Evidence of hydrocarbons in San Juan sub-basin mainly refer to reported seeps inland, near Condoto, on both sides of Middle San Juan Valley, near Buenaventura, and offshore in San Juan River Delta area. Besides, several Cenozoic rocks exposed in the northeastern sector of IDB, to the NE of Condoto show oil impregnations and veins filled with tar.
As detailed below, the main source for hydrocarbon generation in the San Juan sub-basin is the Iró Formation (Figure 7). The thickness of this formation is not well known, but from the seismic profiles it has been estimated to be between 650 m and 1200 m, The Iró Formation may be divided into three segments: Lower, Intermediate and Upper. The Lower and the Upper Segments are composed of thin to medium bedded limestones, interbedded with conspicuous chert beds, bituminous shale and fine grain sands interbeddings. The middle segment consists of medium to fine grained sandstones, containing dark pelitic interbeddings (shales and siltstones).
Based on the results of 1D hydrocarbon generation models, it is possible to propose that during Late Miocene to Pliocene times ,the lower Iró Formation of the San Juan Basin reached the sufficient depths and temperatures needed for the generation and migration of hydrocarbons (Figure 8). The expelled volumes of hydrocarbons from the Iró Formation could exceed those achieved so far in other producing basins in Colombia.
The pre-Late Miocene structures and the possible hydrocarbon generation during the Late Miocene- Pliocene, represents a low-risk timing scenario for entrapment in pre-Late Miocene and younger units.
The Cenozoic lithological sequence of the San Juan Sub-basin contains numerous possibilities for reservoir rocks, such as: Fractured limestones and cherts in the upper and lower segments of Iró Formation, medium grain sandstones in the middle Iró Formation, medium to coarse grained sandstones in Istmina and Condoto formations, conglomerates and sedimentary breccias of the Conglomerados de la Mojarra Formation.
It is represented mainly by shaly and argillaceous intervals in the Istmina and Condoto formations as well as intraformational seals in shales of the Iró Formation.