Image-log Calibration of Fracture-Azimuth and Fracture-Density

8/16/2019 Image-log Calibration of Fracture-Azimuth and Fracture-Density

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16 4 THE LEADIN G EDGE February 2016 Special Section: Imaging/inver sion: Est imatin g the earth model

Image-log calibration of fracture-azimuth and fracture-densityattributes from OVT prestack depth-migrated data

AbstractPotentials and limitations for acquisition and processing are

examined, given the industry’s interest in quantifying orthorhom-bic anisotropy contained in 3D P-P field data. A field-data casepresents one aspect of azimuthal prestack amplitudes and dem-onstrates these limitations and potentials. Careful amplitudeanalysis of azimuth-limited near-offset prestack migrated volumesreveals significant azimuthal var iations that are consistent withcalibration measurements.

IntroductionIn most resource plays, oil production is tied to fracture

density augmented by hydraulic stimulation. Tus, characteriza-tion of the in situ stress state and the natural fractures are im-portant to production rates. However, inconsistent and/or invalidassumptions made in some current migrat ions versus the actualearth through which the waves traveled can lead to “worlds incollision.” Te result of this mismatch is loss of valuable informa-tion and incorrect interpretation of full-azimuth field datademonstrating orthorhombic anisotropy. Te trends in industryfor seismic acquisition are to deploy large channel counts, squarelive-receiver spreads for each shot, and 1C and/or 3C acceler-ometers as receivers. Te square receiver patch and the largechannel count provide full-azimuth, fu ll-offset (offsets equal totarget depths) 3D P-P data, ideal for azimuthal analysis. Full-azimuth illumination is now widely recognized to be beneficialfor imaging complex structure. Although this paper presents a

land-data case history, the methods used here could be used onfull-azimuth marine data.

Seismic data processingOffset vector-tile (OV) binning preserves azimuth and offset

in the prestack migration step so that subsequent steps can quantifythe azimuthal traveltimes and azimuthal amplitudes as attribute

volumes. For advanced processing, OV prestack depth migration

(PSDM) routinely adds value for azimuthal studies, since PSDMcan reduce art ifacts caused by lateral velocity heterogeneities.

Current processing quantifies the azimuthal variation of trav-eltimes and saves this information in attribute volumes. Te basicoutput attribute volumes include: VINfast, VINfast-slow , and azi-

muth of VINfast. Te azimuth of VINfast is interpreted as thestiff direction of the layer bounded by the two reflections. VIN

fast-

VINslow

is customarily interpreted as proportional to the differencebetween the maximum and minimum horizontal stresses. Te

values, not just differences, of VINfast and VINslow are neededto ascertain whether a high-stress environment (elevated VIN)or a low-stress environment (decreased VIN) is present.

After this preliminary work, it is possible to remigrate orperform RMO corrections with the orthorhombic velocity field

to improve the imaging. With the common-image gathers (CIG)

Heloise Bloxsom Lynn 1

reflections now flat in time (or depth), their stack provides a su-perior image of the subsurface. Before stack, amplitude variation

with offset and azimuth, routinely quantified as AVOaz (the AVOgradient change by azimuth), may be calculated and output asattribute volumes. “Azimuth” refers either to OV surface (ac-

quisition) azimuths or to (subsurface) illumination azimuths frommigration. Azimuth as defined via subsurface illumination ispreferred. Te output attribute volumes are as important for furtherinterpretation as the image volume itself.

Te earth models used in this paper use standard terminologysuch as:

• transverse isotropy vertical axis (VI) — layer anisotropy • tilted transverse isotropy (I) related to structura l dip and

Tomsen’s (1986) parameters delta and epsilon to characterizethe layered anisotropy

• horizontal transverse isotropy (HI) — fracture or stressanisotropy, where the azimuthal variation of the far offsettraveltimes indicate an azimuthal VRMS which fits a cosine2θ curve

• orthorhombic — flat layers and one set of vertically alignedfractures (or two sets of orthogonal fractures)

• tilted orthorhombic — dipping layers and dipping faults(fractures), but three 90° right angles still exist among thesymmetry planes

Tis paper maintains that azimuthal variations in traveltimes

and amplitudes are of significant value, due to their sensitivitiesto the in situ horizontal stress field and the natural fracturepopulations.

