Petroleum system

Petroleum system model

Uruguayan offshore and onshore basins have a common evolutionary history, given that the basement highs which separate them constitute archs, with recurrent uplift events along the Fanerozoic eon.

The Permian rocks drilled by the Gaviotín well in the Punta del Este Basin are correlated with the sedimentary sequences from the Norte Basin, suggesting that both basins conformed a single sedimentary environment in the Paleozoic era. Moreover, the Cretaceous and Cenozoic sedimentary sequences have onshore equivalents, such as fluvial and alluvial sequences from the synrift and postrift stages.

The correlations among the onshore and offshore Uruguayan basins and the basin analysis of neighbouring basins allowed the elaboration of an integrated petroleum system model for the offshore of Uruguay.

SOURCE ROCKS

According to the tectono-stratigraphic model established for the Uruguayan offshore basins, potential source rocks are associated to the prerift, synrift and early postrift (transition) sequences.

The sedimentary rocks associated with the prerift sequence (Gaviotín Well, Punta del Este Basin) are correlated with Paleozoic units with excellent source rock properties in the onshore Norte Basin, with TOC up to 3.6% (Devonian shales) and TOC up to 12% (Permian shales).

Cretaceous lacustrine shales of the synrift sequence with good source rock potential are found in the onshore of Uruguay (Santa Lucía Basin), with TOC of 1-2%. Their presence is inferred in the Punta del Este Basin grabens through seismostratigraphic analysis.

The early postrift sequence (transition) shows a clear transgressive character, with the development of marine sequences with very good source rock potential. This presumably Barremian-Aptian sequence in the offshore of Uruguay is equivalent to the transitional sequence of the Orange Basin and other productive South Atlantic basins.

RESERVOIR ROCKS

Proven high-quality reservoir rocks are encountered in the sedimentary record of the Uruguayan offshore basins, with porosity values of around 20%. The most important ones are related to the aluvio-fluvial systems of the synrift sequence and the lowstand deposits of the Late Cretaceous, Paleocene, Eocene and Oligocene sequences.

SEAL ROCKS

In different stratigraphic levels, rocks with both local (e.g., synrift lacustrine pelites) and regional seal (e.g., Paleocene transgression) characters are present.

TRAPS

Different structural, stratigraphic and combined plays are recognized in the Uruguayan offshore basins in variable water depths, from shallow to deep waters.

HYDROCARBON EVIDENCES

In the Uruguayan continental margin there are several direct and indirect evidences of the occurrence of hydrocarbons, which confirm hydrocarbon generation and the presence of an active petroleum system. Among them, the most important are the detection of fluid inclussions of light oil and gas in cuttings from the synrift and Cretaceous post-rift section of the Lobo and Gaviotín wells, the identification of gas chimneys, amplitude anomalies, AVO anomalies and velocity anomalies in seismic lines, and the interpretation of oil seeps in satellite photos. In several intervals of the Gaviotín well inversion of the porosity and neutron logs was recorded in association to mud loss, representing interesting gas evidences which were not tested.