Effects of fine-scale turbidite lobe stratigraphic architecture on dynamic reservoir performance

Abstract

Turbidite reservoirs with lobe stratigraphic architecture are typically encountered in deep-water environments and are present in a large number of basins. In the past, a comprehensive study to identify the key parameters governing the dynamic connectivity of turbidite channel reservoirs formed the basis for the development of the 3D reservoir connectivity factor concept allowing to account for un-modeled fine-scale stratigraphic details by computing geologically consistent field recovery discount factors. Following-up on the success of this approach, it was decided to proceed similarly with turbidite reservoirs containing lobe stratigraphic architecture, by first starting to investigate the parameters driving the performance of these reservoirs. Fine-scale reservoir models are constructed, and a large number of sensitivity-study type flow simulations are performed testing the recovery factor sensitivity to a large number of geological parameters. Analysis of the resulting data indicates that the distributary channel shale-drape coverage and its permeability, together with the lobe axis permeability, are important parameters affecting the connectivity of turbidite lobe reservoirs. Other parameters such as the lobe margin permeability, the lobe infill and lobe complex shale-drape coverages are secondary parameters. The impact of the channel incision depth, the architectural differences between proximal (more channelized) and distal (more lobate) environments and the impact of the chosen well spacing are only noticeable when shale-drape coverages are high, particularly for the distributary channel drapes. The effect of well spacing appears to be stronger in the distal environment.Through extensive visualizations of the simulated oil saturations, we further conclude that the location of by-passed oil depends on the facies permeabilities and on the shale-drape coverage. Our base-case scenario shows that lobe axes are better swept than distributary channels and lobe margins. At 100% channel shale-drape coverage, the oil located within the channels can be totally by-passed for some channels depending on well locations. When all shale-drape coverages are high (at or close to 100%), by-passed oil is observed in every facies. Contrarily to lower drape coverages, this complicated situation does not allow for a per facies localization of by-passed oil. At high shale-drape coverage, it is further noticed that the sweep efficiency of deeply incising channel models is lower than the ones simulated with shallower incising channels. Two main reasons contribute to this effect: first, the significant volume of by-passed oil that is located in the channels (deeply incising channels being bigger, less oil is swept), and second the fact that deep channels compartmentalize the reservoir more than shallow ones at high shale-drape coverages. The ensuing results are compared with those for turbidite channel reservoirs. It is concluded that shale-drape coverage has considerably less impact in lobate than in channelized systems. At equivalent shale-drape coverages, turbidite lobe reservoirs have a greater connectivity than turbidite channel reservoirs.