Seismic Amplitude Versus Offset (AVO) Character of Geopressured

J

Transition Zones

ohn N. Louie and Abu M. Asad

1Seismological Laboratory (168), The University of Nevada, Reno, NV 89557-014

19 March 1993 (final revision 10 August)

T

ABSTRACT

he presence of diagenetically-cemented seals distinguishes between possible.

Tmechanisms for the maintenance of abnormal fluid pressures in sedimentary basins

he velocity gradients and Poisson’s ratio variations surrounding a cemented seal

saffect the pre-stack amplitude versus offset behavior of seismic reflections from theeal. Acoustic synthetic seismograms based on well logs from sealed transitions

ndemonstrate their unusual AVO character. A long-offset COCORP reflection lineear Port Lavaca, Texas shows similar effects from a seal at 6000 feet (1800 m)

-ldepth. Local conductivity logs and regional drilling mud weight compilations estabish the presence of this pressure transition zone. It is associated with a strong, low-

rtfrequency reflector near its base, and displays linear AVO trends markedly strongehan from lower reflections within the overpressured compartment. The fact that a

aseal exhibits such prominent physical property characteristics suggests that AVOnalysis techniques may locate other seals in basins worldwide, where proper condi-

mtions for diagenetic sealing rather than shaliness and subsidence may be needed to

aintain geopressures.

Introduction

Bradley (1975) and Powley (1975) proposed the existence within sedimentary basins worldwide

aof closed but internally-conductive "compartments" containing abnormal pore fluid pressures. Theynd their co-workers compiled accurate downhole pressure measurements into profiles showing

dadramatic changes in pressure gradient with depth. Based on these profiles Bradley and Powley dividebnormally-pressured regions into high-gradient transition zones or "seals" surrounding hydrostatic but

fabnormally-pressured "compartments." Hunt (1990) reviewed this hypothesis and some of the datarom which it originated. This paper briefly explores how seismic reflection surveys may identify such

drpressure seals and transition zones. We studied both models of wave propagation in seals aneflection data from abnormally-pressured oil and gas fields. Our objective is to suggest seismic

-macquisition and analysis strategies that should increase the likelihood of identifying overpressured for

ations in advance of drilling.

In the oil and gas industry, work on the seismic signature of abnormally-pressured formations has-

pconcentrated on the analysis of normal-incidence, stacked reflection data. For many years, the highorosity, reduced-velocity rocks within large overpressured "compartments" have been identified by

,istacking velocity analyses (Bellotti and Giacca, 1978; Bilgeri and Ademeno, 1982). More recentlyndustry specialists have investigated how the presence of abnormal pressures affects the instantaneous

characteristics of stacked reflection wavelets.

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sLouie and Asad AVO Character of Transition Zone

To complement the velocity analysis work, we investigate the seismic properties of the pressure-

oseals or transition zones themselves rather than those of the abnormally-pressured compartment interirs. The seismic character of pressure seals may be similar for both over- and under-pressured com-

mpartments, and may allow the accurate location of vertical as well as horizontal seals. Characterization

ethods based on compartment velocities, on the other hand, whether based on stacking or tomo-

fgraphic velocity analysis, suffer from poor lateral resolution and the inability to identify underpressuredormations.

Bethke’s (1986) modeling of the hydrodynamics of abnormally-pressured sections only requires a

(basin to subside and be relatively shaly to develop large pressure gradients. Bradley (1975), Powley1975), Dewers and Ortoleva (1988), and Tigert and Al-Shaieb (1990) suggested alternatively that

lbands of calcite mineralization observed within transition zones, not necessarily parallel to lithologicayering, provide the sealing mechanism to maintain the overpressures below. The concept of a

sdiagenetically-cemented seal would explain how the many basins listed by Hunt (1990) exhibitinglower subsidence and less shaliness than required by Bethke (1986) could develop abnormal pressures.

cWe base our modeling and data analysis on the results of geological and geochemical efforts to

haracterize sealing mechanisms (this volume). The diagenetic silica or carbonate cementation.

Amechanisms may yield transition zones that form seismic reflectors having identifiable properties

nalysis of reflections before stacking may be a reliable way to detect and measure such properties.

Methods

Our assessment of pre-stack pressure seal reflection properties took two parallel tracks. One-

teffort modeled the properties of pressure seal reflections with synthetic seismograms, to search for disinguishing seismic characteristics. The other effort analyzed seismic reflection and well data to see if

their characteristics matched those of the synthetics.

Modeling— In a sand or shale, the amount of secondary cementation required to retain overpres-

asures also drastically alters the physical properties of the seal rocks. The presence of porosity canffect rock rigidity more than the compressional modulus (Christensen, 1985). Thus, as porosity is

areduced in seals by cementation, rigidity rises faster than incompressibility, leading to distinguishablelterations in the Poisson’s ratio property of the seal. Pre-stack seismic analysis can often detect

reflectors with unusual Poisson’s ratios (Ostrander, 1984).

The higher total incompressibility of the seal may also have other effects on offset seismograms,

adue to velocity gradients above and below the seal. Figure 1 shows the generalized geometry of near-nd far-offset reflection rays from a horizontal seal. At larger incidence angles i, the presence of a

sstrong velocity gradient with depth at the seal can cause rays to bend in directions not predicted byimple reflection theory. While at some angles the rays may bend up, throwing extra energy at some

rrreceiver interval (Figure 1, "turning ray"), at other angles rays may bend down, depriving otheeceivers of returned energy (Figure 1, "diving ray"). We can model this ray bending effect with

purely acoustic synthetics.

Velocity measurements in wells confirm that sufficient gradients exist to produce such ray bend-

sing. We generated synthetic seismograms using velocity and density profiles from wells penetratingeals and abnormally-pressured compartments. Two fields in the south Texas coast provided a suite of

awell logs with complete shale density and sonic transit time information. The completeness of the logsllowed acoustic seismogram modeling with a minimum of assumptions.

eW

Figure 2 shows the log information from two of these wells, which we label the Sarita and thillamar. In the Sarita well, shale density (solid curve on Figure 2) rises to high values between 8000

-land 10,000 foot depths, just above the onset of overpressure. Sonic transit times (dashed curve) simiarly fall to a minimum in this depth range. Thus sonic velocities would reach a maximum also.

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Louie and Asad AVO Character of Transition Zones

SWe call this high-density, high-velocity region within the pressure transition zone a "cap."

everal wells from our south Texas suite exhibited a cap. Some did not, despite the presence of over--

tpressure. One example, the Willamar well, is also shown in Figure 2. Instead of a distinct cap, densiies and velocities simply increase to the top of the overpressured zone. Below, the high compartment

spressures preserve porosity and velocity at nearly constant values in both wells. The presence of a capuggests that the pressure seal is particularly well-cemented.

-g

The velocity gradients above and below such a cap may give the reflections arising from it distinuishing characteristics. These effects will be strongest in the comparison of narrow-angle (i; Figure

ss1) seismograms against wider-angle seismograms, in terms of changes in reflection amplitude versuource-receiver offset (AVO). The velocity gradients may not reflect strongly at short wavelengths

r(relative to the breadth of the gradients) and at normal incidence. However, at wider angles reflectionays may turn in positive velocity gradients, producing strong amplitudes. At certain angles, the

reflections may lose energy downwards through the negative gradients at the bottom of the cap.

To investigate how the cap affects reflection amplitudes relative to incidence angle we generated3

smulti-offset, pre-stack synthetic seismic gathers with acoustic finite-difference methods. Figurehows two of the gathers, from the Sarita and Willamar profiles, respectively, at relatively low frequen-

-bcies. We also made sets of higher-frequency synthetics. Our synthetics include all multiples, revereration, interference, and other full-wave acoustic effects.

na

Data Analysis— We were able to obtain a public-domain seismic reflection data set from abnormally-pressured region. The Consortium for Continental Reflection Profiling (COCORP) program

miat Cornell University provided three lines from the Port Lavaca area of Texas, extending about 100 knland. The COCORP data have several advantages for our study. First, Line 4 of the data set is

iclose to the coast near Port Lavaca (Figure 4), so the expected depth to the onset of abnormal pressures much shallower at 6000 feet (1800 m) than for other regions further inland or offshore (pers.

