Integrated Petrophysical and Sedimentological Characterization of Chlorite-Bearing Late Devonian Reservoir Sandstones, Chaco Basin, Bolivia.

Summary In many parts of the world (Bolivia, Algeria ...), Devonian reservoir sandstones are known to contain chlorite-family minerals. Depending on their disposition (i.e., grain-coating and/or pore-filling) and quantity, this authigenic clay can enhance or degrade the petrophysical characteristics of these sandstones during diagenesis. In the “El Dorado” field (subandean Chaco Basin, Bolivia), the Devonian (Frasnian) “Guanacos” reservoir hosts abundant chloritic sandstones. These sandstones were deposited by shallow marine prograding shoreface system, fed by tide-dominated rivers, within a low subsidence cratonic basin setting. Among studied stratigraphic succession, 7 facies have been identified, using wireline logs cross-plots, where 4 distinctive sandstones facies are recognized by their mineralogical content, defining: “clean”, “chloritic”, “radioactive” and “dirty” facies. These 4 sandy electrofacies were later cross-compared thanks to thin-sections and porosity/permeability plugs results analysis. The “clean” sandstone facies exhibit chlorite-coated grains together with “open” porosity, that both retain good porosity and permeability ranges as the “coating” have prevented the secondary growth of quartz during burial diagenesis. In all other sandstone facies, variable chlorite content is present, occurring mostly as pore-filling authigenic clay cement. This chloritic cement is even more abundant as the grain-size decreases which implies a coeval degradation of permeability without particularly affecting porosity.