This document discusses the challenges of identifying gas reservoirs using traditional bright spot analysis of seismic data. Specifically, it notes that bright spots can be caused by non-reservoir formations, brine zones, or low gas saturation zones rather than high gas saturation gas reservoirs. It proposes that elastic property analysis through pre-stack seismic inversion is a better solution, as high gas saturation reservoirs consistently exhibit low elastic properties like density and Poisson's ratio. The document presents multiple examples from the northern South China Sea where bright spots were found to not indicate gas, and elastic analysis correctly identified the reservoir properties. It concludes elastic prediction is an effective way to identify non-gas bright spots and non-bright spot gas reservoirs.

1. The solution of non-gas bright-spot and non-bright-spot gas identification: Elastic prediction
Jianli Yang*, Xinwei Chang, Elsiwave Reservoir, Zhe Yang, The Ohio State University
Summary
Bright spot (including stacked seismic brigh-spot and
traditional AVO anomaly) does not mean high gas
saturation hydrocarbon (maybe non reservoir, or brine, or
low gas saturation hydrocarbon), and high gas saturation
hydrocarbon maybe not exhibit bright-spot (maybe non-
bright-spot) behavior especially in deep water area or deep
gas situation. But high gas saturation hydrocarbon must
exhibit low elastic properties (Poisson’s ratio, or density,
etc.) behavior in the northern South China Sea and Gulf of
Mexico. Quantitative elastic prediction is the effective
solution of non-gas bright-spot and non-bright-spot gas
identification problem in those areas.
Introduction
In past decades, bright-spot of stacked seismic amplitude
has been used to predict gas (high gas saturation
hydrocarbon) in the northern South China Sea and Gulf of
Mexico. It is commonly accepted that the identification of
commercial gas based on bright-spot technique will take
great risk due to the presence of non-gas bright-spot caused
by low gas saturation hydrocarbon since 1980s.
In recent years, multiple exploratory wells failed in
exploiting gas based on bright–spot technique in deep
water area in the northern South China Sea, since those
bright-spots were caused by non-reservoir, or brine, or low
gas saturation hydrocarbon. Few exploratory wells also
encountered gas zone which didn’t exhibit bright-spot
behavior in the northern South China Sea.
Allen and Peddy (1993) noted that many seismic amplitude
anomalies are not caused by economic hydrocarbon (gas)
accumulation and discovered close relationship between
Poisson’s ratio and Sw (water saturation). It is commonly
accepted that density may reveal crucial information about
fluid saturation. Roderick W. Van Koughnet et al. (2001,
2003) discovered that low density is the main behavior of
gas-sand with high gas saturation in deep water Gulf of
Mexico.
In this paper, we present multiple examples of non-gas
bright-spot and non-bright-spot gas in the northern South
China Sea, and then analyze the elastic property of non-gas
bright-spot and non-bright-spot gas and give the solution
of identification and prediction. Finally, one case study
will be given to prove that elastic prediction is the effective
solution of non-gas bright-spot identification problem in
the northern South China Sea.
Non-gas bright-spot and non-bright-spot gas
Non-gas bright-spot means that those bright-spots are
caused by non-reservoir or brine or low gas saturation
hydrocarbon rather than commercial (high gas saturation)
gas. Non-bright-spot gas means high gas saturation
hydrocarbon which does not exhibit bright-spot behavior.
In this paper, bright-spot means high amplitude reflections
on stacked seismic, P-wave reflectivity (or P-wave
impedance reflectivity) and traditional AVO anomaly
(including Class I, II, III and IV AVO anomaly).
It is not true to be high gas saturation hydrocarbon for
bright-spot
Bright-spot can be caused by non-reservoir
Example 1: In the northern South China Sea, there are two
high amplitude reflections (bight-spot) at target zone in S
Formation in Block C in deep water area, which were
considered as commercial gas before drilling. But those
bright-spots were proven to be seismic response of two
shale zones with low density and low velocity in thick
shale (non-reservoir). Figure 1 shows CRP gather and
Class IV AVO, stacked seismic, synthetic seismic and
geologic column of well C1.
In deep water B Sag in the northern South China Sea, It is
the main reason not to encounter good sand for multiple
exploratory wells failing in exploiting gas (Xiong Pang,
2014).
Statistics show that 37% of total unsuccessful DHI (bright
spot technique) is from non-reservoir (Paul Weimer and
Roger M. Slatt, 2007).
Bright-spot can be caused by brine
Example 2: Bright-spot can be caused by brine in deep
water area in the northern South China Sea. There are two
high amplitude reflections (bright-spot) with Class IV
AVO anomaly at target zone in Y Formation in deep water
A Sag, which were considered as commercial gas before
drilling. But those two bright-spots were proven to be
seismic response of two brine zones. Figure 2 shows
stacked seismic, synthetic seismic and geologic column of
well C2.
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2. The solution of non-gas bright-spot and non-bright-spot gas identification：Elastic prediction
In Block P in the northern South China Sea, two fifths of
total exploratory wells (Five wells) encountered brine
zones with bright-spot behavior, which exhibits both high
stacked seismic amplitude and Class III AVO anomaly
(Weiwei Zhang, 2012).
