Executive Summary of Full Report
FES/NNPC commissioned IBB University Lapai on the 13th November 2017 to produce detailed geological maps of the Bida and Sokoto Basins and to draw cross sections across the basins as a preliminary basis to infer sediment thicknesses and geologically prospective areas. Samples collected during the geological mapping and shallow wells drilled at Kudu and Makera in the Bida Basin were also to be subjected to Rock Eval pyrolysis as a basis for evaluating the source rock potentials in the two basins. The produced geological maps indicate that prospectivities in the Bida Basin increase generally towards the centre of the basin while those in the Sokoto Basin are mainly in the areas towards the border with Niger Republic. Organic matter content in the source rocks is moderate to high, predominantly gas-prone and generally sub-mature to marginally nature. The Executive Summaries for the two studies designated as Milestones 1 and 2 are placed against the report of each of the Milestones.
Executive Summary of Milestone 1
The Bida and Sokoto Basins are a part of the western, central and eastern Africa rifted basins system. In the two basins, the sediment strata young towards the centre of the basins, implying increasing thicknesses in sediment accumulation and prospectivity towards the basinal centres. The two basins have been geologically mapped at higher resolution in this study. Stratigraphical formations and their boundaries have been identified and cross sections drawn to enable delineation of areas of thicker sediment piles. Outcrop and shallow well samples have also been collected, pending analyses. The stratigraphical formations mapped in the Bida Basin comprise the Bida Formation at the bottom, overlain by the Sakpe Formation, Enagi Formation and ending with the Batati Formation at the top. Based on the cross sections constructed, areas of thicker sediment piles in the basin are around the villages of Pattishabakolo, Kandi, Edozigi, Duba, Lemu. Gegu and Agbaja.
In the Sokoto Basin, the encountered stratigraphical formations comprise the Gundumi/Illo Formation at the base, overlain by the Rima Group, the Sokoto Group and the Gwandu Formation at the top. All cross sections across the basins show that sediment thickness vis-à-vis prospectivity increases towards the border with Niger Republic and mainly in the areas covered by the Gwandu Formation. These areas comprise Kiwo-Allah, Balle, Illela, Argungu, Birnin Kebbi, Gwandu, and Koloye, amongst others. The geologically defined prospective areas must be supported by geophysical profiling and source rock data. The maps and cross sections have been produced on power point programmes. Optimum processing parameters are achievable through increasing magnification.
Executive Summary of Milestone 2
Five hundred samples each were collected from the Ahoko and Kudu Shale within the Patti/Enagi Formation in the Bida Basin and from the Dukamaje and Taloka Formation in the Sokoto Basin for the purpose of Rock Eval pyrolysis analysis.
In the Bida Basin, samples were collected from Ahoko (outcrop) (200), Kudu (shallow well) (150), Makera (shallow) (150). Sixty four (64) samples, representing 12.8% of the total number of samples have TOC less than 0.5wt% and were screened out. Of the samples with TOC>0.5wt%, the following Rock Eval results were obtained (averages): Ahoko Shale: TOC = 1.72wt%, HI = 63mgHC/gTOC, Tmax = 426oC; Kudu Shale at Kudu: TOC = 11.04%, HI = 226mgHC/gTOC, Tmax = 431oC; Kudu Shale at Makera: TOC = 9.66wt% , HI = 219mgHC/gTOC, Tmax = 428oC. Integration of the results shows that source rocks in the Bida Basin are generally sub-mature to marginally mature and overwhelmingly gas and oil & gas-prone at deep-seated levels where appropriate maturities may have been attained. A few are oil-prone.
In the Sokoto Basin, samples were collected from Wurno (outcrop) (200), Gada (shallow well, samples already on the surface, uncontrolled depth) (150), Goronyo (Taloka Formation) (outcrop) (150). One hundred and twenty six (126) samples, representing 25.2% of the total number of samples have TOC less than 0.5wt% and were screened out. Of the samples with TOC>0.5wt%, the following Rock Eval results were obtained (averages): Dukamaje Shale at Wurno: TOC = 1.53wt%, HI = 62mgHC/gTOC, Tmax = 427oC; Dukamaje Shale at Gada: TOC = 3.24%, HI = 90mgHC/gTOC, Tmax = 428oC; Taloka Formation coals at Goronyo: TOC = 33.94wt% , HI = 82mgHC/gTOC, Tmax = 434oC. Integration of the results shows that source rocks in the Sokoto Basin are generally sub-mature to marginally mature and overwhelmingly gas-prone at deep-seated levels where appropriate maturities may have been attained. A few are oil & gas prone.
Modelling of the results one-dimensionally on the BasinMod and RockDoc softwares (using only the Tmax values and assumed geoheat flow of 2oC/100m, and deducting corrections) shows that the current maturity as measured in both basins were attained at depths of approx. 3,000m prior to exhumation at about 53 million years ago (Eocene). Source rocks that have remained at that depth and upwards to 1,100m would have matured into the oil/gas window for 53 million years time lag and continuous sedimentation and deposition of overburden. The Sokoto Basin would have experienced further uplift as it is known to be shallower than the Bida Basin.
The result on the organic matter quality is closely similar to the Niger Delta scenario where the hydrocarbon composition is more than 70% gaseous. Maturity in the Tertiary source rocks of the Niger Delta was attained by subsidence and depth of up to 10,000m whereas maturity in the Bida and Sokoto Basins was attained though prolonged time of entrapment, being of Cretaceous age.