avo detection of gas-producing dolomite trends in nonproducing limestone.pdf

7/27/2019 AVO detection of gas-producing dolomite trends in nonproducing limestone.pdf

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The AVO technique has been suc-cessful in hydrocarbon exploration forgas-sand reservoirs such as those in theTexas Gulf Coast, the North Sea, andWest Africa, but there are limited exam-ples of successful AVO application incarbonate exploration.

Commonly in carbonate reservoirs,dolomitized zones have better reser-voir quality than limestones. Becausedolomite has a lower Poissions ratiothan limestone, AVO might be able todetect dolomite trends. In this paper,we use AVO analysis to detect gas-pro-ducing dolomite in the nonproducingBlack River limestone.

The available data are: (1) NMO-corrected CDP gathers for three 2Dlines (lines 1, 2, and 3); (2) log data forfive wells: A (a gas dolomite well @CDP 376) and B (a dry limestone well@ CDP 114) on line 1; C (gas dolomitewell @ CDP 486) and D (dry limestonewell @ CDP 410) on line 2; and E (drylimestone well @ CDP 148) on line 3.

The main zone of interest is the highimpedance Lower Ordovician BlackRiver carbonate (Figures 1-3) at depthsgreater than 8000 ft. This reservoir is

believed dolomitized by hydrothermalfluids moving through the host lime-stone along fractures associated withfaults. Well-log data show that thedolomitized reservoir porosity aver-ages about 5%. The limestone is verytight with average porosity less than2%. Figures 4-8 show the well-log char-acters of the Black River carbonate inthe available wells.

AVO modeling. Modeling was per-formed to determine expected AVO

behavior for dolomite and limestone.Figures 9-13 show NMO-corrected

AVO synthetics for wells A, B, C, D, andE tied to their location on the lines.Shear-wave velocity was calculatedusing Castagnas mudrock equation inclastic intervals but other equationswere used for carbonate intervals.VP/VS for limestone and dolomite wereassumed to be 1.9 and 1.7, respectively.Only minor log editing was performed.Although not affecting the normal inci-dence tie at the target, spikes on thesonic log and washouts on the densitylog had to be edited in order to achieve

good AVO synthetics. Sonic log spikescause ray-traced incident P-waves to gocritical in the modeling algorithm.There was an acceptable qualitativecharacter tie between the synthetic andreal seismic gather. In general, the syn-

thetic AVO responses show the samegeneral trend as the real data althoughdiffering in detail. Dolomite and lime-stone AVO curves for synthetics andreal data show that the intercept (P)and the sign of the gradient (G) can beused to detect gas-saturated dolomite(Figure 14). Gas-saturated dolomite hasa smaller intercept than tight limestoneand exhibits a small positive gradientwhile tight limestone shows larger neg-ative gradient.

Parabolic radon filtering (frequency

range 5-90 Hz and maximum residualmoveout from -10 to +10 ms) wasapplied to NMO-corrected gathers fornoise reduction and light AGC foramplitude balancing was applied usinga 1000-ms gate.

AVO analysis. Our main goal is toestablish a robust intercept (P) versusgradient (G) criteria for gas-saturateddolomite and tight limestone from thewell location to locate gas-saturateddolomite intervals. A velocity modelderived from stacking velocities wasused to calculate local angle of incidentto generate gradient stack sections foreach line. Then the intercept (P) versusgradient (G) plots were generated atthe wells and for the entire line. Figures

462 T HE LEADING EDGE MAY 2003

AVO detection of gas-producing dolomite trends in nonproducinglimestone

MOHAMEDA. EISSA, Tanta University, EgyptJOHNP. CASTAGNA, University of Oklahoma, Norman, U.S.

ALANLEAVER, East Resources

Figure 1. Line 1 showing Black River reflector.

Figure 2. Line 2 showing Black River reflector. Figure 3. Line 3 showing Black River reflector.


