This document discusses different seismic indicators of hydrocarbon reservoirs including: 1) Flat spots which indicate hydrocarbon/water contacts as they are unconformable reflections. 2) Bright spots which result from increased acoustic impedance contrast between hydrocarbon-filled and water-filled reservoirs. They were difficult to identify until use of automatic gain control decreased. 3) Gas chimneys which appear as areas of poor data quality or push-downs, indicating leakage from hydrocarbon accumulations upwards.

1. Ain Shams University
Faculty of science
Geophysics Department
Reservoir identification
Supervision
PROF.DR.ADEELNASER HELAL
1

2. Out lines
 Percent velocity difference between sands
saturated with different fluids
 Flat spot
 Bright spot
 Dim spot
2

3. 3
Fig. percent velocity difference between sands saturated with
different fluids (derived from Gardner, and Gregory, 1974).

4. 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.
4
Fig. dual polarity section showing a flat spot at 1.47 seconds. (Courtesy
geophysical service Inc.)

5. 5
Fig. shows a fine flat spot associated Early observation of this flat
spot on 2-D data (Birtles, 1986).

6. 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
6
Fig .dual polarity section showing bright spot s at 1.62
& 1.72 seconds. (Courtesy geophysical service Inc.)

7. 7
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

8. 8
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

9. 9
Fig. show The magnitudeofacousticimpedancechanges
betweenwater-filledandhydrocarbonfilledsandsand
theresulting observableindications.

10. 10
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

11. 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
11
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 ).