Calibrated petrophysical model for elevated organic matter intervals and mineralogical variability in the Agrio Formation, Neuquén Basin, Argentina

Abstract

Organic carbon-bearing mudstone units of variable lithologies exhibit complex internal heterogeneities in total organic carbon (TOC) enrichment and mineralogy related to stratigraphic and geographic variability. Elevated organic matter-bearing intervals within unconventional reservoirs are commonly contained within these thick (100's of meters), low TOC mudstone units, but poorly delineated. The Cretaceous Agrio Formation is a mixed siliciclastic-carbonate mudstone, known source rock unit, and an emerging unconventional play in the hydrocarbon-rich Neuquén Basin of Argentina with internal and geographic heterogeneity. Source rock intervals within the formation have been identified from a detailed outcrop study; however, the variability of hydrocarbon source rock quality and mineralogy of the unconventional reservoir target intervals in the subsurface is relatively unconstrained. A published petrophysical model for a bimodal clay-silica system was modified to predict intervals of elevated organic matter and ternary clay-silica-carbonate mineralogical compositional trends for distinguishing mudstone complexity in the Agrio Formation. This study uniquely distinguishes carbonate-rich from silica-rich mudstone log response across the paleodepositional profile of the system. The calibrated petrophysical model utilized cuttings programmed pyrolysis and mineralogical XRD data applied to wireline well logs from the southern Mendoza and central Neuquén provinces. Predicted logs were used to correlate elevated organic matter distribution, mineralogical composition, and assess stratigraphic and geographic heterogeneity. Thirteen log-derived elevated organic matter-bearing intervals (wireline TOC >1.25 wt%) were identified using the modified petrophysical model, expanding on the previously interpreted five outcrop source intervals in the Agrio Formation. Organic richness thickness and abundance increases in the northern Neuquén to southern Mendoza province within the study area. The new petrophysical workflow can be applied to improve unconventional target decisions for hydrocarbon exploration efficiency in mudstones worldwide.