Page 82 - Petrophysics

56 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

and used to estimate the temperature of formations at any other
depth D.

EXAMPLE

The bottomhole temperature at 2.2km was found to be 70°C.
The mean surface temperature for the region is 24°C. Determine the
geothermal gradient Gt and a temperature of the formation at a depth of
1,700 meters.

SOLUTION

Solving for Gt from Equation 2.9:

70 - 24
- = 20.9"C/km
-
2.2
The formation temperature at D = 1.7 km is obtained from
Equation 2.9:

There are zones in various locations on the globe where the geothermal
and geopressure gradients are abnormally high. Some areas in the United
States where abnormally high pressures and temperatures have been
reported are: Gulf Coast Basin post-Cretaceous sediments, Pennsylvanian
Period sediments in the Anadarko Basin in Oklahoma, Devonian zone in
the Williston Basin in North Dakota, and the Ventura area of California.
Outside of the United States, geopressure/geothermal zones have been
reported in many areas, e.g., the Arctic Islands, Africa (Algeria, Morocco,
Mozambique, and Nigeria), Europe (Austria, the Carpathians, the Ural
Mountains, Azerbaijan, and Russia), Far East (Burma, China, India,
Indonesia, Japan, Malaysia, and New Guinea), Middle East (Iran, Iraq,
and Pakistan), and South America (Argentina, Colombia, Trinidad, and
Venezuela) [ 19,201. The pressure and temperature gradients range up to
20 kPa/m (0.9 psi/ft) and 30°C/km (1.7"F/lOO ft), respectively, as shown
in Figures 2.11 and 2.12.
Many possible causes for the geopressured zones are presented
in the literature. Fertl and Timko discussed 17 causes [21]. Among
these are rapid sedimentation accompanied by contemporary faulting,
which is apparently the greatest contributing cause of the abnormally