Case history: High-fold full-azimuth P-P 3D datavolume taken through OVT prestack depth migration

A high-fold 3D P-P survey was acquired to characterize acarbonate reservoir, its stress field, and its natural fracture sets. Te flow for conditioning and extracting azimuthal ly variantattributes follows.

ProcessingAlthough the natural bin size was 10 ft × 100 ft, the data were

processed at 100 ft × 100 ft bins, providing 1340 fold for thepreimaging processing. Tis level of fold enables high-qualitydetermination of premigration processing parameters. Data prepa-ration included initial layered anisotropic velocity-model estima-tion, estimating delta using well control, and using the nonhy-perbolic moveout on the far offsets (“hockey sticks”) to estimateepsilon. Five iterations of layered anisotropic tomography andKirchhoff PSDM were used to build the VI (layered) anisotropymodel. Once the model (VP0, delta, epsilon) was built, OVdata were sent through Kirchhoff PSDM. Te data were converted

1Lynn Inc. http://dx.doi.org/10.1190/tle35020164.1.



February 201 6 T HE LE A D I NG E D GE 16 5Special Section: Imaging/inver sion: Est imating the earth model

back to time using a smoothed velocity field. Analysis of azimuthalresidual normal moveout (RNMO) on the migrated OV data,as it fit the cos 2θ variation, gave us the azimuthal interval velocity(VIN) parameters: VINfast, VINslow , azimuth of VINfast,and VINfast-slow (Johnson and Miller, 2013).

Figure 1 shows an example of the P-P common-image gathers,after the OV PSDM and removal of the HI traveltime varia-

tions on the far offsets. Te signal quality is excellent. Tesegathers show near offsets to far offsets, azimuth having beenstacked within each offset range.

Te migrated OV gathers required several further datapreparation steps before interpretation. Tese steps includeddepth-to-time conversion, offset-based random noise attenuation,and sectoring into four azimuths (N15-75E, N55-115E, N95-155E, N135-195E). Next steps were offset-based residual ampli-tude analysis and compensation and finally the formation of four(approximate) incidence-angle (0–15°, 10–25°, 20–35°, and 30–45°)stacks per azimuth sector. In the rest of the paper, these anglestacks are referred to as “< 15°, 17°, 27°, and 37° stacks.” We usedazimuth sectoring for azimuthal amplitude analysis because propercode to evaluate the azimuthal amplitudes in a nonsectored fashion

was not commercia lly avai lable for this project. Te creation of four angle stacks for each of the four azimuth

sectors provided 16 image volumes for the azimuthal amplitudeanalysis. Te azimuth-sectoring process aims at keeping a healthyand balanced distribution of offsets and azimuths among a ll theazimuth sectors, with a minimum of artifacts due to irregularfold or azimuth. Te OV PSDM provided an excel lent preserva-tion of signal, plus removal of artifacts caused by lateral velocityheterogeneities above target. Te azimuthal-amplitude analysis

was performed at the seismic bin spacing, 100 ft × 100 f t. Su-perbinning was not used because it was not necessary due to the

good S/N and because it degrades the spatial resolution of theanalysis. Te larger the bin, the larger the spatial area the inter-preter is assuming homogeneity among the natural fractures.

Interpretation using calibration data Te first set of calibration (production) data used in this paper

is the instantaneous shut-in pressures divided by depth (normalizedISIP): both prefrack ISIP gradients and postfrack ISIPs wereavailable for this study. Previous publications on these field data

documented that VINslow showed a proportional relationship tothe prefrack ISIP and ISIP (Figure 2) (Busanello et al., 2014;Lynn et al., 2014;). Te prefrack VINslow and prefrack ISIP areboth sensitive to the minimum horizontal stress.