In the deep water Gulf of Mexico, up to 62% of brine sand
is corresponding to Class III AVO response, and Class IV
AVO response of brine sand is much more than that of gas
sand (Roderick W. Van Koughnet, 2001).
Statistics show that 35% of total unsuccessful DHI (bright
spot technique) is from brine sand (Paul Weimer and
Roger M. Slatt, 2007).
Bright-spot can be caused by low gas saturation
hydrocarbon
Example 3: Bright-spot can be caused by low gas
saturation hydrocarbon in deep water area in the northern
South China Sea. There are two high amplitude bright-
spots at target zones in L Formation in A Sag, which were
considered as commercial gas before drilling. But those
two bright-spots were proven to be seismic response of two
low gas saturation gas-sand zones. Figure 3 shows stacked
seismic, synthetic seismic and geologic column of well C1.
Statistics show that 22% of total unsuccessful DHI (bright-
spot technique) is from low gas saturation hydrocarbon
(Paul Weimer and Roger M. Slatt, 2007).
It is not true that high gas saturation hydrocarbon
must exhibit bright spot behavior
High gas saturation hydrocarbon may not exhibit bright-
spot behavior. In deep water B Sag in the northern South
China Sea, there are a couple of low amplitude reflections
(non-bright-spot) at target zone in Z Formation (Neogene),
which were considered as non-commercial gas before
drilling. Since people used to think that high gas saturation
hydrocarbon must exhibit bright-spot behavior. Actually
those non-bright-spots were proven to be commercial gas
after drilling (Weiwei Zhang, 2015).
In offshore Mexico and the North Sea, multiple non-bright-
spot gas zones had been discovered (Alistair R. Brown and
William L. Abriel, 2014).
It is really true to exhibit low elastic properties
(Poisson’s ratio, or density, etc.) behavior for high gas
saturation hydrocarbon
Brine/gas sand exhibits high/low elastic properties
behavior
In Example 2, bright-spot is caused by brine sand. First, it
is important fundamental work to conduct elastic property
analysis of different fluids (brine, low gas saturation
hydrocarbon).
Figure 1: Bright-spot is caused by non-reservoir. Bright-spot on
both stacked seismic (left) and synthetic seismic (middle) is
caused by two low density and low velocity shale zones in thick
shale. Density of shale (including shaly sand) is more than
2.300g/cc. AI does not work to differentiate sand from shale.
Figure 2: Bright-spot is caused by brine sand. Two bright-spots
(with Class IV AVO) on both stacked seismic and synthetic
seismic are corresponding to brine zones. Poisson’s ratio is more
than 0.34.
Figure 3: Bright-spot is caused by low gas saturation hydrocarbon.
Two bright-spots on both stacked seismic and synthetic seismic
are corresponding to low gas saturation hydrocarbon.
Compared to other elastic properties, Poisson’s ratio is
more sensitive to fluids. Poisson’s ratio is more than 0.35
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3. The solution of non-gas bright-spot and non-bright-spot gas identification：Elastic prediction
for brine sand (Figure 2) and less than 0.28 for low gas
saturation gas-sand (noted not gas-sand) (Figure 3).
Obviously, brine sand exhibits high elastic properties
(Poisson’s ratio) behavior in A Sag in the northern South
China Sea.
In deep water B sag in the northern South China Sea, rock
physics analysis based on 14 exploratory wells shows that
low density and low Poisson’s ratio are the main elastic
behavior for high gas saturation hydrocarbon (Chengzhi
Yang, 2010).
In deep water Gulf of Mexico, Statistics (19 wells) show
that low density is the main elastic behavior for high gas
saturation hydrocarbon (Roderick W. Van Koughnet,
2001).
The higher gas saturation is, the lower elastic property is
In Example 3, there are two brine sand zones. Water
saturation is 75% for the upper sand zone with Poisson’s
ratio of 0.24 and 81% for the lower sand zone with
Poisson’s ratio of 0.28 (Figure 3). The higher gas
saturation is, the lower Poisson’s ratio is. Poisson’s ratio of
gas-sand should be lower than that of low gas saturation
hydrocarbon.
Figure 4 show the close relationship between water
saturation and Poisson’s ratio based on one exploratory
well in deep water area in the northern South China Sea;
note that the porosity of all samples in left figure is the
same (about 20%). Poisson’s ratio of gas-sand is less than
that of low gas saturation hydrocarbon, whose Poisson’s
ratio is less than that of brine. Obviously, the higher gas
saturation is, the lower elastic property (Poisson’s ratio) is.
Density from AVO fluid substitution increases linearly
with gas saturation increasing in the northern South China
Sea (Figure 5).
Actually, as early as in 1993, Allen and Peddy discovered
an important close relationship between Poisson’s ratio and
gas saturation. Poisson’s ratio decreases substantially when
gas saturation reaches only a few percent, then decreases
linearly with gas saturation increasing (1993, Allen and
Peddy).
