Abstract

The presence of natural seepage slicks and seeps composed of a mixture of cracked and un-cracked lacustrine saline oils suggests the existence of active Barremian hydrocarbon source rock systems in the frontier deep and ultra-deep waters of the Santos Basin, Brazil. Acquisition, interpretation, and integration with meteoceanographic data of 24 Radarsat and 50 Sentinel images resulted in the identification of 78 oil slicks spread over the 700 km² study area from water depths that range from 400 to 3000 m.

The identified seepage slicks resulted from the convergence of optimum tectonic, temporal persistence, and environmental scenarios, confirming that some pre-salt oils are effectively leaking from deep petroleum systems to the sea surface. When integrated with piston core, seismic, geochemical, and 3-D modeling data, it is clear that the seepage slicks are directly related to the salt tectonic processes and the presence of transtensional and listric fault zones, suggesting an origin closely associated with salt windows on the unconformity that defines the top of the sag sequence and seepage features on the seafloor.

The geological framework of the Santos Basin’s ultra-deep waters is different from the proximal areas of the basin. In this sense, four geological domains are interpreted: The first domain occurs in shallow water in the platform region and can be called the seal risk zone; the second corresponds to the platform to salt-ramp region that occurs from shallow to continental slope waters and is a gas/condensate-prone zone; the third includes the salt-ramp to mini-basin domains, comprising the Upper Cretaceous reservoir turbidity fairway associated with the axis of the central rift system located from the deep to ultra-deep-water region; and the fourth is composed of mini basins and stratified layers of the evaporitic sequence in the ultra-deep waters where giant oil accumulations are present in the carbonate pre-salt reservoirs. These four structural domains determine different petroleum habitats that depend on salt thickness and the structural configuration of the rift sequence.

The main hydrocarbon migration model for the identified seeps and oil slicks associated with the salt-weld or salt-wall domain of the Santos Basin’s ultra-deep waters is related to the existence of major fault zones. This type of feature is known as a transtensional fault zone that developed from the basement to the pre-salt reservoirs, with them sometimes functioning as the trigger for the halokinesis-derived listric normal faults that can displace the entire sedimentary wedge and can be observed on the seafloor. The presence of surface slicks is associated with oil seeps detected through piston cores in areas related to these types of faults, which confirms that such systems are the main oil conduits to charge and recharge pre-salt and post-salt oil and gas accumulations. Therefore, this hydrocarbon migration model should be well understood and considered for the exploration for oil and gas, principally post-salt targets.

In summary, the integration of seepage slicks, seismic data, and geochemical analyses of reservoir oils and oil seeps can improve the petroleum system comprehension of frontier areas and, in the case of this chapter, open up a huge exploration frontier for the deep and ultra-deep-water outboard area of the Santos Basin.