Reservoir Modeling using Thin Layer Analysis and Characterization e-tlac™ Approach for Turbiditic Environment

Abstract

Thin turbidity siliciclastic reservoir is a challenging deep-water environment for modeling. In a deep off-shore field in West Africa, sedimentological characterization of these reservoir suggests typical turbiditic sandstones: Arenites with medium granulometry and normal gradation over imposed by plane-parallel sand laminations intercalated by shaly levels (late stage turbidity sandstone beds - characterization by Mutti, 1992). The oil bearing reservoir, is producing both from thick sands and thin layer sands. It is supposed the presence of up to 40% of sand-shale inter-bedded layers with a thickness less than 0.3m. These sands are defined in the reservoir model by curves of petrophysical properties, log facies, characterization of thin bedded intervals and a volume of seismic inversion. Tuning analysis suggest the potential seismic resolution is 16 meters. Seismic inversion was processed to generate a higher resolution driver for modelling. Formation evaluation uses high-resolution logs within "Thin Layer Analysis and Characterization" (e-tlac™) method (Galli et Al., 2002) which helps enhancing thin inter-bedded facies characterization, not captured with conventional logs. The method provides an estimate of the Sand content, Porosity and Saturation logged by the tools with thin lithology. High-resolution logs were acquired only in three wells over twelve drilled in the field. For this reason, a re-calibration of all conventional CPIs including the "e-tlac" output results was necessary to better control the reservoir property distribution all over the grid. This methodology increased capability estimating pay volume close to real value avoiding underestimation of Net Sand and Water Saturation overestimation. The solution to model thin turbiditic sands within the static 3D model is integrating all the above data inputs (stratigraphical environment, seismic inversion volume and "e-tlac" output). Reservoir cores was the input for the sedimentological study; the seismic inversion volume was background for reservoir facies distribution and "e-tlac™" formation evaluation output to assign unbiased reservoir properties to sand and thin layer facies at the well position. As lesson learned, the acquisition of triaxial induction, high-resolution dielectric or image is the key to better characterize the inter-bedded thin levels that are present in similar deep-water environment.