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Chapter 2 – OIL AND GAS RESERVOIR FORMATION                       25






                 very close to each other. At a depth of 2,000 m, the gap between the
                 crystals is about 10 nanometers. By the time the shale is buried to a depth
                 of 5,000 m, this gap has closed to about 1.5 nanometers (1.5 × 10  m or
                                                                               –9
                 0.0000000015 m).
                    Shale has tiny pores that are connected by tiny passages. It takes a long
                 time for the water and the oil produced within the shale source rock to
                 migrate out of the rock, squeezed out by pressure. The actual mechanism
                 by which the oil leaves the source rock is uncertain, but it is thought that
                 the oil is initially in solution in the water under the high pressures that exist
                 in the source rock.


                                     Primary Migration



                    The first two conditions necessary for the birth of a reservoir are the
                 existence of an organic-rich source rock and the conditions necessary for
                 oil to be generated—temperature (the oil window) and time. If the oil
                 cannot migrate out of the source rock, it stays locked within the shale and
                 cannot be produced.

                    The third element required is that the source rock lies next to a
                 permeable rock or a channel that allows the oil to migrate. In most cases,
                 a permeable sandstone deposit provides this conduit, but it can also be
                 provided by fractures in the rock or ancient reefs (limestone structures
                 made up of coral skeletons with very high permeability). Fractures often
                 allow migration vertically upwards, and this mechanism has led to many
                 large oil accumulations, such as those found at shallow depths in Venezuela
                 and northern Iraq.
                    A gently sloping formation bed can carry the oil for long distances
                 horizontally until a trap stops migration and allows accumulation.
                 Therefore, a reservoir can be located many miles away from the source
                 rock that generated the oil.


                                       Structural Traps


                    As the oil and gas undergoes primary migration away from the source

                 rock, it must find a structure that has the right conditions to trap the oil and

                 stop it from reaching the surface (fig. 2–2).





        _Devereux_Book.indb   25                                                  1/16/12   2:06 PM
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