Since azimuthal interval velocity information was availablebefore the drilling of this well (Figure 2), we were able to predictqualitatively (ahead of the bit) that the toe, or maximum depth

drilled, would be easier to frack and the heel, at the start of themain deviation, would be harder to frack. Tis prediction wasfound to be correct. Figure 2 is the first graph of a surface seismicmeasurement showing a linear relationship to a reservoir pressuremeasurement. Te next calibration data are taken from the north-ern portion of the 3D survey. Both of these calibration data setsare shown in Figure 3, in which the colors and contours are topcarbonate structure contoured in depth and the faults are dottedblack lines.

Te standard observations in this area are that the postfrackISIPs (blue boxes, Figure 3) are greater than the prefrack ISIPs(black boxes), because of the increase in pressure from the injectionof frack fluid and proppant. If the postfrack ISIP is less than theprefrack ISIP, as shown in the (northern) purple ellipse, the usualinterpretation is that the hydraulic stimulation broke into a naturalfracture network, which absorbed some of the frack fluid and prop-pant. Tese points are of significant interest to the engineers.

From the azimuthal seismic data, we will see that azimuthalamplitudes can indicate which stages will behave anomalously. Te map shows the purple ell ipse containing the anomalouspostfrack ISIP and northeast-trending natural fractures seen just

west to it. Te interpretation is that these fractures observed inthe resistivity image log are part of the fracture set that causedthe postfrack ISIP to be decreased (relative to the prefrack ISIP).

Tis anomalous postfrack ISIP (decreased from prefrack ISIP)

with its attendant fractures shall be discussed later.In Figure 3, the pink circle indicates the southern fractureicons that are parallel to the northwest-trending fault. Faults areindicated by depth contours becoming much closer together anddotted black lines. Where two identical fracture icons exist (e.g.,blue icons in the pink circle), the horizontal borehole went inbetween two bins. Te observed fractures turn more northeastaway from the fault. Tis change in the fracture azimuth is a keydata point that will be discussed later. Note in Figure 3 the spatial

Figure 1. P-P common-image gathers showing top/reservoir, black arrow.

Figure 2. Graph of the prefrack ISIP (psi) against the VINslow (ft/s)of the carbonate reservoir. Tis well is in the southern area of the 3Dsurvey.




gap where no fracture icons exist (north of the large red icon andsouth of the small blue icon). Tis 400-foot-long spatial gap in-dicates a zone where no fractures were observed in the resistivityimage log. Later in this paper, this spatial gap shall be shown tocorrelate to a seismic spatial gap where no seismic icons are posted.

Tis gap in seismic icons wil l indicate that the azimuthal ampli-tudes display no reliable indication of the presence of one set of

vertica lly al igned fractures.Before azimuth-dependent values are interpreted, it is useful

to review the average measurements in map form. Figures 4 and5 are both azimuth-averaged measurements. Figure 4 is the averagenear-offset (unnormalized) near-offset amplitude and Figure 5 isthe average (unnormalized) AVO gradient of top carbonate.

In Figure 5, the largest red arrow fracture icons (high fracturedensity, wide aperture fractures, trending N20E) are associated with a positive AVO gradient (averaged over azimuth) and adimmed near-offset amplitude (Figure 4). Te interpretation isthat at the highest fracture-density locations, the average valuesof AVO gradient and near-offset amplitude transition to the(anomalous) positive gradient and dimmed near-offset amplitude. Te decrease in P-impedance contrast (near-offset amplitudes)also implies an increase in the V P /V

S of the carbonate, interpreted

here as a decrease in V S greater than the decrease in V P.Inspection of Figures 4 and 5 reveals that the strongly negative

AVO gradients (cold colors, Figure 5) are associated with high-amplitude, near-offset values (cold colors, Figure 4). Also, flat orpositive AVO gradients (warm colors) are associated with dimmednear-offset amplitude values (warm colors). Te color scales shownare constructed to highlight the linear relationship.

