AVO detection of gas-producing dolomite trends in nonproducing limestone

Transcription

1 detection of gas-producing dolomite trends in nonproducing limestone MOHAMED A. EISSA, Tanta University, Egypt JOHN P. CASTAGNA, University of Oklahoma, Norman, U.S. ALAN LEAVER, East Resources The technique has been successful in hydrocarbon exploration for gas-sand reservoirs such as those in the Texas Gulf Coast, the North Sea, and West Africa, but there are limited examples of successful application in carbonate exploration. Commonly in carbonate reservoirs, dolomitized zones have better reservoir quality than limestones. Because dolomite has a lower Poission s ratio than limestone, might be able to detect dolomite trends. In this paper, we use analysis to detect gas-producing dolomite in the nonproducing Black River limestone. The available data are: (1) NMOcorrected CDP gathers for three 2D lines (lines 1, 2, and 3); (2) log data for five wells: A (a gas dolomite CDP 376) and B (a dry limestone CDP 114) on line 1; C (gas dolomite CDP 486) and D (dry limestone CDP 410) on line 2; and E (dry limestone CDP 148) on line 3. The main zone of interest is the high impedance Lower Ordovician Black River carbonate (Figures 1-3) at depths greater than 8000 ft. This reservoir is believed dolomitized by hydrothermal fluids moving through the host limestone along fractures associated with faults. Well-log data show that the dolomitized reservoir porosity averages about 5%. The limestone is very tight with average porosity less than 2%. Figures 4-8 show the well-log characters of the Black River carbonate in the available wells. modeling. Modeling was performed to determine expected behavior for dolomite and limestone. Figures 9-13 show NMO-corrected synthetics for wells A, B, C, D, and E tied to their location on the lines. Shear-wave velocity was calculated using Castagna s mudrock equation in clastic intervals but other equations were used for carbonate intervals. V P /V S for limestone and dolomite were assumed to be 1.9 and 1.7, respectively. Only minor log editing was performed. Although not affecting the normal incidence tie at the target, spikes on the sonic log and washouts on the density log had to be edited in order to achieve Figure 1. Line 1 showing Black River reflector. Figure 2. Line 2 showing Black River reflector. good synthetics. Sonic log spikes cause ray-traced incident P-waves to go critical in the modeling algorithm. There was an acceptable qualitative character tie between the synthetic and real seismic gather. In general, the synthetic responses show the same general trend as the real data although differing in detail. Dolomite and limestone curves for synthetics and real data show that the intercept (P) and the sign of the can be used to detect gas-saturated dolomite (Figure 14). Gas-saturated dolomite has a smaller intercept than tight limestone and exhibits a small positive gradient while tight limestone shows larger negative gradient. Parabolic radon filtering (frequency Figure 3. Line 3 showing Black River reflector. range 5-90 Hz and maximum residual moveout from -10 to +10 ms) was applied to NMO-corrected gathers for noise reduction and light AGC for amplitude balancing was applied using a 1000-ms gate. analysis. Our main goal is to establish a robust intercept (P) versus criteria for gas-saturated dolomite and tight limestone from the well location to locate gas-saturated dolomite intervals. A velocity model derived from stacking velocities was used to calculate local angle of incident to generate gradient stack sections for each line. Then the intercept (P) versus plots were generated at the wells and for the entire line. Figures 462 THE LEADING EDGE MAY 2003

2 Figure 4. Sonic gamma ray, photoelectric, and neutron porosity curves for the Black River Formation in well A. Pe curve (yellow) indicates dolomite lithology. Figure 5. Sonic gamma ray, photoelectric, and neutron porosity curves for the Black River Formation in well B. Pe curve (yellow) indicates limestone lithology. Figure 6. Sonic gamma ray, photoelectric, and neutron porosity curves for the Black River Formation in well C. Pe curve (yellow) indicates dolomite lithology. MAY 2003 THE LEADING EDGE 463

3 Figure 7. Sonic gamma ray, photoelectric, and neutron porosity curves for the Black River Formation in well D. Pe curve (yellow) indicates limestone lithology. Figure 8. Sonic gamma ray, photoelectric, and neutron porosity curves for the Black River Formation in well E. Pe curve (yellow) indicates limestone lithology. Figure 9. NMOcorrected A. The near-offset 464 THE LEADING EDGE MAY 2003

4 Figure 10. NMOcorrected B. The near-offset Figure 11. NMOcorrected C. The near-offset Figure 12. NMOcorrected D. The near-offset MAY 2003 THE LEADING EDGE 465

5 Figure 13. NMOcorrected E. The near-offset Figure 14. for synthetics (red) and real data (blue) in limestone wells (left) and dolomite wells (right) for top of the Black River carbonate. Synthetic and real limestone data show an amplitude decrease with offset; as the gas-filled dolomite show an amplitude increase with offset. Figure 15. plots and intercept (P) sections at well B (dry limestone) (left) and at well A (gas dolomite) (right). 466 THE LEADING EDGE MAY 2003 MAY 2003 THE LEADING EDGE 0000

6 Figure 16. (bottom) for line 1, showing limestone and dolomite trend. Figure 17. plots (top) and intercept (P) sections at well D (dry limestone) (left) and at well C (gas dolomite) (right). Figure 18. (bottom) for line 2, showing limestone and dolomite trend. MAY 2003 THE LEADING EDGE 467

7 Figure 19. (bottom) at well E (dry limestone). Figure 20. (bottom) for line 3, showing limestone and dolomite trend. 15, 17, and 19 show the intercept (P) versus plots and the intercept section at the well locations for lines 1-3. It is obvious that the gas-saturated dolomite has different intercept (P) versus behavior than tight limestone. Gas-saturated dolomite tends toward a lower intercept (P) than tight limestone and tends toward a low positive while tight limestone usually has higher negative gradient (P). Points representing tight limestone are red, and points representing gas-dolomite are yellow. Figures 16, 18, and 20 show the intercept (P) versus plots and for lines 1-3 with zones of dolomite (yellow) and limestone (red). 468 THE LEADING EDGE MAY 2003 Conclusion. modeling and analysis enable discrimination of gas-producing dolomite and tight dry limestone in the Black River carbonate. intercept (P) versus attribute plotting is particularly effective. Gas-saturated dolomite tends toward lower intercept than tight limestone while exhibiting low positive gradient as compared to high negative gradient for tight limestone. Suggested reading. AVA analysis and interpretation of a carbonate reservoir: northwest Java basin, Indonesia by Adriansyah and McMechan (GEOPHYSICS, 2001). Rock physics the link between rock properties and response by Castagna et al. (in Offset Dependent Reflectivity: Theory and Practice of Analysis, SEG, 1993). Recent advances in application of in carbonate reservoirs: Calibration and interpretation by Li et al. (SEG 2002 Expanded Abstracts). Comparison of P-wave techniques for locating zones of fractured dolomite within nonreservoir limestone by Ho et al. (SEG 1992 Expanded Abstracts). and Devonian reef exploration: Difficulties and possibilities by Lu and Lines (TLE, 1995). Direct determination of carbonate reservoir porosity and pressure from inversion by Pigott et al. (SEG 1990 Expanded Abstracts). TLE Acknowledgments: The authors thank with gratitude Fortuna Energy for releasing the data for this publication. Corresponding author: castagna@ou.edu