Deepwater Uruguay: Reducing uncertainties in the Cretaceous stratigraphic play with an improved TL-FWI velocity model
FWI velocity model uplift
The advancement in Full-Waveform Inversion (FWI), both in efficiency and quality, has been phenomenal, especially since the introduction of Time-Lag Full-Waveform Inversion (TL-FWI) in 2018, which has led to data reimaging opportunities aimed at achieving more accurate velocity models and improved seismic images. In May 2023, Viridien launched a 25,000 km2 reimaging program for all non-multi-client data offshore Uruguay. Reimaging of two priority sub-regions (OFF-6 and 7) was completed in May 2024, and the remaining two sub-regions (OFF-4 and 5) are expected to be completed by the end of this year. Figure 1 shows a dip line comparison between legacy imaging and Viridien’s new reimaging. As expected, the velocity model from the reimaging program is more geologically sound, revealing previously unseen details of interest to exploration geologists and reducing uncertainties on low-relief stratigraphic traps.
Exploration history
The Uruguayan Continental Margin has been briefly tested by only three wells: Lobo x-1 and Gaviotin x-1 (1976) and, most recently, Raya x-1 (2016).
Raya x-1 confirmed the presence of good Miocene / Eocene sandstone reservoir intervals (135 m thick and 24 % porosity). However, the underlying
Paleocene appears clay-marl dominated with continuous parallel seismic facies with no visible faults to enable hydrocarbon migration from a potential deeper Aptian source rock as seen across the conjugate margin of Orange Basin, Namibia. This scenario naturally steers future exploration in Uruguay to target deeper reservoirs at Cretaceous levels. Reimaging in depth will further help to eliminate charge-migration risk and to pursue and test Namibia’s play equivalents in deepwater Uruguay.
Mitigating risks of a cretaceous petroleum system
Source Rock: Exploration in Namibia’s Orange Basin has proven a prolific world-class source rock of Aptian age deposited on top of seaward dipping reflectors (SDRs)(Figure 3). The soft-top, tabular geometry of this key source rock horizon can be clearly identified in the newly reimaged 3D seismic data in the lesser explored conjugate Uruguayan Margin. Viridien’s new TL-FWI data-driven velocity model played a key role in risk reduction relating to source rock presence by increasing the vertical resolution of the velocity model and removing depth undulating artifacts. Notably, it highlighted three key velocity reversals: one in the lower Cretaceous sequence (likely of Barremian age), another in the Aptian sequence, and a third associated with a potential Cenomanian-Turonian source rock interval (Figure 1 bottom).
Uncertainties regarding Aptian source rock maturity can also be mitigated. Viridien’s basin modelling studies indicate that postulated Aptian Source Rocks reached maturities ranging from oil mature to gas mature across the basin in Uruguay.
The thicker overburden in parts of the Uruguayan margin also brings into play the Albian and Ceno-Turonian (A-CT) source rocks which are immature in Namibia. If the A-CT source rock is considered in the South American case, then Uruguay could offer multiple petroleum systems to charge prospective reservoirs.
Reservoir: Risk of reservoir presence is expected to be low in the Cretaceous offshore Uruguay. Just as in Namibia’s successful case, many channel / fan systems can also be identified in Upper, Mid and Lower Cretaceous levels in Uruguay in all the offshore blocks OFF-4, OFF-5, OFF-6 and OFF-7. Envelope attributes extracted at multiple levels in Viridien’s newly reimaged 3D seismic data shows the scale and variable distribution of Cretaceous channels and fans (Figures 4a, 4b and 4c) indicating many promising targets. The Late Cretaceous seems particularly prospective based on its burial depth, lying in a good drilling range and likely to have retained good reservoir quality.
Viridien’s 3D reimaging of deepwater Uruguay has revealed the promising prospectivity of an under-explored basin along the Atlantic margin.
This extensive data set, coupled with key learnings taken from successful exploration in the Orange Basin of Namibia, gives an insight into the key geological uncertainties in terms of source rocks and reservoir presence, and serves as an invaluable tool to future explorers for decision-making and exploration risk reduction in this highly prospective basin.
