Abstract. Petrophysical properties of drill core and drill cuttings samples from both bore holes of the German Continental Deep Drilling Program (KTB) measured at atmospheric pressure and room temperature in the field laboratory are presented, along with data of core samples measured at simulated in situ conditions by other laboratories. Most of the petrophysical properties show a bimodal frequency distribution corresponding to the two main lithologies (gneiss and metabasite), except electrical resitivity and Th/U ratio which are lithology independent (monomodal distribution). Low resistivities are mainly associated with fractures zones enriched in fluids and graphite. The most abundant ferrimagnetic mineral is monoclinic pyrrhotite. Below 8600 m, hexagonal pyrrhotite with a Curie temperature of 260øC is the stable phase. Thus the Curie isotherm of the predominant pyrrhotite was reached (bottom hole temperature about 265øC). The highest values of magnetic susceptibility are linked with magnetite. Microcracks grow due to pressure and temperature release during core uplift. This process continues after recovery and is documented by the anelastic strain relaxation and acoustic emissions. The crystalline rocks exhibit marked reversible hydration swelling. Anisotropy of electrical resistivity, permeability, P and $ wave velocity is reduced significantly by applying confining pressure, due to closing of microcracks. Fluids within the microcracks also reduce the P wave velocity anisotropy and P wave attenuation. Anisotropy and shear wave splitting observed in the field seismic experiments is caused by the foliation of rocks, as confirmed by laboratory measurements under simulated in situ conditions. The petrophysical studies provide evidence that microfracturing has an important influence on many physical rock properties in situ.
Summary During the course of the German continental deep drilling project (KTB) two scientific drill holes were drilled, the KTB Vorbohrung down to 4 km and the KTB Hauptbohrung down to 9.1 km, both intersecting several cataclastic shear‐zones. As few drill cores were available in the KTB Hauptbohrung, most of the petrophysical and geochemical data are based on drill cuttings investigations. We present an analysis of drill cuttings data, addressing the question of what relationship between cataclastic shear‐zones and petrophysical and geochemical data can be revealed. For that purpose we developed a regression model with the amount of cataclastic rocks in drill cuttings as a dependent variable and the petrophysical and geochemical variables as regressors. We use depth related data from two sections of the KTB Hauptbohrung with cataclastic shear‐zones in gneiss (1738–2380 m) and in metabasite (4524–4908 m). The variables are selected by estimating and testing a linear regression model taking into account the autocorrelation of the data due to the depth structure. The variables which characterize the cataclastic shear‐zones in gneiss according to our model are the contents of carbon and crystal water and the thermal conductivity, each with positive coefficients. This model explains, in total, 57 per cent of the variance of the observed data. For cataclastic shear‐zones in metabasite the content of crystal water and the magnetic susceptibility with positive coefficients and the content of chromium with a negative coefficient are the significant variables. The explained variance in this model is 60 per cent. Being significant in both lithologies, the content of crystal water is an important variable for cataclastic shear‐zones. The prediction of shear zones is feasible by our methods, but the results of our study should be confirmed and widened by investigations of other data sets.
The heat production rate of continental crystalline rocks was determined from cores and drill cuttings recovered from depths up to 9.1 km in boreholes drilled by the Continental Deep Drilling Program of Germany (KTB). The drilled rocks are mainly gneisses and metabasites. Based on a large amount of data, the comparison of laboratory and in situ determinations by logging shows good agreement. The loss of radioactive elements in drill cutting samples at the shaker tables was minimized by using a broad range of available fragment sizes. The data reveal a clear lithological dependence of the heat production rate of the metamorphic rocks at KTB. The lithologically caused variations of the heat production rate have a wavelength in the order of the drilled depth and therefore, they cover up any decreasing trend if there is one. Below 6 km, tectonic elements such as fault zones play also an important role in the observed vertical distribution. Despite the availability of data for the upper third of the crust, models to predict temperatures for greater depths still contain uncertainties.
Magnetic susceptibility was measured at 2 m depth intervals on drill cuttings from the main drill hole of the German Deep Drilling Project KTB. Metamorphic rocks (metabasites and gneisses) were the rock types most frequently found down to a depth of 9101 m. Petrophysical (susceptibility, density), geochemical (element concentrations), lithological and petrological data (ore mineral concentrations, lithological components, alteration index) were used for a statistical analysis. The histograms of magnetic susceptibility show nearly log‐normal distributions with two distinct peaks depending on the lithology. The most frequent susceptibility values are 0.266 × 10−3 SI for gneissic rocks and 0.847 × 10−3 SI for metabasic rocks (mainly amphibolites). The higher level of metabasite susceptibility is caused by higher contents of paramagnetic silicates such as hornblende. A theoretical paramagnetic susceptibility was calculated from the iron and manganese contents derived from X‐ray fluorescence (XRF) measurements. The ferrimagnetic susceptibility was determined by subtracting the theoretical para‐magnetic susceptibility from the measured susceptibility. Cross‐plots of the ferrimagnetic susceptibility versus density are used to discriminate between samples with predominantly magnetite or pyrrhotite as the main ferrimagnetic mineral. Samples containing mostly pyrrhotite show susceptibilities not exceeding 6 × 10−3 SI, whereas the highest measured susceptibilities of 66.5 × 10−3 SI correspond to zones exclusively with magnetite. A factor analysis was applied to investigate the background factors representing the data variabilities. The factor analysis reduces 13 original variables from the complete depth section to five independent initial factors. These explain in total 66.2 per cent of the total data variance. The most significant factor, 1, correlates with metabasite content, density and paramagnetic susceptibility and it anticorrelates with gneiss content. The next significant factor, 2, correlates with ferrimagnetic susceptibility and magnetite content. Factor 3 correlates with the amount of cataclastic rocks, factor 4 with hornblende gneiss and factor 5 with pyrrhotite
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