Ultra-Deep 3D Electromagnetic Inversion for Anisotropy, a Guide to Understanding Complex Fluid Boundaries in a Turbidite Reservoir

Abstract

In mature fields, fluid contacts undergo vertical depth shift and lateral variation due to production and injection. Variability in these contacts can be exaggerated by depositional, structural, and lithological elements, acting as baffles and conduits. In turbidite deposits, where sand sequences are intercalated or draped by shales, fluid movement can be influenced by these lithological changes. Shales can range from thin layers to units several meters thick, thus occurrences can in some cases be identified from seismic data or detected with shallow to ultra-deep logging-while-drilling (LWD) tools. However, thin shales can go undetected in electromagnetic (EM) data if resistivity contrasts are low. In a water-filled reservoir, shales can be indistinguishable from water-flooded sands, unless directly penetrated. Inverting for formation resistivity alone leaves a question unanswered: are the low-resistivity zones water-filled sands or shales? One defining factor is anisotropy; inverting for anisotropy provides a tool to differentiate shales from water-filled sand. Clean, water-filled sand will be relatively isotropic compared to laminated shale. This paper investigates identifying shales at distance through 3D EM inversion for anisotropy, and the deeper reservoir understanding gained through identifying how lithological distribution impacts fluid movement. In a trilateral well, initial 1D inversions for resistivity clearly show variable, low-resistivity contacts above and below the reservoir. Above the well is the overlying shale caprock. Below the well, correlation with offset data indicates that this is the oil-water contact (OWC). However, this lower boundary is not confined to a single horizontal plane and has a more complex morphology than expected, given the properties of the reservoir. The boundary was only penetrated in one area towards the toe, where both shales and water-filled sands were encountered. Ultra-deep 3D EM inversion shows inverted resistivity values consistent with the anticipated water below, but also close to the resistivity value for shale. 3D inversion for anisotropy shows a thin layer with high anisotropy bounding the OWC, which has a similar value to thin shale layers and lenses penetrated along the well and identified on gamma logs. The thin shale layer extends over 1000 m (3280 ft) MD. This layer appears to be acting as a baffle, limiting the movement of water within the reservoir. A potential interpretation is therefore that the depositional morphology and post-depositional deformation of this thin shale layer appears to be responsible for the unusual, convoluted OWC.