1comm. with R. Spiller of Maxxus Energy). Second, the COCORP line was recorded using offsets to2 km, so it provides wide incidence angle coverage. Moreover, COCORP acquisition parameters

saensure good coverage of relatively low reflection frequencies between 10 and 35 Hz. Frequenciebove 35 Hz are not included in the data, however, limiting resolution to features comparable in size to

the ∼80 m seismic wavelength.

Figure 4 shows a map of COCORP Texas Line 4 near Port Lavaca. The numbered dots indicate-

cthe locations of wells for which we obtained all available geophysical logs. Our search revealed prinipally conductivity logs. Figure 5 displays COCORP’s stacked seismic section for Line 4, converted

klto a depth scale using velocities from regional type logs and COCORP velocity spectra. The thicines superimposed on the section show shale conductivities we picked from eight of the nine wells.

sFigure 5 suggests that a relatively low-frequency reflector at about 6000 foot (1800 m) depth across theection is at the base of a gradual rise in shale conductivity. The presence of a shale diapir below the

-ssoutheast end of the section provides a mechanism for a relatively shallow transition to fluid overpresures, as shale diapirism is often a result of the buoyancy of an under-compacted, less dense shale sec-

ption. These observations suggest that the shale conductivities in Figure 5 are increasing through aressure transition zone 1000 to 2000 feet (300-600 m) thick.

,p

Offshore information on the depth to overpressure from reported drilling mud weights (R. Spillerers. comm.) supports the presence of a transition zone near the depth of the low-frequency reflector.

(The wells in this compilation closest to Line 4 reported overpressures as shallow as 6100 to 6300 feet1860-1920 m). Further offshore, depths to the onset of geopressure in this region range from 4700

-tfeet (1400 m) to 15,000 feet (4600 m), clustered around 8000 feet (2400 m). The local shale conducivities together with the offshore drilling information together establish the existence of a geopressured

section beginning ∼6000 feet (1800 m) below COCORP Texas Line 4.

The low-frequency reflection may arise at the top of the seal or at its base, where it meets the,overpressured compartment. Assuming that velocity gradients over 300 foot (100 m) depth intervals

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s

a

Louie and Asad AVO Character of Transition Zone

s in Figure 2, control the reflectivity of the seal, the seal boundaries could appear as reflections having-

fa generally lower frequency content than the reflections from sharper stratigraphic intervals. The lowrequency reflection we observe in COCORP Texas Line 4 at the 6000 foot (1800 m) depth may be a

direct seismic characteristic of a fluid-pressure transition zone.

After identifying the reflections associated with the top of overpressure, we analyzed the pre-stackt

edata for amplitude changes with incidence angle. We used the methods of Louie (1990), which mosffectively find AVO changes for reflections that are strong and flat-lying in a stacked section. Figure

-h6 shows our own stack of the southern portion of Line 4 (box on Figure 5), emphasizing the nearorizontal structures. To develop this section we began with COCORP correlated field tapes, and

-mapplied quantile equalization, spherical divergence correction, mutes, and crooked-line common

idpoint sorting before normal-moveout correction and stack. Stacking velocities were picked from a-

tset of constant-velocity stacks. Bayesian statistics applied after stack enhance the reflections most coninuous before stack. We applied similar techniques also to the northwestern portion of Line 4 (not

shown).

In the manner of Louie (1990), we developed linear AVO statistics for our stacked sections,

nusing Figure 6 from the southern portion of Line 4. We also developed the AVO statistics for aorthwestern portion of Line 4, which we do not show here. The true-amplitude stack of Figure 6

dfidentifies the strong, continuous reflections above the shale diapir (Figure 4). Louie (1990) showerom COCORP data across Southern California that such events are the only ones amenable to analysis

with AVO trend stacks.

Figure 7 overlays the wiggle traces of the amplitude trend stack on top of the variable-densityg

rtraces of the portion of the stack boxed on Figure 6. Wiggles to the right represent increasineflection amplitude with offset, whereas wiggles to the left are on top of reflections having decreasing

-iamplitude with offset. In examining reflections that are strong and laterally continuous in the underlyng variable-density traces, the wiggle traces show the corresponding linear trend of amplitude versus

loffset. Where seismic velocity gradients in the seal produce turning or diving rays, we expect to seearger absolute values for the AVO trend, from the resulting rapid changes in reflection amplitude with

.Hoffset. Inconsistent AVO trends appear in low-fold "wedges" near the top of the section (Figure 7)

igh-fold reflections above 1.5 s, near the bottom of the pressure transition zone, show strong,

nlaterally-consistent AVO changes. Reflections below 1.5 s, within the overpressured compartment, doot show such strong variations in AVO.

