Abstract
The Vaca Muerta Formation, one of the most prolific unconventional play in South America, exhibits concentrations of total organic carbon (TOC) of up to 16% with variations in TOC paralleling the general facies evolution of the prograding system. This study examines the applicability and accuracy of predicting TOC from two methods of quantifying organic-rich zones from wireline logs: the relationship of uranium from spectral gamma-ray to TOC, and the δlogR technique for calculating TOC.
A database from extensive outcrop geochemical sampling (n = ~1900) was correlated to the uranium signal from a handheld spectral gamma-ray logging tool. The U/TOC relationship derived from the outcrop samples shows high variability between outcrop sections measured across the formation. The U/TOC ratio increases from proximal to distal positions within the basin.
A second dataset, consisting of core samples, was also analyzed and compared to the TOC calculated using a combination of the sonic and resistivity data obtained from downhole logging. This second method of TOC prediction reliably and accurately reproduces laboratory measurements, and is the far superior choice when estimating organic richness from wireline log signatures. To further test this method of TOC prediction, a subset of sidewall plugs were analyzed for petrophysical and geochemical measurements and using the δlogR technique, was able to produce TOC values that compared to discrete laboratory measurements with a correlation coefficient of R2 = 0.83.
The time-tested method of determining organic richness from wireline logs can be successfully applied in the Neuquén Basin and could be attempted in other basins once all available data has been carefully considered.
•The scalar relationship between U/TOC is highly variable across the basin.•The δlogR method of Passey et al. (1990) for TOC estimation from wireline logs reliably match laboratory values.•Further testing yields a robust and direct correlation between geochemical analysis and estimated TOC values.
