Origin of bleaching zones in the redbeds mudstones of Portezuelo Formation, Neuquén Basin, Argentina.

PONS, MARIA JOSEFINA; FRANCHINI MARTA; DANIEL BEAUFORT; PATRICE PATRIER; AGNES IMPICCINI,; RAINOLDI ANA

Santa Rosa, La Pampa

Congreso; VII Congreso Latinoamericano de Sedimentología y XV Reunión Argentina de Sedimentología; 2016

Asociación Argentina de Sedimentología

The Portezuelo Formation (upper Turonian-lower Coniacian) belongs to the Cretaceous fluvial red-bed deposits of 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 caused by hydrocarbon and copper-rich fluid circulation (Pons et al., 2015; Rainoldi et al., 2014, 2015). At the contact with the most permeable bleached sandstones, mudstones of the Portezuelo Formation are also discolored, turning from reddish 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 hydrocarbon reservoir. 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 Lata and Aguada Baguales oil and gas fields.Portezuelo Formation consists in a fining-upward succession of medium- to coarse-grained sandstones and conglomerates 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 oxides and 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 the red 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 subcircular shapes, mimicking the coalescence of two or more drops. Whole rock analyses (EDS) show a strong decrease in total FeO from the red to the white mudstones, indicating leaching of iron oxides. XRD analyses permitted to identify clays in both mudstones, which consist in smectite and traces of chlorite, the latter recorded only in the white mudstones. Chemical analyses of smectite coating indicate a decrease in Fe, Ca, Na, K, and Si and increase 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 upward strata are entirely bleached due to hydrocarbon circulation, the white spots present in the studied mudstones result from dissolution of iron oxide cements by the circulation of hydrocarbon and associated reducing fluids. The organic acids formed by hydrocarbon redox within the redbeds might have leached the most mobile elements from interlayer sites of clays. Upward migration of hydrocarbons and associated fluids into the shallow rocks could increase the temperature 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 model for hydrocarbon migration in non-conventional reservoir rocks.