The solution of non-gas bright-spot and non-bright-
spot gas identification: Elastic prediction
How to identify non-gas bright-spot and non-bright-spot
gas? Bright-spot technique using stacked seismic or
traditional AVO analysis does not work. The effective
solution is elastic prediction. Elastic Prediction is the key
to open the door of non-gas bright-spot and non-bright-
spot gas identification problem.
Figure 4: In the northern South China Sea, there is close
relationship between Poisson’s ratio and water saturation. High
gas saturation hydrocarbon exhibits low Poisson’s ratio behavior,
and low gas saturation exhibits high Posson’s ratio behavior (left).
All samples in left figure are from those fluids with the same
porosity (three red circles)(right). Noted that AI does not work to
discriminate sand/gas from shale/low gas saturation hydrocarbon.
Figure 5: Density from well C1 increases linearly with gas
saturation increasing in the northern South China Sea.
Elastic prediction means the prediction of reservoir and
hydrocarbon using elastic properties based on pre-stack
seismic inversion without well, which is quantitative
prediction method of reservoir and hydrocarbon. In this
paper, pre-stack seismic inversion technique used is full
gather based elastic direct inversion without well.
The strategy of high gas saturation hydrocarbon prediction
is: reservoir prediction first, high gas saturation
hydrocarbon prediction next.
In Example 1, bright-spot is caused by non-reservoir
(shale). Density is the most sensitive to lithology. Density
is more than 2.39g/cc for shale and less than 2.25g/cc for
sand (Figure 6). Therefore quantitative pre-stack density
inversion for reservoir prediction is the solution of non-gas
bright-spot (caused by non-reservoir) identification
problem.
In Example 2 and Example 3, bright-spot is caused by
brine or low gas saturation hydrocarbon. Based on rock
physics analysis of well C1 and well C2, Poisson’s ratio is
more than 0.35 for brine and less than 0.28 for low gas
saturation hydrocarbon (between 0.22 and 0.28).
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4. The solution of non-gas bright-spot and non-bright-spot gas identification：Elastic prediction
Furthermore, the higher gas saturation is, the Poisson’s
ratio is. Poisson’s ratio of gas-sand should be lower than
about 0.2. Therefore, quantitative pre-stack Poisson’s ratio
inversion for high gas saturation hydrocarbon prediction is
the solution of non-gas bright-spot (caused by brine or low
gas saturation hydrocarbon) identification problem.
Figure 6: In A Sag in the northern South China Sea, density is
more sensitive to lithology than other elastic properties. Density is
more than 2.39g/cc for shale and less than 2.25g/cc for sand.
Quantitative pre-stack density inversion for reservoir prediction is
the solution of non-gas bright-spot (caused by non-reservoir)
identification problem.
Case study
In Block E in the northern South China Sea, there are three
high amplitude bright-spots at target zone (Figure 5a),
which were considered as commercial gas before drilling.
Figure 5b shows quantitative Poisson’s ratio inversion
section, in which bright-spot A and C are corresponding to
high Poisson’s ratio (between 0.22 and 0.26) zones, which
indicate non-gas sand based on rock physics analysis
(Figure 4). Bright-spot B is corresponding to low Poisson’s
ratio (<0.21) zone, which indicates gas-sand with high gas
saturation. Well E2 encountered brine sand at bright-spot
A and gas-sand at bright-spot B. Bright-spot C is below
gas-water contact.
Conclusions
It is not true to be high gas saturation hydrocarbon for
bright-spot; It is not true that high gas saturation
hydrocarbon must exhibit bright-spot behavior; It is really
true to exhibit distinct low elastic properties (Poisson’s
ratio, or density, etc.) behavior for high gas saturation
hydrocarbon. Bright-spot technique based on qualitative
stacked seismic amplitude and traditional AVO analysis is
ambiguous, complicated and dangerous for hydrocarbon
detection. Quantitative elastic prediction based on pre-
stack seismic inversion is definite (less uncertainty), and it
is the effective solution of non-gas bright-spot and non-
bright-spot gas identification problem in the northern
South China Sea and Gulf of Mexico.
It is also feasible to predict high gas saturation
hydrocarbon using quantitative elastic prediction method
in other area. But the elastic behavior (how low) of high
gas saturation hydrocarbon needs to be re-confirmed in a
new area.
Figure 7a: In Block E in the northern South China Sea, there are
three high amplitude bright-spots at target zone on stacked seismic.
All three bright-spots indicate high gas saturation hydrocarbon?
Figure 7b: In Block E in the northern South China Sea, of three
bright-spots on Figure 5a, one is high gas saturation hydrocarbon
with low Poisson’s ratio (<0.21); Other two bright-spots are brine
sand with high Poisson’s ratio (>0.22). Elastic prediction is the
solution of non-gas bright-spot (caused by brine) identification
problem.
ACKNOWLEDGEMENTS
The authors would like to thank Mr. Jun Li and Ms. Weijia
Fan for their help,.
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5. EDITED REFERENCES
Note: This reference list is a copyedited version of the reference list submitted by the author. Reference lists for the 2015
SEG Technical Program Expanded Abstracts have been copyedited so that references provided with the online metadata for
each paper will achieve a high degree of linking to cited sources that appear on the Web.
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