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SILICA CONTENT AND GAMMA RAY LOGS IN VOLCANIC ROCKS

Valgardur Stefansson, Steinar Thor Gudlaugsson, and Asgrimur GudmundssonOrkustofnun, Grensasvegur 9, 108 Reykjavik, Iceland

Key words: gamma ray, volcanic rocks, silicaconcentration,

Abstract

Chemical analyses of 254 rock samples fromIceland are used to establish a relationship betweenthe natural gamma ray intensity (expressed as APIgamma ray units, or API GU) and the silicaconcentration of Icelandic rocks (% SiO2). For eachsample, the gamma response is calculated based onconcentrations of K, Th, and U, using linearequations established by Belknap (1959). The SiO2concentration in this sample population is shown tobe linearly related to the K, Th, and U contents.Combining these equations, we obtain the followinglinear relation: API GU = (2.63 0.10)*SiO2- (102 6). This result agrees with a previous calibrationwhere actual gamma ray logs were compared withchemical analyses of cores and cuttings (Stefanssonet al. 1982). The relation obtained makes it possibleto express the natural gamma ray logs as percentageof the SiO2 in the Icelandic rocks. Furthermore, asthe matrix thermal conductivity of volcanic rocksseems to correlate with their silica content, it mightbe possible to obtain continuous profiles of thethermal conductivity from the gamma ray logstogether with a suitable porosity log.

Introduction

Different concentrations of the radiogenic elements,uranium, thorium, and potassium, in variousgeological formations make it possible to usenatural gamma ray logs (GRL) to map different

geological units in wells. The GRL has been in usefor some 60 years and is considered one of thesimplest logs presently in use. As most of the wellsin the world have been drilled into sedimentaryrocks, the interpretation methods applied to GRLare based on the radiogenic properties ofsedimentary rocks. The classical approach is to usethe GRL to distinguish between pure sandstone andshale. Interpretation of GRLs in igneous rocks hasreceived less interest.

Natural gamma ray logs have been usedsystematically in Iceland for the last 20 years. Earlyin this period, it was demonstrated that a relation

exists between the intensity of the GRL and thechemical composition of Icelandic crystalline

volcanic rocks (Stefansson et al., 1982). Anapproach was proposed, where the GRL could beexpressed as the concentration of the SiO2 of therocks. These results were mainly based on acomparison of a detailed GRL and extensiveanalyses of the continuous core obtained in theIRDP research borehole in Reydarfjrdur in EasternIceland (Flower et al., 1982) together with a similarstudy of one well in Krafla, Iceland.

At present, the National Energy Authority ofIceland (Orkustofnun) is carrying out an extensiveresearch programme on the properties of Icelandicreservoir rocks, including petrophysical(Stefansson., 1997), petrochemical, and petrological(Franzson et al., 1998) studies. Orkustofnunparticipates in the Nordic heat conduction project(Middleton et al., 1998). Detailed chemicalanalyses of some 250 rock samples from Icelandhave been carried out as a part of this researchprogramme. These analyses provide an opportunityto recalibrate, in an independent way, the intensityof the GRL as a function of the chemicalcomposition of the rocks. The present paperdescribes the new determination of the relationbetween the GRL (in API GR units) and theconcentration of SiO2(in %). Results obtained withthe new calibration method are in agreement withprevious results (Stefansson et al., 1982).

Classification of Icelandic rocks

Icelandic rocks are divided into petrographicalprovinces according to their chemistry and they aresystematically correlated with the active volcaniczones. The provinces are closely related and it maybe difficult to distinguish between basaltic types inhand specimens from different localities. The rockclassification is therefore primarily based onchemistry and secondly on mineralogy. Thenomenclature of igneous rocks has been based ondifferent methods of classification. The most widelyaccepted method for classification is based on theconcentration of the main constituents, that is morebasic ( richer in Fe, Mg, Al, Ca ) to more acidic(richer in Na, K, SiO2). By using the alkali andsilica contents, three main basaltic differentiationtrends are obtained, that is tholeiite-, transitional-and mild-alkalic basalt trends. The more evolved therock is, the richer it is in silica and alkalicomponents. Sodium and potassium increase with

silica and their amount in the rock constitutes the

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rock type index. The terminology most widely usedin Iceland and adopted here is shown in Fig. 1.