Modeling presented by Lynn (2014a) documented that thesteeply negative AVO gradients (cold colors) are associated withthe lithologic contrast of V P, V S, and density, as measured in the log

values, with the contrast in V P /V S ratio at the boundary being2.31/1.69. Other work, not shown, has established a strong l ineartrend between near-offset intercept and gradient. Te AVO gradientis governed by the contrast in the V P /V S ratio at the boundary. Anygiven AVO gradient is accompanied by a characteristic intercept value that is defined by the contrast of the P-impedance at theboundary. If the AVO gradient changes, then the intercept changes

too. Te AVO gradient is affected by the far-offset amplitudes, which are known to be sensitive to the contrast of shear-wave velocityat the boundary (or, the contrast of V P /V S at the boundary). Whenthe overlying shale has minimal spatial variation of V P, V S, anddensity, then the underlying carbonate will cause the rapid lateral

variations shown in Figures 4 and 5. Te linear relationship just

Figure 3. Calibration data for the northern area (north is up). wo hori-zontal wells drilled from same pad; heel in the north (top), toe in thesouth (bottom, off the map). Calibration data: normalized ISIPs: blackbox size is proportional to prefrack ISIP; blue box proportional to post-frack ISIP. Te fractures in the resistivity image log (colored arrows calledicons) are shown binned at the seismic bin size (100 ft × 100 ft). Icon sizeis observed fracture density; azimuth is observed fracture azimuth; color isobserved fracture aperture (blue small aperture, red wide aperture). Telog started at the blue icon in the south (pink circle) and was logged to thenorth (to the heel). Locations south of the pink circle were not logged.

Figure 4. Mean (azimuth-blind) near-offset amplitude (< 15° angle of

incidence) top carbonate. Contours show top carbonate structure (CI:100 ft). wo horizontal boreholes posted with calibration data (Figure3). Te shale-carbonate interface should be high amplitude (coldcolors, values 2500+). Dimmed amplitudes (warm colors) are anoma-lous and are observed at faults and at negative curvature locations. Tedimmed amplitude locations are interpreted as indicating that thecarbonate acoustic impedance has decreased, possibly due to fractures.

Figure 5. Mean (azimuth-blind) AVO gradient, top/carbonate.Strongly negative AVO gradients (cold colors) are expected due to theshale-carbonate interface. wo horizontal boreholes posted with cali-bration data (Figure 3).




seismic indication of one set of vertically aligned fractures. Techange in fracture azimuth from northwest (fault parallel, south)to northeast (away from the fault) also is observed in the seismicicons. Te magnitude of the azimuthal amplitude variation islargest (red icons) at the largest red arrow (highest fracture density). Te smallest magnitudes of the azimuthal amplitude variation(green icons) are observed at the smallest fracture icon arrows(the least fracture densities). Te magnitudes of the azimuthal

variat ions are often large: 100–300%. Te two southernmost blue fracture icons (pointing northwest)

are matched only on the western bin. Although the horizontal

borehole ran between the two bins, only the western bin’s seismicresponse matched; heterogeneity in the fracture azimuths is in-terpreted present. Observe the good match between the fractureicon and the near-angle azimuthal amplitudes in 11 of 14 locations.For the AVOaz measurement (Figure 6), a good match exists at10 of 14 locations. Te above implies a slightly better performanceof the azimuthal near-angle amplitudes to match the calibrationdata compared to the AVOaz attribute.

o emphasize the similarity in the azimuth measurementbetween the two azimuthal amplitudes measurements, a corender-ing of these two sets of icons is presented in Figure 8.

In Figure 8, bins containing three icons indicate fracture(image) icon plus two seismic attributes. Bins containing twoicons indicate only seismic attributes. Concurrence between the

two seismic icon sets’ azimuths is typically seen, both in this mapand throughout the survey. Tere are also areas of disagreement

of azimuths, usually 10% or less of any examined map area.“Disagreement” is defined as “an azimuthal difference greaterthan 30 degrees.” Te postfrack decreased ISIP location (purpleellipse, Figure 3) has seismic icons agreeing that northeast-trending

discussed implies that a decrease in the carbonate’s V S (due tofractures) is accompanied by a decrease in V P.

