the transformation of the East China Sea Basin from a depression along the continental margin to wide-developed rift. Affected by tectonic migration, the lacustrine source rocks of the Yueguifeng Formation exhibit variations in scale, quality, and hydrocarbon expulsion intensity between the East and West Subsags. The East Subsag features larger scale source rocks of superior quality and higher hydrocarbon expulsion intensity, whereas the West Subsag contains smaller scale source rocks with increased maturity. Additionally, the West Subsag is characterized by the presence of terrestrial marine source rocks of the Lingfeng Formation, which are larger in scale and exhibit higher maturity levels. Therefore, the primary emphasis of the exploration in East Subsag should be directed towards the oil and gas system originating from the hydrocarbon source kitchens of the Yueguifeng Formation, with a predominant focus on oil exploration. Conversely, exploration activities in the West Subsag should encompass a comprehensive evaluation of the two source rock formations, namely the Yueguifeng Formation and the Lingfeng Formation, with a particular emphasis on both oil and gas reservoirs, with gas as a key focal point.