Abstract

The ANDRILL (Antarctic Geological Drilling Program) Southern McMurdo Sound Project (SMS) drilled a 1138-m-deep borehole (AND-2A) within Neogene strata in the southern Victoria Land Basin of the West Antarctic Rift in the Ross Sea, Antarctica, during the 2007–2008 austral spring. An extensive downhole logging program was carried out in the SMS borehole that included temperature and spectral gamma ray logs. These logging data were combined with lab measurements of the thermal conductivity of core samples to deduce several thermal parameters of the borehole country rock. The results indicate a bottom-hole temperature of 57.8 °C and an average temperature gradient of 51.9 K/km. Thermal conductivities on core samples range from 1.22 to 2.95 W/mK and average 1.57 W/mK. Thermal conductivities coupled with the average temperature gradient yield an average heat flow of 81.5 mW/m2, a minor amount (1.1 mW/m2) of which is generated by radiogenic heat production. Temperature, gradient, and heat flow are considered as possible minimum values. The average heat flow determined for the SMS borehole therefore provides important confirmation that heat flows are locally above average values for continental crust in the southern Victoria Land Basin of the West Antarctic Rift system. This new heat flow constraint is consistent with extension and volcanism within the Terror Rift, which was active in Neogene time.

INTRODUCTION

The thermal properties of boreholes are important for understanding tectonic history (Sclater et al., 1980), yet there are few temperature logs from boreholes in Antarctica, even though tectonic records from the continent are poorly known and of widespread interest (Blackman et al., 1987; Bücker et al., 2001; Morin et al., 2010). As part of the Antarctic Geological Drilling Program (ANDRILL) to better understand Antarctic climate and tectonic history, the Southern McMurdo Sound Project (SMS) cored Early and Middle Miocene sedimentary rocks in the Victoria Land Basin at the western edge of the West Antarctic Rift system to the east of the Transantarctic Mountain front in the austral spring of 2007–2008 (Figs. 1 and 2; Florindo et al., 2008–2009). Seismic studies indicate that the West Antarctic Rift system within this area of the western Ross Sea records at least two distinct periods of Cenozoic rifting (Paleogene and Neogene), the most recent of which produced the Terror Rift and was associated with the production of volcanic rocks of the McMurdo Volcanic Group (Fig. 1; Cooper et al., 1987; Kyle 1990a; Salvini et al., 1997; Davey et al., 2006; Henrys et al., 2007; Fielding et al., 2008).

The SMS drill site was located on an 8.5-m-thick sea ice drilling platform above a 380-m-thick water column (Falconer et al., 2008–2009). Total core recovery was ∼98% and the total depth of the AND-2A well was 1138 m below seafloor, which is particularly deep for boreholes previously drilled in the region. The SMS project involved a comprehensive logging program, which included a series of temperature and spectral gamma ray log runs (Wonik et al., 2008–2009) and subsequent laboratory work on core samples to determine porosities and thermal conductivities. Our intent is to present the results of these analyses, which provide important new constraints on thermal parameters (temperature gradient, heat flux density, radiogenic heat production) that have general bearing for understanding the geodynamic evolution of the Victoria Land Basin within this sector of the West Antarctic Rift (e.g., Huerta and Harry, 2007).

MEASUREMENTS

The earliest measurements of borehole temperature (n = 41) were made using the GyroSmart™ core orientation tool during AND-2A drilling. The measurements were made immediately after drilling a core run and before recovery of the inner core barrel (Fig. 3). The tool and battery pack were installed in a 1.2-m-long brass barrel, which was housed within a 1.5 m pressure barrel. This pressure barrel was located below the latch head assembly and screwed into the upper part of the wireline recoverable inner core barrel. Thus, during the drilling of a core run, the GyroSmart™ tool was located above the base of the borehole by a length that was equal to the length of the inner core barrel (3 m for PQ drilling and 6 m for HQ and NQ drilling; PQ, HQ, and NQ refer to tube diameters). Drilling logs indicate that the time spent drilling the core runs ranged from 36 to 195 min and averaged 103 min. The time the tool spent downhole prior to recording the temperature allowed the tool and its housing to equilibrate to the downhole temperature of the drilling mud that was passing the tool on the way to the drill bit.

