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

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 mu m), which exhibits dull cathodoluminescence (CL), is the most abundant with mean delta O-18 value of -7.2 +/- 1.3 parts per thousand (VPDB) and delta C-13 of -1 +/- 3.3 parts per thousand (VPDB) and deemed to have deposited in tropical warm shallow-water settings. The equant calcite cement (C2, 25 mu m-140 mu m) exhibits red CL and is associated with recrystallization of bioclasts and micritic matrix. The fibrous calcite (Cb, 35 mu m-125 mu m) fills parallel-to-bedding antitaxial veins (beefs) and shows bright orange CL. Its delta O-18 and delta C-13 values are more depleted (-10.7 +/- 2.2 parts per thousand VPDB and -4.8 +/- 3.9 parts per thousand 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 mu m-5 mm) showing bright orange CL, which has been interpreted as precipitated during deep burial conditions at temperatures around 118 +/- 26.8 degrees 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.