Reservoir quality and petrophysical evaluation of the Paleocene paralic sandstones in the Ranchería sub-basin, Colombia

Abstract

The Ranchería sub-basin is an unexplored basin which gives a valuable opportunity to find new perspectives about the hydrocarbon potential in Colombia. The sandy facies of the Paleocene Cerrejón Formation accomplish with the characteristics within the petroleum system, to be considered as a potential reservoir rock. This work aims to evaluate the reservoir quality of the Cerrejón Formation using three cored wells, through the integration of well logs, core sedimentological descriptions, porosity and permeability lab data, and thin-sections analysis. Using GR log patterns and sedimentary cycles, four facies associations (A,B,C,D) were defined associated to an environment of a deltaic plain complex. The well correlation was performed using the sedimentary cycles methodology. Five petrophysical rock-types (mudstone (M), coal (P), sandy mudstone (sM), muddy sandstone (mS), and sandstone (S)) were identified, according to textural and lithological features from the core descriptions, adjusted with well logs analysis and NTG ratio. A supervised neural network model of electrofacies was generated with the GR, RHOB, NPHI, and DT logs combination, using as training curves the wells B and C. The blind test validation was executed in the well C with an 82% of correlation. Petrophysical evaluation was performed calculating shale volume, effective porosity, permeability and water saturation. Reservoir rock properties are fair to moderate with porosities between 0.3-22.9 %, and permeability values that range from 0.02mD to 47mD. The interest intervals average water saturations vary between 0.3 and 0.5. Four hydraulic flow units were estimated with the Winland’s R35 method for the assessment of reservoir quality. There is not a clear relationship between F.A. and HFUs for the Cerrejón Formation, however, the F.A. B comprises the HFUs with the best petrophysical properties. The reservoir quality of Cerrejón Formation is controlled mainly for diagenetic and compositional processes than related to primary deposition. The diagenetic controls in reservoir properties are the calcite cementation, quartz overgrowth, pseudomatrix presence, and feldspar and calcite dissolution. This integrated study provides a good approximation to the understanding of the Cerrejón Formation and the evidence of a moderate reservoir quality.