The Tuy-Cariaco region is located between La Orchila and Blanquilla islands, to the north, and the coast zone between Cabo Codera and the Araya peninsula, to the south. It covers an area of about 45,000 square kilometers. Twelve wells have been drilled during the exploratory campaign carried out since 1979 to 1982. Of these, four wells tested oil and gas, two tested gas, and six were dry wells.

The sequence stratigraphy and structure of regional seismic transects is calibrated with wells and surface geology data. The integration of all this information establishes a tectono-stratigraphic model for the region, leading to definition of exploration plays and to estimation of possible amounts of generated hydrocarbons.

Geologically, two different basins occur in this region: the Tuy-Cariaco Basin, to the south of the Margarita and La Tortuga Islands; and the La Orchila-Blanquilla Basin, to the north. The sedimentary column of these basins unconformably overlies an igneous-metamorphic basement, associated with Cretaceous and older rocks of the Caribbean Mountains.

The age of the Tuy-Cariaco Basin sediments ranges from Oligocene (?) to Recent. This basin can be subdivided in several areas separated by major faults: 1) Barcelona Bay; 2) Cariaco Trough; 3) Cubagua Depression; 4) Structural high to the east of La Tortuga island; and 5) Margarita Depression. Seismically, two sequences are identified in most of the area: one sequence of Late Miocene-Pleistocene (?) age and another one of Pleistocene-Recent age. In the Cubagua Depression an additional sequence of probable Oligocene age is present.

The La Orchila-Blanquilla Basin shows a Middle Eocene to Recent sedimentary column. Two areas separated by the Margarita Oeste fault have been differentiated in this basin: 1) north of La Tortuga, and 2) northwest of Margarita. Four sequences have been identified using sequential interpretation with the following age ranges: a) Middle Eocene-Early Miocene, b) Early Miocene- Pliocene, c) Plio-Pleistocene, and d) Pleistocene-Recent.

Based on the seismic interpretation two fault patterns are identified as affecting the basement top (nappes top): 1) East-West faulting, represented by the San Sebastian-El Pilar, Carupano (North Fault of Trinidad), Charagato-El Yaque, and La Tortuga faults, and 2) Northwest-Southeast faulting, which includes the La Costa-Sta. Ines, Urica, and Margarita Oeste faults. The fault pattern affecting the Tertiary sequence is mainly the same, since most of the faults cut the complete sedimentary sequence.

We propose four major tectonic phases to explain the geologic evolution of this area: 1) Extensional Phase, from the Late Cretaceous to the Middle Eocene (100 My - 52 My); 2) Compressional Phase, associated to the nappes emplacement during the Middle Eocene (52 My 40 My); 3) Extensional Phase, from approximately Late Eocene to Early-Middle Miocene (40 My - 15 My); and 4) Transpressive Phase, from Middle Miocene to present (15 My - 0 My).

Using geochemical data from wells samples two source rock types are identified in the Tuy-Cariaco region. The first type characterizes the La Orchila-Blanquilla Basin and is found in marine sediments of Late Eocene-Early Miocene age drilled by the MTC-2X well. TOC ranges between 1.02 and 2.67, the terrestrial organic matter content increases with depth, and Ro equivalent ranges between 0.8 and 1.5.

The second type of source rock characterizes the Tuy-Cariaco Basin and is associated with the Late Miocene marine transgression. TOC ranges between 1.7 and 4.0. In the wells, this source rock is inmature, but probably it is mature in the small but potent depocenters of the Late Miocene-Pleistocene sediments.

The integration of all the geological and geochemical data allow us to identify two different plays in the La Orchila-Blanqnilla Basin, which could trap important amounts of hydrocarbons. One is a stratigraphic play and it is formed by turbiditic fans surrounded by shales in Early Miocene sediments. The other is a structural play and is represented by folds related to reverse faulting that affects rocks of Eocene to Miocene age.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas