Page 25 - Origin and Prediction of Abnormal Formation Pressures

8 E.C. DONALDSON, G.V. CHILINGAR, J.O. ROBERTSON JR. AND V. SEREBRYAKOV
Basinward D,

~ : Sand

~ ~ . . . . . , .:...,...,~: ~,... .............

............... . .,.:.. ...... ....... ..............
~~~ABNORMAL~

Salt

Fig. 1-3. Schematic section through a piercement salt dome showing modification of abnormal pressure sur-
face. (Modified after Harkins and Baugher, 1969, p. 964. Courtesy of the Society of Petroleum Engineers.)

salt and the overlying sediments (Johnson and Bredeson, 1971). In Fig. 1-3, the steep
boundary demarcation of the abnormally pressured lower zone reflects earlier (before
development of the salt dome) topography associated with the uplift of the region.
Harkins and Baugher (1969) illustrated the abnormal pressures associated with the
sheath in Fig. 1-3. A well drilled into Formation C would encounter high pressures in
the Formation D rocks. The sheath deposits are out of place, having been dragged into
their present position by the dome.
Growth of salt domes in the Gulf Coast region of the United States was contempora-
neous with the sedimentation (see Fig. 1-4).
Deep shale beds also undergo plastic flow when subjected to high overburden
pressures, forming diapiric masses with the same characteristics as those of salt beds
(low bulk density, high pressure gradients, and low electrical resistivity) (Gilreath,
1968). This condition probably occurs when low-density, low-permeability formations
are rapidly loaded by sediments; this occurs in major river deltas such as the Niger, Nile,
Mississippi, Amazon, etc. where shales are rapidly loaded by sands (Murray, 1961).

Geothermal temperature

Another contributor to the fluid pressure is the temperature increase that occurs
within the geopressured zone. The overlying, normally pressured, sediments that are
compacted possess a lower thermal conductivity and act as a 'blanket', decreasing the
transfer of heat from the deep mantle. A leak-proof permeability seal is required in
order to have a closed system, and the heat trapped by the blanket effect above the
geopressured zone produces an abnormally high temperature in the formation. This
contributes another incremental pressure increase to the fluid (Kreitler and Gustavson,
1976). The approximate subsurface temperature gradients are illustrated in Fig. 1-5. The
temperature gradient increases from the normal gradient of 18.2~ (1.0~ ft)
to about 30~ (1.7~ ft) in the geopressured zone at a depth of about 3000 m.