ABSTRACT:The study was carried out to investigate the dissimilar seismic amplitude responses observed in sandstone reservoirs with the same fluid saturation. This challenge now informed the analysis of different amplitude responses from the 'Jay' Field in order to verify the reservoirs fluids around and away from well location based on the integration of Amplitude Variation with Angle (AVA) and seismic inversions. The well log data provided were used to identify hydrocarbon bearing zones and Poisson Ratio analysis. Anomalies from the AVA analysis were investigated using the elastic impedance inversion of the near and far volumes. Crossplots of Lambda-Mu-Rho inversion were produced to relate reservoir incompressibility and rigidity modulus for lithology and fluid determination. Sand E exhibited amplitude increase with angles at well point and also away from well location (red triangle). The approximate elastic impedances of the inverted seismics are almost similar to the log estimates, an indication of good correlation. Integration of incompressibility (Lambda-Rho) and rigidity (Mu-Rho) modulus analyses reveals the presence of gas saturation in reservoir Sand G because low Lambda-Rho coincides with high Mu-Rho from both log estimates and inverted seismics. In the case of Sand D, high Lambda-Rho coincides with high Mu-Rho because it is oil saturated. This study has helped to differentiate the fluids in Sands D and G despite having similar AVA responses. The methods adopted in this work can be useful in hydrocarbon detection from seismic data in fields with similar geological setting. Seismic inversion technique helps in the determination of reservoir properties away from well location (Simm, 2005). For instance, inverted seismic data enhances the identification of subtle properties in conventional seismics (Contreras et al., 2006). Reservoir properties between wells can be determined from inverted seismic results. This was demonstrated by Adekanle and Enikanselu, (2013) where the vertical and lateral extent of rock properties such are porosity, lithology and density were determined. However, seismic data have limited frequency content such that thin layers may not be resolved (Simm and Bacon, 2014).The integration of high frequency log information with seismic data in model based inversion was adopted to mitigate this effect. Moreover, model based inversion technique of iterative forward modeling and comparison procedure does not suffer from defective wavelet and over simplification of the subsurface associated with recursive and sparse spike inversions methods respectively (Cooke and Schneider, 1983;Veeken and Da Silva, 2004;Simm and Bacon. 2014).The integrated approach of Lambda-Mu-Rho inversion and Rock Physics modeling by Ekwe et al., (2012) was used to delineate hydrocarbon charged reservoirs in a Niger Delta field. In gas hydrates region where well information is most often sparse, Amplitude Variation with Angle (AVA) was the method used for free hydrate gas assessment as discussed by Javaherian et al., (201...
Seismic interpretation and petrophysical assessment of borehole logs from seven wells were integrated with the aim of establishing the hydrocarbon reserves prior to field development which will involve huge monetary obligation. Four hydrocarbonbearing sands, namely Pennay 1, 2, 3 and 4 were delineated from borehole log data. Four horizons corresponding to near top of mapped hydrocarbon-bearing sands were used to produce time maps and then depth structural maps using checkshot data. Three major structure-building faults (F2, F3 and F5 which are normal, listric concave in nature) and two antithetic (F1 and F4) were identified. Structural closures identified as rollover anticlines and displayed on the time/depth structure maps suggest probable hydrocarbon accumulation at the upthrown side of the fault F4. Petrophysical analysis of the mapped reservoirs showed that the reservoirs are of good quality and are characterized with hydrocarbon saturation ranging from 56 to 72%, volume of shale between 7 and 20% and porosity between 25 and 31%. Pennay 2 and 3 have a better relative petrophysical ranking compared to other mapped reservoirs in the study area. Dissimilarity in the petrophysical parameters and the uncertainty in the reservoir properties of the four reservoirs were considered in calculating range of values of gross rock volume (GRV) and oil in place volume. This research study revealed that the discovered hydrocarbon reserve resource accumulations in the Pennay field for the four-mapped reservoir sand bodies have a total proven (1P) reserve resource estimate of 53.005MMBO at P90, 59.013MMBO at 2P/P50 and 65.898MMBO at 3P/P10. Reservoir C, the only interval with a gas cap, has a volume of 7737MMscf of free gas at 1P, 8893.2MMscf at 2P and 10185.2MMscf at 3P. These oil and gas volumetric values yield at 1P/ P90 total of 137.30MMBOE, 154.9MMBOE at 2P and 171.515MMBOE at 3P. Reservoirs B and D have the highest recoverable oil at 1P, 2P, and 3P values of 5.265MMBO and 10.70MMBO, 12.053MMBO and 5.783MMBO, 13.557MMBO and 6.244MMBO, respectively.