Borehole temperatures were also recorded at the end of three drilling phases (see Fig. 4) using a combined salinity and temperature tool, which logged points every 0.05 m with a ∼0.1 °C accuracy. Detailed descriptions of all of the logging tools utilized in the AND-2A borehole are available in Wonik et al. (2008–2009) and Cape Roberts Science Team (1999, 2000). Due to time constraints, it was not possible to wait long enough for the restoration of temperature equilibrium after drilling operations. Measurements had to be made only a couple of hours after drilling operations were suspended. Four temperature logs were recorded in total: one each during phases 1 and 3, and two during phase 2. Figure 3 shows the temperature data but leaves out the second log in phase 2 for the sake of simplicity. Both logs from phase 2 are similar in shape and differ primarily in absolute temperature values due to time elapsed between measurements.

The first well log using the salinity and temperature tool was run after the borehole had reached a depth of 1012 m below seafloor (mbsf). During phase 1 drilling (22 November 2007), logging could only be conducted down to ∼620 mbsf because the well was blocked by a bridge at this point. After removal of the bridge during phase 2 drilling, logging was conducted down to a depth of 1000 mbsf (23 November 2007). During phase 3 drilling, the well was drilled down to a total depth of 1138 mbsf and the last temperature measurement was recorded after drilling activity had been terminated (30 November 2007).

For all of the logging phases, the upper 230 m of the well was permanently cased with a conductor pipe. The drill string was also kept in place to prevent the walls of the borehole from caving in. The borehole was open below 400 mbsf during phase 1 logging, below 650 mbsf during phase 2, and below 1000 mbsf during phase 3 (Fig. 4). This means that most of the temperature measurements were conducted within the drill pipe, and above 230 mbsf, also within the surrounding conductor pipe. Although this does not influence the temperature measurements, it disrupts the movement of fluids between the well and the formation, and therefore influences the reestablishment of the natural temperature field in the formation, which is disrupted by the drilling process. The frictional heat generated by the drill bit can be ignored, but the mud circulated through the borehole during drilling has a significant influence on the temperature field. Pumping the mud generates an almost constant temperature profile along the borehole (Wohlenberg, 1979). The longer the mud is circulated, the deeper the artificially created temperature signal penetrates into the formation (Zschocke, 2005). The temperature of the drilling mud was ideally 25 °C when it was first made up from seawater. The drilling mud may have cooled to 20 °C once it was pumped down the borehole (Alex Pyne, 2008, personal commun.) and may have cooled further while transiting through the water column before it was once again heated while transiting the surrounding bedrock. The time required for the natural temperature field to reestablish depends largely on the geometry of the borehole, the physical properties of the formations penetrated by the borehole, and the thermal properties of the formations and the drilling mud. It is assumed that the temperature is least affected at the deepest point of the borehole and that this is the first place where the temperature field reaches equilibrium again. Nevertheless, the cooling of the formation by the drilling mud means that the calculated temperature gradients and heat flow densities should be regarded as probable minimum values.

A spectral gamma ray logging tool was also used to measure the natural gamma radiation of the formation as a dependence on its energy via a bismuth-germanium scintillator. The tool was run a total of three times, once in phase 1, phase 2, and phase 5, respectively, covering the borehole from the seafloor down to 982 mbsf. Thus, parts of the measurements were carried out within the conductor pipe and drill string, which have a damping effect on the signal. The software splits the total signal into discrete energy windows. This allows determination of the proportion of the total radiation generated by potassium, uranium, and thorium. Since data were available for both cased hole and open hole logging at several intervals, a correction for the damping effect was feasible. It was not possible to correct for the influence of the drilling mud because its precise clay composition was unknown. However, it can be assumed that the proportion of the total radiation associated with the drilling mud was constant down the entire length of the borehole at the time the logging was carried out. Differences between the respective logging runs were taken into account.

The borehole had an inclination of 2°–2.5°. The difference between the log depth and the true vertical depth of the borehole can therefore be ignored with respect to the interpretations made herein (e.g., calculations of temperature change per unit depth).

RESULTS

Temperature Logs, Gradient, and Reduced Temperature

To find the true bottom-hole temperature (BHT) of the formation, the measured BHT was corrected following the Horner method (e.g., He et al., 2008). Because only one measurement was available for the total depth (1138 mbsf), the measurements recorded during phase 2 drilling were used to correct for the depth of 1000 mbsf. This type of correction assumes a continuous linear source during the drilling process, causing heat to flow into the borehole at a rate √ (W/m). The relation between the measured temperature Tlog (°C) and true BHT, Ttrue (°C) is then given by: 
graphic
where λ = the thermal conductivity of the surrounding rock at the deepest point of the borehole (W/mK), tm is the time that has elapsed from the end of drilling mud circulation and the temperature measurement (hr), and td is the time from the end of the drilling process to the end of mud circulation (hr).

