Sandstones characterization in a redox front related to paleo-hydrocarbon migration

RAINOLDI ANA; DANIEL BEAUFORT; PATRICE PATRIER; FRANCHINI MARTA

Río de Janeiro

Conferencia; XV International Clay Conference; 2013

AIPEA

Located in the central part of the Neuquén Basin (Argentina), Los Chihuidos High represents a smooth anticline structure generated by tectonic inversion of the basement faults. In the east flank of Los Chihuidos are exposed subhorizontal sandstones belonging to the Huincul Formation (Neuquén Group; Cenomanian-Campanian). The basin underwent multiple episodes of uplift and inversion during the Early-Middle Tertiary, favoring the migration of hydrocarbons. Differences in the clay mineralogy of the Late-Cretaceous Huincul sandstones depends on the degree of interaction between hydrocarbons-bearing solutions released from?.and the sandstones and the relative position of the roll front developed during this process. The Huincul Formation consists of feldspatic litharenite progressively cemented during the burial diagenesis by fine-grained hematite, secondary quartz, and albite overgrowths, pore-filling kaolinite, and late calcite. The red colour of the regional sandstones is due to hematite crystallization which attest of a diagenetic  process under oxidizing conditions. In the areas affected by the hydrocarbons paleo-migrations, the sandstones are pervasively bleached (light grey or white colour?)  and contain bitumen impregnations. The contact between both the red and bleached sandstones is frequently marked by small haloes in which the colour of sandstones turn to dark grey or brown. The main differences between the sandstones with different colours concern porosity as well as the qualitative and quantitative mineralogy of clay minerals and associated oxides minerals.  The bleached sandstones represent the most reduced facies in which all the hematite has been dissolved by the reducing hydrocarbons-bearing solutions. Albite overgrowths still persist and both the feldspars and the diagenetic kaolinite have been  partially dissolved and replaced by dioctahedral smectite (montmorillonite).The grey and brownish sandstones are representative of the redox front between both the regional oxidizing diagenetic solutions and some infiltrated reducing hydrocarbons-bearing solutions. In the grey sandstones the dissolution of feldspars and diagenetic kaolinite is more intense and smectite (montmorillonite-type) predominates with subordinated chlorite-smectite mixed layer minerals (commonly associated to microgranular quartz. In the brownish sandstones chlorite-smectite mixed layer minerals  predominate as clay coating over all the detrital grains. The brownish sandstone could be representive of alteration of both detrital grains and digenetic cements in more reducing conditions than in the gray facies. Secondary hematite crystallized in association with the chlorite-smectite mixed layer minerals in both the grey and brown sandstones. The high contents in vanadium, copper and manganese which have been measured as well in  secondary clay minerals (smectite and chlorite-smectite mixed layers) and the associated oxides (secondary hematite) of both grey and brown sandstones enhance the role of the redox front on the  precipitation of such metallic elements which speciation is very sensible to Eh conditions.