Autigenic chlorite and chlorite-smectite mixed layer as indicator of increasing reducing condition in the Huincul Formation: Neuquén Basin, Argentina

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

Edimburgo

Conferencia; Euroclay 2015 Conference; 2015

European Clay Groups Association and The Clay Minerals Society

The Huincul High (HH) and Dorso de los Chihuidos High(DCH) are two regional morpho-structures of the Neuquén Basin, characterized by the occurrence of hydrocarbon fields, regional structures at depth and the presence of solid hydrocarbons impregnating the foreland deposits of Cretaceous rocks (Neuquén Group). Huincul Formation belongs to the fluvial and lakered-bed deposits of the Neuquén Group.Some of the structuresof HH and DCH were reactivated during the Tertiary Andean orogeny, and acted asvertical channels connecting the reservoir fluids with the sub-horizontal paleo-channel of the Huincul Formation. The circulation of these fluids in the clastic rocks is documented by the presence of bitumen associated with baroque calcite, pyrite and clay minerals rims (smectite, chlorite and chlorite-smectite mixedlayer). The pervasively greenish gray clay coating is a common feature of the sandstones and conglomerate from the Huincul Formation. This presentation establishes the relationship between diagenetic processes, hydrocarbons migration and their implication in the composition of clay coating precipitation.During the early diagenesis stage, multiple episodes of clay infiltration into the unconfined meandering river deposits of the lowersection of Huincul Formation resulted in the thick (2-3 µm) clay coating present in some volcanic rich litharenite sandstone and conglomerate. Progressive burial caused the reddening of the whole Huincul Formation, documented in the reddish brown and pink color of the mudstone and sandstone layers duethe presence of hematite pigment. This hematite derived from infiltration of meteoric water following breakdown of ferromagnesian silicate minerals and iron bearing clay mineral.  The oil emplacement is documented in the intense bitumen impregnation of sandstone and conglomerate and at micro scale in calcite and barite cements that host numerous organic rich fluid inclusions. The greenish grayfacies documents hydrocarbon migrations in the Huincul Formation where previous kaolinite and hematite coating were transformed to dioctahedral smectite, chlorite-smectite mixed layer or chlorite and pyrite, respectively. These changes from smectite to chlorite-smectite mixed layer or chlorite is interpreted as a signature of increasing reducing conditions. The direct transformation of smectite to chlorite in some of the greenish gray facies located at the lower part of the Huincul Formation, involved fast rate of growth nucleation at expense of previous smectite suggesting a high fluid/rock ratio (Robinson et al., 2002). This condition prevailed in the permeable layers of lower section of the sedimentary unit that yields a focused flow pathway. In the rest of the sedimentary unit, the presence of chlorite-smectite mixed-layer suggest chloritization of previous smectite. The partial transformation of dioctahedral smectite to chlorite-smectite mixed layer or trioctahedral chlorite produced excess of silica resulting in the micro quartz precipitation (Mosser-Ruck et al., 2010). Addition of Mg is required to form chlorite-smectite mixed layer or chlorite from the dioctahedral smectite and could be taken from the alteration of detritus maficminerals and/or Mg rich water formation that migrate with hydrocarbons.