Geochemical and petrophysical source rock characterization of the Vaca Muerta Formation, Argentina: Implications for unconventional petroleum resource estimations

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The Vaca Muerta Formation is currently regarded as the most prolific source rock interval for unconventional petroleum exploration in Argentina. In this study, we integrate recent CT-scanner, nuclear magnetic resonance (NMR) T1-T2 and Rock-Eval Shale Play® analytical techniques, combined with classical geochemical, petrological and petrophysical methods to characterize Vaca Muerta source rocks along a vertical profile. The proposed analytical workflow was tested here on 4 samples derived from one core: the LJE-1010 borehole drilled in the lowermost Jurassic Vaca Muerta member (Neuquén Basin – Argentina), to evaluate the unconventional hydrocarbon potential and pore network attributes. First, entire rock samples were analyzed by dual energy CT scanner to evaluate possible heterogeneities; this also helped in the selection of representative samples. Then the organic matter properties were investigated by open-system pyrolysis techniques, organic petrography and biomarker analyses as a function of the core depth. A comparative approach between open-system pyrolysis methods (e.g. Rock-Eval Basic/Bulk-Rock & Shale Play® methods vs. Source Rock Analyzer) is also illustrated in this work. Finally, petrophysical characteristics and pore network attributes in shale were estimated by gas permeability (steady-state method corrected for Klinkenberg effect), porosity and NMR methods. The investigated samples have oil-window maturity and are characterized by a network of solid bitumen. Based on new Rock-Eval Shale Play® data, the potentially producible oil present in the rock samples was estimated using the modified oil saturation index (OSI=Sh0+Sh1 peaks×100/TOC). Results indicate that the oil crossover effect and potential productive oils occur within intervals showing higher TOC values (~3 to 8wt%). We also illustrated here that, for unconventional shale play perspectives, Rock-Eval Shale Play® parameters (Sh0 & Sh1) allow to obtain both a better quantification of free and retained hydrocarbons in source-reservoir rock samples and correct original oil in place (OOIP) estimations in early exploration campaign for shale oil and shale gas assessment. For Vaca Muerta rock samples, OOIP estimations range from 60 to 160bbl oil/acre-ft, approximately. We also demonstrated that the solid bitumen, oil and organic matter are clearly distinguished in the studied samples using advanced NMR T1-T2 maps obtained at different temperatures. The solid bitumen was clearly evidenced as a peak with T1/T2 ratio~14 on NMR T1-T2 maps. Concerning the matrix bulk rock permeability, the obtained results showed that the most deep sample is characterized by a permeability of about 140nanoDarcy (nD) whereas the shallower sample containing also higher amounts of organic matter is more permeable (~213nD), indicating that TOC values play a main control on Vaca Muerta poromechanical characteristics. The maximum total porosity values range between 6.5 and 21.8% and were calculated in this work integrating NMR and helium porosity values. Finally, global interpretations of these obtained results suggest that the Vaca Muerta Formation could be considered as a prolific shale play interval for unconventional petroleum exploration in the near future.