zeng etal 2008

Ls

H

bcitfoftsflcB

2

b©

GEOPHYSICS, VOL. 73, NO. 5 �SEPTEMBER-OCTOBER 2008�; P. A27–A31, 6 FIGS.10.1190/1.2949255

inear amplitude patterns in Corpus Christi Bay Frio Subbasin,outh Texas: Interpretive pitfalls or depositional features?

ongliu Zeng1, Robert G. Loucks1, and Ursula Hammes1

hsflmsp

sbdspats

wtsrmpcrssett

rCpHs

ts. Man

onomic

ABSTRACT

Linear amplitude patterns on stratal slices in the CorpusChristi Bay area of Texas are important seismic geomorpho-logical features that reflect sediment dispersal patterns.These amplitude patterns are oriented in both strike and dipdirections. Some of the linear amplitude patterns are relatedto faults; however, most are related to orientation of sand-stone bodies. Faulting may make the depositionally relatedlinear amplitude patterns more fragmented, but faults do notdestroy the overall orientation or geomorphologic signifi-cance of these patterns. Amplitude patterns on stratal slicesshould be interpreted as unbiased, general, sediment-dispers-al patterns unless the patterns can be tied directly to a struc-tural feature. In-depth understanding of structural and depo-sitionally related amplitude patterns leads to more accuratestratal slicing interpretation in facies/reservoir prediction us-ing poststack 3D seismic data.

INTRODUCTION

In south Texas, Oligocene �Frio Formation� fault-controlled sub-asins have been prolific petroleum targets for many years and areurrently a focus of deep-gas exploration. These basins formed dur-ng third-order �1–1.3 million years� Tertiary depositional cycles inhe Gulf Coast �Brown et al., 2004�. Controlled by regional growth-ault systems, the subbasins are filled with third-order lowstand dep-sitional systems tracts. Transgressive and highstand systems tractsormed above the subbasin and completed the third-order deposi-ional sequence. The overall depositional history is a progradationaluccession from deepwater, highly aggradational lowstand basin-oor and slope fans to shallow-water lowstand, prograding-wedgeomplexes �Vail, 1987; Sangree et al., 1990; Mitchum et al., 1993;rown et al., 2004� and terminally to on-shelf transgressive and

Presented at the 77thAnnual Meeting, Society of Exploration Geophysicis5 March 2008; published online 30 July 2008.

1University of Texas at Austin, School of Geosciences, Bureau of [email protected]; [email protected] Society of Exploration Geophysicists.All rights reserved.

A27

Downloaded 21 Feb 2012 to 202.173.95.254. Redistribution subject to S

ighstand systems tracts �coastal barrier bar and lagoon, and deltaicediments; Galloway et al., 1982, 2000�. Sediments in the basin-oor fan are sandy, whereas slope-fan systems are predominantlyuddy and have thin intervals of sandstones and siltstones. Sand-

tones and siltstones in the slope fan are inferred to be turbidites de-osited in submarine channels and levees.

The prograding deltaic complex is composed of interbeddedandstones and shales interpreted to have been deposited as over-ank deposits along deltaic distributaries and at mouths of lowstandeltaic distributaries. Strong interaction between sea-level cycle,ediment supply, and growth-fault tectonics was recorded in multi-le higher-order �fourth and fifth� sequences that reflect organizednd relatively predictable sediment-dispersal patterns. A model ofhe higher-order sequence development is a current subject of re-earch.

Because the Frio Formation in these subbasins is deeply buriedith only sparse well control, 3D seismology has been an important

ool for exploration. One aspect of seismic interpretation is imagingandstone distribution to predict depositional systems, lithofacies,eservoir distribution, and reservoir quality. A study of seismic sedi-entology in Corpus Christi Bay Subbasin by Zeng et al. �2007� ap-

lies stratal slicing to high-frequency sequence stratigraphy and fa-ies delineation.Astratal slice �Zeng, Backus et al. 1998; Zeng, Hen-y et al. 1998� is a seismic attribute display following a depositionalurface or geologic time surface; it can be a time slice, a horizonlice, or a proportionally interpolated slice between two referencevents, depending on structural and thickness trends of the forma-ion. Proportional slices were used in this study to calibrate a largehickness gradient in the subbasin.

