The acquisition of openhole logging data is not always guaranteed because of difficult drilling environments. In such cases, formation evaluation, and thus the completion program, becomes a real challenge. The situation becomes more complex when dealing with unconventional reservoirs with very tight carbonates and organic carbon-rich formations. This paper presents a method to measure the total organic carbon (TOC), which, in this paper, represents the organic carbon in the matrix (kerogen and coal), and to estimate oil saturation in such a challenging environment.

A suite of wireline tools (gamma ray (GR), Spectralog, density, neutron, nuclear spectroscopy) was run through 7.625-in. casing to evaluate the formation and to quantify TOC and the oil in the pores. The nuclear spectroscopy tool, which was the master tool, measures the total carbon in the formation. Part of this carbon is attributed to the inorganic matrix (carbonates). Another part is attributed to the organic matter in the matrix (kerogen). The remaining carbon, or excess carbon, is mainly the carbon inside the pores. The process consists of integrating the conventional logging data, spectroscopy data, core data, and some geological constraints to estimate corrected porosity, mineralogy, and TOC in the kerogen-rich intervals. The excess carbon, which is not attributed to the matrix and TOC, is used to estimate oil saturation. Finally, core data are used to validate the analysis results.

The presented methodology has been applied to a casedhole well with no openhole data previously acquired due to drilling issues. The primary target of the well, in the deep section, produced water; then, the operator decided to revisit the second target and complete it for testing. It has to be pointed out that over the well-cemented intervals, the results showed a very good matching of the corrected total porosity and the core total porosity. Relying on TOC and saturation analysis results, the operator selected the most promising intervals to be tested. Testing results have shown excellent matching between production results and oil saturation analysis results.

TOC and oil saturation quantification using nuclear spectroscopy technology and core data results showed its success in both tight carbonates and organic carbon-rich reservoirs. This method will be a solution to evaluate and complete any wells with no openhole data acquired, and also to evaluate and complete unconventional formations where the conventional methods have shown their limitations.

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