Page 76 - Origin and Prediction of Abnormal Formation Pressures

5 8 G.V. CHILINGAR, J.O. ROBERTSON JR. AND H.H. RIEKE III

(2) conduction through mineral grains and interstitial fluids, and (3) radiation. Jones
(1969, p. 807) listed several factors that have a direct beating on the heat flux in
sediments:
(1) thermal conductivity and composition of (a) the mineral grains that form the rock
matrix and (b) interstitial fluids;
(2) specific heat of the pore fluids and solids;
(3) porosity and pore distribution in the shales and sands;
(4) density, viscosity and thermal expansion of the pore fluids;
(5) thermal expansion of solids;
(6) absolute temperature.
Lewis and Rose (1970) and Jones (1969) observed that in the Gulf Coast region
the overpressured zones have abnormal temperature gradients. Jones (1969, p. 804)
found no relationship between the average geothermal gradient and pressure/depth
ratio (geostatic ratio) in the Gulf Coast Tertiary sediments after studying 175 south
Louisiana overpressured reservoirs above a depth of l 1,000 ft. Nevertheless, the
occurrence of abnormal pressures is commonly associated with a sharp increase in
the geothermal gradient in the sealing clay member of the reservoir (C.E. Hottman,
personal communication, 1966; in Jones, 1969, p. 804). According to Lewis and Rose
(1970), the abnormally pressured shale zones constitute thermal barriers, because they
are undercompacted and have high porosity compared to the adjoining sediments.
Reduction in the upward flow of water in these zones greatly reduces the rate of upward
flow of heat and, consequently, the overpressured zones become heat storage areas.
In addition, the insulating effect of water is three times greater than that of the shale
matrix. The larger the amounts of fluid stored in the overpressured shales, the greater
is the insulating value of the zone. Whenever there is an insulating layer in the Earth's
crust there can be a buildup of heat beneath this layer. Thus, the geothermal gradient
is steepest in the portion of the beds above a permeable reservoir. Jones (1969, p. 805)
reported gradients as high as 6~ ft in such settings.
The steepness of the geothermal gradient varies inversely with the thickness of
unconsolidated sediments in the structural basins (Jones, 1969, p. 807). Geothermal
gradients are large in the undercompacted shales overlying the reservoir sands and
are very much reduced in the aquifers. The thermal conductivity of sediments varies
inversely with the geothermal gradient, if the geothermal flux is uniform over broad
areas. Langseth (1965) stated that the thermal conductivity of clay varies inversely with
its water content, and Zierfuss and van der Vliet (1956) discovered that the thermal
conductivity of sand increases with porosity owing to the occurrence of convective heat
transport in the wider pores. As pointed out by Bogomolov (1967), water plays a major
role in the redistribution and subtraction of heat in the geothermal field of the Earth's
sediments.
Jones (1969) stated that convective and conductive heat flow is important in the
low-temperature range above depths of 10,000 ft in the northern Gulf of Mexico Basin.
Water temperatures in this area are greater than 250~ at depths ranging from 10,000 to
14,000 ft (Jones, 1969). Lewis and Rose (1970) showed a range in average geothermal
gradients from 1.6~ to 2.2~ ft for the Texas Gulf Coast.