Hunting for Cenomanian-Turonian source rocks in the South Atlantic

Like pieces of a jigsaw puzzle, key risk elements to any play (source, reservoir and trap) need to be located and joined together. On seismic, some source rocks stand out, impossible to ignore and in the South Atlantic, the omnipresent Cenomanian-Turonian (CT) interval deposited during a global anoxic event – Oceanic Anoxic Event (OAE2) belongs firmly in this cat­egory, at least, most of the time.

Globally, the CT OAE2 has generated world-class marine source rocks; from Venezuela’s La Luna Formation to the organic-rich Canje Formation offshore Guyana, this interval has powered some of the most prolific petroleum systems on the planet. Across the South Atlantic, evidence of CT source rocks is widespread: From the MSGBC Basin to Brazil’s Pelotas Basin, and into the conjugate Walvis and Orange Basins, where TOCs can reach double digits. Although the CT interval has often been dismissed as not being buried deeply enough to be mature for hydrocarbon generation, when present, it still tends to announce itself with a signature seismic swagger – a strong low-frequency amplitude event, dimming at far angles, the unmistakable fingerprint of an AVO Type IV anomaly. Thick, organic-rich, and often hydrocar­bon-charged, these intervals light up seismic sections and sweetness attributes that boost high amplitude, and low frequency amplifying this signal.

So naturally, when the first 3D seismic dataset arrived in the Pelotas Basin, the expectation was clear: Use the seismic to define the source rock distribution in the basin. Whilst this worked perfectly for the Early Cretaceous Aptian Source Rock (as it had in the conjugate margin associated with the of Venus in Namibia), the Late Cretaceous CT interval was not playing ball and displayed a very different set of characters.

Source hunting with 3d seismic in the Pelotas Basin

Searcher’s and partner Shearwa­ter’s newly acquired 3D seismic (2023-2026) revealed a well-de­fined Aptian source rock package emerging clearly from the data, correlating confidently with the proven system in the Orange Ba­sin across the Atlantic. So far, so good, demonstrating this basin is incredibly prospective.

When the top of the CT se­quence was tied using calibration from wells on the shelf, a distinct sequence boundary appears be­neath a Coniacian channel system (Figure 1). This channel complex, heavily reworked by contourite currents, forms a striking ge­omorphological feature in the central portion of the seismic line in Figure 1. Up-section, in the CT sequence, the contourite bedforms pass into more parallel, lower-am­plitude reflections. And yet the expected Cenomanian-Turonian AVO Type IV source rock indicator anomaly? Well, this is nowhere to be seen.

When the signal goes missing

One explanation for the missing seismic signature lies in the contour­ite system itself. Persistent bottom currents may have reworked the CT interval so extensively that only fragmented remnants remain – thin, discontinuous slivers of what was once a regionally extensive source rock. RMS amplitude maps within this sequence reinforce this interpre­tation. The dominance of contourite bedforms, coupled with subdued amplitudes, suggests lower-energy currents reworking fine-grained sediments (Figure 2). In such an environment, the original seismic expression of a thick, organic-rich interval could easily be erased – or at least obscured.

CT source rock seismic buster

A less celebrated – but equally pow­erful – diagnostic of generative source can be fluid pressure disequilibrium re­vealed from seismic velocity inversions (decrease in velocity with increasing depth). Within the parallel seismic package overlying the contourite bedforms, a clear velocity inversion emerges. Subtle at first glance, but persistent and regionally coherent (Foldout Figure).

This is no ordinary anomaly. Or­ganic-rich source rocks are known to exhibit reduced P-wave velocities. High kerogen content lowers density, softens the rock frame, and reduces elastic stiffness. As maturity increases, kerogen transforms into hydrocarbons – fluids that are more compressible than wa­ter – further decreasing velocity. Add overpressure into the mix – common in under-compacted, fine-grained systems and velocities drop even more dramatically. Elevated pore pressure weakens grain contacts, reducing stiff­ness and amplifying the velocity anom­aly. In short, low velocity can mean high potential.

The observed inversion in the Pe­lotas Basin can be related to:

• organic richness (high TOC);
• hydrocarbon generation;
• overpressure;
• elevated porosity relative to depth.

Furthermore, this interval correlates with a proven Turonian source rock in well 2-BPS-0006A, where TOCs reach up to 4 %. And in addition, a thermal maturity model, constrained by a BSR-derived geothermal gradi­ent, places the interval firmly within the oil window – approximately 100 to 115° C.

Given the highly favourable cur­rent location of the inferred CT source rock, generated fluids can move both vertically and laterally into adjacent clastic reservoirs, so the mapped AVO Type II and III anomalies outboard and laterally connected to the CT source suddenly take on new significance (Foldout Figure).

The velocity of adaptation: Rethinking source rock detection

For years, seismic source rock charac­terization has leaned heavily on the identification of AVO Type IV anoma­lies. When present, they are powerful indicators – clear, confident, and easy to map. The Aptian system in the Pelotas Basin fits this model perfectly.

But the CT interval in the Pelotas Basin tells a different story. Here, the absence of a classic seismic response nearly led to a missed opportunity. It took a shift in perspective – from amplitude to velocity – to reveal what had been hiding in plain sight.

Not all source rocks shout – some whisper through velocity fields, subtle inversions, and secondary attrib­utes. They require a different kind of listening.

Final call

So, is the CT source rock working in the Pelotas Basin of the southern South Atlantic?

The evidence is mounting. If con­firmed by future drilling, it could reshape our understanding of the region’s petroleum systems – bringing it closer in line with the prolific basins further north. Because sometimes, when the amplitudes go quiet and the character you are looking for disap­pears, you don’t abandon the hunt; you just pick up another tool.