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MAY 2003 T HE LEADING EDGE 46

Figure 5. Sonicvelocity, density,

gamma ray, photo-electric, and neu-tron porositycurves for the BlackRiver Formation inwell B. Pe curve(yellow) indicateslimestone lithology.


gamma ray, photo-electric, and neu-tron porositycurves for the BlackRiver Formation inwell C. Pe curve(yellow) indicates

dolomite lithology.


gamma ray, photo-electric, and neu-tron porositycurves for theBlack RiverFormation in well

A. Pe curve (yel-low) indicatesdolomite lithology.


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gamma ray, photo-electric, and neu-tron porositycurves for theBlack RiverFormation in wellD. Pe curve (yel-low) indicateslimestone lithol-

ogy.


gamma ray, photo-electric, and neu-tron porosity

curves for theBlack RiverFormation in wellE. Pe curve (yel-low) indicateslimestone lithology.

Figure 9. NMO-corrected AVOsynthetic for well

A. The near-offsettie is better thanthe AVO tie.


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Figure 10. NMO-corrected AVOsynthetic for wellB. The near-offsettie is better thanthe AVO tie.

Figure 11. NMO-

corrected AVOsynthetic for wellC. The near-offsettie is better thanthe AVO tie.

Figure 12. NMO-corrected AVOsynthetic for wellD. The near-offsettie is better thanthe AVO tie.


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466 T HE LEADING EDGE MAY 2003 MAY 2003 T HE LEADING EDGE 00

Figure 13. NMO-corrected AVOsynthetic for wellE. The near-offsettie is better thanthe AVO tie.

Figure 14.AVOfor synthetics (red)and real data (blue)in limestone wells(left) and dolomitewells (right) for topof the Black Rivercarbonate. Synth-etic and real lime-stone data show anamplitude decreasewith offset; as the

gas-filled dolomiteshow an amplitudeincrease with offset.

Figure 15.Intercept (P) ver-sus gradient (G)

plots and intercept(P) sections at wellB (dry limestone)(left) and at well A(gas dolomite)(right).


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plot (top) andintercept (P) sec-tion (bottom) forline 1, showinglimestone anddolomite trend.

Figure 17.Intercept (P) versus

gradient (G) plots(top) and intercept(P) sections at wellD (dry limestone)(left) and at well C(gas dolomite)(right).




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plot (top) andintercept (P) sec-tion (bottom) atwell E (dry lime-stone).



15, 17, and 19 show the intercept (P)versus gradient (G) plots and the inter-cept section at the well locations forlines 1-3. It is obvious that the gas-sat-urated dolomite has different intercept(P) versus gradient (G) behavior than

tight limestone. Gas-saturated dolomitetends toward a lower intercept (P) thantight limestone and tends toward a lowpositive gradient (G) while tight lime-stone usually has higher negative gra-dient (P). Points representing tightlimestone are red, and points repre-senting gas-dolomite are yellow.Figures 16, 18, and 20 show the inter-cept (P) versus gradient (G) plots andintercept (P) section for lines 1-3 withzones of dolomite (yellow) and lime-stone (red).

Conclusion. AVO modeling and analy-sis enable discrimination of gas-pro-ducing dolomite and tight drylimestone in the Black River carbonate.AVO intercept (P) versus gradient (G)attribute plotting is particularly effec-

tive. Gas-saturated dolomite tendstoward lower intercept than tight lime-stone while exhibiting low positive gra-dient as compared to high negativegradient for tight limestone.

Suggested reading. AVA analysis andinterpretation of a carbonate reservoir:northwest Java basin, Indonesia byAdriansyah and McMechan (GEOPHYSICS,2001). Rock physicsthe link betweenrock properties and AVO response byCastagna et al. (in Offset DependentReflectivity: Theory and Practice of AVO

Analysis, SEG, 1993). Recent advancesin application of AVO in carbonate reser-voirs: Calibration and interpretation byLi et al. (SEG 2002 Expanded Abstracts).Comparison of P-wave AVO techniquesfor locating zones of fractured dolomite

within nonreservoir limestone by Ho etal. (SEG 1992 Expanded Abstracts). AVOand Devonian reef exploration: Difficult-ies and possibilities by Lu and Lines(TLE, 1995). Direct determination of car-

bonate reservoir porosity and pressurefrom AVO inversion by Pigott et al. (SEG1990 Expanded Abstracts). TLE

Acknowledgments: The authors thank with grat-itude Fortuna Energy for releasing the data forthis publication.

Corresponding author: [email protected]

avo detection of gas-producing dolomite trends in nonproducing limestone.pdf

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