Results

For our modeling study, Figure 3 shows that the presence of a high-density, high-velocity cap at

nthe pressure transition zone will result in strong AVO variations. The AVO is complex in that it can-ot be explained in terms of reflection coefficients from a single interface. Instead, it is the product of

nreflections turning in the positive velocity versus depth gradients and leaking energy down through theegative gradients. The left panel of Figure 3 shows how as offset increases, strong reflections within

tthe cap will have sudden increases and decreases in amplitude. The amplitude of the reflection fromhe velocity gradient at the top of the cap in the Sarita model increases at larger offsets because of

-tturning rays (Figure 1), producing a strong linear AVO trend. In contrast, reflections within a transiion zone that is not well enough cemented to have developed a cap (right panel of Figure 3) have a

e3smooth and monotonic AVO. Reflection amplitudes from the Willamar model, lacking a cap (Figur, right inset) show only the weakest AVO trend.

We have identified similar AVO phenomena in the COCORP Texas Line 4 data set, which passes

bover a relatively shallow seal. Shale conductivities and regional mud weight reports show the seal toe associated with a relatively low-frequency reflection near 1 s time and 6000 feet (1800 m) depth on

)the stacked sections of Figures 5, 6, and 7. Figure 7 and stacks of other parts of Line 4 (not shown

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Louie and Asad AVO Character of Transition Zone

ndicates that this reflection has large AVO variations. Below it, however, the AVO variations become-

pmuch smaller. The lack of strong AVO trends suggests that these deeper reflections, within the overressured compartment, do not exhibit the degree of Poisson’s ratio variation or the velocity gradients

C

associated with the almost complete cementation of the pressure transition zone above.

onclusions

Our work leads to several recommendations for identifying pressure seals with seismic reflection-

twork. Essentially, high-quality reflections are needed over a large range of incidence angle. In addiion, relatively low frequencies may be more diagnostic than the high frequencies that must be used for

-qdevelopment and stratigraphic work. To acquire a survey to locate seals, we suggest the use of freuencies below 50 Hz and maximum offsets of at least 60% of the target depths. In processing such

,cdata, true-amplitude techniques (such as described by Louie [1990]) need to be maintained. Finallyareful interpretation must compare the AVO trend section with the strength and quality of the stacked

etreflections, and against other geophysical and geochemical information suggesting the depth of thransition zone.

This work has only examined a top seal from a small area of the U.S. Gulf Coast. However, our

tverification that strong reflection AVO effects are associated with the pressure transition zone suggestshat it embodies steep velocity gradients or unusual variations in Poisson’s ratio. Coincident with an

adobserved pressure transition zone or seal, either observation would point to the presence ofiagenetically-cemented seal as proposed generally by Bradley (1975) and Powley (1975). While

fgregional shaliness and subsidence as advanced by Bethke (1986) could assist in the formation oeopressures in the Gulf Coast, diagenetic sealing would allow many other basins worldwide to

A

develop abnormal fluid pressures. Seismic reflection techniques can locate and characterize such seals.

cknowledgments

We thank the Industrial Associates of the Gas Research Institute for generously providing seismics

sand borehole data, and Cornell University for providing COCORP seismic sections. This study waupported by the Gas Research Institute under Contract No. 5088-260-1746, while the authors resided

nAat the Department of Geosciences of The Pennsylvania State University at University Park. Shelto

lexander and Terry Engelder of Penn State provided valuable insights and guidance to this work.

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References

ellotti, P., and Giacca, D., 1978, Seismic data can detect overpressures in deep drilling: Oil and Gas

B

Jour., v. 76, no. 34 (Aug. 21), p. 47-52.

ethke, C., 1986, Inverse hydrologic analysis of the distribution and origin of Gulf Coast-type geopres-

B

sured zones, J. Geophys. Res., v. 91, p. 6535-6546.

ilgeri, D., and Ademeno, E. B., 1982, Predicting abnormally pressured sedimentary rocks: Geophysi-

B

cal Prospecting, v. 30, p. 608-621.

radley, J. S., 1975, Abnormal formation pressure: Am. Assoc. Petrol. Geol. Bull., v. 59, p. 957-973.

