In petrophysical evaluation one of the most common procedures of zoning into shale and sand layers is to use GR logs. In this study an alternative zoning approach was proposed, using the Vp/Vs log curves. First, Vp and Vs were calculated based on the sonic slowness log data (DT and DTS). The correlations between GR, Vp/Vs and Vsh (shale content) were investigated using Pearson's correlation coefficient. Zoning by GR and Vp/Vs was performed. Two data sets, one taken from a published reference and another of real data from the Cuu Long basin, were analyzed. The former data set was used to show the steps of zoning procedure using Vp/Vs and the latter data set was used to show some issues that may encounter in practical petrophysical evaluation. The Vp/Vs-based technique could be a helpful tool for zoning, especially where the GR values may fluctuate with depth due to many factors besides the shale content. Its application needs also further investigations.
Cuu Long basin is the most important petroleum basin in Vietnam until now. Hydrocarbon reservoirs in this basin are Middle Miocene to Oligocene sands and fractured granite basement (FGB). For the last 25 years most HC production came from the famous fractured granite basement reservoirs of the Cuu Long basin, however there is a clearly decreasing production of HC from the FGB, and in the same time more and more HC is produced from the overlying clastic reservoirs (Giao et al., 2012), for which lithological zoning is always the first important step in well log analysis.
Using natural GR logs for lithological zoning and calculation of shale volume (Vsh) has been the most predominant technique by now since the day one in log analysis as well as it continues to be so in future. But it is recognized in practice that there are many situations when the relationship between GR values and the shale content of the clastic formation depends also on other factors, one of which is high radioactivity. One example was mentioned by Giao and Anh (2015) by the shale points of high GR values of the very thick Upper Oligocene shale comparing to the overlying Miocene shale in the Cuu Long basin.
