Three-Dimensional Assessment of Oil Migration in a Japanese Basin by Two-Dimensional Basin Model "SIGMA-2D"

Akihiko Okui, Mamoru Hara, Hideki Matsubayashi

The assessment of oil migration should be conducted in three-dimensional segment integrated with pressure distribution in basins. However, sophisticated three-dimensional basin modeling is still under development. We instead apply two-dimensional basin modeling to several cross-sectional slices throughout basin to evaluate oil migration in three-dimensional direction.

Japan National Oil Corporation's SIGMA-2D is used as the two-dimensional basin model. SIGMA-2D is a two-phase fluid flow simulator, which can model hydrocarbon generation, migration and accumulation. In SIGMA-2D, hydrocarbon generation is calculated by kinetic model based on the temperature distribution in the basin. Oil migration and sealing are simulated by Darcy's law based on the pressure distribution in the basin. Tectonic fracturing/faulting is also modeled by simple strain analysis.

In offshore Niigata basin, which evolved as Tertiary back-arc basin in Japan, main oil producing reservoir is turbidite sandstones in Pliocene. Geochemical data reveal that accumulating oil sourced from shales in middle Miocene. Several cross sections across this basin were modeled. The simulations indicate that the combination of deeply buried source rock, tectonic fracture zone and turbidite fan appears to be essential factor for oil accumulations. Lacking of any one element can not accumulate oil.

A series of these two-dimensional simulations enables us to visualize three-dimensional migration pattern in this basin and identify the drainage area. The simulations indicate that the drainage area is not as large as that evaluated from simple geometrical analysis.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995