Abstract
An appropriate velocity model from well logs is a key issue in the processing and interpretation of seismic data. In a deep borehole located in the central part of the Polish Outer Carpathians, the sonic measurements were inadequate for seismic purposes due to the poor quality of data and gaps in the logging. Multiple regression (MR) and a modified Faust equation were proposed to model the velocity log. MR estimated the P-wave slowness as a dependent variable on the basis of sets of various logs as independent variables. The solutions were verified by the interval velocity from Check Shots (CS) and by the convergence of synthetic seismograms and the real seismic traces. MR proved to be an effective method when a set of other logs was available. The modified Faust method allowed computation of P-wave velocity based on the shallow resistivity logs, depth, and compaction factor. Faust coefficients were determined according to the lithology and stratigraphy divisions and were calibrated with the use of the velocity previously determined in the MR analysis. The modified Faust equation may be applied in nearby old wells with limited logging data, particularly with no sonic logs, where MR could not be successfully applied.
Highlights
Modern seismic processing and interpretation is focused on improving the evaluation of hydrocarbon prospection on the basis of detecting lithology and fluid anomalies
Slowness computed from the Check Shot (CS) interval velocity; DT_reg—result of Multiple regression (MR) in the depth sections of the same lithological solution, based on DTc; DTP_reg—result of MR without the Istebna beds, based on DTP; DTP_reg_dCALI_subtr—result of MR excluding dCALI, based on DTP; DTP_reg_full—result of MR in full depth sections, based on DTP; DTS, DTSx, DTSy—S-wave slowness measured by the full waveform sonic log with monopole source, the XX and YY cross-dipole source, respectively; DTS_reg—result of MR in full depth sections, based on DTS
The presence of organic matter in rocks is a serious limitation to the application of the Faust method since it significantly increases the values of the modeled P-wave velocity
Summary
Modern seismic processing and interpretation is focused on improving the evaluation of hydrocarbon prospection on the basis of detecting lithology and fluid anomalies. In the deep D-1 borehole, located in the central part of the Polish Outer Carpathians area of complex geological structure, a few sonic logs were available They were as follows: raw and corrected P-wave slowness from standard sonic logging—DT and DTc, respectively; synthetic P-wave slowness based on the litho-porosity–saturation petrophysical interpretation—sDT; P- and S-wave slowness from the full waveform sonic logging—DTP and DTS, respectively. In the shallow part of the D-1 borehole, the DTc log presented an overly low Pwave velocity and overly high contrasts at the boundaries between the reservoir sandstones horizons and interbedded shales These sections correlated with significant caliper enlargements, so the problem with the DTc curve was related to the poor borehole conditions and poor quality of sonic logging. It reveals that the proven statistical techniques made the archival data still useful
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