Page 55 - Geology and Geochemistry of Oil and Gas

32 OIL- AND GAS-BEARING ROCKS
where P a ¼ the total potential energy of the accumulation, P wr ¼ the water potential
in the reservoir rock, Dg ¼ the difference in fluid densities, G o ¼ the initial oil pres-
sure gradient in the reservoir rock, and H ¼ the height of accumulation column.
Maximum possible height of accumulation column may be determined from
equality of the breakthrough potential and the accumulation energy by solving to-
gether the above two equations. Provided the reservoir fluid densities are constant,
the maximum height (H) of the oil column is equal to
P a P wr þ G o ah þ P c
H ¼ (2.16)
Dg G o
Pore water pressure in compacting argillaceous beds is always greater than the
pressure in the adjacent reservoir beds. As a result, sealing capability of the Type I
caprocks is determined by hydraulic sealing, by the amount of capillary pressure,
and by the pressure at which water begins to flow through caprocks. Just the cap-
illary pressure alone in such caprocks (groups A and B, of Khanin, 1976) may exceed
2
100 kg/cm . This means that the Type I caprocks is capable of confining an oil
accumulation having almost any column height. Dobrynin and Kuznetsov (1993,
p. 106) proposed the following correlation between the accumulation column height
Dh a and caprock thickness Dh seal :
Dh a ¼ f ðDh seal Þ (2.17)
where f depends on temperature. At 401C in gas accumulations, f 4, whereas in oil
accumulations, f 17. At 601C in gas accumulations, f 2 and in the oil accu-
mulations, f 7.
It appears that a statement by Buryakovsky et al. (1990, p. 186) that ‘‘sealing capabil-
ity of argillaceous caprocks does not depend on their thickness’’ describes only the
aforementioned caprock type. Similar statements were made by Kontorovich et al.
(1975) and Nesterov and Rylkov (1986).
Type II caprocks are associated with rocks compacted beyond the plasticity limit
and having lost ability to swell on contact with water. Such rocks do not contain
swelling clay minerals, and interstitial water contains surfactants. Consequently,
pore water in these rocks does not have initial pressure gradient. This type of cap-
rocks is encountered mostly in the Paleozoic and Mesozoic sediments of young and
old platforms. There are no clear-cut overpressure environments there, but there is a
relatively clear hydrodynamic subdivision in the section. The breakthrough potential
(P b ) for the Type II caprocks is a sum of capillary pressure (P c ) and hydrodynamic
forces:
P b ¼ P w:u:layer þ P c (2.18)
where P w.u.layer is the water potential of the upper layer (i.e., layer above the cap-
rock).
Maximum accumulation column height H can be determined as follows:

P w:u:layer P w:l:layer þ P c
H ¼ (2.19)
Dg G w