Many clay correction theories and models have been proposed to improve the quantification of hydrocarbons in shaly sands. They are normally based on laboratory measurements. Such measurements are an excellent benchmark but limited in number due to the difficulty and time required to make them. Large volumes of data for justifying different interpretation models are available in the form of wireline logs. Using this data is generally not done because of the qualitative nature of key log information, such as shale index. Shale indices are difficult to compare quantitatively to laboratory measurements such as Qv (clay counterion charge per unit pore volume) or CEC (Cation Exchange Capacity).
of the geochemical log interpretation model has brought the potential of continuous in-situ information on clay types and volumes from which one can infer continuous values for CEC and Qv. It provides information on rock minerals and elements, primarily from density and spectral measurements. Such extensive data furnishes the vehicle necessary to judge the different published shaly sand interpret at ion models. Clay mineral (not shale) corrections give good water saturation answers using theoretically derived parameters in accordance with the Waxman-Smits or Dual Water models, provided the water salinities are above 30 KPPM. Below 30 KPPM neither model fully explains observed field results. Wells drilled with fresh muds have invaded zones that are low in salinity. Since most logs respond to this invaded zone it is import ant to understand the low salinity response region. Using theoretical mineral end points in the interpretation of a fresh water heavy oil shaly sand formation has brought a better understanding on the magnitude of clay correction required and how to use CEC information. It points to the possibility of different electrical and chemical CEC for smectite. A long term goal has been to use theoretical values in log interpretation, eliminating the need for arbitrary crossplot techniques and guesswork in the calibration of clay indicators. The field example demonstrates that theoretical values can be used directly for minerals, though not for clay conductivities. Theoretical clay conductivities can be used as a guide but must be reduced in very freshwater, hydrocarbon bearing sands to provide a realistic water saturation.
