Abstract
The conventional synthetic seismogram is created with a sonic and a density log; however, the sonic log can be replaced with the resistivity, neutron, gamma-ray or spontaneous potential log to produce a pseudo-sonic (PS) log. More recent techniques involve combining an SP log and a GR log to produce a PS log. In the past, a drawback in using GR logs for the PS is the presence of highly radioactive and often organic-rich layers possessing abnormally high GR readings. To improve the pseudo-sonic log produced from the gamma-ray log, a technique was developed to statistically treat the outliers from the wells in the Hugoton Embayment that encountered predominantly shale, sandstone, limestone, and dolomite and whose logged sections included both normal and abnormally high GR readings. To demonstrate a wider-range application of our method, the procedure was applied to wells from the Hugoton Embayment, Central Kansas Uplift, Sedgwick Basin, Salina Basin, Forest City Basin and Nemaha Uplift. The correlation coefficients between the PS and the conventional sonic for the six basins were 0.75, 0.92, 0.86, 0.91, 0.77, and 0.70, respectively. Also, the match between the resulting conventional synthetic seismogram and the pseudo-synthetic seismogram from a blind test well for each area was quite good. Provided the outliers have been properly treated, the GR log is a viable tool for creating pseudo-sonic logs and pseudo-synthetic seismograms for exploration in oil and gas basins where there are few wells with sonic logs or where sonic log quality is poor.
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Acknowledgements
The authors express their gratitude to Pierre Keating for his professional editorial help and two anonymous reviewers for their suggestions that helped to improve the manuscript. We are deeply indebted to Kansas Geological Survey for free use of well data from their internet site, and we are also thankful to Mr. Carl Schaftenaar for use of purchased Geotools software (QuickSyn).
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Appendix
Appendix
This section includes cross-plots for the gamma ray versus sonic ITT, as well as cross-plots for resistivity versus Sonic ITT for a total of 72 wells from six basins/uplifts in Kansas (Figs. 15, 16, 17, 18, 19) demonstrate that the empirical correlation between the gamma-ray versus sonic ITT is overall as good (or better) in Kansas than that of the much more commonly used resistivity versus sonic ITT (Tables 4, 5).
In the plots that follow, a gamma ray maximum cut off value of 160 API according to the box-and-whiskers plot (Fig. 3 in text) was used for all structural elements and all wells. We combined the Sedgwick and Salina basin into one structural element and counted it as Sedgwick-Salina Basin (Table 4) because there were fewer wells available in the Salina Basin. The minimum depth of each log was approximately 300 m. We cross-plotted the gamma ray against the sonic (ITT) (Figs. 15a, 16a, 17a, 18a, 19a), deep induction resistivity (ILD) versus sonic (ITT) (Figs. 15b, 16b, 17b, 18b, 19b) and the shallower induction resistivity (RLL3) against sonic (ITT) (Figs. 17c, 18c). The gamma ray versus sonic (ITT) in all structural elements were plotted on a linear scale. By contrast, the deep induction resistivity (ILD) versus sonic (ITT) and the shallower induction resistivity (RLL3) versus sonic (ITT) were plotted on a semi-logarithmic scale. Then, we calculated the \(R^{2}\) values for all cross-plots. A comparison of the results is given in Table 5.
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Quadir, A., Lewis, C. & Rau, RJ. Generation of Pseudo-synthetic Seismograms from Gamma-Ray Well Logs of Highly Radioactive Formations. Pure Appl. Geophys. 176, 1579–1599 (2019). https://doi.org/10.1007/s00024-018-1979-6
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DOI: https://doi.org/10.1007/s00024-018-1979-6