Diagenesis of the Vaca Muerta Formation, Neuquén Basin: Evidence from petrography, microthermometry and geochemistry

Abstract

The Tithonian Vaca Muerta Formation (500 m-thick marine carbonates) in the Pampa de Tril area is a potential unconventional hydrocarbon reservoir but earlier studies rarely focused on its diagenetic history. It constitutes the upper part of the lower Mendoza Group in the Neuqu´en Basin, western Argentina. Petrographic examinations reveal four generations of calcite (C1, C2, Cb and C3). The microbial lime mudstone (C1, <4 μm), which exhibits dull cathodoluminescence (CL), is the most abundant with mean δ18O value of − 7.2 ± 1.3‰ (VPDB) and δ13C of − 1 ± 3.3‰ (VPDB) and deemed to have deposited in tropical warm shallow-water settings. The equant calcite cement (C2, 25 μm–140 μm) exhibits red CL and is associated with recrystallization of bioclasts and micritic matrix. The fibrous calcite (Cb, 35 μm–125 μm) fills parallel-to-bedding antitaxial veins (beefs) and shows bright orange CL. Its δ18O and δ13C values are more depleted (− 10.7 ± 2.2‰ VPDB and − 4.8 ± 3.9‰ VPDB, respectively) relative to those of C1, which is consistent with the occurrence of hydrocarbon fluid inclusions and may imply precipitation during a mid-to late stage of diagenesis within the oil window. The fracture-filling C3 consists of coarse subhedral crystals (40 μm–5 mm) showing bright orange CL, which has been interpreted as precipitated during deep burial conditions at temperatures around 118 ± 26.8 ◦C based on fluid-inclusion microthermometry. The relative depletion of Fe and Mn from C1 to C3 and the low concentration of U, may imply a relatively closed system of low water/rock interaction and restricted meteoric alteration. The shale normalized (REESN) patterns of C1, Cb and C3 generally exhibit parallel patterns with a distinct positive Eu anomaly and progressive enrichment towards the MREE and HREE. The positive Eu (Eu/Eu*)SN anomalies reflect the impact of dissolution of plagioclase feldspars by circulation of the diagenetic fluids through the surrounding siliciclastics in the basin. The Ce (Ce/Ce*)SN and La (La/La*)SN anomalies of Cb and C3 are consistent with precipitation under reducing conditions of deep burial settings. The Vaca Muerta carbonates are enriched in organic matter (up to 12%) that fills the pores but permeability is insignificant.