Cenozoic volcanic rocks in the Pearl River Mouth and Southeast Hainan Basins of South China Sea and their implications for petroleum geology

Drilling, gravity, magnetic observation and seismic facies are combined to identify the Cenozoic volcanic rocks in the Pearl River Mouth Basin and the Southeast Hainan Basin in the northern South China Sea, and to analyze how their lithology, distribution characteristics and volcanic activity influence on hydrocarbon accumulation. As a result, more than 70 volcanic bodies are ascertained. They are divided into two types: (1) laminated lava and pyroclastic rocks in the Cenozoic sedimentary sequence; and (2) conoid, mounded or wedged basalt with rigid basement under the Cenozoic sedimentary sequence. In the early Cenozoic (E1-E2), the acidic, neutral and basic volcanic rocks developed, while in the mid-late Cenozoic (E3-Q), single basic rocks existed intensively. The volcanic lithofacies mainly include the lower and middle effusive subfacies and the pyroclastic flow deposit explosive subfacies. Spatially, from the Southeast Hainan Basin to the Pearl River Mouth Basin, the Cenozoic volcanic activity gradually enhances; from shallow water area to the Central Basin of the South China Sea, the scale of Cenozoic volcanic rocks increases gradually, and the age tends to be younger. These activities improve the formation temperature, reduce the hydrocarbon generation threshold, and promote the hydrocarbon accumulation. Volcanic rocks in Wenchang Formation (E2w) to Zhujiang Formation (N1z) provide favorable conditions for hydrocarbon accumulation, so they are important targets in the deep exploration.