Influence of mobile shale on thrust faults: Insights from discrete element simulations

The Portezuelo Formation is part of a red bed sequence in the Neuquén Basin assigned to the Neuquén Group (lower Cenomanian–middle Campanian). In outcrops of the Portezuelo Formation in the Barda González region, iron oxide and hydroxide cements are only preserved in the less permeable claystone layers. Paleomigration of hydrocarbons and formation waters along the most permeable layers bleached the rocks due to partial to total dissolution of cements and grains and formation of several authigenic minerals: (1) montmorillonite and pyrite in the mediumto fine-grained sandstones and mudstones, and (2) multistage calcite and pyrite in the coarse-grained sandstones and conglomerates. The δSCDT (Canyon Diablo troilite) of −24.4 to −60‰ values in pyrite point to microbial reduction of a sulfate precursor. The oxidation of hydrocarbons and fluctuations in pH and carbon dioxide pressure caused by organic acids resulted in multiple stages of dissolution and precipitation of calcite. Calcite crystals host hydrocarbon-bearing fluid inclusions (± pyrite) with variable fluorescence color, indicating that hydrocarbon composition changed with time. The intermediate δCPDB (Peedee belemnite) (−8.11‰ to −8.52‰) and δOPDB(−9.76 to −9.83‰) isotope composition of calcites may have resulted from mixtures of 13C-rich CO2 after the dissolution of local calcrete with 13C-poor CO2 from oxidizing hydrocarbons. This contribution provides for the first time key information to predict the distribution of authigenic minerals in reservoir rocks Copyright ©2015. The American Association of Petroleum Geologists. All rights reserved. Manuscript received September 12, 2013; provisional acceptance January 10, 2014; revised manuscript received February 10, 2014; revised manuscript provisional acceptance May 5, 2014; 2nd revised manuscript received May 13, 2014; final acceptance August 13, 2014. DOI: 10.1306/08131413170 AUTHORS M. J. Pons ∼ Consejo Nacional de Investigaciones Científicas y Técnicas; Centro Patagónico de Estudios Metalogenéticos, Universidad Nacional del Comahue, Buenos Aires 1400, CP8300, Neuquén, Universidad Nacional de Río Negro, Argentina; josefina. pons074@gmail.com María Josefina Pons obtained a bachelor’s degree in geology and Ph.D. in natural sciences, specialized in economic geology at Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata. Her current positions are mineralogy professor at Universidad Nacional del Comahue and Río Negro and assistant researcher of CONICET. Her main research lines are ore deposits related to Miocene Andean magmatism and sediment hosted Cu-V-U deposits associated to oil migration. A. L. Rainoldi ∼ Consejo Nacional de Investigaciones Científicas y Técnicas; Centro Patagónico de Estudios Metalogenéticos, Universidad Nacional del Comahue, Buenos Aires 1400, CP8300, Neuquén, Universidad Nacional de Río Negro, Argentina; Departamento de Geología, Universidad Nacional del Sur, Bahía Blanca, Argentina; analaurarl@hotmail.com Ana Laura Rainoldi belongs to the Centro Patagónico de Estudios Metalogenéticos (CPEM), is a Ph.D. candidate with a scholarship of CONICET, and a teaching assistant in sedimentology at the Universidad Nacional del Sur (Bahía Blanca, Argentina). Her Ph.D. project consists of the study of the cretaceous sedimenthosted stratiform Cu (V-U) deposits and its relationship with hydrocarbons in the Neuquén Basin, Argentina. M. Franchini ∼ Consejo Nacional de Investigaciones Científicas y Técnicas; Centro Patagónico de Estudios Metalogenéticos, Universidad Nacional del Comahue, Buenos Aires 1400, CP8300, Neuquén, Universidad Nacional de Río Negro, Argentina; mfranchini@ speedy.com. ar Marta Franchini received her Ph.D. from Córdoba University and her postdoctoral training at Washington State University. She has worked as a researcher from CONICET leading metallogenic projects in several regions of Argentina. Her most important contributions include the development of new models for metal exploration in Argentina, the use of phyllosilicates in the exploration of porphyry Cu, providing useful information for processing of metals, the discovery of AAPG Bulletin, v. 99, no. 3 (March 2015), pp. 525–554 525 in the basin and improves understanding of the diagenetic history of the Portezuelo Formation.