Composition and effectiveness of marine hydrocarbon sources in the Lower Yangtze area

Tectonic activity in the Late Indosinian-Yanshanian period was the most important event for marine hydrocarbon generation and accumulation in the Lower Yangtze area. The evolution of source rocks could be classified into early and late (secondary) stages of hydrocarbon generation. Accordingly, 3 hydrocarbon accumulations inmarine sequences in the Lower Yangtze area were formed in the Caledonian, Indosinian and Himalayan periods, respectively. There were obvious differences among hydrocarbon sources in the different accumulation periods. The late hydrocarbon accumulation in the Himalayan period was mainly sourced from the secondary generation of the Permian and Silurian source rocks and the thermal cracking of ancient oil pools. The thermal history of the source rocks indicates that the potential location for secondary hydrocarbon generation was limited to the area where R_o was < 1.3% at the end of the Middle Triassic. Oil and gas migration direction and possible accumulations were identified in the light of paleo-fluid potential for the top of the Lower Paleozoic sequence. Combined with the analysis of preservation conditions of the Silurian mudstone seals after the Yanshanian movement, the distribution of oil accumulations could be predicted. Therefore, the effective source rocks might be determined and the potential late accumulations that were formed in their nearby areas identified.