Background: Aeromagnetic data of the Ikogosi warm spring region was used to calculate the basal depth of the magnetic layer (Curie point depth) in the region. The warm spring issues from a crossing of fractures from a metasedimentary suite of Effon Psammite formation which form part of the Precambrian basement complex in Nigeria. Method: The adopted computational method transforms the spatial data into the frequency domain and provides a relationship between radially average power spectrum of the magnetic anomalies and the depths to the respective sources. Heat flow density and equivalent depth extent of heat production from radioactive isotopes in the area were also evaluated. Results: The average Curie point depth for the Ikogosi warm spring area is 15.1 ± 0.6 km and centres on the host quartzite rock unit. The computed equivalent depth extent of heat production provides a depth value (14.5 km) which falls within the Curie point depth margin and could indicate change in mineralogy. The low Curie point depth observed at the warm spring source is attributed to magmatic intrusions at depth. This is also evident from the visible older granite intrusion at Ikere -Ado-Ekiti area, with shallow Curie depths (12.37 ± 0.73 km). Conclusions: Results indicate that the area is promising for further geothermal explorations.
In reply to Geothermal Energy 2:11 comments about possible errors in our recent paper Geothermal Energy 2:6, 1-21 with title 'Spectral analysis of aeromagnetic data for geothermal energy investigation of Ikogosi Warm Spring -Ekiti State, southwestern Nigeria', we show that there are no errors in the published paper. Our choice of 55 × 55 km block dimension slide across the magnetic anomaly map is consistent with the derived Curie point depths. This choice was adopted because of complexity of the geology in the area and the need to sample more data points while preserving the spectral peak. All depth estimates were carefully and thoroughly performed and assessed using tectonic framework, geological and geophysical evidence, heat flow, seismicity, and other independent information.
We present results and a technique for imaging the subsurface structures of a geothermal field with particular focus on the Ikogosi geothermal field in Nigeria. The intent was to provide an understanding of the subsurface structural setup in the region and assess its viability for further geothermal resources exploitation. High-resolution aeromagnetic and gravity data were used for the study. A constrained 2D forward modelling technique was applied to these datasets to map the shape and corresponding depths of geologic structures in the region. This study has gone deeper to ascertain the basement structure and configurations and how it influences the heat source of the Ikogosi Warm Spring (IWS) region. The dominant host quartzite rock unit at the IWS location reaches average depths of 2.5–3.0 km and is located directly on an intruded high-density geologic formation in the subsurface. Fault structures traversing the IWS source have also been uncovered. We infer that these structural setups are central to the geothermal system of the IWS. Valid reflection responses from the profile model have been retrieved from randomly induced noise sources, using a passive seismic interferometry technique. Pre-stack depth migration of the reflected responses suitably imaged the reflectors within the subsurface of the IWS region, tracing fault boundaries and delineating intruded geological structures. This has provided pre-survey insights into the subsurface seismic imagery of the region. Results derived from this study could assist informed decision making regarding geothermal exploration and exploitation in the region.
The Electrical Resistivity Tomography (ERT) data was acquired within the area suspected to have high potential for bitumen occurrence using the Wenner-Schlumberger configuration in Agbabu, southwestern Nigeria. PASI 16GL-N Earth resistivity meter instrument was used to acquire data along five (5) traverses with 5m electrode spacing and traverses length of 150m. The apparent resistivity values obtained was processed using RES2DINV software which helped to automatically obtain the 2D inversion model of the subsurface. This study has shown the occurrence of bitumen between the depth of 13.4m and 9.93m for Traverses 1, 2, 3 and Traverses 4, 5 respectively in a 2-Dimensional electrical resistivity images for boreholes with a depth of about 18m. The results indicate that the bitumen is characterized by good lateral continuity and is sufficiently thick for commercial exploitation.
Hydrocarbon play assessment of any field involves the evaluation of the production capacity of hydrocarbon reservoir unit in the field. This involves detail study of the reservoir petrophysical properties and geological interpretation of structures suitable for hydrocarbon accumulation in the field as observed from seismic reflection images. This study details the assessment of hydrocarbon play in OSWIL field onshore in Niger Delta, with the intent of appraising its productivity using a combination of seismic, well logs, petrophysical parameters and volumetric estimation using proven techniques which involves an integrated methodology. Two reservoir windows ‘R1’ and ‘R2’ were defined from five wells OSWIL-02, 04, 06, 07 and 12. The top and base of each reservoir window was delineated from the wells. Structural interpretation for inline 6975 revealed two horizons (X and Y) and eight faults labelled (F1, F2, F6, F8, F10, F16, F17 and F18). Five faults (F1, F6, F10, F17 and F18) were identified as synthetic faults and dip basin wards while three faults (F2, F8 and F16) were identified as antithetic faults and dips landwards. Time-depth structural map at top of reservoirs R1 and R2 revealed structural highs and closures. These observations are characteristics of growth structures (faults) which depicts the tectonic style of the Niger Delta. Results of petrophysical evaluation for reservoirs ‘R1’ and ‘R2’ across the five wells were analysed. For reservoir ‘R1’ effective porosity values of 27%, 26%, 23%, 20% and 22% were obtained for wells OSWIL-04, 12, 07, 06 and 02 respectively with an average of 23.6%, while for reservoir ‘R2’ effective porosity values of 26%, 22%, 21%, 24% and 23% for wells OSWIL-04, 12, 07, 06 and 02 were obtained respectively with an average of 23.2%. This porosity values correspond with the already established porosity range of 28-32% within the Agbada formation of the Niger Delta. Permeability index of the order (K > 100mD) were obtained for both reservoirs across the five wells and is rated very good. Hydrocarbon saturation (Shc) across the five wells averages at 61.6% for reservoir ‘R1’ and 67.4% for reservoir ‘R2’. Result of petrophysical model for porosity, permeability and water saturation reveal that the reservoir system in R1 and R2 is fault assisted and fluid flow within both reservoirs is aided by presence of effective porosity and faulting. Volumetric estimation for both reservoirs showed that reservoir R1 contains an estimate of 455 × 106 STB of hydrocarbon in place, while reservoir R2 contains an estimate of 683 × 106 STB of hydrocarbon in place. These findings impact positively on hydrocarbon production in the field and affirm that the two reservoirs R1 and R2 are highly prospective.
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