Reservoir Characterization Challenges Due to the Multiscale Spatial Heterogeneity in the Presalt Carbonate Sag Formation, North Campos Basin, Brazil

摘要

Textural and compositional heterogeneities found in the presalt Aptian carbonates of Macabu Formation, also known as the Sag Unit in the Campos Basin, Brazil, pose unique challenges with respect to achieving a meaningful and robust reservoir description. In this paper, we present a case history based on our evaluation of conventional cores and e-logs acquired from three wells drilled in the North Campos Basin. Lithofacies description of the core shows that the original depositional textures and fabric of these carbonates have been altered severely because of the subsequent episodes of diagenesis. The Sag Unit carbonates are characterized by very complex heterogeneities caused by millimeter- to decimeter-scale variations in their petrophysical properties, such as composition of solid matrix, porosity distribution and pore-size variation. Comprehensive well-data acquisition was planned and implemented, e.g. cutting a full-diameter conventional core from the Sag Unit, acquiring fit-for-purpose modern e-logs, and tailoring the protocols for analyses of the cores. Multiscale petrophysical and geological measurements are needed to capture the details of heterogeneities in the sag carbonates in terms of their lithology, textures, pore space and fluid characteristics. This enabled us to acquire direct measurements of the reservoir parameters of these carbonates and compare them with those parameters derived from e-log interpretation. This paper presents a summary of reservoir description as well the challenges posed by the presalt Sag carbonates for obtaining direct measurements of reservoir parameters needed for core-to-e-log integration and upscaling in reservoir modeling. Our experience points out certain inadequacies and inconsistencies in the available state-of-the-art technologies and techniques deployed for the evaluation of core and e-logs. We conclude that development of new "fit-for-purpose" technologies and approaches are needed to achieve a meaningful integration of the multiscale measurements of rock properties into the reservoir description of heterogeneous carbonates of the presalt Sag Unit. This has significant implications for the estimation of hydrocarbon reserves and produceability of not only the presalt Sag reservoirs but also the other heterogeneous carbonate reservoirs elsewhere in the world.