AAPG Bulletin, Vol.103, No.4, 931-950, 2019
Paleothermometry and distribution of calcite beef in the Vaca Muerta Formation, Neuquen Basin, Argentina
The term "beef' describes bedding-parallel calcite veins found commonly in the organic-rich matrix of unconventional resource plays. Although some authors have interpreted beef to be an early diagenetic feature, these calcite veins are commonly attributed to precipitation at high temperatures and localized overpressure during hydrocarbon generation. The temperature at which the beef formed is thus crucial to ascertain the process of beef genesis. We use the novel methodology of clumped isotope analysis to constrain both the temperature at which beef forms and the isotopic composition of fluids present during formation. For this study, we use beef from basinal sections of the Vaca Muerta Formation in the Neuquen Basin, where veins are commonly up to approximately 10 cm (similar to 4 in.) thick and are laterally continuous over 1 km (0.6 mi). The calcite veins occur in isolation or in association with concretions and ash layers. Sequence stratigraphic boundaries have little influence on distribution, and only a low correlation between beef and total organic content or beef and ash layers exists. The internal crystal structure of beef varies largely, suggesting both syntaxial and antitaxial growth forms. The delta O-18 values of beef range from approximately -12 parts per thousand to - 9 parts per thousand, and the delta C-13 values vary between approximately -1 parts per thousand and 1 parts per thousand. The surrounding mudstone and concretion fracture fills (calcite) show little difference isotopically when compared to the beef itself The delta O-18 values of nearby concretions range from approximately - 3.5 parts per thousand to 1 parts per thousand, and the delta C-13 values vary between approximately 6 parts per thousand and 11 parts per thousand. Clumped isotope analysis of beef in the Vaca Muerta Formation indicates temperatures between approximately 140 degrees C and 195 degrees C, whereas the surrounding mudstones vary from approximately 120 degrees C to 150 degrees C. The corresponding formation fluid delta O-18(w) values range from 8.5 to 14.5 parts per thousand. These temperature data are higher than the maximum temperatures suggested by published studies modeling the basin's thermal and burial histories. If these models are correct, the clumped isotope data indicate that the growth of beef in the Vaca Muerta Formation required the input of hydrothermal fluids from greater depths. Alternatively, the geothermal gradient or burial depth was underestimated in these models.