The development of a total organic carbon log will open the way for a new quantitative approach to wireline source rock evaluation. The derivation of the total organic carbon profile will provide the opportunity to make a preliminary survey of the well for organic-rich shales. This survey can provide a solid basis for sample selection, and it should reduce the number of cuttings or samples taken for preliminary laboratory analysis. It will also provide a rapid means of zoning and evaluating the well in terms of source rock richness which will in turn be valuable data for basin analysis. The method for deriving total organic carbon uses log measurements of carbon : oxygen ratio and density porosity. The model partitions the formation into a solid and a pore space on the basis of density porosity and then treats the solid as minerals and treats the pore space as water. The environmentally-corrected carbon : oxygen ratio is multiplied by an approximation of the formation oxygen to obtain formation carbon. In formations containing carbonate minerals the inorganic carbon content is estimated and subtracted from the total carbon to provide total organic carbon. This approach is quite different from the current practice of using some combination of gamma ray, sonic, resistivity, neutron, and density logs to either identify or quantify organic matter in potential source beds. Its advantages are:

  1. it is sensitive to very low amounts of organic carbon,

  2. it does not require calibration with core data on either a single well or regional basis, and

  3. it does not require extensive log interpretation to produce an answer.

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