Quantitative Lithology: An Application For Open And Cased Hole Spectroscopy

A new quantitative lithology interpretation is based on elemental concentrations available from logs. The concentration logs are obtained from single, induced-neutron gamma ray spectrometers, thus differentiating this work from earlier geochemical interpretations which required additional sondes for the measurement of aluminum and potassium. The new interpretation offers considerable advantages over conventional logs. This new interpretation is based on a recently acquired data base, Fourier transform infrared mineralogy and chemical compositions were measured on over 400 samples to examine the relationships between lithology and a number of geochemical signatures, including gamma ray, the gamma ray components Th, U, and K and other loggable elements. The results show that in individual wells, gamma ray correlates roughly with total clay content, but a closer analysis exposes the inherent weaknesses in clay estimation. Breaking gamma ray down into its individual components brings little improvement to the clay estimation compared to that possible from a different suite of geochemical logs. The study reveals a strong linear relationship between aluminum and total clay concentrations. It is characterized by a near-zero intercept and a common slope. Unfortunately, the measurement of aluminum by logging devices has proven to be difficult and expensive. This study introduces a technique whereby the elements silicon, calcium and iron can be used to produce as accurate an estimation of clay as from aluminum. A general algorithm for predicting clay from these three elements is presented. An example of how the algorithm can be optimized is also provided. Carbonate concentrations are determined from the calcium concentration log with an accuracy that is not available from any other logging devices. Finally, the remainder of the formation is composed of quartz, feldspar, and mica minerals. Examples of the new lithology interpretation are provided for both open and cased hole environments.