Page 328 - Origin and Prediction of Abnormal Formation Pressures

ABNORMALLY LOW FORMATION PRESSURES 297

Earth's crust, whereas the second one is a local process in some areas. But both of these
processes are associated with well-compacted rocks. In this chapter, the authors discuss
only the second phenomenon. In order to estimate and compare compaction of rocks in
different basins and regions, the writers used the coefficient of irreversible compaction
of rocks /3(t, T), where t is time and T is temperature. The change in porosity of a
sedimentary rock with depth can be presented as follows (Dobrynin, 1970):

0(~p = /3 (t, T) x d(~ - p) (11- l)
(1 -- ~p)~p
where a is the overburden pressure, p is the pore pressure, (or - p) is the effective stress,
and ~bp is the value of porosity in the highest part of the interval of interest. Effective
stress can be estimated as follows:

(or - p) = g(Pr - pw)h (11-2)
where g is the gravitational acceleration, fir is the average rock density, Pw is the average
density of water, and h is the depth. Using fir - - 2.5 g/cm 3 and Pw -- 1.1 g/cm 3 as the
average density of rocks and water, respectively, one can estimate the average effective
stress for a geological section as (c~ - p) = 0.014 h, and determine the coefficient of
irreversible compaction/3 (t, T) as:
1 Aq~p
~(t, T) ~ (11-3)
0.014(1 -- ~bp)~bp Ah
where A~bp/Ah is the porosity gradient in the depth interval of interest.
One can estimate and compare the coefficient of irreversible compaction of shales
/3(t, T) in different basins. On comparing shale porosity-depth relationships of the
Gulf Coast (Dickinson, 1953), the Oklahoma Basin (Athy, 1930), the West Kuban
Depression (Popov, pers. commun., 1970), the North Caspian Basin (Dobrynin and
Serebryakov, 1978) and the Powder River Basin, one can observe that shale porosity-
depth relationships in various basins are quite different (also see Rieke and Chilingarian,
1974). Using these data and Eq. 11-3, one can estimate the coefficient of irreversible
compaction for these four basins (Table 11-1). In three of these basins (Gulf Coast,
Oklahoma and West Kuban Depression), the coefficient of irreversible compaction
varies greatly with depth. There is a fourfold change with a depth of 2 km in the
Gulf Coast, almost twice in Oklahoma, and one and a half times in the West Kuban
Depression. This variation shows that rocks in these basins are not fully compacted
and that the compaction of these rocks may be continuing even now. Only two basins
in Table 11-1, the North Caspian and Powder River, have highly compacted rocks, as
shown by the fact that the coefficient of irreversible compaction is not changing much
with depth.
Compaction of these rocks has stopped. The probability of creating abnormal
pressure in the more compacted, isolated rocks of these basins is significantly greater,
because the influence of temperature on pore pressure in these rocks is significantly
greater.
Unusual underpressured hydrocarbon traps, and the seals that isolate them, result
from global temperature change at the Earth's surface or from local temperate change