A unified model to evaluate shaliness in compacted and soft formations using downhole GR log

- A new shaliness model suitable for formations with different compaction degree is proposed.
- Two case studies are provided to demonstrate the practical value of the new model.
- Theoretical derivation and numerical simulation show that GR index model is not a valid one, especially in soft formations.

:The conventional GR (gamma ray) index model is widely used to calculate shale volume in a drilling hole. It is just based on the raw downhole GR log. However, it is found that formation porosity and density have effect on the measurement of GR log. This effect is not obvious in compacted formations, but is nonnegligible in soft formations. In this work, a new model based on the concept of matrix GR which is immune to the effect of porosity and density is proposed. We apply the new model and GR index model to two distinct wells, one is ODP Site 1173, representing soft formations, and another is a hydrocarbon exploratory borehole, representing compacted formations. The results of the new model are in better consistent with shale volumes from X-ray diffraction analysis in Site 1173 and well-log inversion in the exploratory hole. At last, we give an explanation to a long-term problem in GR index model that GR index is generally larger than the actual shale volume.