ORBITAL FORCING OF ORGANIC MATTER QUALITY AND QUANTITY IN A SOURCE-ROCK FORMATION: THE CASE OF THE VACA MUERTA FORMATION, TITHONIAN-VALANGINIAN, NEUQUÉN BASIN, ARGENTINA
Résumé
The Vaca Muerta Formation (Jurassic-Cretaceous transition) from the Neuquén Basin is an important
petroleum source-rock formation in Argentina. Several cores from exploration wells were fully analysed. A
cyclostratigraphic study was also successfully conducted, based on a set of continuous data from several
neighbour cored wells. Among various variables successfully tested, the Total Organic Content (TOC), either
continuously measured with the Laser Induced Pyrolysis System (LIPS) method (1 cm spacing) or derived
from Rock-Eval 6 (1 m averaging window), has shown the control of orbital forcing on the variation of organic
matter distribution. Moreover, and of particular interest, the integration of the Rock Eval Hydrogen Index (HI)
and Oxygen Index (OI), providing useful information on the quality of the source rock, were also tested on
164 m long cores from the Lower Vaca Muerta in order to understand the climatic conditions which have
favoured the deposition and preservation of organic matter in the basin. In all four signals, cyclic changes were
attributed to astronomical forcing. The data shows that the main driver of the slight oxygenation change of the
basin is the obliquity. We attribute this link to changes in the sea level due to fluctuations in ice coverage in
the ice-house climate of the late Jurassic – early Cretaceous. Moreover, HI, OI and TOC correlate with the
precession, the long and the short eccentricity. We thus suggest the following depositional scenario. With
falling sea level, the connection between the Neuquén basin and the proto-Pacific gets shallower, resulting in
stratification of the basin with warm fresh water supplied from the south of the basin sealing off the deeper
basin. With rising sea level, the connection to the ocean gets wider and the stratifications weakens. Nutrients
are mainly supplied by continental run-off from the south of the basin due to monsoonal activity during winter,
which varies with the precessional cycle. Warm, hypersaline waters are in addition formed in the south-east of
the basin, during the parallel formation of the sabkha deposits of the Loma Montosa Formation during summer.
This would lead to the formation of a hypersaline lens in the deepest part of the basin and would thus cut it off
from oxygen supply.