Petrophysical Implications From a Microtectonic Evaluation of Villeta Formation Cores in the Putumayo Basin (Colombia)

The HC‐producing Putumayo Basin is located in Southwestern Colombia, forming the northern part of the Ecuadorian Oriente Basin. Traditional reservoirs are sandstones of the Caballos Fm. (Aptian–Albian) and also the ‘U and M2 Sandstones’ of the Villeta Fm. (Late Albian–Maastrichtian). To provide a frame for our observations, a review of the main tectosedimentary aspects and petroleum systems considered in the literature was carried out. To evaluate the potential of the carbonate layers as reservoirs, in addition to the traditional ones, a microtectonic study was carried out on some available carbonate‐bearing cores from wells drilled in the Villeta Fm. Although not properly oriented in space when acquired, when available, dipmeter logs have contributed to the approximate 3D positioning of these cores. The recognized microstructures belong to two main tectonic events progressively superimposed in time from Upper Cretaceous to Recent and in space from West to East: (i) early features due to vertical compaction by overburden following deposition that develop gently deformed stratification and lamination surfaces around more competent objects generated by differential compaction, steep‐angle normal faulting in competent levels with slickensides, subhorizontal stylolites and subvertical calcite veins with cuneiform patterns; (ii) later features due to ENE–WSW subhorizontal convergence by the action of the post‐Eocene Andean convergence producing dextral transpressional reactivations on the pre‐existing N–S normal faulting. The early compaction is usually associated with the onset of HC maturation and subsequent migration towards contemporaneous potential traps. The carbonate veins confirm that dissolution and precipitation processes have decreased the primary porosity and permeability of the reservoirs by calcareous cementation, especially in the deeper parts of the basin. If the Villeta Fm. is to be considered a potential unconventional or tight reservoir, the more efficient direction for future drilling is subhorizontal NNW–SSE. These studies on available cores are only complementary to the usual, more thorough and expensive methodologies of petrophysical reservoir analysis and mainly aspire to extract additional information considering the high costs of obtaining them.