Abstract
This study aims at evaluating the influence of the presence of shale on the quality of reservoir sand in “CAC-Field”, Coastal swamp Niger Delta by integrating suites of well logs and 3D pre-stack seismic data. Shales in the reservoir pose interpretation challenges as they form baffles to fluid flow and reduce effective porosity. The data used included well logs (density, gamma ray, neutron, resistivity) and 3D seismic data. Petrel and Interactive Petrophysics software were adopted for the analyses. The Vclay/effective porosity cross-plots were used to determine the clay distribution patterns hence the influence of shale on the petrophysical properties of the hydrocarbon reservoir. Result of the well correlation yielded 12 reservoirs with 4 (RES 4 - RES 7) being hydrocarbon bearing and laterally continuous across the 4 wells, (CAC-1 - CAC-4) forming the focus of the study. Evidence of an NW-SE trending delta progradation in the CAC field is represented by the increasing sandiness downdip, at both intermediate and the shallow horizons. Thickening of the reservoir in some instances may be structurally controlled due to faulting. The results from the petrophysical evaluation show Vclay ranges of 13% - 21% and good to very good porosity values that vary from 15% - 25%. The permeability range from 240.49 - 2406.49 mD except for the sands in RES 7, CAC-3 well where the permeability was low (91 mD). Additionally, the Vclay/Effective Porosity cross-plots indicate essentially laminated and structural clay types with few dispersed clay in RES 7, CAC-3 well. The existence of these 3 clay types did not significantly influence the quality of the sands containing the hydrocarbon in the area, except in RES 7, CAC-3. The compartmentalizing effect of the laminated clay/shale could only possibly affect the vertical flow due to possible baffles to the vertical flow, but the horizontal flow may not have been impeded significantly. The study of the type and pattern of clay has helped to better evaluate the quality and mobility trend of the hydrocarbon in the CAC field.
Highlights
Petrophysical study deals with the physical and chemical properties of rock and their interaction with fluids [1]
This study aims at evaluating the influence of the presence of shale on the quality of reservoir sand in “CAC-Field”, Coastal swamp Niger Delta by integrating suites of well logs and 3D pre-stack seismic data
A clean sandstone with a good porosity of value of 30% can have the porosity reduced to 0% if the pore spaces of the sandstone are clogged by dispersed clay, but if the clay/shale is a lamina only within the clean sandstone, about two-thirds of the permeability could be retained
Summary
Petrophysical study deals with the physical and chemical properties of rock and their interaction with fluids [1] Some of these properties include lithology, porosity, net-to-gross thickness, water saturation, permeability and shale volume content. Reference [3] described three common distribution patterns of clay heterogeneity observed in reservoirs, namely; structural, laminated and dispersed clays each exhibiting different effects on reservoir properties. Reference [6] identified growth fault and rollover anticline as the major petroleum traps in the Niger Delta He described them as arising from compressional uplift very close to the slope and having extensional growth faults which were developed on the modern shelf and slope
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