Analysis of array-sonic logs from the Medicine River field, Alberta
Susan L. M. Miller
Array-sonic logs from four wells in the Medicine River field of Alberta are analyzed for relationships between Vp (compressional-wave velocity), Vs (shear-wave velocity), Vp/Vs, lithology, shaliness and porosity. Vp/Vs in conjunction with Vp effectively discriminates between sandstone, limestone and shale lithologies in the sampled intervals. Vp appears to increase linearly with Vs in sandstones, limestones, and mixtures of clastics and carbonates. The average Vp/Vs value of 1.61 for sandstone agrees with the literature values of 1.6 - 1.7 obtained from core analysis. The average Vp/Vs value for the limestone is 1.86 which fails between the literature values of 1.8 for dolomite and 1.9 for limestone. This is attributed to the slightly dolomitic nature of the limestone formation from which the data are taken. Mixed lithologies have Vp/Vs ratios which fall between values for their component lithologies. Vp/Vs has been found to vary in the Basal Quartz formation between two adjacent wells. A drop in Vp/Vs may be due to an increase in porosity, a decrease in shaliness, a change in pore fluid or some combination thereof.
Both Vp and Vs decrease as porosity increases in the sandstones and in the Nordegg, Shunda, and Detrital formations, which are carbonate/clastic mixtures. With the exception of the Nordegg, Vp is more sensitive to porosity than Vs which results in an overall decrease in Vp/Vs with increasing porosity. This is the opposite trend to that which is reported by several other investigators. The Nordegg does not exhibit any Vp/Vs trend with changing porosity. In the limestone there are no trends for Vp, Vs or Vp/Vs with respect to porosity.