Figure 6 displays the azimuthal variations in the AVO gradient(referred to as AVOaz), the current industry metric for azimuthalamplitude analysis. Te gap of no fractures seen in the image logcorresponds to a gap of seismic icons: in the gap, there is no reliableseismic indication of the presence of one set of vertically alignedfractures. Te change in fracture azimuth from northwest (faultparal lel, south) to northeast (away from the fault) is also observedin the seismic icons. Te magnitude of the AVOaz is largest atthe largest red arrow (highest fracture density). Te smallestmagnitudes of the AVOaz (green icons) are observed at the smallestfracture icon arrows (the least fracture densities). Te seismic iconmatches the fracture icon in 10 out of the 14 bins where indepen-dent evidence of fracture azimuth and/or fracture density ex ists. Te postfrack decreased ISIP location has two seismic iconspresent, indicating the likely presence of northeast-trendingfractures. Te magnitudes of the AVOaz are large: values of200–600% are visible in Figure 6.

Te azimuthal amplitudes are affected by the natural fracturesas follows (Lynn, 2014a, 2014b; Lynn, 2015): a) the azimuth ofthe most negative AVO gradient, and the azimuth of the highestamplitude < 15° angle of incidence, correlate to the fracture azi-muth; b) the magnitude of the AVOaz, and the azimuthal variationof the near-offsets amplitudes, give a qualitative prediction offracture intensity. Tese observations are for a model with oneset of vertically aligned fractures.

Figure 7 presents the icon map of the azimuthal variation ofthe < 15° angle of incidence amplitudes, after their fit to the cos2θ. As before, the gap of no fractures seen in the image log cor-responds to a gap in seismic icons, which indicates no reliable

Figure 6. Azimuthal var iations in AVO Gradient (AVOaz), presentedas a field of (bar) icons (north up). wo horizontal boreholes posted with calibration data (F igure 3). Azimuth of icon: azimuth of mostnegative gradient, interpreted as fracture-parallel (see Lynn, 2014a).Length = “Reliability,” here defined as (Max AVO gradient – min AVO gradient) / (rms error in fit of model to data). Reliabil ities > 2 areplotted on the map. Low reliability data are not displayed. Color:(Max-Min)/Mean. Spacing of icons: 100 ft × 100 ft (one per bin). Teseismic azimuthal amplitude icon matches the fracture icon (imagelog) in 10 out of 14 bins where independent fracture evidence exists.

Figure 7. Te azimutha l variat ion of the near-offset (< 15°) amplitudes(triangle icon). wo horizontal boreholes posted with calibration data(Figure 3). Te seismic azimuthal amplitude icon matches the fractureicon (image log) in 11 out of 14 bins where independent fractureevidence exists. Notice also the gap; no seismic indication of one set of vertica lly al igned fractures where the image log recorded no fractures.Reliabilities > 2 are plotted on the map. Low-reliability data are notdisplayed. Te postjob decreased ISIP location (purple ellipse, Figure3) contains two seismic icons, indicating the likely presence of north-east-trending natural fractures.




fractures likely are present. Te gap of no fracture icons has noseismic icons. Te change from northwest fractures (at the fault)to northeast fractures is a lso observed in the seismic icons. Teconclusion, so far, is that the azimuthal variation of the near-offsetsamplitudes tracks the AVOaz measurements. Te size of the iconsis similarly constructed: reliabilities greater than 2 are requiredin order to be shown on the map.

Te CIG bin of the largest red arrow (fracture icon with azi-muth N20E, Figure 3) will next be examined. Te AVOaz responseof this bin is shown in Figure 9.

In Figure 9, the line fit to each azimuth is displayed. Te R 2 ranges from 0.65 to 0.97, with average R 2 = 0.86. For field data,the fit of the AVO gradient (and intercept) to the observed am-plitudes is good. Tis figure shows the AVO gradient changing

with azimuth. It also shows that the azimuth of the most negativeAVO gradient has the highest amplitude intercept and is fractureparallel (Lynn, 2014a). From Figure 9, one can observe the azi-muthal variation of the near-offset amplitudes has a relationshipto the AVOaz measurement; this relationship is also visible in

Figure 8. Also, the most positive AVO gradient has the lowestamplitude intercept; this azimuth is fracture perpendicular, knownfrom calibration data and modeling efforts.

