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Reservoir Architecture, Ichnology and Sedimentology of the Upper Cretaceous Gacheta Formation, Las Maracas Field, Llanos Basin, Colombia

Abstract

The Upper Cretaceous (Barremian) Gacheta Formation constitutes one of several mudstone units that make up the world class source rock of the La Luna-Gacheta system in northwestern South America. Recent discoveries have documented significant hydrocarbon resources from reservoir-quality, sand-rich intervals in the historically unconsidered Gacheta Formation in the Llanos Basin (eastern Colombia). However, exploration and development is challenging, mainly due to its complex internal lithological relationships and the laterally discontinuous nature of individual reservoir-quality sandstone bodies. The thickness and the constraints that represent electrical log interpretation in mud-rich, highly interbedded heterogeneous deposits may cause these reservoirs to be bypassed. This presentation is part of an ongoing study of the petrophysical properties, distribution and internal architecture of the reservoir facies within the Las Maracas field. The study integrates sedimentology, ichnology, log-derived petrophysical data, well testing interpretations and reservoir performance analysis. The Gacheta Formation study area is subdivided into four recurrent stratigraphic packages (i.e. Gacheta-A through D). The Gacheta-C reservoir is revealed to have the best quality reservoir sands through petrophysical interpretation, testing analysis and reservoir performance. Detailed core logging of this interval allowed identification of four lithofacies: 1) weakly burrowed muddy tidal-flat deposits; 2) moderately burrowed sandy tidal-flat deposits; 3) estuarine channel deposits; and, 4) storm-influenced offshore shales. Overall, a trace fossil assemblage consistent with a brackish-water suite (e.g. Planolites, Paleophycos, and Thalassionoides) that resembles the impoverished Cruziana ichnofacies, along with sedimentary structures (e.g. flaser bedding, cyclical double mud drapes, inclined heterolithic stratification IHS) and vertical stacking characteristics of tidally influenced settings, typify the Gacheta-C sandstone. This is also interpreted as representing a shallowing or shoaling trend within an overall trangressive sequence. Relative sea level falls allowed the establishment of marginal marine brackish-water systems prior to a significant phase of deepening marked by a transgressive surface of erosion (TSE) located on top of the Gacheta-C reservoir. This laterally continuous surface provides a useful stratigraphic reference for future exploration and development plans.