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Geological and geophysical uncertainties account for most of the challenges encountered during the placement or geosteering of high-angle and horizontal wells in deepwater environments. Structural uncertainties could result from the targeted subsurface structure that is folded, undulating and faulted. Lateral discontinuity of sand bodies, lateral variations in sand thickness, multiple beds, and formation heterogeneities are some of the more common sedimentological uncertainties. Geophysical uncertainties include the vertical depth of the seismic data and seismic reservoir characterization. These uncertainties make increasing the likelihood of success during geosteering not only dependent on the integration of geologic and seismic reservoir characterization techniques, but also on the application of a robust reservoir navigation scheme. In this paper, we present a case study of the geosteering of a horizontal producer well in a complex reservoir in the deep offshore Niger Delta. The reservoir consists of highly faulted channelized turbidites. The lateral discontinuity of sand bodies and the variations in sand thickness have been calibrated by other producer wells in the field. For efficient geosteering, geological and geophysical well planning was complemented by the availability of scenario modeling, a suitable drilling strategy, the availability of fit-for-purpose drilling and formation evaluation tools, robust software, and a multidisciplinary team with the right mix of experience for effective reservoir navigation. An extra-deep reading azimuthal propagation tool was used, and the inversion was performed with Multi-Component While Drilling (MCWD) software that utilized an algorithm to perform real-time processing of any combination of the deep and extra-deep logging-while-drilling (LWD) resistivity measurements, both coaxial and azimuthal [Sviridov et al., 2014]. The case study primarily reviews the geological and geophysical strategies employed during the geosteering, examines the role the extra-deep azimuthal resistivity inversion modeling and borehole imaging played in understanding the nature of the reservoir and checking the effect of formation anisotropy on depth of detection. The study highlights some peculiarities of the depositional environment of the area and shows the benefits of having extra-deep azimuthal propagation resistivity tools in the bottom hole assembly.

The knowledge of structural geology and depositional environment plays an important role in the success of any geosteering job. The bedding type, bedding dip and dip direction, faulting, localized folding, lateral facies change and continuity of sand bodies, sand-shale ratio of the deposits are some critical factors to consider in the planning of a geosteering job. These factors determine the predrill geosteering strategy and the real-time decision-making steps. The resolution of seismic data, the limited number of offset wells and their lack of proximity to the planned well, contribute uncertainty to the knowledge of these factors before drilling commences. This is a case study describing the geosteering of a complex deepwater turbidite reservoir in the Niger Delta. The reservoir is a sandy lobe deposit and is faulted. The well objective was to drain the required intervals in the sand compartments and geosteering was needed to support real-time decision making by estimating the throw of major faults, determining the presence of sub-seismic faults, interpreting near-wellbore structural geology, stating the proximity of conductive boundaries and describing reservoir heterogeneity. The geosteering was successful with the trajectory draining all planned sand bodies, providing improved structural interpretation of the reservoir geology in real time and achieving over 500m of net sand. The aim of this paper is to review the real-time decisions taken to achieve the well objective of placing a drain section in a sandy lobe deposit, describe the near-wellbore structural geology of a turbidite reservoir in the Niger Delta, and review some geological factors to consider while geosteering in deepwater Niger Delta reservoirs.