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A Compaction Index for Mudstone in the Vaca Muerta Formation, Neuquén Basin, Argentina

Abstract

Compaction values (n=193) were calculated using marine concretions as an "uncompacted" reference thickness in mudstone of the Vaca Muerta Formation. Concretions form early in the diagenetic history and coupled with the thickness of adjacent equivalent compacted beds provide a natural strain gauge. The compaction measured at several field sites, in both proximal and distal settings, had values ranging mostly from 80%-95% and a modal class of 85%-90% compaction. The mean value for all data was 84.7% with a standard deviation of 6.8%. Basinal mudstones showed slightly higher average compaction than mudstones from more proximal sites. The median value of the proximal sites was 82.5%, whereas the median value of the basinal site was 88.2%. A comparison of compaction between calcareous mudstone (50-70% CaCO3) and more argillaceous mudstone (40-50% CaCO3) at the basinal site (Puerta Curaco) produced a similar range (85%-90%) and had median values of 88.2% and 87.5%, respectively. Compaction in this dataset had relatively little co-variance with concretion[G1] attributes (width, thickness) or host rock attributes (TOC, percent carbonate). A relatively strong (R=0.67) co-variance occurs between isolated concretion thickness and width. Paleoporosity of the mudstones (mean 46.2%, median 46.7%) was calculated from the compaction strain values using a 70% primary porosity (mean of petrographic estimates of cement) for all concretions. These porosities are consistent with those measured from early dewatering and compaction and likely equate to the timing of concretion formation. Uncompacted mud accumulation averaged less than 0.1 mm/yr based on ammonite zonation ages. This average rate indicates low sedimentation during the accumulation of the organic-rich Vaca Muerta Formation.