Paleoenvironmental reconstruction of the Berriasian organic-rich interval of the Vaca Muerta Formation (Neuquén Basin, Argentina): insights for the characterization of unconventional hydrocarbon shale reservoirs

CAPELLI, I.; SCASSO, R.A.; SPANGENBERG, J.; KIETZMANN, D.A.; PRÁMPARO, M.; ADATTE, T.

MARINE AND PETROLEUM GEOLOGY

ELSEVIER SCI LTD

Lugar: Amsterdam; Año: 2024 vol. 163

0264-8172

The 50 m-thick Berriasian organic-rich interval (BORI) of the Vaca Muerta Formation (Neuquén Basin, Argentina) represents one of the most attractive stratigraphic intervals for unconventionalhydrocarbon exploration in the unit. Nevertheless, compared to other organic-rich intervals of theVaca Muerta Formation (VMFm), little is known about the paleoenvironmental conditions that prevailed during the accumulation of the sediments.To unravel the main causes that favored the accumulation and preservation of organic matter (OM)in this interval, a multiproxy analysis of the BORI in Puerta Curaco section was done by studying themicrofacies, palynology, calpionellid–calcisphere dinocysts, bulk and clay mineralogy, spectralgamma-ray, major and trace elements, Rock-Eval pyrolysis and stable carbon isotopes of whole rock carbonate (δ13Ccarb) and associated OM (δ13 Corg). The BORI presents a late Early to early Late Berriasian age and is constituted mainly by marl-limestone alternation with interbedded microbial bindstones, calcite concretions, and ash beds. Its 25.6 m-thick lower interval (LI), primarily formed by radiolaritic wackestones, presents higher total organic carbon (TOC) content (4.2 wt% on average) mainly in the form of amorphous organic matter (AOM), with variable contribution of opaque phytoclasts, whereas the geochemical analyses (Si, Ni, Cu, Mo, U, V) indicate high productivity of the water column and overall sea bottom anoxia. Oppositely, the 24.4 m-thick upper interval (UI), mostly constituted by intraclastic/peloidal packstones/grainstones, is characterized by a lower TOC content (1.5 wt% on average), OM composed of a higher content of opaque phytoclasts, with variable contribution of AOM and palynomorphs, whereas a decrease of the productivity in the water column and an increase of sea bottom oxygenation is recorded on the basis of the trace element analyses. The high OM content of the BORI was generated by the combination of high productivity of the water column and sea bottom anoxia. These paleoenvironmental conditions were reached in the Neuquén Basin during the late Early Berriasian by the combination of two processes: a sea level transgression and a worldwide paleoclimatic change towards more humid conditions. The sea level transgression favored the stratification of the sea water by decreasing bottom-water ventilation and so enhacing anoxia in the sea bottom, whereas the worldwide paleoclimatic change towards more humid conditions in the hinterlands may have enhanced continental runoff and nutrient supply into the ocean, triggering the productivity of the water column. The results of our study indicate that the low bulk clay mineral content (23 wt% on average), the high quartz content (29 wt% on average), mostly of biogenic origin, and the presence of former smectite may have contributed to adequate geomechanical and petrophysical properties, thus positioning the BORI as a highly-attractive stratigraphic interval for unconventional hydrocarbon production in the VMFm in neighbors-subsurface hydrocarbon blocks.