Petrophysical evaluation and lithology delineation using cross-plots analysis from some onshore wells in the Nigerian-delta, west Africa
Keywords:Cross-Plot, Porosity, Reservoir, Sand, Well.
Reservoir rock attributes such as porosity, permeability, pore-size geometry and net-to-gross ratio can grossly be affected by inaccurate delineation of the sand intervals. It is therefore pertinent that well log cross-plot is utilized to accurately delineate the sand body and correctly evaluate the petrophysical properties of the mapped sandstone intervals. Three wells (A, B and C) were studied from which analysis of various cross-plots were done. The cross-plots of the density and gamma ray, Acoustic impedance and VpVs ratio with various colour indicators such vertical depth, porosity, resistivity, water saturation etc. were generated using the Hampson Russel software. The hydrocarbon interval in the area occurs between 5870ft to 8900ft for well-A, 5500ft to 5910ft for well-B and 5700ft to 7230ft for well-C as interpreted from the well logs with an average porosity range from 26 to 39%. Cross-plot analysis was carried out to validate the sensitivity of the rock attributes to reservoir saturation condition. The cross-plot results clusters shows two major lithologies of sandstone and shale with occasional intercalation of sand and shale units. For the three wells considered, ten reservoirs were observed. Fluid detection analysis shows that reservoirs A1-A2 (well-A), B1-B2 (well-B), C3-C4 (well-C) were found to contain oil, while reservoir A3-A4 (well-A) and C1-C2 (well-C) contains gas. This study has shown that the cross-plots approach can be used to accurately delineate reservoirs for further formation evaluation. It therefore means that an outright estimation of petrophysical properties on wrongly delineated reservoirs can signiï¬cantly affect the porosity, permeability, pore-size geometry and net-to-gross ratio of the reservoir units.
 Adewoye O, Amigun J, O and Afuwai C, G. (2015). Lithostratigraphic interpretation and Seismic Attributes Analysis for reservoir characterization in some parts of Niger Delta. Pet Coal 57(1):76â€“84.
 Anderson P. F and Gray F. D. (2001) Using LMR for dual attribute lithology identiï¬cation. Expanded abstracts. SEG, San Antonio.
 Anyiam, O. A., Mode, A. W. and Okara, E. S. (2017). The use of cross-plots in lithologydelineation and petrophysical evaluation of some wells in the western Coastal Swamp, Niger Delta. J Petrol Explor Prod Technol. ISSN 2190-0566. Pg 1-11. https://doi.org/1.17/s13202-017-0364-9.
 Chatterjee R and Paul S (2012) Application of cross-plotting techniques for delineation of coal and non-coal litho-units from well logs in Jharia Coalï¬eld, India. Geomaterials 2:94â€“104. https://doi.org/10.4236/gm.2012.24014.
 Ekine A S and Ibe A A (2013) Delineation of hydrocarbon bearing reservoirs from surface seismic and well log data (Nembe Creek) in Niger Delta oil ï¬eld. J Appl Phys 4(3):26â€“30. https://doi.org/10.9790/4861-0432630.
 Gray F and Andersen E C (2000) Case histories: inversion for rock properties. European Association of Geoscientists and Engineers (EAGE). In: 62nd Conference and technical exposition.
 Hunze S and Wonik T (2007) Lithological and structural characteristics of the Lake Bosumtwi Impact Crater, Ghana: interpretation of acoustic televierwe images. Meteorit Planet Sci 42(4â€“5):779â€“792. https://doi.org/10.1111/j.1945-5100.2007.tb01074.x.
 Lamont M G, Thompson T A and Bevilacqua C (2008) Drilling success as a result of probabilistic lithology and ï¬‚uid prediction: a case study in the Carnarvon Basin, WA. APPEA 48:1â€“12. https://doi.org/10.1071/AJ07004.
 Ohakwere-Eze M C and Adizua O F (2014) Qualitative Delineation of Hydrocarbon bearing Reservoir from Preliminary study of Well Log Data over an Offshore Niger Delta Fieldâ€. PacificJournal of Science and Technology. 15(2):293297.
 Omudu M L, Ebeniro J O and Olotu S (2007) Optimizing Quantitative Interpretation for Reservoir Characterization: Case Study Onshore Niger Delta: A paper presented at the 31st Annual SPE International Technical Conference and Exhibition in Abuja, Nigeria.