Formation Temperature and Geothermal Gradients

Estimating Formation Temperature

The simplest formation temperature estimate uses a linear gradient model:

T_formation = T_surface + (G × D)

• T_formation = estimated formation temperature (°F or °C)
• T_surface = mean annual surface temperature (°F or °C)
• G = geothermal gradient (°F/100ft or °C/km)
• D = depth (feet or km, matching gradient units)

The normal geothermal gradient is approximately 1.0–1.5°F per 100 feet (25–30°C per km) in most continental settings. This is often quoted as a rule of thumb of 1.2°F/100ft. However, actual gradients vary widely: from 0.6°F/100ft in deep sedimentary basins with thick shale sequences (low thermal conductivity) to 3.0+°F/100ft near volcanic or geothermal areas.

In offshore environments, the surface temperature used is the mudline temperature, which depends on water depth. In deep water (5,000+ ft), the mudline temperature can be 35–40°F, significantly cooler than onshore surface temperatures, which affects the temperature profile throughout the well.

Regional Gradient Variations

Geothermal gradients vary significantly by geological setting:

• Stable continental interiors: 0.8–1.2°F/100ft. Thick sedimentary sequences with normal heat flow. Examples: Michigan Basin, Williston Basin.
• Passive margins: 1.0–1.5°F/100ft. Gulf Coast, offshore Brazil, West Africa. Gradients may increase with depth due to radiogenic heat from thick shale sections.
• Rift basins: 1.5–2.5°F/100ft. Thinned crust produces higher heat flow. Examples: North Sea, East African Rift, Rhine Graben.
• Volcanic/geothermal areas: 2.5–5.0+°F/100ft. Proximity to magmatic heat sources. Examples: Iceland, Indonesia, parts of the western United States (Basin and Range).
• Subduction zones: 0.5–0.8°F/100ft. Cold oceanic plate being subducted suppresses heat flow. Examples: Cascadia, offshore Japan trench side.

Salt bodies complicate temperature prediction because salt has much higher thermal conductivity (~3×) than surrounding sediments. Salt conducts heat efficiently, creating cool anomalies below salt bodies and warm anomalies above and beside them. Sub-salt drilling in the Gulf of Mexico often encounters lower-than-expected temperatures below thick salt canopies.

Thermal Maturity and Hydrocarbon Windows

Organic-rich source rocks generate hydrocarbons when exposed to sufficient temperature over geological time. The oil window occurs at approximately 150–300°F (65–150°C), where kerogen in the source rock thermally cracks into liquid hydrocarbons. The gas window begins above approximately 300°F (150°C), where previously generated oil cracks into natural gas (dry gas at temperatures above about 390°F / 200°C).

The geothermal gradient determines the depth range of these windows. In a basin with a normal gradient (1.2°F/100ft) and surface temperature of 70°F, the oil window extends from roughly 7,000 to 19,000 feet. In a high-gradient rift basin (2.0°F/100ft), the same oil window is compressed to 4,000–11,500 feet. This has major implications for exploration. In high-gradient basins, deep targets may be over-mature (only dry gas), while in low-gradient basins, even deep prospects may be in the oil window.

Vitrinite reflectance (R_o) is the standard measure of thermal maturity in sedimentary rocks. R_o values of 0.5–1.3% indicate the oil window. Values of 1.3–2.0% indicate the wet gas / condensate window. Values above 2.0% indicate dry gas to over-mature conditions. R_o data from offset wells or outcrop samples helps calibrate the geothermal gradient and predict maturity at untested depths.