SEG Hydrates FRL

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714 The Leading Edge June 2009

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Seismic evidence and geological distinctiveness related to gas

hydrates in Mexico

R esearch for determining the existence of natural gas

hydrates in sediments can range from exploring the sea-floor to detect structures associated with hydrate formation tospecial seismic processing and interpretation.

When only seismic data are available, reflection anomaliesand direct hydrocarbon indicators can help in the evaluationof this resource.

Gas hydrates are a manifestation of natural gas that typi-cally occurs under low temperature and high pressure, eitheron land with permanently frozen soils or underwater at depthsgreater than 500 m in continental shelves.

Gas hydrates are characterized by a kind of chemicalcompound called clathrates, which consist of a network of unstable molecules, characterized by open cavities. Te guestmolecule is trapped by the clathrate, and together they canform a solid ice-like structure. Tese structures form differentarrangements, one of which is often methane. Decomposi-tion of 1 m3 of methane hydrate produces 164 m3 of meth-ane gas and 0.8 m3 of water. Consequently, gas hydrates may contain enormous amounts of carbon, more than double thatof the world’s combined coal and conventional oil and gasreserves. Tis has led several countries, including Mexico, toconduct research and exploration to understand the behaviorof hydrates, identify accumulations, and develop methods of exploitation.

Tis article describes some significant findings from gas-

hydrate research on the Mexican side of the Gulf of Mexico.

Prospecting

Seismic surveys can detect gas hydrates via anomalies calledbottom-simulating reflectors (BSR). Tey are characterizedby their large amplitude and reversed polarity compared withthe sea-floor reflection. Reflected seismic waves are producedin the layer of gas immediately below the zone of gas-hydratedeposits. BSRs are roughly parallel to the topography of thesea floor and produce a similar reflection.

BSRs occur several hundred meters below the sea floorand indicate the limit of the lower boundary of the stability zone of hydrates; the upper boundary is the sea floor. Te

sediments confining the hydrates represented by the BSR arein effect a seal that prevents part of the gas from escaping toupper layers. omography, AVO, and waveform inversion arealso useful for quantification of deposits of hydrates and as-sociated gas.

Resistivity anomalies help identify gas hydrates in areas where seismic data show no indication of their presence. Tepresence of hydrates above the BSR increases the resistivity orthe seismic velocity, while a small amount of free gas underthe zone of sediments with hydrates can reduce seismic veloci-ties considerably.

For the determination of probable accumulations and geo-

F RANCISCO J. R OCHA-LEGORRETA, Instituto Mexicano del Petroleo

physical and geological evidence of gas hydrates, we used 3Dpoststack seismic data without postprocessing or any methodthat could modify the stacked amplitudes.

Occurrence and distribution in Mexico

Mexico’s gas-hydrate research program is composed of twostages: the first will identify the occurrence and extent of the

BSR anomalies, and the second will determine the probableenergy resource contained by the hydrates (with considerationgiven to the geological hazards associated with deepwater op-erations). Tis paper describes advances in the first phase.

Geologic setting . Te study area, in the Gulf of Mexico Ba-sin, involves a sequence of clastic sediments from the UpperPleistocene and Pliocene that covers a long block correspond-ing to a rollover structure limited by synthetic faults merging in a deep regional listric system. No more geological informa-tion is available, and only seismic data are used in this re-search. Te seismic data cover 4100 km2.

Te sea floor map extracted from the seismic made it pos-sible to identify parallel structural highs capable of storing

these resources (Figure 1). We believe this is a possible con-tinuation of structural elements identified by the USGS inthe northern Gulf of Mexico that could maintain continuity in these southern latitudes with the same conditions, sincemost northern Gulf of Mexico hydrates deposits are in an area below the 500 m isobaths. Te seismic volume is between the800 and 2000 m isobaths.

Examples of BSR . A BSR is on the seismic lines because of the contrast in velocity between two materials with differentdensities, cemented sediments with gas hydrates and storage, with high acoustic impedance. Velocity is lower below thisarea due to water-filled pores in the underlying material that

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Figure 1. Sea-floor topography across the seismic volume in the study area. Note the main parallel structures (A, B, C, D) and a circular

feature interpreted as an expulsion crater.



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may also contain free gas trapped by the low permeability of the upper gas hydrates. Te contrast of acoustic impedances be-tween the two areas produces a very strong reflection, generally parallel to the seabed, and consequently identified as a BSR.Compared to the ocean floor, this is a characteristic reverse-po-larity reflection due the decrease in seismic velocity (Figure 2).

Identifying a BSR is very important because it represents thelower limit of the gas-hydrates stability zone and the upper limitof a zone of possible trapped gas. Figure 3 shows how this re-flector crosses the stratigraphy, suggesting that the formation

of gas hydrates occurredafter the sedimentationof the rocks that sur-round it. Note the “boostup” on the right, whichis associated with gas mi-grated from deeper layers

that deforms this wholepackage of rocks and thatalso supplies material key to the formation of gashydrates at this perim-eter.

