CALCULATION OF ELASTIC PROPERTIES IN LOWER PART OF THE KOLA BOREHOLE FROM BULK CHEMICAL-COMPOSITIONS OF CORE SAMPLES

In-situ elastic properties in deep boreholes are controlled by several factors, mainly by lithology, petrofabric, fluid-filled cracks and po res. In order to separate the effects of different factors it is usefu l to extract lithology-controlled part from observed in-situ velocitie s. For that purpose we calculated mineralogical composition and isotro pic crack-free elastic properties in the lower part of the Kola boreho le from bulk chemical compositions of core samples. We use a new techn ique of petrophysical modeling based on thermodynamic approach. The re asonable accuracy of the modeling is confirmed by comparison with the observations of mineralogical composition and laboratory measurements of density and elastic wave velocities in upper crustal crystalline ro cks at high confining pressure. Calculations were carried out for 896 core samples from the depth segment of 6840-10535m. Using these result s we estimate density and crack-free isotropic elastic properties of 5 54 lithology-defined layers composing this depth segment. Average synt hetic P- wave velocity appears to be 2.7% higher than the velocity fro m Vertical Seismic Profiling (VSP), and 5% higher than sonic log veloc ity. Average synthetic S-wave velocity is 1.4% higher than that from V SP. These differences can be explained by superposition of effects of fabric-related anisotropy, cracks aligned parallel to the foliation pl ain, and randomly oriented cracks, with the effect of cracks being the predominant control. Low sonic log Velocities are likely caused by dr illing-induced cracking (hydrofractures) in the borehole walls. The ca lculated synthetic density and velocity cross-sections can be used for much more detailed interpretations, for which, however, new, more det ailed and reliable seismic data are required.