Clay Estimation From Gr And Neutron -Density Porosity Logs

Clay estimations from gamma-ray logs and neutron-density porosity logs are commonly used techniques. However, failure to recognize the difference between shale and clay leads to confusion when comparing weight percent clay estimated from well logs with weight percent clay estimated by x-ray diffraction analysis. The principal error in clay estimation stems from the assumption that shales are composed of 100 percent clay. Our study shows that shales are commonly composed of 50 to 70 percent clay, 25 to 45 percent silt- and clay-sized quartz, and 5 percent other minerals that include feldspars and carbonates. The non-clay minerals in the shale do not commonly affect the total gamma-ray count or the neutron-density log separation. The estimation of weight percent clay can, therefore, be corrected by multiplying the GM (gamma ray index) by a factor (C), which is weight percent clay of average shale adjacent to the zone of interest. Typically this factor ranges from 50 to 70. This approach will significantly improve the weight-percent clay estimations from the gamma-ray log, but will not correct for the inaccuracies associated with the distribution of heavy minerals. We developed empirical relationships that will account for errors associated with clay content of shale as well as distribution of heavy minerals. A similar approach is applicable to clay estimation from neutron and density porosity logs. Neutron porosity is unaffected by the common non-clay minerals such as quartz, feldspar, carbonates and pyrite, which are present in shales. Therefore, the estimation of volume percent clay can be corrected by multiplying the conventional clay-estimation equation by the factor C as discussed above. In the neutron-density crossplot, Schlumberger's (1989) "Clay Point" is redefined as a "Wet Shale Point". The area between the clean sand line and the Wet Shale Point is rescaled from 0 to 60 volume percent dry clay, and a new "Clay Point" is established to represent 100 percent wet clay. A neutron-density crossplot shows the locations of the Wet Shale Point, the new Wet Clay Point, the Dry Clay Point, and the Dry Shale Point.