Blanking . A signifi-cant feature in geom-etries associated with gashydrates is blanking—a marked decrease in seis-mic amplitudes above a BSR. Some authors pro-pose that this decreasein amplitudes is due to a reduction in impedancecontrast across sedimen-tary interfaces due topresence of hydrates andcementation of the strata.Te reduction in imped-ance contrasts causesacoustic “homogeniza-tion” of strata that resultsin lesser or weaker reflec-

tions.Figure 4 shows an ex-ample. Te area betweenthe sea floor reflector andthe BSR along most of the profiles in this seismicvolume is clear due tothe change in polarity. A strong reflector indicatespossible hydrate deposits,and the 250-ms intervalabove this reflector show the typical reduction of

amplitude, or blanking,along the profiles.

Associated gas accumulations . Examination of differentsegments of the seismic volume reveal geometries associatedto possible free gas trapped below the BSR. So far it has notbeen possible to calculate the concentration of this accumu-lation and this is a topic for future research. Figure 5 showsa mound associated with gas migrating from deeper strata probably via a vertical fault system. A reflection of reverse po-larity below the BSR may represent a gas-water contact thatlimits the free gas accumulation within the hydrate zone. Onthe top of the mound is an apparent open conduit where free

Figure 2. BSR evidence from reverse polarity of the reflectors. Te BSR runs across the profile parallel to the

sea-floor reflector.

Figure 3. BSR running parallel to the sea floor and crossing the stratification. Note, on the right, chaotic behavior that corresponds to a “boost” of gas from deeper accumulations (suggesting it is the source for the shallow gas) and a possible escape route to the surface.



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gas may be released.Geological structures associated with gas hydrates . Struc-

tures that indicate the occurrence of gas hydrate are cratersand mounds, the latter caused by the thrust behind the flow of gas into the upper sediments (Figures 1, 5, and 6). Figure6 shows an expulsion crater with a diameter of about 2 km ata depth of about 250 m. A major tectonic event related to a fault system appears to be the trigger that released free gas andformed this expulsion crater. Figure 6 also shows a mound

Figure 4. Example of reduction in amplitude or blanking between the sea-floor reflection and the BSR.

Figure 5. Reflection of apparent free gas with gas-water contact, where the reflection at the base of the free- gas interval is opposite polarity to the reflection at the top of the free-gas interval.

above the BSR, possibly theresult of gas arriving fromlower levels. Figure 4 showsa possible open conduiton top of the folded struc-ture. Tis conduit is possi-bly formed by gas released

through vertical faults.

Conclusions

Te examples in this articlehave significant implica-tions for exploration of gashydrates in Mexico. Te3D seismic data volumeprovides indirect evidenceof the existence of sedi-mentary material with gashydrates. Tis evidence isinferred from BSRs with well-defined features. Seis-mic strata show a polarity reversal of the reflection atthe top of the hydrate zonecompared to the top of theunderlying gas zone. Teexistence of such struc-tural elements as blank-ing, expulsion craters, andmounds associated with gasflow confirm the evidence.

Future research is needed

to determine if seismic imag-ing and pressure/tempera-ture relations can establishthe gas or mixtures of gases(methane, ethane, etc.) thatare related to these hydrates.

Key petrophysical pa-rameters are needed to per-form estimations of in-placevolume of free gas and hy-drates. Critical parametersthat need to be estimatedare porosity, gas saturation,

and hydrate saturation.

Suggested reading. “Energy density of deepwater gas hydrate” by Hardage (Search and Dis-covery article 40241, 2007). Economic Geology of Natural Gas Hydrate by Max et al. (Springer, 2006). “Economic geology of offshore gas hydrate accumulations and provinces” by Milkov and Sassen ( Marine and Petroleum Geology , 2001). “Practicalphysical chemistry and empirical predictions of the methane hy-drate stability” by Peltzer and Brewer (in Natural Gas Hydrate in Oceanic and Permafrost Environments , Kluwer AcademicPublishers, 2000). “Seafloor reflectivity—an important seismic



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Figure 6. Massive expulsion crater associated with the BSR to the right. Note the probable gas flow from deeper levels and, above the BSR, where

response from the gas pressure creates more deformation on the sedimentary layers.

property for interpreting fluid/gas expulsion geology and thepresence of gas hydrate” by Roberts et al. (LE , 2006). “Seis-mic evidence for widespread possible gas hydrate horizons oncontinental slopes and rises” by Shipley et al. (AAPG Bulletin,1979). Clathrate Hydrates of Natural Gases by Sloan and Koh(CRC Press, 2007). “Direct seismic indicators of gas hydrates inthe Walker Ridge and Green Canyon areas, deepwater Gulf of Mexico” by Wei-Huu et al. (LE , 2007).

Acknowledgments: I thank PEMEX Exploracion y Produccion for permission to publish this work. I am indebted to MarcoVazquez-Garcia, geophysics manager, for access to seismic data and bits of help along the way. Tanks to Bob Hardage (BEG) for his comments. Support from the Instituto Mexicano Del Petroleo is gratefully recognized.

Corresponding author: [email protected]

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