Authigenic chlorite and chlorite-smectite mixed layers as an indicator of increasing reducing conditions in the Huincul Formation: Neuquén Basin, Argentina

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

Edimburgo

Encuentro; EUROCLAY; 2015

European Clay Groups Association, The Clay Minerals Society, International Natural Zeolite Association, Geological Society

The Huincul High (HH) and Dorso de los Chihuidos High (DCH) are two regional morpho-structures of theNeuquén Basin, characterized by the occurrence of hydrocarbon fields, regional structures at depth and thepresence of solid hydrocarbons impregnating the foreland deposits of Cretaceous rocks (Neuquén Group). TheHuincul Formation belongs to the fluvial and red-bed lake deposits of the Neuquén Group.Some of the structures of HH and DCH were reactivated during the Tertiary Andean orogeny, and acted as verticalchannels connecting the reservoir fluids with the sub-horizontal paleo-channel of the Huincul Formation. Thecirculation of these fluids in the clastic rocks is documented by the presence of bitumen associated with baroquecalcite, pyrite and clay minerals rims (smectite, chlorite and chlorite-smectite mixed layers). The pervasively greenishgray clay coating is a common feature of the sandstones and conglomerate from the Huincul Formation. Thispresentation establishes the relationship between diagenetic processes, hydrocarbons migration and their role incontrolling the composition of the clay coating.During early diagenesis multiple episodes of clay infiltration into the unconfined meandering river deposits of thelower section of Huincul Formation resulted in the formation of thick (2-3 μm) clay coatings in some volcanic richlitharenite sandstones and conglomerates. Progressive burial caused the reddening of the whole Huincul Formation,whose mudstone and sandstone layers display a reddish brown or pink color due the presence of a hematitepigment. The hematite is derived from the infiltration of meteoric water following breakdown of ferromagnesiansilicate minerals and iron bearing clay minerals.Oil emplacement is demonstrated by the intense bitumen impregnation of sandstone and conglomerate and atmicro scale via calcite and barite cements that host numerous organic rich fluid inclusions.The greenish gray facies document hydrocarbon migrations in the Huincul Formation where previous kaoliniteand hematite coatings were transformed to dioctahedral smectite, chlorite-smectite mixed layers or chlorite andpyrite. These changes from smectite to chlorite-smectite mixed layers or chlorite is interpreted as indicatingincreasingly reducing conditions. The direct transformation of smectite to chlorite in some of the greenish gray facieslocated in the lower part of the Huincul Formation, involved a fast rate of growth nucleation at the expense ofprevious smectite, suggesting a high fluid/rock ratio (Robinson et al., 2002). This condition prevailed in thepermeable layers of the lower section of the sedimentary unit that yields a focused flow pathway. In the rest of thesedimentary unit, the presence of chlorite-smectite mixed-layers suggest chloritization of previous smectite. Thepartial transformation of dioctahedral smectite to chlorite-smectite mixed layers or trioctahedral chlorite producedan excess of silica resulting in the micro quartz precipitation (Mosser-Ruck et al., 2010). The addition of Mg isrequired to form chlorite-smectite mixed layers or chlorite from the dioctahedral smectite and this could be takenfrom the alteration of residual mafic minerals and/or Mg-rich formation water that migrated with the hydrocarbons.