Geochemical variations in Icelandic rocks

Fig. 2shows the correlation between K2O and SiO2of the samples used for the calibration in this paper.The best fit line through the data points is alsoshown in the figure. There is considerable scatter inthe data, but there is a clear positive correlationbetween the parameters.

The positive correlation between Th and SiO2 ispresented in Fig. 3. The graph for thorium issimilar to that of potassium. It is clear from thefigure that the samples are mainly of two types, onecluster of data points for basalt with a silica content

of about 50% and one cluster for rhyolite with asilica content of about 75%. The scatter isconsiderable, especially for the rhyolite.

Fig. 4 shows the correlation between uranium andSiO2for the samples used for the calibration. Thereis larger scatter in this graph than in Figures 2and3. This is most likely due to higher mobility ofuranium than for potassium and thorium.

In spite of the fact that the data points in Fig. 2-4show considerable scatter, it is clear that there is apositive correlation between the elements K, Th,and U on the one hand and the concentration of

SiO2on the other hand. Therefore, the geochemicalvariations in the Icelandic rocks suggests that thereshould be a positive correlation between the naturalgamma ray intensity emitted from the rocks and thesilica content.

Calibration of GRL in SiO2units

The specific intensity of gamma rays from extendedrock formations can be considered a linear functionof the amounts of uranium, thorium, and potassiumpresent in the rocks (Belknap et al., 1959):

A = AU* CU+ ATh* CTh+ AK* CK (1)

where

A is the specific activity of the rockAi is the specific activity of the elements U, Th, andK respectively andCi is the concentration (mass fraction) of the sameelements.

For all practical purposes in well logging, it can beassumed that the source material is uniformlydistributed throughout the rock and that the

radiogenic elements are in radioactive equilibrium.

The specific activities of the elements are (Belknapet al.,1959):

AU = 2.8 * 104photons / sec gram U

ATh = 1.0 * 104photons / sec gram Th

AK = 3.4 photons / sec gram K

If the concentration of U, Th, and K is known for a(rock) material, the gamma ray intensity can becalculated from equation (1).

Natural gamma ray logs are usually calibrated inAPI gamma ray units (API = American PetroleumInstitute). This intensity unit is traceable to a testpit in Houston in Texas, where a specially madeblock of concrete with the following composition:

24 ppm Th13 ppm U

4 % K

has been defined as environment emitting 200 APIGU (Belknap et al., 1959).

Equation (1) and the concentration of the radiogenicelements in the test pit gives that

200 API GU = 72 * 10 -2 photons / sec gram

This means that the gamma ray intensity in API GUcan be calculated from the chemical analyses of therocks. We have used chemical analyses of 254 rocksamples from Iceland to calculate the gamma ray

intensity in API GU. The result is given in Fig. 5where the gamma ray intensity is shown as functionof the silica content (% of SiO2).

A linear fit through the data points gives thefollowing equation:

API GU = (-102 6 ) + ( 2.63 0.10 ) * SiO2(%)

The correlation coefficient for the line is 0.86.

The previous calibration (Stefansson et al., 1982)was based on direct comparison of gamma ray logsin two wells with the determination of the SiO2 in

the core in one case, and cuttings from the well inthe other case. The best fit equations obtained forthe wells were:

API GU = (- 108 18 ) + ( 2.7 0.3 )*SiO2for well KJ-16 (cuttings)

and

API GU = ( - 144 11 ) + ( 3.6 0.2 ) *SiO2for the IRDP well (core)

There is a good agreement between the twocalibration methods. The calibration line obtained

from the chemical analyses is almost identical with

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the line obtained for well KJ-16. As two differentand independent calibration methods are givingsimilar results, it is assumed that the relationbetween the API gamma ray intensity and the silicacontent of Icelandic rocks is reliable.

