Download Geothermal Gradients and Thermal Conductivity

Geothermal Gradients and Thermal Conductivity
Temperature, modified by time, has been instrumental in the formation of most
major petroleum accumulations. During drilling, formation temperature can be
measured by lowering self-recording thermometers into the borehole. When this is
done for various depth levels, the geothermal gradient can be determined.
The worldwide average geothermal gradient, which measures the increase in the
earth's temperature with depth, is about 26 °C/km (14°F/1000 ft). Gradients
measured in sedimentary basins around the world typically range from lows of
about 18 °C/km to highs of 55 °C/km.
A low geothermal gradient causes the first formation of oil to begin at fairly deep
subsurface levels, but it also causes the oil window to be quite broad ( Figure 1 ).
Figure 1
In contrast, a high geothermal gradient enhances the early formation of oil at
relatively shallow burial depths, but it causes the depth range of the oil window to
be quite narrow. Overall, however, the oil-forming process is more efficient in
young source rocks, where there is a high geothermal gradient and oil can form
early at shallow depths (Klemme, 1975).
The magnitude of a petroleum basin's geothermal gradient is most often directly
related to the earth's heat flow; it will be high where heat flow is high ( Heat Flow =
Geothermal Gradient Thermal Conductivity ). Consequently, high geothermal
gradients are often found in basins that are associated with active deformation, sea
floor spreading and mountain-building (tectonic) processes. Gradients will usually
be low in basins associated with old, stable interiors of the continents, the craton.
Gradients will also tend to be low in areas insulated by cool underlying rocks or
thick, rapidly deposited sediments.
Locally, the geothermal gradient will be influenced by the subsurface rocks through
which the earth's heat must pass. The thermal conductivity of rocks, is inversely
related to the geothermal gradient ( Heat Flow = Geothermal Gradient Thermal
Conductivity ). It varies both with the rock type or lithology, and the kinds and
amounts of pore-filling fluids. Thus, the geothermal gradient will normally vary
vertically through a stratigraphic sequence ( Figure 2 ), and temperature will have a
nonlinear relationship to burial depth.
Figure 2
The present-day geothermal gradient may be of less importance to maturation than
paleogeothermal conditions, particularly in areas that have undergone large-scale
uplift and erosion. The chemical reactions completed at higher temperatures are
normally not reversible. It is therefore most important to be able to establish the
highest temperature attained at some time in the geological past. Various
measurement methods, or paleothermometers, have been devised to determine the
maximum formation temperature of a source rock.