ORIGIN OF BLEACHING ZONES IN THE REDBEDS MUDSTONES OF PORTEZUELO FORMATION, NEUQUÉN BASIN, ARGENTINA

PONS, JOSEFINA ; FRANCHINI, MARTA; BEAUFORT DANIEL; PATRIER PATRICIA; IMPICCINI, AGNES; RAINOLDI, ANA LAURA

Santa Rosa

Congreso; VII CONGRESO LATINOAMERICANO DE SEDIMENTOLOGÍA y XV REUNIÓN ARGENTINA DE SEDIMENTOLOGÍA; 2016

Portezuelo Formation (upper Turonian-lower Coniacian) belongs to the Cretaceous fluvial red-bed depositsof the Neuquén Group. This group was extensively studied in the stratigraphic point of view by its fossil content(Garrido, 2010 and reference there in) and by the widespread alteration of sandstone and conglomerate layers causedby hydrocarbon and copper-rich fluid circulation (Pons et al., 2015; Rainoldi et al., 2014, 2015). At the contact withthe most permeable bleached sandstones, mudstones of the Portezuelo Formation are also discolored, turning fromreddish brown to gray or white (Pons et al., 2015). Such bleaching has been studied in detail for the most permeablesandstones (Rainoldi et al., 2014, 2015; Pons et al., 2015), which had been acted as carrier beds for hydrocarbonreservoir. In this contribution, we present a detailed study on the nature of the discoloration of mudstone at thecontact with the bleached sandstones in Barreales and La Cuprosa areas, located just above the Giant Loma La Lataand Aguada Baguales oil and gas fields.Portezuelo Formation consists in a fining-upward succession of medium- to coarse-grained sandstones andconglomerates interbedded with mudstones that have been exposed to shallow burial conditions (~ 700 m depth).Sandstones are composed mainly by K-feldspar and quartz grains, with subordinate mica, tourmaline, Ti-Fe oxidesand zircon. Conglomerates are monolithic and rich in mudchips. Mudstones are mostly formed by the same detritusgrains of sandstones and are pervasively cemented by iron oxides and hydroxide and clay minerals, which give thered color to this facies. Close to the contact (15 to 20 cm) with the pervasively bleached sandstone layers, themudstones show a zone of white to gray colored lenses and spots (2-10 % of total volume). Some spots show subcircularshapes, mimicking the coalescence of two or more drops. Whole rock analyses (EDS) show a strongdecrease in total FeO from the red to the white mudstones, indicating leaching of iron oxides. XRD analysespermitted to identify clays in both mudstones, which consist in smectite and traces of chlorite, the latter recordedonly in the white mudstones. Chemical analyses of smectite coating indicate a decrease in Fe, Ca, Na, K, and Si andincrease in Mg from the red to the white mudstones.Taken into account that iron is more mobile under reducing conditions (Brown, 2005) and that the upwardstrata are entirely bleached due to hydrocarbon circulation, the white spots present in the studied mudstones resultfrom dissolution of iron oxide cements by the circulation of hydrocarbon and associated reducing fluids. The organicacids formed by hydrocarbon redox within the redbeds might have leached the most mobile elements from interlayersites of clays. Upward migration of hydrocarbons and associated fluids into the shallow rocks could increase thetemperature and create the reducing conditions require for the subsequent transformation of smectite to chlorite.The morphology and distribution of reduced spots in the studied redbed mudstones could be used as a natural modelfor hydrocarbon migration in non-conventional reservoir rocks.