Characterization and origin of the "3 -Dedos" marked bed (Santonian) of the Campos Basin

Abstract

The "3-Dedos" Marker bed, which appears in the Santonian pelitic section of the Ubatuba Formation in the SE of the Campos Basin, is characterized in electrical well-Iogs by three peaks of low resistivily and very high porosity. Each peak corresponds to a 1 to 2 m thick bed of greenish gray, highly hygroscopic, homogeneous and nonbioturbated claystone. These beds are separated by 5 to 10 m of intensely bioturbated shales and siltstones representing the usual marine sedimentation. Contrasting whit these polymineralic pelitic rocks, the claystones are almost exclusively composed of homogeneous, highly expandable and disordered dioctahedral illitel/smectile mixedlayers. The high free water content of these mineraIs is responsible for lhe low resistivity values that were determined. The anomalous porosity corresponds to the microporosity created by the random distribution of the minute crystals. 
The mode of occurrence of the claystones suggests their origin from the deposition of trachytic volcanic ashes that are deeply altered. For these reasons, based on compositional and genetic criteria as well, such claystones can be classified as bentonite. The "3-Dedos" Marker bed was originated by successive brief events and represents an ideal horizon for chronostratigraphic correlation. It was possible to trace this marker over an extensive area of the Campos Basin lhat covers, up to the present, the Marimbá and Piraúna Gil fields, the area of the RJS-116 wildcal well, and part of the Enchova, Pampo and Bonito oil fields.