The measured temperature is plotted against ln[1 + (td/tm)] and a linear regression is then carried out (Fig. 5). The intersection of the regression line with the y axis gives the true formation temperature after an infinite waiting time. The difference to the true formation temperature at 1000 mbsf is 1 °C, a rather small difference. However, a comparison of the GyroSmart™ data and the logging data revealed that the formation temperature approached the equilibrium state at the time logging took place (Fig. 3). Therefore, it can be assumed that the difference between measured temperature and undisturbed formation temperature at the total depth (1138 mbsf) is in the same range as at 1000 mbsf. This results in a corrected BHT of 57.8 °C at 1138 mbsf. However, it has to be taken into account that the Horner plot is based on only two data points. This leads to a significant uncertainty of the regression.

In addition to each of the temperature logs, Figure 3 also shows the temperature gradients calculated from these logs. These are gradients derived from the uncorrected temperature values. The lowest temperature value measured at the seafloor is –1.2 °C (phase 2 logging), which is slightly warmer than temperatures reported by Morin et al. (2010) or Barrett et al. (2005), who obtained temperatures at the seafloor of –1.68 °C and –1.89 °C, respectively. From the temperatures at the seafloor and the bottom of the hole, we obtain an average temperature gradient of 51.9 K/km.

Figure 3 also shows the reduced temperature profile for the borehole, which is calculated by subtracting the average temperature gradient (taken from the uncorrected measurements) from the measured temperatures. Thus, the reduced temperature profile shows the depths at which there are significant differences between the predicted temperature gradient and the actual measured temperatures. We identified a total of 6 anomalies in the AND-2A well in the interval from 300 mbsf to 700 mbsf (Fig. 3). These anomalies have much lower temperatures than expected and appear in several successive temperature logs. These temperature minima are located at 341 mbsf, 471 mbsf, 531 mbsf, 591 mbsf, 626 mbsf, and 658 mbsf. The shallowest temperature minima occurs at the same depth (341.71 mbsf) as a pressurized brine interval in unconsolidated gravels and sands that flowed back into the borehole and 27 m up the barrel and drill pipe on 30 October 2007 (Falconer et al., 2008–2009; Frank et al., 2010). There are no indications of any caving of the borehole wall at the other temperature minima depths. Density measurements are available for all of the aforementioned depths. Caliper data are also available for 658 mbsf. The borehole televiewer log shows steep fractures at 531 mbsf, 591 mbsf, and 626 mbsf. These fractures dip to the south and southeast at inclinations of 60°–80°. It is possible that colder fluids flowed into the well via the fractures at these depths and therefore reduced the temperatures recorded in the well. No reasons could be found for the temperature anomaly at 658 mbsf. However, the zone from 626 mbsf to 648 mbsf could have been influenced by the additional drilling activity required to remove the bridge at ∼620 mbsf. The additional mud circulation across this zone could have led to greater cooling of the formation. This hypothesis is supported by the fact that the anomalies had almost disappeared when the last temperature log was run.

Thermal Conductivity and Heat Flow

The thermal conductivity of 25 core samples was analyzed to help determine heat flow at the drill site. The core samples were taken from the entire length of the core down to 932 mbsf (Fig. 6). The conductivity measurements were carried out on the samples after they were saturated with water. We used an optical conductivity scanner, which provides very precise non-contact measurements (Popov, 1997). The saturated samples were heated with a continuous, focused, and mobile heat source, and the conductivity of the heat in the sample was measured using two infrared sensors.

The samples yielded conductivities that ranged from 1.22 to 2.95 W/mK; the average value was 1.57 W/mK, and relative errors in each case were <15%. These results are similar to the conductivities determined by Morin et al. (2010) and Bücker et al. (2001) for Neogene–Paleogene samples recovered by the ANDRILL McMurdo Ice Shelf and Cape Roberts drilling projects. The thermal conductivities do not differentiate between the different types of rock penetrated by the well. The only value above 2 W/mK came from a sandstone sample. The greater conductivity in this case could be due to extensive cementation of the rock by dolomite, which has a conductivity λ of ∼5.5 W/mK (Schön, 2004).

