Generation of Pseudo-Synthetic Seismograms from Spontaneous Potential and Gamma-ray Well Logs

Abstract

The “conventional” synthetic seismogram (CSS) that is used in oil and gas interpretation is generated from sonic and density logs Alternatively gamma-ray and spontaneous potential logs can be combined with a density log to create a pseudo-synthetic seismogram (PSS) by using different methods as demonstrated from the logs of wells in Kansas USA Herein we introduce several new methods for constructing pseudo-synthetic seismograms by editing (Method-I) by considering major shifts in the velocity log curve (Method-II) by deleting the outliers both from gamma-ray and sonic (ITT) logs by statistical formulas (Method-III) the borehole corrections of gamma-ray logs (Method-IV) (due to extremely high gamma-ray readings) by combining the gamma-ray (GR) and self-potential (SP) logs to produce the spontaneous ray (SR) log (Method-V) and by deleting the outliers by using the statistical formulas resembling Method-III but applied only to the outliers of the gamma-ray log (Method-VI) The values were cross-plotted against the sonic ITT values to determine a linear transform for producing a pseudo-sonic (PS) log and ultimately a pseudo-synthetic seismogram After generating the pseudo-sonic logs there were several ways to compare the pseudo-sonic with the sonic ITT The first four methods (Method-I to Method-IV) were compared visually and the second two were compared statistically between the conventional and pseudo-sonic logs The Nash-Sutcliffe efficiency (NSE) formula was used in Method-V and the Pearson product-moment correlation coefficient (PCC) was used in Method-V and Method-VI to compare between the pseudo-sonic and the sonic (ITT) (Quadir et al 2018a; Quadir et al 2018b) The range for the Nash-Sutcliffe efficiency (NSE) acceptable value for the pseudo-sonic logs of three wells was from 78% to 83% (Method-V) This technique (Method-V) was tested on three wells one of which was used as a blind test well with satisfactory results The Pearson product-moment correlation coefficient (PCC) value between the composite PS (SR) log with low density correction and the conventional sonic (CS) log was 86% (Method-V) To demonstrate a wider-range application of our method (Method-VI) the procedure was applied to wells from the Hugoton Embayment (HE) Central Kansas Uplift (CKU) Sedgwick Basin (SGB) Salina Basin (SB) Forest City Basin (FCB) and Nemaha Uplift (NU) The correlation coefficient between the PS and the conventional sonic (ITT) was 0 75 0 92 0 86 0 91 0 77 and 0 70 respectively Also the match between the resulting conventional synthetic seismogram (CSS) and the pseudo-synthetic seismogram (PSS) from a blind test well for each area was quite good Because of the common occurrence of spontaneous potential and gamma-ray logs in many of the hydrocarbon basins of the world this inexpensive and straightforward technique (Method-V) could hold significant promise in areas that are in need of alternate ways to create pseudo-synthetic seismograms for seismic reflection interpretation Provided the outliers have been properly treated (Method-VI) 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 The economical and straightforward technique presented here for creating the pseudo-synthetic seismograms from the gamma-ray and SP logs can be useful for exploration in oil and gas basins where there are few wells with sonic logs and where high gamma-ray log may cause anomalous pseudo-velocities

Date of Award	2018 Oct 11
Original language	English
Supervisor	Ruey-Juin Rau (Supervisor)