AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

Unravelling Paradigms in Vaca Muerta Formation, Neuquén Basin, Argentina: The Construction of Geochemical Wellbore Images by Geostatistical Integration of Geochemistry and Conventional Log Data With Wellbore Resistivity Images


In this article the problem of paradigm change in the study of unconventional reservoirs is analyzed. A paradigm change involves not only a deconstruction process of category analysis, but also an intellectual and collective effort of scientific community of the whole local oil industry. The ways to do that are rethinking old techniques, incorporating new ones, combining like elements differently, and adding contributions from other disciplines. We describe a methodology based on the geostatistical integration of organic geochemical (TOC content) and inorganic geochemical data (major and trace element concentration), wireline log (density log), used as hard data and resistivity wellbore image used as soft data. The integration method used was the Sequential Gaussian Simulation with Markov model Type II with hard and soft data mentioned above. Well data belongs to drilled well at Cerro Vagón block, southeast of Neuquén basin, Argentina, where Vaca Muerta Fm. has a thickness of 110 m. Chemical analysis was performed on 45 cutting samples for both organic (destructive pyrolysis analysis) and inorganic (nondestructive X Ray fluorescence analysis with rapid handheld equipment). Moreover, 18 m of core was analyzed for TOC by 12 chip samples irregularly spaced and 180 readings with nondestructive X ray fluorescence analysis, separated 10 cm from each other. As a result of the stochastic simulation process several wellbore images were obtained to a 0.2 inch of vertical resolution involving properties such as density, calcium carbonate and molybdenum concentration together with TOC content. Vaca Muerta Fm. has a pattern of molybdenum and vanadium metals that correlate with TOC and pyrite content and hence, the anoxic level of the sedimentary environment. Pyrite is very precisely evaluated by means of high correlation showed by iron and sulfur. Conventional methods for TOC thickness evaluation (Passey model, among others) are based on high transit time, low density and high resistivity properties of organic matter. In this case the pyrite concentration decreases the resistivity values and underestimates the real TOC thickness values. High resistivity values are related to calcium carbonate concentration which overestimates the TOC thickness as observed in wellbore images. As a conclusion, an accurate TOC thickness evaluation must be made with geochemical and wireline logs geostatistically integrated.