Determination of thermal conductivity from GRL

and porosity logs

The influence of porosity on thermal conductivity ofvolcanic rocks is poorly known. Stefansson (1997)showed that for a limited number of basalticsamples from Iceland and published data on rocksamples from California and Mexico, thegeometrical average seemed to give the bestdescription of the effect of porosity on thermalconductivity. There is, however, large scatter in the

available data and a special Nordic project ispresently being carried out in order to determineunambiguously the effect of porosity on the thermalconductivity of volcanic rocks (Middleton et al.,1998).

It was also suggested by Stefansson (1997) that thematrix thermal conductivity of volcanic rocks mightbe described by one parameter only, the silicacontent of the rock. A possible relation betweenthese parameters is given by Stefansson (1997) andis reproduced here as Fig. 6. If this hypothesis turnsout to be valid, it is clear that a unique relation canbe established between the GRL and the matrix

thermal conductivity of volcanic rocks. Togetherwith a suitable porosity log (neutron-neutron, sonicor gamma-gamma) it would be possible todetermine a continuous profile of the thermalconductivity in a well drilled into a volcanic rockpile. If such relation can be established, it wouldopen up new dimensions in heat flow studies involcanic regions.

Conclusions

A new calibration method, which relates the

intensity of the gamma ray logs (GRL) with thechemical composition of Icelandic rocks, isdescribed in the paper. The method is based on thedefinition of the API gamma ray unit together withchemical analyses of 254 rock samples fromIceland. The results obtained are in excellentagreement with a previous calibration where actualGRL in two wells in Iceland were compared withdeterminations of SiO2of core or cuttings from theactual wells.

If the matrix thermal conductivity of volcanic rockscan be described by the silica content of the rocks, itmay be possible to obtain continuous profiles of thethermal conductivity in wells from GRL togetherwith a suitable porosity log.

Acknowledgments

We thank Gudmundur mar Fridleifsson forproviding the chemical analyses of rhyolite samplesfrom Torfajkull for this work. Also we thankMitch Stark and an anonymous reviewer forvaluable comments on the manuscript.

References

Belknap, W. B., Dewan, J. D., Kirkpatric, C. V.,Mott, W. E., Pearson, A. J., and Rabson, W. R.,1959: API calibration facility for nuclear logs,API. Reprinted as paper E in SPWLA reprintvolume Gamma ray, neutron and density logging,SPWLA, 1978.

Flower, M. F. J., Pritchard, R. G., Brem, G., Cann,J. R., Delaney, J., Emmerman, R., Gibson, I. L.,Oakley, P. J., Robinson, P. T., and Schmincke, H.-U., 1982: Chemical stratigraphy, Iceland ResearchDrilling Project Reydarfjrdur, Eastern Iceland. J.Geophys. Res. Vol. 87, pp. 6489-6510.

Franzson, H., Fridleifsson, G. ., Gudmundsson,., and Vilmundardttir, E., 1998: Reservoirparameters. Report on the status of petrologicalinvestigations in the end of 1997. Report ofOrkustofnun1998. (In Icelandic)

Middleton, M., P. Starzec, K. Malaga, N. Springer,V. Stefansson, and W. Fjeldskaar, 1998:Petrophysical Properties of Selected IcelandicBasalts. Paper presented at the 4th NordicSymposium on Petrophysics, 29-30 January 1998.

Stefansson, V., 1997: The relation between thermalconductivity and porosity of rocks. NordicPetroleum Technology III, Ed. M. Middleton, pp.201-220.

Stefansson, V., A. Gudmundsson, and R.Emmerman, 1982: Gamma ray logging in Icelandicrocks. The Log Analyst, November-December1982, pp. 11-16.

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Fig. 6 Possible relation between thermalconductivity and concentration of silica in volcanicrocks

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