Stratal slices make it possible for interpreters to see 10-m-thickeservoir features that are detected but not resolved vertically. In theorpus Christi Bay Subbasin, stratal slices imaged many linear am-litude patterns associated with slope fans and lowstand deltas.ere, we discuss the origin of the linear amplitude patterns in low-

tand deltas and analyze whether they are depositional features that

uscript received by the Editor 12 February 2008; revised manuscript received

Geology, Austin, Texas, U.S.A. E-mail: [email protected];

EG license or copyright; see Terms of Use at http://segdl.org/

mt

ofqpetaf

sttppimui�

pHqwao�T�fppssont

rsKtedfp

usftta

2sb

FgAp

Ft

A28 Zeng et al.

ay be important exploration targets or interpretive pitfalls relatedo faulting and mispicking.

POSSIBLE INTERPRETIVE PITFALLS OFLINEAR AMPLITUDE PATTERNS

Stratal slices reveal distinctive amplitude patterns in successionsf Frio prograding deltaic complexes. For example, in a stratal sliceollowing a higher-order �fourth or fifth� prograding deltaic se-uence �Figure 1�, linear, strike-oriented amplitude patterns, whicharallel the main boundary fault, are merged with linear, dip-orient-d amplitude patterns, indicating a lowstand deltaic system thatransported sandy sediments in normal, dip-oriented lobate systemsnd in strike-oriented accommodation space created by growthaulting and rollover.

These patterns are common in the Corpus Christi Bay Frio Subba-in. Because they occur repeatedly through the thick �1500 m�,hird-order lowstand Frio systems tract interval, they are not a mis-ake in picking a thin, higher-order sequence. However, the com-lexity of seismic events and apparent association of some of theatterns with faults �Figure 2� and clinoforms make the credibility ofnterpreted depositional systems a subject of debate. Possible seis-

ic interpretive pitfalls include �a� imaging at the top of an angularnconformity with basinward-dipping underlying strata, �b� imag-ng through a set of clinoformal events instead of a single event, andc� fault separation of an otherwise not-so-linear amplitude pattern.

The first case is unrealistic, given the sequence stratigraphic inter-

igure 1. Amplitude stratal slice of a higher-order, lowstand, pro-rading deltaic sequence in the Corpus Christi Bay Frio Subbasin.mplitude patterns: 1�unfaulted; 2�faulted; 3�inconclusive. Theosition of the stratal slice is indicated in Figures 2 and 3.


retation of well data �Brown et al., 2004; Brown et al., 2005;ammes et al., 2007; Zeng et al., 2007� that third-order Frio se-uences were mostly subaqueous deposits. Submarine erosionould occur during the early higher-order lowstand, but only in local

reas around sediment ridges, and it would rarely cut into more thanne underlying sequence. Consequently, as seen in a vertical sectionFigure 2�, a stratal slice would follow mostly conformable events.he second case exists in limited areas close to the boundary fault

1–2 km into the subbasin, Figure 2�. Because thin, shingled clino-orms in the Corpus Christi Frio Subbasin indicate mostly basinwardrogradation or accretion, they generate linear, strike-oriented am-litude patterns that resemble those unrelated to clinoforms. As a re-ult, although clinoformal events may suggest subtle differences inandstone architecture, general sediment dispersal patterns revealedn stratal slices should be similar. The third case is more realistic andeeds the most attention. Some of the linear features may be relatedo faults �Figures 1 and 2�. The question is, how many?

FAULTS OR DEPOSITIONAL FEATURES?

Acareful analysis of seismic attribute patterns on stratal slices andelated vertical seismic characteristics is necessary for us to under-tand the potential influence of faults on seismic geomorphology.ey is a comparison of linear patterns with and without faulting in

he same systems tract on stratal slices, to see whether a cause-and-ffect relationship exists between faults and linear patterns. In thisata set, the seismic coherency attribute failed to differentiate smallaults from linear depositional features; alternatively, only the am-litude attribute could be analyzed.

Three observations can be made. First, linear amplitude patternsnrelated to faults �Figures 1 and 2� are imaged from a relativelymooth, continuous seismic event and are clearly unaffected by anyaulting �or at least seismic-scale faulting�. The linearity in ampli-ude distribution, if any, originated from sandstone distribution pat-erns. About half the linear amplitude patterns observed �Figure 1�re not fault related.

