Eutrophication mechanisms and hydrocarbon potential of Cenozoic excellent source rocks in offshore China: A critical review

Development of excellent source rocks in offshore China is closely associated with eutrophication, which provides an important foundation for the hydrocarbon generation. This review analyzes the origins of lacustrine and marine excellent source rocks in offshore China, focusing on six major eutrophication mechanisms: hydrothermal fertilization, volcanic fertilization, granite weathering coupled with fluvial transport, moderate salinization, closed to semi-closed bays, and upwelling currents. These mechanisms promote plankton blooms, particularly those of algae, by supplying abundant nutrients (e.g., P, Fe, Mn, Zn, N, and Si) to aquatic environments or by creating favorable environmental conditions (e.g., low-energy hydrodynamic settings, anoxic conditions, and moderate salinity), thereby providing the material basis for the development of organic-rich source rocks. Previous studies indicate that the widely developed lacustrine excellent source rocks in offshore China (e.g., the Shahejie Formation in the Bohai Bay Basin, the Wenchang Formation in the Pearl River Mouth Basin, and the Liushagang Formation in the Beibu Gulf Basin) developed in semi-deep to deep lacustrine environments formed during intense rifting, together with nutrient-rich hydrothermal activity, volcanic activity, and terrigenous inputs that facilitated the proliferation of dominant algal communities. In contrast, the formation of marine excellent source rocks (e.g., the Yacheng Formation in the Qiongdongnan Basin) is closely related to nutrient accumulation in closed to semi-closed bays and the upward transport of nutrients by upwelling currents. These environmental conditions promoted the flourishing of aquatic organisms and enhanced the quality of source rocks. This review highlights the influence of eutrophication in controlling hydrocarbon potential, providing a theoretical basis for understanding the development of excellent source rocks in offshore petroliferous basins and offering new insights into the formation of giant gas fields, biogenic gas, and natural gas hydrates.