Figure 9 shows that the unnormalized far-offset (high-angle)azimuthal amplitude variation is approximately 1500; the unnormal-ized near-offset amplitude variation is about 3000, or double thefar-offset variation. Tere is a midoffset, somewhere between 30°and 35°, where the azimuthal variation is very small. Te authorinterprets these observations as competing anisotropies: Te azi-muthal variation of V P is present on the near offsets, while the azi-muthal variation of V S is on the far offsets. Te azimuthal variationof V P in the presence of one set of vertically aligned fractures is

widely accepted; perhaps this near-offset amplitude variation is a

manifestation of the same phenomenon. Also note that the highest

Figure 8. Te corendering of the AVOaz icon (lower bar icon) withthe < 15° angle of incidence icon (upper triangle icon). wo horizontalboreholes posted with calibration data (Figure 3). Eleven of 14 loca-tions of independent evidence of fracture orientation and/or fracturedensity show agreement with the AVOaz icon and/or the < 15° angleof incidence icon. Te two seismic icons match in azimuth in ~ 80% ofthe bins on the map.

Figure 9. Te azimuth-dependent reflection amplitudes (y-axis)plotted against sine of incident angle (squared), at the location of thelargest red arrow (Figure 3). Te resistivity image log shows this loca-tion to have high f racture density, wide apertures, and N20E.

amplitude azimuth is fracture parallel on the near offsets, but thehighest amplitude azimuth is fracture perpendicular on the faroffsets. Tis change of the highest amplitude azimuth with increasingangle of incidence also is not predicted by any published theory.

Discussion Te above case history documents that the expected AVOaz

effect is present and ties calibration data. Te industry metric of AVOaz has once again proven useful. Te decreased postfrackISIP near the well heel shows seismic indicators of one set of vertically aligned fractures trending northeast: this is anotherpositive calibration point.

Tere are four out of 13 locations of documented fractures,seen in the 8 inch borehole traversing the 100 ft bin, that are not observed in the seismic measurements (arising from the averageof a 100 ft × 100 ft lateral area). Tis discrepancy can be explainedby pointing out that the earth is often heterogeneous. An 8 inch× 100 ft measurement may not be a representative sample of the100 ft × 100 ft area. However, if one is principally focused upon

the borehole flow measurements (affected by hundreds of feet ofrock away from the borehole), then the author suggests that theseismic measurements provide a more likely representation of theaverage response of the rocks inside a 100 ft × 100 ft bin. In myopinion, the most meaningful calibration data should include theproduction data, measured stage by stage. However, this type ofcalibration data is not available in this study.

Te above case history also documents azimutha l variation

of near-offset amplitudes, a phenomenon the author has observedin field data during the last 20 years as well as in the l iterature. Te theory used to explain azimuthal amplitudes needs to beupdated to take into account field-data observations like these.Classical inversion for azimuthal S-impedance variations predicts

the response to be at large incident angles. Small-angle variationsneed to be addressed in a manner adequate to explain or modelthe field data in this and other surveys. Tat said, criticisms ofthe above data analysis have arisen due to many (other) data setshaving poor S/N on the near offsets (< 15°).




Summary and future work Advanced pract ice can accomplish rel iable interpretation of

azimuthal VIN and azimuthal amplitudes with sufficient cali-bration data and high S/N seismic data. Tere are multiple calibra-tion data sets for natural fractures and in situ stress that are beyondthe scope of the article. If a processing contractor is handed azi-

muthal P-P seismic with no support data, then the processor can,at best, only report back upon the seismic signal itself. It is theinterpreter that must perform calibration of the data,

Te cause of an observed near-offset azimuthal-consistentamplitude anomaly may include:

• some heretofore unrecognized systemic industry error inprocessing

• an inadequacy in the near-offset acquisition (not present inthis study)

• azimuthal P-impedance variation arising from verticallyaligned macrofractures capable of flowing fluids and/or stress-aligned microfractures due to present-day unequal horizontal

stress field• a local strain (vertically aligned fractures) oriented by thecurvature of the local rocks

• a lateral velocity heterogeneity overlying target that is causingan art ifact in either the traveltimes or the amplitudes or both

• some combination of the above

We may be able to discern among these potential causes withcareful acquisition and processing coupled with sufficient inde-pendent calibration data, and with appropriate modeling.