A total of thirty-two (32) vertical electrical sounding (VES) data were acquired using R 50 d.c. Resistivity meter within the Precambrian basement geology of part of Akure metropolis southwestern Nigeria. The VES data were interpreted; eight type curves were identified, from the geoelectric section the subsurface was characterized into four lithologies namely topsoil, weathered layer (clay /lateritic clay), weathered / fractured basement and bedrock. Thus, two major aquifers were mapped these are weathered/partially weathered layer and weathered basement/fractured basement aquifers. These aquifers are characterized by thick overburden, found within basement depressions and exhibit moderate to relatively high values of coefficient of anisotropy, λ, (0.97-1.11) with depth. Also, ancient river channel trending approximately NE-SW was mapped. The assessment of the materials above the aquifers showed that longitudinal conductance (S), values ranged from 0.0035 to 0.17 mhos; thus the S values are generally low suggesting that the materials above the aquifers are loose and porous thus having less capacity to protect aquifers in the study area. Thus the aquifers are poorly protected, and by implication vulnerable to infiltration.
Groundwater potential assessment using the vertical electrical sounding (VES) technique was carried out at at Ewuloja and its environs in Simawa Southwestern, Nigeria with a view to solving the problem of incessant failure of groundwater boreholes in the study area. A total of 40 VES were carried-out using Schlumberger electrode configuration. The data was interpreted quantitatively using the partial curve matching and computer iteration techniques to generate the first order geoelectric parameters. The borehole lithological log was used to constrain the conventional VES interpretation. The VES results aided by the borehole lithological log reveal four to seven distinct subsurface geologic layers which correspond to Top soil, Lateritic clayey sand, Sand, Sandstone and Consolidated sandstone. The analysis shows that sands at the upper layer within the depth of 2.3 -76.2 m are unsaturated while sands at depth not less than 100 m in some sampling points are saturated and denote the aquifer units in the study area. Boreholes/deep wells could be cited at these points at depth interval 150 -210 m. This study has provided an insight to the subsurface condition of the aquifer systems and delineated areas for probable groundwater development in Simawa community.
This study focuses on the use of some geostatistical tools to spatially distribute reservoir parameters in order to identify the bypassed prospects from the earlier seismic interpretation that was carried out in the field using 3D seismic data. Four wells and seismic data were used to generate the interpreted input horizon grids, fault polygons and to carry out detailed petrophysical analysis. Structural and property modeling which include; facies, net to gross, porosity and water saturation were distributed stochastically within the constructed 3D grid using Sequential Gaussian Simulation (SGS) algorithm. The reservoir structural model show system of different oriented growth faults F1 to F9. Faults F1, F2, F3 and F4 were the major growth faults, dipping towards south-west and are quite extensive almost across all the seismic section. A rollover anticline formed as a result of deformation of the sediments deposited on the downthrown block of fault F1. The other faults were minor fault (synthetic and antithetic). The trapping mechanism is a fault assisted anticlinal closure. Results from well log analysis and petrophysical models shows Godwin reservoir to be a moderate to good reservoir in terms of facies, with good net to gross, porosity, permeability and low water saturation. This study has also demonstrated the effectiveness of 3D geostatical modeling technique as a tool for better understanding the distribution with respect to space of continuous reservoir properties. It will also provide a framework for the future prediction of reservoir qualities and yield rate of the reservoirs.
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