Montmorillonite as indicator of intense bleaching of the Cretaceous Portezuelo redbeds in the

JOSEFINA PONS; VALENTIN ROBIN; RAINOLDI ANA; FRANCHINI MARTA; DANIEL BEAUFORT; PATRICE PATRIER

Río de Janeiro

Conferencia; XV International Clay Conference; 2013

AIPEA

The Barda Gonzalez region located in the Dorsal de Huincul structure of the Neuquén Basin, is characterized by the occurrence of oil fields and regional structures at depth and the presence of solid hydrocarbons impregnating the outcrops of the Cretaceous (LateTuroniano -Early Coniaciano) fluvial rocks of the Portezuelo Formation (Neuquén Group: Cenomanian-Campanian). Some of the structures of the Huincul system were reactivated during the Tertiary Andean orogeny, and acted as vertical channels connecting the reservoir fluids with the subhorizontal paleochannel of the Portezuelo Formation. The circulation of these fluids in the clastic rocks is documented by the presence of bitumen associated with calcite, pyrite and clay minerals rims. This presentation establishes the relationship between the diagenetic processes, the migration of hydrocarbons and their implication in the clay mineral precipitation. The Portezuelo Formation in the Dorsal de Huincul region only could reach shallow burial conditions considering that it was covered by a sedimentary column of ~ 700 m thick. These deposits form a fining-upward sequence composed of a medium-grained sandstones and subordinate conglomerates layers (0.4 to 2.5 m thick each) rich in intraclasts of claystone and siltstone, interbedded with beds (<1 to 5 m thick each) of massive- micaceous sandstones and mudrocks. The shallow burial indicates that compaction of these rocks was mainly mechanical, producing the expulsion of fluids and the precipitation of early iron oxides and hydroxides and calcite cement along with the ductile deformation of mica grains and mudchips. In contrast to the reddish brown color of the Neuquén Group rocks at regional scale in the basin, in the area of Barda González, the bleaching (white to grey white color) of the rocks is a common feature. It is due to the lixiviation of the iron oxides and hydroxides cement, only preserved in some of the less permeable reddish brown and thickest claystone layers. The hydrocarbons paleo-migrations in the most permeable layers generated the partial to total dissolution of the previous cements and grains and the precipitation of authigenic montmorillonite with pyrite (anhedral-euhedral). Anhedral pyrite is also filling fractures with bitumen. Synchronically, the oxidation of the hydrocarbons during the fluid-rock interaction produced the precipitation of calcite cement which occluded both the primary and secondary porosity. This calcite shows concentric zones of non-luminescent and luminescent alternated which is an bands defined by the absence or occurrence of organic fluid inclusions ± pyrite, respectively. Mn activator agent of luminescence in carbonates, could have been incorporated in calcite under reducing condition given by the presence of hydrocarbons. The pervasive bleaching of the Portezuelo Fomation in this area, without any spatial differences in its clay mineralogy, is evidence that a huge volume of fluids may have circulated through the Portezuelo Formation without having preserved the roll fron