Mineralogical and chemical variations in clay minerals as key to decipher hydrocarbon migration in siliciclastic rocks, Neuquén Basin (Argentina)

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

Edimburgo

Conferencia; Euroclay 2015 Conference; 2015

European Clay Groups Association and The Clay Minerals Society

The Neuquén Basin,represent the main gas and second oil producing basin in Argentina with 2075.5million barrels of oil and 17 tcf (trillion of cubic feet) of gas of discovered resources (Giusiano et al., 2011). Hydrocarbons are produced in conventional and unconventionalreservoirs ranging from Late Triassic to Tertiary in age. The Late Cretaceous(Cenomanian-Campanian) deposits of the Neuquén Group comprises a thick continentalred-bed sequence, which is an important reservoir for hydrocarbons in thenorthern part of the Neuquén Basin. Along the Los Chihuidos and Huincul highs,two of the most important morphostructural units of the basin, the red-bedsequence is bleached and the clay mineralogy is further modified were thehydrocarbons have passed through. The objective of this contribution is to decipher the clay variations, including mineralogy and chemical transformations, after the circulation of hydrocarbons. The Neuquén Group is made up of seven formations, in this case we will focus on the basal Candeleros and Huincul Formations. The Candeleros Formation, deposited influvial and swamp environments, contains Na-rich corrensite whereas the HuinculFormation deposited in meandering and braided fluvial systems, under more humidand acidic conditions than the Candeleros Formation, lacks of corrensite and insome places contains well-ordered authigenic kaolinite. Both formations were deposited under oxidizingconditions and contains hematite coatings as well, giving the red coloration to the sedimentary sequence. In the areas affected by the hydrocarbons paleo-migrations, thesandstones are pervasively bleached and contain bitumen impregnations. In theCandeleros Formation, corrensite is partially dissolved and its interlayercharge is satisfied by Ca instead of Na as in corrensite from the redsandstones whereas, in the Huincul Formation,kaolinite is partially dissolved and smectite (montmorillonite) and randomly interstratified chlorite-smectite mixed layer have precipitated. In the bleached sandstones, Ca-rich corrensite,montmorillonite and chlorite-smectite mixed-layer contain minor amounts of Cu; Vhas also been measured in montomorillonite and in randomly interstratified chlorite-smectite mixed layer. The incoming of hydrocarbons and their associated reducing fluids generated cation exchange between the infiltrated extraformational fluids (Ca-rich) and the earlier corrensite (Na-rich) whereas, in the Huincul Formation precipitated new additional authigenic minerals including montmorillonite and chlorite-smectite mixed-layer;chlorite-smectite mixed-layer probably precipitated from an incipient chloritization process of previous montmorillonite in response to more reducing local conditions. Bleaching of the sandstones is due to the redox reactions between hematite and the reducing waters associated to hydrocarbons where the hematiteis reduced and the Fe2+ produced is fixed in iron sulfides or removed, promoting the bleaching of the sandstone. V is commonly related to thehydrocarbons whereas Cu seems to be incorporated in the clay minerals after its precipitation.The behavior of clay minerals plays an important role in the study ofthe hydrocarbon paleo-migrations. The complex clay mineral paragenesis,contemporaneous with bleaching of the red beds, can be potentially used to provide evidence for hydrocarbons migration, including in those cases where early hematite is not present or where the original red color has been completely removed, making more difficult the recognition ofthe hydrocarbons passage.