The heat flow, which is the product of the thermal conductivity and the temperature gradient, has an average value of 81.5 mW/m2 for the drill site.

Radiogenic Heat Production

The decay of potassium, thorium, and uranium radionuclides is responsible for the generation of radiogenic heat. The amount of heat generated by the decay process can be determined from the spectral gamma ray log of the borehole. The concentrations of potassium, thorium, and uranium are calculated using the following expression (Rybach, 1986): 
graphic
where A is the heat production in μW/m3, ρ is the raw density of the rock in kg/m3, and cU, cTh, and cK are the concentrations of radioactive elements in the rock for thorium and uranium (ppm) and potassium (%), respectively.

Radiogenic heat production and gamma radiation are attributable to the same causes and therefore show comparable plots versus depth (Fig. 7). Radiogenic heat production ranges from 0.3 μW/m3 to 3.4 μW/m3 and averages 1.1 ± 0.2 μW/m3. An interval of increased heat production (1.1 μW/m3 and 1.8 μW/m3) occurs below 770 mbsf in a 130-m-thick clay and siltstone section. The radiogenic heat production integrated along the entire borehole is 1.1 mW/m2, which corresponds to 1.3% of the average heat flow (81.5 mW/m2) at the seafloor.

DISCUSSION AND CONCLUSION

The temperature measurements with the GyroSmart™ tool highlight that the temperature in the SMS borehole rapidly approached the temperature of the undisturbed state, and that this equilibrium condition was nearly reached during the last temperature log. The thermal gradient and heat flow values derived from our measurements are therefore reliable even though they are probably minimum values. Our temperature gradient (51.9 K/km) and heat flow (81.5 mW/m2) results for the AND-2A borehole are consistent with the range of temperature gradients (28.5–76.7 K/km) and heat flows (60–115 mW/m2) previously reported for other locations in the southern Victoria Land Basin and the Transantarctic Mountains (Fig. 2; Blackman et al., 1987; Bücker et al., 2001; Morin et al., 2010, and references therein). Even if these values are taken as minimum approximations, this result is still valid. The error in the bottom-hole temperature is estimated to be 10% of the measured value. That would lead to a maximum BHT of 62.6 °C, a maximum gradient of 56 K/km, and a heat flow of 88 mW/m2. The heat flow is also affected by changes in the thermal conductivity. Conductivity data are quite sparse. However, samples were taken from a wide variety of lithologies and all but one have conductivities of ∼1.51 ± 0.23 W/mK; therefore, it can be assumed that the aforementioned average thermal value of 1.57 W/mK reflects the distribution of the thermal conductivity appropriately.

The SMS borehole results therefore provide important confirmation that heat flows are above the average value of continental crust (57 mW/m2; Sclater et al., 1980) in places within the southern Victoria Land Basin. High heat flows such as these typify areas of recent continental extension (see review in Talarico and Kleinschmidt, 2008) and are consistent with Neogene–Pleistocene faulting (Henrys et al., 2007; Wilson et al., 2007; Fielding et al., 2008; Granot et al., 2010), anomalously thin crust (∼20 km; Bannister et al., 2000; Lawrence et al., 2006), Neogene to active volcanoes (i.e., the Erebus Volcanic Province; Kyle, 1990b), and elevated mantle temperatures (Watson et al., 2006) that occur associated with the Terror Rift in the southern Victoria Land Basin. The heat flow results reported herein also provide an important new constraint for future modeling studies that aspire to understand the thermal evolution of the West Antarctic Rift system.

This work was supported by the German Research Foundation DFG (WO 672). ANDRILL (Antarctic Geological Drilling Program) is a multinational collaboration between the Antarctic programs of Germany, Italy, New Zealand, and the United States. Scientific studies are jointly supported by the U.S. National Science Foundation; New Zealand Foundation for Research, Science and Technology; the Italian Antarctic Research Program; the DFG; and the Alfred Wegener Institute for Polar and Marine Research. We thank Alex Pyne (drilling science manager), the drilling team, the science laboratory personnel, and Marshall Pardey. We thank two anonymous reviewers for constructive comments that improved this manuscript. We also thank Rüdiger Schellschmidt and Matthias Halisch (Leibniz Institute for Applied Geophysics) for fruitful discussions and support of the thermal conductivity measurements, and Rich Jarrard for pointing out the significance of the GyroSmart™ tool data to Paulsen.