DMNS-Denver Basin Project

Temperatures in the Kiowa Drill Hole
by Shari Kelley

For several years, I have been using an age-dating technique called apatite fission-track thermochronology to study the uplift and erosional history of the Colorado Front Range. Bob Raynolds asked me to apply this technique to the sedimentary rocks encountered in the Kiowa drillhole so that we could gain a better understanding of the source of the sediments as the Front Range formed. Bob and I chose some sandstones from the Kiowa core for fission-track analysis. In addition, we have sampled core from a well in Castle Pines.

A piece of information that is very useful in interpreting fission-track data is the temperature distribution in the well. Temperature data in boreholes are also helpful in looking at the hydrology of an area. Temperatures generally increase with increasing depth in the earth as heat from deep inside the earth escapes. The geothermal gradient is a measure of how rapidly the temperature increases as a function of depth. Because the Denver Basin area is currently relatively quiet in terms of volcanic and earthquake activity, one might expect the average geothermal gradient to be about 30 degrees C/km. This means that, on average, the temperature increases 30 degrees C for every kilometer drilled into the Denver Basin. The Kiowa drillhole bottomed out at about 0.67 km, so the temperature should increase about 20 degrees C between the earth's surface and the bottom of the well.

However, as you might expect, nature is not that simple. Water flowing through sedimentary rocks rearranges the temperature distribution in a basin, as illustrated in the diagram below. Aquifers, such as the Fox Hills Sandstone, are porous rock units that permit water to move through the earth fairly easily. Near the mountain front, where rain and snowmelt from the mountains enters the aquifer, the water seeps downward, cooling the rocks. Thus, near the mountain front in the recharge area, the geothermal gradient is below average. The water moves down and is heated by thermal energy escaping from the earth. If the heated water is brought back toward the surface at a place where the aquifer reappears on the other side of the basin, the geothermal gradient is above average in the discharge area.

Figure 1 - Click for enlarged diagram.

The pattern of low geothermal gradient near the western mountain front and high geothermal gradient on the eastern margin of the basin on the plains has been well documented adjacent to the Rocky Mountains in Canada. This pattern has not been documented in the Denver Basin, largely due to a lack of data. Some temperature data exist for an area north of Denver known as the Wattenburg thermal anomaly, a zone of elevated geothermal gradient, but that's another story. Very little thermal data is available for the part of the basin south of Denver. We could not pass up the chance to measure the temperatures in the Kiowa drillhole!

Temperatures in the Kiowa well were measured about 24 hours after the last section of core had been extracted from the well and about an hour after the geophysical logs were run. Drilling a well disturbs the ambient temperatures of the rocks. Fluids used during drilling heat up the upper section of a hole and cool the deeper portions of a well. Consequently, immediately after drilling, the temperatures in the borehole are out of thermal equilibrium and it can take up to year for the temperatures to return to "normal." Although the well was very much out of thermal equilibrium, we decided to log the well immediately after drilling because the well was going to be plugged back to 800 feet, and we wanted to get some idea of the deeper temperatures. The first log of the well was measured by taking readings every 5 meters using a calibrated thermistor attached to about 2,500 meters of cable. The thermistor was lowered using a hand crank. Data from the top 560 meters were collected inside the drill pipe and in open hole below that point. The drill pipe was left in the hole above the top of the Fox Hills Sandstone to keep the hole from collapsing during logging. The fluid level in the hole was about 15 meters below the ground surface at the time of logging.

The hole was completed to a depth of 244 meters (800 feet) in late April 1999. The upper part of the well was re-logged approximately three months after the initial logging run using a truck-mounted system operated by two researchers from Southern Methodist University. Data were collected every 0.1 meters. The water level in the hole was around 100 meters at the time of second log.

The temperature and geothermal gradient data from the two logging runs are shown in Figure 2. The temperature data from an April 1999 run are surprisingly smooth. Fluids moving around at the base of the drill pipe caused the rather large disturbance at 560 meters (1,837 feet). The truck-mounted logging system has a hard time equilibrating in air, so there is an offset in the temperatures measured in July 1999 at the water table. Note that, below the water table, the temperatures have decreased 3 degrees C in the upper part of the well since the April run. Thermal equilibrium was returning!

Figure 2 - Click for enlarged diagram.

The geothermal gradient plot as a function of depth is rather interesting. An average gradient of 30 degrees C/km is shown as a dashed line for reference. Notice that there are some sections of the hole where the interval geothermal gradient is above 30 degrees C/km and other sections where the gradient is below the average value. These deviations from the average value are, in part, related to the type of rocks present in each section.

Heat flow, which is a gauge of the amount of thermal energy coming out of the earth, is calculated by multiplying geothermal gradient by thermal conductivity. Each rock type has a different thermal conductivity, a measure of how well a material conducts heat. Rocks that are rich in quartz, like sandstone, have a high thermal conductivity, indicating that heat readily passes through sandstone. Rocks that are rich in clay or organic materials, like shale and coal, have low thermal conductivity, meaning that heat passes less readily through these intervals. If the heat flow is constant throughout a drillhole (i.e., water is not flowing up or down the hole), then it stands to reason that low-conductivity shale intervals will have a higher geothermal gradient compared high-conductivity sandstone intervals. For example, the gradient jumps from about 30 to 50 degrees C/km at 220 meters. This jump roughly corresponds to an interval in the D1 sedimentary rocks that contains many low thermal conductivity coal layers. Also, note the high geothermal gradient associated with the clay-rich paleosol on the most recent log of the well. The correspondence of rock type with geothermal gradient is likely to become more clear as the well reaches equilibrium.