Studies on Hydrocarbon Migration in Tertiary Formations in The Chengbei Step-Fault Zone, Bohai Bay Basin, China

Luo, Xiaorong¹, Likuan Zhang¹, Qianjin Liao², Junqing Su², Shuqin Yuan², Haiming Song¹, Bo Zhou¹, Ping Hou¹, Changhua Yu² (1) Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China (2) Dagang Oilfield Company, CNPC, Dagang, China

Recent researches demonstrate that hydrocarbons migrate heterogeneously in carriers, and migration pathways occupy only a small proportion (1~10% in volume) of carriers. More quantitative studies are needed to understand the hydraulic potential and the migration process linking source and traps. Based on percolation method, We constructed a model to simulate migration pathways by coupling hydraulic potential and heterogeneous pore spaces of carriers. The characteristics of migration pathways were simulated within Tertiary formations in Chengbei Step-Fault Zone on the southern slope of Qikou Sag, a half-graben in Huanghua Depression. We focused on construction of the stratigraphic architecture of carriers in the studied area, as the hydraulic potential in carriers can be easily obtained through basin modeling.

Hydrocarbons migrate southward to the studied zone from depocenters since Miocene, mainly along porous and permeable sandstones of the Eocene Shahejie Formation. We found that the basal conglomerates and sandstones of Lower Minghuazhen Formation, onlapping the pre-Neogene rocks, may have also been carriers. A fault opening coefficient, C=P/(sRss), is proposed to characterize statistically the degree of hydraulic connection at each unit on fault planes, by combining the normal stress (s), the formation pressure (P,), and the sandstone/mudstone ratio (Rss).

Simulation results show that migration direction is controlled by hydraulic potential in the studied area, but migration pathways expand preferentially in high-permeability carriers, especially in open fault segments. The simulated pathways clearly point out favorable areas of hydrocarbon accumulation. As a result, areas of future prospects can be predicted with the consideration of only traps conditions.