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

PONS M. J.; FRANCHINI M. ; BEAUFORT D.; PATRIER P. ; IMPICCINI A.; RAINOLDI A. L.

Sant Rosa

Congreso; 7°Congreso Latinoamericano de Sedimentología y 15° Reunión Argentina de Sedimentología; 2016

Asociación Argentina de Sedimentología

Portezuelo Formation (upper Turonian-lower Coniacian)belongs to the Cretaceous fluvial red-bed deposits of the Neuquén Group. Thisgroup was extensively studied in the stratigraphic point of view by its fossilcontent (Garrido, 2010 and reference there in) and by the widespread alterationof sandstone and conglomerate layers caused by hydrocarbon and copper-richfluid circulation (Pons et al., 2015; Rainoldi et al., 2014, 2015). At thecontact with the most permeable bleached sandstones, mudstones of thePortezuelo Formation are also discolored, turning from reddish brown to gray orwhite (Pons et al., 2015). Such bleaching has been studied in detail forthe most permeable sandstones (Rainoldi etal., 2014, 2015; Pons et al., 2015), which had been acted as carrier beds forhydrocarbon reservoir. In this contribution, we present a detailed study on thenature of the discoloration of mudstone at the contact with the bleachedsandstones in Barreales and La Cuprosa areas, located just above the Giant Loma La Lata and Aguada Baguales oiland gas fields. Portezuelo Formation consists in a fining-upwardsuccession of medium- to coarse-grained sandstones and conglomerates interbeddedwith mudstones that have been exposed to shallow burial conditions (~ 700 m depth). Sandstones arecomposed mainly by K-feldspar and quartz grains, with subordinate mica,tourmaline, Ti-Fe oxides and zircon. Conglomerates are monolithic and rich inmudchips. Mudstones are mostly formed by the same detritus grains of sandstonesand 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 pervasivelybleached sandstone layers, the mudstones show a zone of white to gray colored lensesand spots (2-10 % of total volume). Some spots show sub-circular shapes,mimicking the coalescence of two or more drops. Whole rock analyses (EDS) showa strong decrease in total FeO from the red to the white mudstones, indicating leachingof iron oxides. XRD analyses permitted to identify clays in both mudstones,which consist in smectite and traces of chlorite, the latter recorded only in thewhite 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 underreducing conditions (Brown, 2005) and that the upward strata are entirelybleached due to hydrocarbon circulation, the white spots present in the studiedmudstones result from dissolution of iron oxide cements by the circulation ofhydrocarbon and associated reducing fluids. The organic acids formed by hydrocarbonredox within the redbeds might have leached the most mobile elements frominterlayer sites of clays. Upward migration of hydrocarbons and associatedfluids into the shallow rocks could increase the temperature and create the reducingconditions require for the subsequent transformation of smectite to chlorite. Themorphology and distribution of reduced spots in the studied redbed mudstonescould be used as a natural model for hydrocarbon migration in non-conventionalreservoir rocks.