2.5D Adaptive Finite Element Forward Modeling And Inversion Of Synthetic Marine Controlled-Source Electromagnetic Data Of Nigeria Niger Delta Oil Field

Mapping hydrocarbon reservoirs with sufficient resistivity contrasts between them and the surrounding layers has been demonstrated using Marine Controlled Source Electromagnetic (MCSEM) techniques in this study. The methodology was applied to the Niger Delta hydrocarbon province where resistive targets are located in a wide range of depths beneath variable seawater depths in the presence of heterogeneous overburden. An efficient 2.5D adaptive finite element (FE) forward modeling code was used to delineate the characteristics of the MCSEM responses on moderately realistic geological models of the Niger Delta and to establish the suitable transmission and detectable frequencies for targets with variable seawater and burial depths. The models consist of multiple resistive hydrocarbon layers of 100 Ωm resistivity, two of which overlain each other. This presents an opportunity to study and understand the 2.5D Marine CSEM parameters such as the transmission frequency, transmitter-receiver-target geometry, seawater depth and burial depth of the resistive hydrocarbon layers that is characteristics of the region. We found that MCSEM response of two vertically-placed thin resistors is higher than that of the individual resistive layer, which could be a valuable tool to identify the two reservoirs, which would have been previously identified by seismic, as possible hydrocarbon layers. Modeling various seafloor depths shows that detectability of the resistive hydrocarbon increases for the deeper seafloor models but decreases for the shallow depths (305-m and 500-m). This is noticeable for all offsets in the electric filed (E-field) amplitude responses but obvious and distinct for the long range E-field amplitude. The modeling results also indicate that lower frequencies produce high E-field amplitude though higher frequencies generate higher anomaly measured as normalized amplitude ratio (NAR). Generally, it was deduced that expanded frequency spectrum will be needed to significantly resolve thin resistive layers owing to the wide range of burial depths and sharply variable seawater depths in the region.Topography distortions in bathymetrically acquired MCSEM responses are capable of misleading interpretation to the presence or absence of the target if not corrected for. For this reason, the effects and correction of bathymetry distortions on the deep and shallow seafloor MCSEM responses of Nigeria Niger Delta Oil province were equally examined. MCSEM responses of realistic Niger Delta geological models using 2.5D adaptive finite element forward modeling code were computed. In both water cases, the bathymetry distortions in the electric field amplitude and phase were found to get smaller with increasing Tx-Rx offsets and short-wavelength components in the amplitude curves persist at all Tx-Rx offsets. In the deep water, topographic effects on the reservoir signatures are not significant, but as water depth reduces, bathymetric distortions become more significant with airwave effects, masking the target signatures. The correction technique employed produces a good agreement between the flat-seafloor reservoir model and its equivalent bathymetric model in the deep water at 0.25 Hz, while in shallow water, the correction only shows good agreement at shorter offsets but becomes complicated at longer offsets due to airwave effects. Transmission frequency was extended above and below 0.25 Hz in the frequency spectrum and the correction was applied. The bathymetry correction at higher frequency (1.75 Hz) is not effective in removing the topographic effects at both deep and shallow water. At 0.05 Hz for both seafloor scenarios, we obtained the best corrected amplitude profiles, removing completely the distortions from both topographic undulation and airwave effects in the shallow water model. The correction method applied on single reservoir model shows difference in E-field based on the position of the hydrocarbon layer in the model. Overall, the work shows that the correction technique is effective in reducing bathymetric effects in deep water at medium frequency and in both deep and shallow water at a low frequency of 0.05 Hz.The apparent resistivity psuedosection of the Niger Delta oil and background models reveal resistivity distribution in the sub-seafloor materials, though at exaggerated depth and thickness. The 2D smooth inversion image provides depth and geometric control of the resistive layers as the 1st hydrocarbon target was successfully recovered. At low transmission frequency, the 1st reservoir was successfully recovered at almost its correct position and the 2nd reservoir was partially recovered and the underlying Akata shale was well separated from the target zone.