Second, linear amplitude patterns related to faults �Figures 1 and� can be identified clearly by abrupt, systematic displacement alongeismic events, although the definition of systematic is arbitrary. Toe practical, we interpret faults in which displacement can be ob-

igure 2. Dip seismic sections, showing relationship of amplitudeso faults. See Figure 1 for positions and label explanation.


spmtu

sscGhmc

lmts�

vtsptpnwsatu

tptPasic

mvshaa

dpcc

fscfqtcswa

Fisitm

Linear amplitude patterns in Frio Subbasin A29

erved along at least four consecutive events �two peak-trough cou-lets�. In the Frio interval, except for the boundary fault, faults areostly compensational, small �10–50 m throw�, and postdeposi-

ional. Roughly 20% of the linear amplitude patterns observed �Fig-re 1� are fault related.

Third are inconclusive amplitude patterns �Figures 1 and 2�. Inome areas, seismic events are more-or-less discontinuous but fail tohow systematic, downthrown displacement of events. As a result,lear judgment is difficult, and interpretation is subject to user bias.iven the complexity of the depositional systems in the subbasin,owever, most subtle changes in continuity and smoothness of seis-ic events should be stratigraphic and depositional �sand shoals,

hannels, slumps, etc.�, not structural or tectonic.In summary, although faulting can lead to linear seismic artifacts,

inear amplitude patterns may occur without faults. Faults may haveade some linear patterns more segmented, but they do not change

he pattern’s overall orientation or seismic geomorphologic relation-hip because most parallel long axes of unfaulted amplitude patternsFigure 1�.

CALIBRATION OF AMPLITUDE PATTERNSWITH WIRELINE-LOG FACIES

Sparse well coverage in the area of 3D seismic technology pro-ides necessary calibration for seismic sedimentological interpreta-ion of stratal slices. On wireline-log seismic sections �Figure 3�, thetratal slice �Figure 1� depicts a sandy unit of 5–30 m. Spontaneousotential �SP� logs at the stratal-slice interval �Figure 3� show a pat-ern of upward coarsening followed by upward fining, indicating arograding deltaic system �prodelta/delta front/distributary chan-el�, followed by transgression. General thickness trends thin basin-ard in the dip section �Figure 3a� but are more consistent in the

trike section �Figure 3b�. Coinciding with linear, strike-orientedmplitude patterns �Figure 1�, the seismic event is much more con-inuous in the strike section �Figure 3b� than in the dip section �Fig-re 3a�.

Further study revealed a quantitative relationship between ampli-ude �Figure 1� and sandstone thickness in wells �Figure 4�. An am-litude tuning curve drawn from well measurements is comparableo the modeled one, with a tuning thickness �23 m� clearly defined.enetrating both high-amplitude and low-amplitude linear patterns,ll wells show the presence of sandstones and no signs of faulted-outections along either linear pattern. Therefore, amplitude variationsn linear patterns most likely indicate thickness changes related to fa-ies rather than faulting.

Comparing wireline-log facies and seismic anomalies shows thatany linear-amplitude bodies along the stratal slice in Figure 1 and

icinity are actually distributary-channel sandstones. On sometratal slices �e.g., Figure 5�, such channel features are selectivelyighlighted with clear outlines. On vertical seismic sections, theyre barely resolved and show up only as single-event incisions ormplitude anomalies �Figure 2�.

IMPROVING RESERVOIR PREDICTION

In the south Texas Frio Formation, a good understanding of theistribution of slope fan and prograding deltaic sandstones hasroved crucial to deep-gas exploration, especially stratigraphic andombined traps. Where dense well control is lacking, stratal slicesan help in drawing more realistic, high-resolution sandstone maps


or accurate reservoir prediction. For example, for the higher-orderequence represented by the stratal slice in Figure 1, only a roughontour map can be drawn using sandstone thicknesses measuredrom available wells alone �Figure 5a�. However, by applying theuantitative relationship between sandstone thickness and ampli-ude value �Figure 4�, seismic amplitude trends on the stratal slicean be integrated into the contouring process. In this way, a newandstone isopach map can be made �Figure 5b� that satisfies bothireline-log and seismic data while providing a more convincing,

lthough quite different, view of reservoir distribution �Figure 6�.

a)

b)

igure 3. Wireline-log seismic sections, showing wireline-log faciesntegrated with seismic facies. �a� Dip-oriented section revealingandstones of upward-coarsening-then-fining pattern that thin bas-nward. Seismic events are discontinuous. �b� Strike-oriented sec-ion illustrating sandstones with similar wireline-log patterns with

ore consistent thicknesses and more continuous seismic events.


osSSfppdIpti

pfittw

seatLmsibots

FtwfiZeng et al., 2007�.

