Page 37 - Drilling Technology in Nontechnical Language

28 Drilling Technology in Nontechnical Language Second Edition

Chapter 1 described the possibility of drilling into a formation that had
a greater pressure than would be expected for the depth of the formation.
A seal rock gives one of the two conditions necessary for overpressure to
occur—a pressure barrier.

Secondary Migration

In secondary migration, the oil droplets move about within the
reservoir to form pools. Secondary migration can include a second step
during which crustal movements of the earth shift the position of the pool
within the reservoir rock (fig. 2–3).

Accumulations can be affected by several, sometimes conflicting,

factors:
1. Buoyancy causes oil to seek the highest permeable part of the
reservoir. Capillary forces direct the oil to the coarsest grained

area first, then successively into finer grained areas later.
2. Any impermeable barriers in the reservoir channel the oil into
somewhat random distribution.
3. Oil accumulations in carbonate reservoir rocks are often erratic,
because part of the original void spaces have been plugged
by minerals introduced from water solutions after the rock
was formed.
4. In large sand bodies, barriers formed by thin layers of dense shale
may hold the oil at various levels. With crustal movement of the
earth, accumulations are shifted away from where they were
originally placed.
5. Faults (where part of the rock body has moved along a crack)
sometimes cut through reservoirs, destroying parts or shifting
them to different depths.
6. Uplift and erosion bring accumulations nearer to surface, where
lighter hydrocarbons may evaporate.
7. Fracturing of the cap rock may allow accumulations to
migrate upwards.
Wherever differential pressures exist and permeable openings provide
a path, petroleum will move.

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