Ain Shams University Faculty of science Geophysics Department

Reservoir identification

Supervision PROF.DR.ADEELNASER HELAL

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Out lines Percent velocity difference between sands

saturated with different fluids Flat spot Bright spot Dim spot

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Fig. percent velocity difference between sands saturated with different fluids (derived from Gardner, and Gregory, 1974).

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Flat spot The flat spot is easily identified by its flatness, and because it is unconformable

with adjacent reflections. Hence it is a good indicator of the hydrocarbon/ water

contact. Nevertheless there is no high amplitude associated with it.

Fig. dual polarity section showing a flat spot at 1.47 seconds. (Courtesy geophysical service Inc.)

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Fig. shows a fine flat spot associated Early observation of this flat spot on 2-D data (Birtles, 1986).

Bright spot Bright spots were not commonly identified until the early 1970s because of the

extensive and industry-wide use of automatic gain control, which obscured the

amplitude effects of hydrocarbon accumulations

A bright spot primarily results from the increase in acoustic impedance contrast

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Fig .dual polarity section showing bright spot s at 1.62 & 1.72 seconds. (Courtesy geophysical service Inc.)

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Fig. Same section showing the interpreted position of the gas reservoir and demonstrating a phase change between the reflections from the gas sand and the water sand. (Courtesy Geophysical Service Inc.)

• The reflection from the top of the reservoir (Figure 3) changes from a peak to a trough across the fluid contact and this again implies a significant change in acoustic properties between the gas sand above the hydrocarbon/water contact and the water sand beneath it

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fig. Schematic diagram of the zero-phase response of hydrocarbon reservoirs for different acoustic contrasts between the reservoir and the embedding medium. Note these diagrams are drawn for European polarity

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Fig. show The magnitudeofacousticimpedancechanges betweenwater-filledandhydrocarbonfilledsandsand theresulting observableindications.

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fig .illustrates other Gulf of Mexico bright spots and flat spots. These data are also zero phase, but the polarity is American. Hence, in Figure 5-9 flat spots are black and reflections from the top of gas reservoirs are red. Note particularly the prominent reservoir reflections between times of 1.5 and 1.6 on Line 49 and on Line 51 and on horizontal section 1.520 in the lower right against a salt dome

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Gas chimney A subsurface leakage of gas from a poorly sealed hydrocarbon accumulation. The gas can

cause overlying rocks to have a low velocity. Gas chimneys are visible in seismic data as areas of poor data quality or push-downs. A chimney cube is a 3D volume of seismic data, which highlights vertical chaotic behavior of seismic characters

The figure shows shallow fluid-flow systems in this area. Gas chimney ( chaotic area with no coherent seismic signals within red polygon in the seismic section ) indicate the fluid flow path (migration) $ the cross ponding bright spot s above indicate the gas accumulation (Vadakkepuliyambatta et al., 2013 ).