Hydrocarbon downward accumulation from an upper oil source to the oil reservoir below in an extensional basin: A case study of Chezhen Depression in the Bohai Bay Basin

• Published in: Marine and petroleum geology Vol. 103; pp. 516 - 525
• Main Authors: Lao, Haigang, Wang, Yongshi, Shan, Yixian, Hao, Xuefeng, Li, Qin
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.05.2019
• Subjects: Downward hydrocarbon migration; Extensional basin; Oil-source correlation; Overpressure; Upper source-lower reservoir; Downward hydrocarbon migration; Upper source-lower reservoir; Oil-source correlation; Overpressure; Extensional basin
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2019.03.009

There is a disputable theory that hydrocarbon can be injected into the reservoir, and migrate downward in an oil reservoir with an upper oil source, which is still questioned now. However, although some examples are previously found, questions still remain about the theory, regarding oil sources, oil migration pathways, and migration forces that related to downward hydrocarbon migration. For this reason, the Chezhen Graben in the Bohai Bay Basin is selected to study all the problems above. Geo-chemical evidence show that lower Es3 source rock (T6 in seismic profiles) has a close genetic relationship with the underlying Es4 (T7 in seismic profiles) and Ordovician oil. Therefore, the “upper source-lower reservoir” type can be formed inside the lower gentle slopes and sag zones, and inside the Cambrian and Permian reservoirs. The fluid inclusion experiment shows that the lower ES3 source rock underwent 3 over-pressure evolution stages. Meanwhile, there is a huge pressure difference between the lower ES3 source rock and underlying ES4 reservoirs, which is the migration force of downward migration. In the lower areas of gentle slopes and sags, the early-generated and early-ended faults are terminated at the source rock of Es3 Member, and can be re-opened under the over-pressure influence of the source rock and regional tensile stress. Then they become the pathways of the so-called downward migration. The distance of downward migration can be calculated through the accumulation dynamic, which is obtained by hydrocarbon pressure subtracting the reservoir hydrostatic pressure, pressure attenuation, and displacement pressure. The research result on downward migration process is of great significance to broaden the hydrocarbon exploration fields. •Downward migration can occur in the lower gentle slope and sag zones, in which tectonic activity is not active.•Pathways of downward migration are mainly the “early generating and early ending” faults disappearing source rock upward.•The large paleo-pressure difference between source rock and its underlying reservoir becomes downward migration force.