Abstract: Cretaceous/Tertiary Petroleum System in the Tala-Urica Area, EVB

Vivas, María Auxiliador Martínez; Sanchez, Humberto - PDVSA; Santos Neto, Eugenio Vaz - Petrobras/Cenpes

Integration of geological data and geochemical analyses from surface and subsurface (rocks, gas and oil samples), suggest the presence of an upper Cretaceous-Tertiary petroleum system in the Tala-Urica Area, located in the Northwestern region of the Eastern Venezuela Basin (Fig. 1).

The study area is characterized by a passive margin phase from Cretaceous to middle-late Oligocene, and by an active margin phase from late Oligocene to present. A compressive tectonic regime, associated to the collision of the Caribbean plate with the South American plate is responsible for the presence of imbricated thrusts and duplex structures to the East of the NW-SE Urica fault system. Recently acquired seismic data has allowed the interpretation of a potential structural trap represented by a triangular zone, formed by a tectonic wedge of Cretaceous and Tertiary sediments, which was generated during the upper Miocene-Pliocene (Fig. 2).

Oils from this petroleum system were accumulated in sandstones of the San Juan Formation (upper Cretaceous), Naricual, Merecure and Oficina Formations (late Oligocene to middle Miocene) during the development of the foreland basin. Shales of the Oficina and Carapita Formations constitute the seals for the accumulations. Sediments overthrusted during the collision of the Caribbean and South American plates as well as the thick sequence of sediments of the Carapita Formation represent the overburden rocks.

The main compositional characteristics of the oil samples analyzed are: high API degrees (>38°), dominance of low molecular weight n-alkanes (<n-C₁₅), d¹³ c (whole oil) lighter than -28‰ low sulphur contents (<0.1), high pristane/phytane ratio (>2.5), high oleanane indices (>0.6), low C₃₅/C₃₄ hopanes ratios, C₂₉ steranes more abundant than their C₂₈ and C₂₇ counterparts, and low sterane/hopane ratios (<0.2). The abundance of oleanane suggest that the source rocks for these oils are Tertiary and have a high input of Angiosperms. The values of d¹³C (whole oil) are similar, which confirm the generation of these oils from source rocks of the same origin. Similar geochemical characteristics were found in oil extracts from Tertiary rocks of the Neogene from outcrops located in the Northeastern region of the study area ( Mundo Nuevo area), which were deposited under marine-deltaic conditions. These results are in agreement with studies that have been published previously by Meneven-Gulf (1982), Audemard et al. (1986), Stoufer et al. (1996) and Alberdi et al. (1996) from oils from this area of the Eastern Venezuela Basin.

The high ratios of the aaa/(aaa+abb) and 2OS/(20S+20R) isomers of the C₂₉ steranes correspond to a level of thermal maturity of at least peak generation. The abundance of low molecular weight n-alkanes (<n-C₁₅) supports the thermal evolution of the oils analyzed. Based on this stage of thermal maturity it is proposed that the pods of active source rock are buried at depths greater than 10,000 feet, located to the NW of the area (due to stratigraphic overburden), and beneath the Serranía del Interior (due to tectonic loading).

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil