Characterization of Fault Sealing for Hydrocarbon Migration
- A Case Study in the Chengbei Step-Fault Zone, Bohaiwan Basin, China
Likuan Zhang1, Xiaorong Luo1, Dunqing Xiao2, Junqing Su2, Shuqin Yuan2, and Changhua Yu2
1Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China
2Research Center of Exploration and Development, Dagang Oilfield Company, CNPC, Tianjin, China
Faults may act as pathways or seals during hydrocarbon migration
and accumulation and, thus, are critical elements in a petroleum system. The Chengbei Step-Fault Zone, Bohaiwan Basin, China, is ideal to study the role of faults in
migration
and accumulation because syn-depositional faults controlled not only sedimentation during Paleogene but also hydrocarbon
migration
and accumulation during early Neogene. We identified three factors critcal to fault sealability: pore pressure (P) in shale, shale gouge ratio (SGR) in the vicinity of faults, and normal stress (σ) on fault plane, and defined a fault opening coefficient (FOC), which is directly proportional to P and inversely to and SGR. By dividing a fault plane into small zones, the value of FOC can be calculated. A larger FOC indicates a greater possibility of an open fault. Whether a fault was open as a pathway or closed as a seal at a specific zone (as indicated by sealing probability, Ps) can be verified by the presence or absence of oil in the reservoirs above this zone. A larger Ps indicates a larger probability of fault closing. The results were statistically analyzed to establish a relationship between FOC and Ps during
migration
: Ps tends to be 1 when FOC is smaller than 1; a power relationship exists between FOC and Ps when FOC is between 1 and 3.5; and Ps tends to be 0 when FOC is larger than 3.5. The relationship suggests that faults opened variably during hydrocarbon
migration
in early Neogene, and provided important pathways for vertical
migration
in the Chengbei Step-Fault Zone.
AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas