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TOC Distribution and Prediction in the Vaca Muerta Formation, Neuquén Basin, Argentina


Detailed geochemical analyses of outcrop and the subsurface provide insight to the distribution of organically rich source rocks in the Vaca Muerta Formation in the Neuquén Basin. The repeated occurrence of TOC-rich intervals at certain sequence stratigraphic levels determined in outcrop combined with a calibration of δLogR method of TOC prediction provides a robust tool for evaluating the source potential within the central Neuquén Basin. At Puerta Curaco, a nearly 800 m thick section is utilized as a reference outcrop section whereas 415 m of core and corresponding well logs from a nearby block provide the subsurface data set. In the Puerta Curaco section TOC varies from <0.1% to a maximum value of 9.7%. Four thin intervals of >7% TOC exist, but there are 300 m of strata (from 20 - 320 meters) where the average TOC value is above 2.9%. TOC in the subsurface ranges from <0.3% to 11.5%, with an average value of 5.45%. While the entire cored interval is organically rich, there are several high TOC intervals that coincide stratigraphically with those observed in Puerta Curaco. Both TOC and δ13Corg can be used to correlate the outcrop to the subsurface. Similar fluctuations to those seen in outcrop corroborate the relationship of TOC to stratigraphic position. In outcrop the highest average TOC (2.52%) are located within transgressive hemi-cycles of transgressive portions of the sequence. In the subsurface TOC also increases towards the maximum flooding surfaces and decreases in the more regressive portions. The addition of subsurface data to our outcrop dataset definitively confirms our previous finding that high TOC is not tied to a unique stratigraphic level but is a recurring pattern within the prograding depositional system. In the outcrop section a high-resolution sequence stratigraphy is possible that allows for a high-resolution geochemical analyses and the assessment of the concomitant fine-scale variability TOC, which subsequently can be recognized in the subsurface data. This outcrop calibration provides invaluable insight to the evolution and distribution of the high TOC intervals in the Neuquén basin that would not be immediately apparent with well data alone. Additionally, the measured subsurface TOC confirms the validity of the δLogR method for determining organic richness from wireline log information (Passey et al., 1990).