-Christensen, N., 1985, The influence of pore pressure and confining pressure on dynamic elastic properties of Berea sandstone: Geophysics, v. 50, p. 207-213.

dDewers, T., and Ortoleva, P., 1988, The role of geochemical self-organization in the migration antrapping of hydrocarbons: Appl. Geochemistry, v. 3, p. 287-316.

-Hunt, J. M., 1990, Generation and migration of petroleum from abnormally pressured fluid compartments: Am. Assoc. Petrol. Geol. Bull., v. 74, p. 1-12.

tLouie, J. N., 1990, Physical properties of deep crustal reflectors in southern California from multioffseamplitude analysis: Geophysics, v. 55, p. 670-681.

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Louie and Asad AVO Character of Transition Zones

fOstrander, W. J., 1984, Plane-wave reflection coefficients for gas sands at nonnormal angles oincidence: Geophysics, v. 49, p. 1637-1648.

.

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Powley, D. E., 1975, Course notes used in Am. Assoc. Petrol. Geol. Petroleum Exploration Schools

igert, V., and Al-Shaieb, Z., 1990, Pressure seals: their diagenetic banding patterns: in Ortoleva, P.,

GHallet, B., McBirney, A., Meshri, I., Reeder, R., and Williams, P., Eds., Self-Organization of

eological Systems, proc. of workshop 26-30 June 1988 at Univ. Calif., Santa Barbara; Earth

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Science Rev., v. 29, p. 227-240.

igure Captions

Fig. 1— Geometry of multi-offset reflection rays from a horizontal seal at a range of incidence anglesf

ii. Positive velocity gradients with depth in the top of the seal lead to turning rays at some angles oncidence. At other angles, the negative velocity gradients at the bottom of the seal bend diving rays

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down and away from the receivers.

ig. 2— Shale density (solid curves) and sonic transit time (dashed curves) measurements from twos

atypical wells on the south Texas coast that penetrate pressure transition zones. The Sarita well exhibit

high-velocity, high density "cap" at the top of the overpressured section. The Willamar well shows

F

no cap, but exhibits constant velocity and density within the overpressured section.

ig. 3— Synthetic acoustic shot records computed from the density and velocity profiles of the Saritar

t(with cap, left) and Willamar (no cap, right) wells, of Figure 2. The arrows point to a reflection neahe top of the seal in each model. The insets show the amplitude of each reflection versus offset. The

tiamplitude of the reflection from the velocity gradient at the top of the cap in the Sarita model (lefnset) increases at larger offsets because of turning rays (Figure 1), producing a strong linear AVO

Ottrend. Reflection amplitudes from the Willamar model without any cap (right inset) show little AVrend.

Fig. 4— Location map of COCORP Texas Line 4 near Port Lavaca, Texas. The numbered circles-

plocate wells providing shale conductivity measurements. The rectangle shows the range of stack midoints near the line of section included in Figure 5.

Fig. 5— Approximately depth-converted COCORP stacked section for Texas Line 4. The superim-

tposed curves give shale conductivities picked from the numbered wells. Greater conductivities are tohe right. Where the curves are constant no shale conductivities could be picked. The box identifies

lthe area shown in Figure 6. A thick section of shale with increasing conductivity overlies a relativelyow-frequency reflector at ∼6000 feet (1800 m) depth.

4dFig. 6— Stacked section derived from the southern portion of the pre-stack COCORP Texas Lineata. Note that the N-S sense of this section is reversed relative to that of Figure 5. This image

iemphasizes strong, flat reflectors having the most predictable normal moveout before stack. The boxdentifies the higher-fold region analyzed for reflection amplitude versus offset trends.

sLFig. 7— Linear amplitude versus offset trends (wiggle traces) superimposed on our COCORP Texa

ine 4 reflection section (variable density traces). Inconsistent AVO trends appear in low-fold-

s"wedges" near the top of the section. High-fold reflections above ∼1.4 s, near the bottom of the presure transition zone (dashed line), show strong, laterally-consistent AVO trends. Reflections below

∼1.4 s, within the overpressured compartment, show only weak AVO trends.

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