Characterization of cataclastic shear-zones of the KTB deep drill hole by regression analysis of drill cuttings data

Abstract

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.

P-spline Varying Coefficient Models for Complex Data

“…These authors modelled the mean and dispersion structure of the amount of cataclastic rocks by focusing on a subset of drill samples ranging from 1000 to 5000 meters, which led to the identification of possible depth breakpoints and potential outliers. Further, Winter et al (2002) investigated the relationship between the amount of cataclastic rocks to several geological variables using standard regression methods for two specific cataclastic zones within two lithologies: gneiss (1738-2380m) and metabasite (4524-4908m).…”

“…The goodness-of-fit measures associated with P-spline VCM shows promise for applications to the KTB data. For example, the models of Winter et al (2002) that target specific zones, only using a depth range of several hundred meters, reported R 2 values between 0.57-0.60. Our VCM approach initially show a 12% -21% improvement, while using the entire 9.1 km range over all data zones.…”

“…For the KTB data, K = 17 specific knots locations (at depths in meters) are chosen based on prior knowledge of lithologies (Winter et al 2002(Winter et al ), with values: 0, 290, 552, 1183(Winter et al , 1573(Winter et al , 2384(Winter et al , 2718, and 9101 m. The complete B-spline basis (for q = 0, 1, 2, 3) using the above knots locations is provided in Figure 4. Using the B-spline bases displayed in …”

Geothermal Reservoir Simulation

“…Several analyses of the tectonic features at the KTB site have been undertaken, particularly (Hirschmann, 1996) and the KTB work group under Hirschmann provided much detailed fracture picking information. (Winter et al, 2002) analyzed drill cuttings from the KTB main hole from 1700 to 2400 m and from 4500 to 5000 m. Their results indicate that cataclastic shear zones can be found in the cuttings extending well over 100 m thickness. Table 5.3 presents a summary of information gained from the literature on the geological fault zones recognized at the KTB site, along with their orientation and dip.…”

“…The arrows in Figure 5.28 refer to the calculated crossing points of the geological fault zones (Table 5.3) represented as a single plane with the main borehole. From Figure 5.28 it can be seen that the identification of a single fracture corresponding to the geological fault zones of Table 5.3 is impossible, rather areas of increased fracture density, with a thickness of a few hundreds of meters can be defined, as per (Winter et al, 2002). This interpretation is naturally dependent on a number of factors including uncalibrated measurements of the effect of changing drilling techniques; however, a general pattern can be observed.…”