Generation of Pseudo-synthetic Seismograms from Gamma-Ray Well Logs of Highly Radioactive Formations

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.