--> When the Syn-Depositional Climatic Variations Influence My Source Rock Properties – The Case Study of the Vaca Muerta

AAPG ACE 2018

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When the Syn-Depositional Climatic Variations Influence My Source Rock Properties – The Case Study of the Vaca Muerta

Abstract

The Vaca Muerta Fm is a world class source rock developed in the Neuquén basin of Argentina at the Jurassic Cretaceous boundary. Several continuous high resolution acquisitions were performed on the Vaca Muerta Fm in order to thoroughly characterize this heterogeneous source rock.

Core spectral gamma-ray and density as well as LIPS (Laser Induced Pyrolysis System for TOC acquisitions at a resolution of one cm) were acquired over complete sections of cores. This set of data enabled a cyclostratigraphy study that revealed the control of orbital forcing on the TOC quantity and quality of the source rock. Imbricated sequences from 1.2 MY down to 20 KY (around a meter thick) were successfully defined on many wells where this set of data was acquired.

Besides these previous acquisitions, automated micro-XRF spectrometer and Automated Fourier Transform Infra-Red spectroscopy (FTIR) were tested on portions of cores for high resolution continuous mineralogical determinations. An impulse hammer probe was also used alongside the FTIR for elastic stiffness determination.

The results show that all rock properties vary along with the paleo-climatic variations, independently of the cycle duration. In general, the carbonate content is negatively correlated with the silica and clay contents. Next to Maximum Flooding Surfaces, the TOC is the highest whereas the calcite content is the lowest. The corresponding source rock facies is a mudstone with few radiolarians. During subsequent progradational periods (or HST), the TOC is gradually decreasing whereas the calcite is increasing until the Transgressive Surface is reached. The mudstones become wackestones incorporating gradually more radiolarians, bioclasts or also develop cementation (nodules). During Transgressive periods (TST), the inverse variations are observed with increasing TOC and decreasing carbonate content. As a consequence, the Brittleness Index (BRI) computed from these high resolution mineralogical acquisitions also regularly vary with the sequence stratigraphy. Vertical trends independently obtained with the Impulse Hammer exhibit strong similarities with this BRI. The integration of all this data set helps us to derive a detailed geological model of the Vaca Muerta along with all its heterogeneities, facies and associated properties.