Oil geochemical characterization in the Cretaceous Napo Formation reservoir (Ecuador) and its relationship with
the interbedded shales as hydrocarbon source rock
The Napo Formation from the Cretaceous age is located in the Oriente basin in Ecuador and is
composed of limestones, shales, and sandstones, the «U» and «T sandstones», which are the most important
reservoirs in the basin. The formation was deposited in fluvial, transitional and marine environments, and
their correlation with the sequence stratigraphy may be associated with the fluctuation of the eustatic sea
level. This work presents the analyses of the organic composition and the thermal maturity of the shales
interbedded in the sandstones reservoir. Furthermore, the geochemical characterization of oils from the
«U» and «T» reservoirs was preformed. The analyses were performed to establish the contribution of the
shale as a source rock in the generation of the hydrocarbons. The spatial trend of the TOC in the shales
varies from 1,78% in the Eden-10 well, located in the eastern part of the basin to 0,56% in Jivino-1 well
located in the center of the basin. The hydrogen index (IH) varies from 116 to 414 mg/g, and the thermal
maturity expressed as Tmax shows values of 445ºC. Oil studies show the relationship between tricyclic
terpanes (C20/C23) and trisnorhopane/trisnoneohopane (Ts/Tm) among others biomarkers. These data suggest
a marine origin and the thermal maturity is compatible with foundin thats the shale shows (reflectance
equivalent of 0,6% - 0,9%, deduced from the analyses of the regular steranes isomerization). The
interpretation of all the analyses suggests that the interbedded shales in the «U» and «T» sandstones could
have contributed to the source of the hydrocarbon oil in the Oriente basi