Abstract

The aim of this research is to determine the percentage of clay minerals by X-Ray Diffraction (XRD) technique and compare with the range of CGR log data. The random orientation of the samples is more suitable for analysis of clay minerals using the X-Ray diffraction technique. Evaluation of clay minerals in shaley zones has long been a difficult task. Presence of shale and shaley zones in some of the reservoir formations can introduce uncertainties in determination and evaluation of petro physical parameters. Percentage of clay minerals, shale types and shale volume has to be considered exactly in this case. Existence of shale reduces porosity and permeability of the reservoir to some extent. Shale distributed in sand and carbonates in many different types. Each of these shale types has different effects on porosity, permeability and water saturation of a reservoir formation. Dispersed shale reduces porosity and permeability to a great extent, but laminar and structural shale have less effect on petro physical parameters. In this investigation shale types, shale volume and effective porosity of shaley reservoir formations have been determined from well log data based on cross plotting of the CGR log. In other words, a triangle Density-Neutron porosity cross plot is used to determine and correct the above parameters. The area of study lies in central oil fields of Iran, where well log data and core samples are used (Tabnak Field, Well-C). Well-C selected to study the Kangan Formation (Upper Khuff) at different depths interval. A comparison between the clay minerals determined by XRD and by cross plotting of log data is satisfactory. This study illustrates that distribution of shale types in Well-C is mainly dispersed shale with few of laminar shale and the percentage of effective porosity (φe) decreases with increasing depth along well-C in Kangan Formation. The great match between the shale type determined by the cross plotting of neutron vs. density porosities after correction for gas effects and by the result of Scanning Electron Microscopy (SEM) for the same core sample is interesting. Finally, we found that the cross-plotting techniques used in this investigation can determine shale types, shale volume, clay minerals and effective porosity with acceptable accuracy.

Keywords: Percentage of Clay Minerals; XRD Evaluation; Cross Plotting of Log Data; Comparison