Page 27 - Origin and Prediction of Abnormal Formation Pressures

10 E.C. DONALDSON, G.V. CHILINGAR, J.O. ROBERTSON JR. AND V. SEREBRYAKOV

(20 ft3/bbl) (Kharaka et al., 1977). Organic matter which is a substantial part of
freshly deposited muds decomposes during diagenesis as a result of biochemical and
thermochemical processes. The resulting methane gas which is released during the
transformations can create, or accentuate, the overpressured, undercompacted, state
of the compacting mud sediments in two ways: (a) by building up additional pore
pressure; and (b) by further impeding the expulsion of interstitial pore water through
the development of a second gas-fluid phase. Gas bubbles dispersed in water reduce the
permeability of the rock to either phase (Chilingarian et al., 1995).
The mechanism of temperature increase (aquathermal expansion) as a possible cause
of overpressures has been questioned by several authors (e.g., see Swarbrick and
Osborne, 1998). The main objection is the absence of practically impermeable seals.

Phase changes that produce abnormal pressures

Berner (1980) described two phases in early diagenesis. The first one consisted of
two stages: (1) the initial stage which is regulated by the chemistry of water; and (2)
the early burial stage which is controlled by the entrapped pore water that is chemically
modified by bacteria and bioturbation of surface organisms. During the initial stage,
the clay minerals undergo a gradual change of their ionic exchange capacity, and

Normal thermal
gradient 18.2~
(I.O~ ft.)
.-. 6
if3
i
c)
x
I-
uJ 9 Geothermal zone
bJ ,,=, /~30.OOC/km
a (I.7~ ft.)
-r-
l--
o.
w
ol2
\
\
16 \ \
\
- 50 I00 150 2_00
TEMPERATURE ~
I , I ~ I I I I
0 I00 200 300
TEMPERATURE ~
Fig. 1-5. Approximate average subsurface temperature gradients.