Chocó (Atrato Sub-basin)


Petroleum geology

Hydrocarbon Evidences

Five exploration wells have been drilled In the Atrato sub-basin: Buchadó-1, Pacurita-1, Nécora-1, Opogadó-1 and Urodó-1. In the Buchadó-1 well manifestations of oil and gas were found. Also, numerous oil and gas seeps have been reported mainly in the area extending from Quibdó to Buenaventura, clearly indicating the existence of an active petroleum system. Although, the state of knowledge for the Atrato sub-basin may look little encouraging, the new data acquired from airborne geophysics suggests the existence of extensive “kitchen areas” for organic matter in those sectors (elongated in the north-south), where source rocks have reached depths greater than 5000 m.

Source Rock

Although results are not conclusive, it is considered that the Clavo and Salaquí formations constitute lateral equivalents of Iró Formation of the San Juan sub-basin. According this, and given the obvious lithofacies similarities, the Clavo and Salaquí formations may have a generation potential quite similar to the Iró Formation. The Salaquí Formation is described as a sequence of gray to light brown siliceous limestones, interbedded with irregular thin layers of black chert and with marls in the less siliceous intervals(Figure 4).

Migration

Presence of source rocks in San Juan and Atrato sub-basins is documented from geochemical analysis of samples derived from the Iró Formation in the Istmina-Condoto high area. As indicated by the results from hydrocarbon generation models (pseudo- well, 1D modeling) involving the Salaquí and Clavo formations, it appears that these units have undergone important processes of hydrocarbon generation and expulsion during the Late Miocene-Pliocene(Figure 5).

The occurrence of pre-late Miocene structuring and the possible stratigraphic traps in the Atrato subbasin generates a low-risk timing scenario for possible economic hydrocarbon accumulations in pre - Late Miocene and even younger units.

Reservoir Rock

Most likely units to contain reservoirs in Atrato subbasin are Uva, Napipí and Sierra formations, which contain significant conglomeratic and sandy intervals, especially on the eastern side. Apparently, on the western side, the Cenozoic sequence contains a greater volume of marine source rocks (limestones, cherts and dark mudstones) which, under favorable conditions, may lead to formation of potential traps in fractured zones.

Seal Rock

Major seals are shaly levels in Napipí, Sierra and Munguidó formations. As an example, the silty claystones and calcareous and carbonaceous siltstones found towards the top of the Sierra Formation may well represent good seal rocks. Also, in Napipí Formation calcareous gray claystones are found, which can also become barriers to migration.

Traps

The following types of entrapment are expected in the Atrato sub-basin:

  1. Stratigraphic traps inside the Salaquí and Uva formations, as they wedge out against both flanks of the basin.
  2. Structural traps, such as anticlines associated with reverse faulting, expected on the western side of Atrato River valley.
  3. Structural traps associated with possible mud diapirs, especially in central and western flank of the sub-basin.

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