Te “worlds in collision” fast approaching us are the assump-tions made in the worlds of prestack migration, prestack elasticinversion, and full waveform inversion, compared to the actual

earth through which the waves traveled. Processing in each ofthese arenas holds in mind an earth model that may, or may not,be representative of the earth model. Te good news is that prestackmigration is usually applied to well-sampled OV binned data,thus permitting the preservation of azimuthal traveltimes andazimuthal amplitudes. Data processing also must provide forcareful preservation of amplitudes.

Prestack elastic inversion typically is accomplished on azimuth-blind (stacked over azimuth) common-angle gathers (or stacks).

Tis obliterates any azimuthal information on the prestack am-plitudes. Perhaps even worse, standard AVOaz programs assumethat only the far offsets have any azimuthal amplitude informationat all, and so tr y to assign all azimuthal amplitude variation into

the S-impedance term.Currently, azimuthal amplitudes are evaluated prestack to

estimate the azimuthal AVO gradient (Rueger, 1997). Currenttheory based on simplified fracture structures (HI, one set of

vertica lly aligned fractures) predicts that the azimuthal variationof the near offset (< 15° angle of incidence, usually offsets lessthan half of target depths) P-P reflection amplitudes is muchsmaller than the maximum difference in amplitude to be found

on the far offsets (> 30° angles of incidence, usually offsets equalingtarget depth). P-P field data published herein and elsewhere implythat current theory is inadequate to explain field-data observationsof substantial azimuthal change of amplitude on the near offsetsat calibrated bins. Azimuth-dependent prestack elastic inversionis needed to establish the relationships among azimuthal interval

velocity, azimuthal P-impedance, and azimuthal S-impedance,thereby overcoming limitations in current theory.

Finally, this ar ticle is written to encourage more geophysiciststo look at their own seismic data and calibration data, discussthe possible causes, and publish their interpretations. An ex-panded version of this article is ava ilable to download by contact-ing the author.

AcknowledgmentsI gratefully thank our client for permission to publish.

Corresponding author: [email protected]

ReferencesBusanello, G., Z. Chen, X. Lei, R. Li, M. Egan, . Heesom, B.Liang, H. B. Lynn, A. Poole, P. van Baaren, F. Xiao, 2014, Landseismic surveys for challenging reservoirs, Oilfield Review, 26,no. 2, 32–47.

Johnson, G. M., and P. Miller, 2013, Advanced imaging and inversionfor unconventional resource plays: First Break, 31, no. 7, 41–49.

Lynn, H., 2015, Azimuthal 3D seismic data, ISIP’s, and micro-seismicdata evaluated for unconventional completions in a fracturedcarbonate reservoir, 85th Annual International Meeting, SEG,Expanded Abstracts, 366–371, http://dx.doi.org/10.1190/segam2015-5803861.1.

Lynn, H., 2014a, Fracture densities and fracture azimuths evident inthe azimuthal amplitudes from the top of a fractured carbonate oil

reservoir: 84th

Annual International Meeting, SEG, Expanded Abstracts, 341–345, http://dx.doi.org/10.1190/segam2014-0393.1.Lynn, H., 2014b, Field data evidence of orthorhombic media: Changes

in the P-P bright azimuth with angle of incidence: 84 th AnnualInternational Meeting, SEG, Expanded Abstracts, 463–467,http://dx.doi.org/10.1190/segam2014-0394.1.

Lynn, H., W. Lynn, J. Obilo, and V. Agarwall, 2014, Azimuthalprestack depth migration for in-situ stress evaluation, in a fracturedcarbonate oil reservoir: Predrill prediction of instantaneous shut-inpressure gradients: 84th Annual International Meeting, SEG,Expanded Abstracts, 301–305, http://dx.doi.org/10.1190/segam2014-0392.1.

Rüger, A., 1997, P-wave reflection coefficients for transversely isotropicmodels with vertical and horizontal axis of symmetry: Geophysics,

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Editor’s note: Use of “frack” as an abbreviation for the term hydraulicfracturing in this publication is current SEG style.

Documents

Image-log Calibration of Fracture-Azimuth and Fracture-Density