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Depositional Environment and Permeability Distributions in a Heterogeneous Reservoir: A Case Study from the Upper Eocene to Oligocene C7 Member, Carbonera Formation, Dorotea Field, Llanos Basin, Colombia


The Upper Eocene to Oligocene C7 Member represents one of the four regressive cycles within the Carbonera Formation and hosts significant hydrocarbon resources across the Llanos Foreland Basin (eastern Colombia). Despite its economic importance, relatively little work has been directed towards the understanding of the distribution and quality of this reservoir unit. This study documents a multidisciplinary approach integrating ichnology, sedimentology and production data in order to characterize the distribution, fluid storativity and deliverability of the C7 reservoir in the Dorotea field. The aims of this study are to: 1) assess the stratigraphic stacking patterns; 2) understand the potential for lateral connectivity; and, 3) build conceptual models of the permeability distributions of the C7 lithofacies. Overall, the C7 strata comprises unburrowed to sparsely burrowed very-fine to medium-grained sandstones and mudstones containing a diminutive, low-diversity ichnofossil suite that resembles a stressed expression of the proximal Cruziana ichnofacies. This trace fossil assemblage along with primary sedimentary structures (e.g. flaser bedding, cyclic double mud drapes) suggest brackish water conditions within a tidally-influenced setting. Core-plug permeability measurements, XRD analysis, and petrographic assessments show that permeability distributions are strongly influenced by the nature and percentage of clay content. Core description and well log interpretation suggest that amalgamated tidal channel deposits display the best reservoir properties, whereas muddy tidal-flat deposits limit the porosity and permeability. Petrophysical analysis indicates that sandy intervals with abundant clay content are prone to be bypassed as zones of low-resistivity pay. Thus, clay distribution is interpreted to be the primary control for reservoir quality. This study improves the prediction of permeability distributions in the C7 reservoir. Furthermore, it provides basis for future numerical simulation in the Dorotea field and similar deposits in the study area.