ABSTRACT
A coherent oil-migration model based on geomechanical considerations includes both the high-molecular kerogen structure and the capillary properties of source rocks. Oil is squeezed from kerogen by compaction following oil generation. This squeezing effect should be created by the differential stress (maximum compressive stress minus least compressive stress) acting on the kerogen which has been chemically broken up by oil formation. In sedimentary bodies whose water is at hydrostatic pressure, the migration of oil seems to involve two processes: (1) lateral transfer, by channeling into the more coarsely microporous layers of the source rocks, from the oil generation site toward the geologic structure or lower pressured zone; and (2) vertical transfer from source rock to reservoir by the opening or reopening of vertical fractures in the few areas, such as structural tops, where the least compressive stress is slightly greater than or equal to the pore pressure, and where the capillary pressure increment (2γ/R) of oil in the microporosity exceeds the tensile strength of the rock.
In sedimentary bodies whose water is overpressured, the pore pressure should be governed by the least compressive stress and thus migration should begin by oil transfers in a system of small open fractures, and eventually in larger fractures.
The theory demonstrates the impossibility of oil being transferred to the reservoir under true tensile conditions (negative effective compressive stress) and thus explains the large asphaltic veins of southeastern Turkey and the well-known bitumen veins of the Uinta basin.
