The striking arrangement of colorful facies in the sandstones outcrops of the Los Chihuidos high, Argentina, has been attributed to the surface expression of hydrocarbon-induced alteration. The Huincul Formation (late Cenomanian-early Turonian) is part of the thick continental red-bed-dominated Neuque ´n Group, which consists of meandering fluvial deposits developed in a foreland basin. Visual distinctions reveal four diagenetic facies: red, white, gray, and minor brown sandstones. Differences in the diagenetic mineralogy among sandstones facies are interpreted to depend on the degree of interaction with hydrocarbon-bearing solutions and the relative position of the redox roll front developed during this process. The red facies represents the original oxidized sandstone, with hematite, kaolinite, quartz and albite overgrowths, and calcite cement precipitated under oxidizing conditions during the regional burial diagenesis. The white facies formed during the interaction of red beds with reduced fluids (in presence of hydrocarbons or organic acids), where clasts and cement were partially dissolved, and iron was reduced to Fe 2+ due to redox reactions and removed, resulting in the sandstone decoloration and in a marked development of the secondary porosity. Minor montmorillonite replaced kaolinite and detrital feldspars, and a poikilotopic calcite precipitated, filling the pores. The gray and the brown sandstone bound the redox front, which is characterized by the presence of abundant clay minerals and oxide minerals enriched in vanadium and copper. Montmorillonite and secondary hematite are dominant in the more oxidized gray sandstones whereas interstratified chlorite-montmorillonite is more abundant in the more reduced brown sandstones. The coexistence of these minerals indicates metastable reducing-oxidizing conditions at the redox front. Based on diagenetic characteristics, tectonic framework, maturation timing of source rocks, and petroleumemplacement history, hydrocarbons could have migrated into the Huincul Formation during the Tertiary uplift, but later erosion could have released the hydrocarbons and exhumed the reservoir.This multi-scale investigation of diagenetic features related to burial history and to structure-controlled upflow of hydrocarbons in a succession of siliciclastic rocks enhances modeling of a regional-scale redox system worldwide. Our study offers new insight into the application of authigenic minerals to trace hydrocarbon pathways along red beds and to evaluate the reservoir quality, in the understanding the importance of fluid compositions, mixing, and fluid-rock interaction along a major redox system.

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 medium-to fine-grained sandstones and mudstones, and (2) multistage calcite and pyrite in the coarse-grained sandstones and conglomerates. The δ 34 S CDT (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 δ 13 C PDB (Peedee belemnite) (−8.11‰ to −8.52‰) and δ 18 O PDB (−9.76 to −9.83‰) isotope composition of calcites may have resulted from mixtures of 13 C-rich CO 2 after the dissolution of local calcrete with 13 C-poor CO 2 from oxidizing hydrocarbons.This contribution provides for the first time key information to predict the distribution of authigenic minerals in reservoir rocks

Bleaching of red beds by the migration of reducing pore fluids is a widespread phenomenon, with most previous work on the topic focused on detailed studies in the southwestern United States, and to a lesser extent Germany. Herein, we report on widespread bleaching of Cretaceous red beds in the Neuquén Basin and the relationship of this bleaching to hydrocarbon migration. In the Cerro Granito area of the Huincul High, Neuquén Basin, the basal deposits of the Neuquén Group (Candeleros and Huincul Formations) are bleached. This alteration was apparently controlled by fluid and host-rock composition as well as by the intensity and span of the fluid-rock interaction. Red, fine-grained sandstones and mudstones of the Candeleros Formation were deposited in fluvial and swamp environments and contain authigenic hematite, Na-rich corrensite, micro-and mesoquartz, calcite, and analcite. Coarse-grained sandstones and mudstones of the Huincul Formation, which overlie the Candeleros Formation, were deposited in a braided fluvial system, under more humid and acidic conditions than the Candeleros Formation. Sandstones are similar in composition and texture to the Candeleros sandstones, except they contain less basic volcanic detritus and lack corrensite. Bleached facies in both formations contain bitumen and clusters of calcite + bitumen concretions, and partially dissolved detrital clasts and most cements, including early Fe-oxides, which resulted in an increase in secondary porosity and bleaching of the red beds. Alteration of the Candeleros Formation is confined to medium-to coarse-grained sandstone in which corrensite is partially dissolved and its interlayer charge is satisfied by Ca instead of Na, as in corrensite from the red sandstones. Extensive dissolution occurred in the Huincul Formation, which favored the precipitation of new authigenic minerals such as smectite, mixed-layer chloritesmectite, and pyrite. Bleaching and associated alteration most likely resulted from interaction of hydrocarbons with the red beds. Hydrocarbon paleomigration in the Cerro Granito area occurred in higherpermeability portions of the Candeleros and Huincul Formations. The seal provided by the Lisandro Formation was breached during uplift driven by Tertiary tectonism, resulting in the exhumation of the Huincul and Candeleros Formations. This study highlights the importance of a variety of alterations, including complex clay mineral paragenesis, contemporaneous with bleaching of the red beds. Such alterations can potentially be used to provide evidence for the passage of chemically reducing fluids associated with hydrocarbons in other regions, including cases where the original red color has been completely removed. Chachil Plutonic Complex+ + 39° 09′ 39° 09′ S S 1m Figure 4. (A) Geological map of the area showing the location of altered and unaltered levels in the Candeleros and Huincul Formations. (B) Orientation of fractures, faults, and elongated carbonate concretions within the Cerro Granito area. (C) Panoramic view showing the unconform...

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