Sandstones characterization in a redox front related to paleo-hydrocarbon migration in Los Chihuidos High, Neuquén Basin, Argentina

RAINOLDI, ANA LAURA; FRANCHINI, MARTA; BEAUFORT DANIEL; PATRIER PATRICIA

Rio de Janeiro

Conferencia; XV International Clay Conference; 2013

AIpea, Associacao Brasileira de Geologos de Petroeleo y Grupo Brasileriro de Argilas

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; CenomanianCampanian). Los Chihuidos underwent multiple episodes of uplift and inversion during the EarlyMiddle Tertiary, favoring the migration of hydrocarbons. Differences in the clay mineralogy of the Huincul sandstones depends on its degree of interaction with hydrocarbonsbearing solutions released from deeper reservoirs and the relative position of the roll front developed during this process. The Huincul Formation consists of feldspathic litharenite progressively cemented during the burial diagenesis by finegrained hematite, secondary quartz, and albite overgrowths, porefilling kaolinite, and late calcite. The red colour of the regional sandstones is due to hematite crystallization which attests of a diagenetic process under oxidizing conditions. In the areas affected by the hydrocarbons paleomigrations, the sandstones are pervasively bleached (to 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 colors 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 hydrocarbonsbearing solutions. Albite overgrowths still persist and both the feldspars and the diagenetic kaolinite have been partially dissolved and replaced by dioctahedral Mgrich smectite (montmorillonite).The grey and brownish sandstones are representative of the redox front between both the regional oxidizing diagenetic solutions and some infiltrated reducing hydrocarbonsbearing solutions. In the grey sandstones the dissolution of feldspars and diagenetic kaolinite is more intense and the smectite (montmorillonitetype) predominates with subordinated chloritesmectite mixed layer minerals, commonly associated to microgranular quartz. In the brownish sandstones chloritesmectite mixed layer minerals predominate as clay coating over all the detrital grains. The brownish sandstones could be representative 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 smectite and the chloritesmectite 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 chloritesmectite 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.