Abstract:

The Bohai Bay Basin hosts dozens of sags with relatively independent petroleum systems, in which large-scale hydrocarbon migration and accumulation occurred at various geologic times and stages. A systematic investigation is conducted on the timing of hydrocarbon accumulation as well as the main controlling factors on the differences in geologic times and stages of hydrocarbon accumulation in the major reservoirs of these sags in the basin through combining a large number of statistical data and the analyses of hydrocarbon generation history of source rocks with previous studies. Results show that hydrocarbons generated by the Paleogene source rocks probably accumulated in two stages, namely the end of Paleogene (early stage) and middle Neogene-Quaternary (late stage but more important), respectively. During the whole process, the predominance of early accumulation has been gradually taken over by the late accumulation from the basin edge sag inward to the Bozhong sag, and the accumulation stages feature in changing from single stage of early accumulation to two stages with early accumulation predominance, to two stages with late accumulation predominance, and to single stage of late accumulation. Vertically from deep to middle and further to shallow reservoirs, hydrocarbon accumulation stages change from single stage of early accumulation to two stages and further to single stage of late accumulation. The subsidence patterns of the sags during the Cenozoic are suggested to be the major control over the thermal evolution of source rocks, which in turn controls the differences of hydrocarbon accumulation stages, resulting in a good correspondence between the subsidence patterns and the dominant accumulation stages. The denuded thickness at the end of Paleogene and the sedimentary thickness of Neogene-Quaternary are believed to be the determining factor on which accumulation stage being predominant. Observation shows that the greater the compensation thickness, the more obvious the characteristics of late accumulation stage. The main hydrocarbon generation periods determine the main hydrocarbon accumulation stages. Hydrocarbons sourced from the Kongdian Formation source rocks is likely to accumulate mainly in the early stage, while that from the source rocks in the first member of Shahejie Formation to the third member of the Dongying Formation is more likely to be related to the late accumulation. Hydrocarbons from source rocks in the upper fourth and third members of Shahejie Formation shows characteristics of two stage accumulation.

Key words: early stage accumulation, late stage accumulation, evolution of source rock, hydrocarbon accumulation stage, Neogene, Bohai Bay Basin