Formation Temperature Calculator: Geothermal Gradient at Depth

How It Works

1. Enter Surface Temperature

   Input the mean annual surface temperature for the well location. This is the stable ground temperature below seasonal variation, typically measured at 30-50 feet depth. Not the air temperature.

2. Enter Geothermal Gradient

   Input the local geothermal gradient in degrees F per 100 ft or degrees C per km. Use local well data or published regional values. The global average is about 1.5 degrees F per 100 ft (25-30 degrees C/km).

3. Enter Target Depth

   Input the true vertical depth of the formation or zone of interest in feet or meters.

4. Review Formation Temperature

   See the estimated static formation temperature at depth. Use this for mud system selection, cement design, logging tool ratings, and geothermal resource assessment.

Built For

• Drilling engineers selecting mud systems and additives rated for the expected bottomhole temperature
• Cementing engineers designing slurries with correct retarder concentrations for downhole temperature conditions
• Logging engineers verifying tool temperature ratings before running wireline or LWD tools to TD
• Geothermal engineers assessing resource temperature at target drilling depths for power generation feasibility
• Geologists estimating thermal maturity of source rocks for hydrocarbon generation analysis
• Completion engineers selecting elastomer and packer materials rated for the formation temperature
• Well planners budgeting for temperature-related equipment and material upgrades on deep or hot wells

Features & Capabilities

T = T_surface + Gradient x Depth

Linear geothermal gradient formula. Simple and effective for most planning purposes. Enter values in your preferred units.

Regional Gradient Reference

Quick lookup for typical gradients by region: Basin and Range (2.5-4.0 degrees F/100 ft), Gulf Coast (1.5-2.0), Permian Basin (1.2-1.5), Stable Shield areas (0.8-1.2).

Multi-Layer Gradient

Enter different gradients for different depth intervals. Useful when the gradient changes across major lithology boundaries (shale vs sandstone vs carbonate).

Equipment Rating Check

Compare calculated temperature against common tool and material ratings: standard elastomers (275 degrees F), polymer muds (350 degrees F), standard logging tools (350 degrees F).

Unit Conversion

Switch between degrees F and degrees C for temperature, degrees F per 100 ft and degrees C per km for gradient, feet and meters for depth.

PDF Export

Export formation temperature analysis for well planning documents, AFE support, or equipment requisitions.

Assumptions

• Geothermal gradient is linear from the surface datum to the depth of interest.
• Surface temperature is the mean annual temperature at the wellsite or a user-entered value.
• Default geothermal gradients use regional averages (e.g., 1.0-1.5 F per 100 ft for most basins).
• No convective heat transfer effects from fluid flow or artesian zones.

Limitations

• Real geothermal gradients vary with lithology — shales conduct heat differently than sandstones.
• Does not apply Horner correction to bottom-hole temperature (BHT) measurements from wireline logs.
• Ignores localized thermal anomalies from salt domes, igneous intrusions, or fault zones.
• Deep wells may have non-linear gradients due to changing thermal conductivity with depth.
• Not suitable for geothermal energy applications where detailed thermal modeling is required.

References

• AAPG Studies in Geology — geothermal gradient maps of North American basins.
• SPE technical papers on bottom-hole temperature measurement and Horner correction methods.
• Beardsmore and Cull, Crustal Heat Flow: A Guide to Measurement and Modelling.
• State geological survey geothermal gradient databases.

Frequently Asked Questions