FgtSv�

Fal4

A30 Zeng et al.


CONCLUSIONS

In the Corpus Christi Bay Frio Subbasin, linear amplitude patternsn stratal slices are not interpretive pitfalls but are predominatelyeismic geomorphologic expressions of depositional features.tratal slices that image along multiple unconformities do not exist.ome linear patterns may follow thin clinoforms near the boundaryault, but they still reflect the general orientation of sediment dis-ersal patterns. On the other hand, even though some patterns are ap-arently associated with faults, most come from smooth or slightlyisturbed seismic events that show no convincing signs of faulting.n a sand-rich, high-frequency sequence, all wells penetrating linearatterns reveal sand development, with predictable relationships be-ween sandstone thickness and amplitude — a phenomenon unreal-stic for faulted amplitude patterns.

Linear patterns indicate widespread, repeated development ofrograding deltas. Many linear sandbodies are distributary channellls, which are important exploration targets in the subbasin. De-

ailed sandstone distribution can be evaluated by converting ampli-ude stratal slices to sandstone thickness maps with at least sparseell control.

ACKNOWLEDGMENTS

This study was supported by the state of Texas Advanced Re-ource Recovery �STARR� program. The authors thank associateditor Klaus Holliger, reviewers Qiang Sun and Robert Brown, andn anonymous reviewer for their constructive comments. The au-hors also extend gratitude to WesternGeco for use of seismic data.andmark Graphics Corporation provided software via the Land-ark University Grant Program for the interpretation and display of

eismic data. Figures were prepared by Jana Robinson. Lana Dieter-ch edited the text. Partial support for this publication was providedy the Jackson School of Geosciences and the Geology Foundationf the University of Texas at Austin. The publication is permitted byhe director, Bureau of Economic Geology, Jackson School of Geo-ciences, the University of Texas atAustin.

a) b)

igure 6. Sandstone thickness maps �a� without and �b� with guid-nce of seismic amplitude patterns on a stratal slice �Figure 1�, fol-owing the quantitative amplitude-thickness relationship in Figure. See Figure 1 for map position.

igure 4. Amplitude tuning curve as an indicator of lithology andhickness. The actual tuning curve �dashed line� is drawn from SPireline-log measurements of clean sandstones encased in shale

rom wells in a higher-order sequence �Figure 1�. The modeled tun-ng curve is from a wedge model with a 25-Hz Ricker wavelet �after

igure 5. Amplitude stratal slice of a higher-order, lowstand, pro-rading deltaic sequence adjacent to the sequence in Figure 1. Dis-ributary channels are highlighted by negative �red� amplitudes.ome channels show single-event incisions on vertical seismiciews, with many others depicted only as isolated, patchy eventsFigure 2�.


B

B

G

G

H

M

S

V

Z

Z

Z

Linear amplitude patterns in Frio Subbasin A31

REFERENCES

rown, L. F., Jr., R. G. Loucks, and R. H. Treviño, 2005, Site-specific se-quence-stratigraphic section benchmark charts are key to regional chro-nostratigraphic systems tract analysis in growth-faulted basins: AAPGBulletin, 89, 715–724.

rown, L. F., Jr., R. G. Loucks, R. H. Treviño, and U. Hammes, 2004, Under-standing growth-faulted, intraslope subbasins by applying sequence-stratigraphic principles: Examples from the south Texas Oligocene FrioFormation: AAPG Bulletin, 88, 1501–1522.

alloway, W. E., P. E. Ganey-Curry, X. Li, and R. T. Buffler, 2000, Cenozoicdepositional history of the Gulf of Mexico Basin: AAPG Bulletin, 84,1743–1774.

alloway, W. E., D. K. Hobday, and K. Magara, 1982, Frio Formation of theTexas Gulf Coast Basin — Depositional systems, structural framework,and hydrocarbon origin, migration, distribution, and exploration potential:The University of Texas atAustin, Bureau of Economic Geology Report ofInvestigations 122.

ammes, U., R. G. Loucks, L. F. Brown, R. H. Treviño, P. Montoya, and R. L.Remington, 2007, Reservoir geology, structural architecture, and se-quence stratigraphy of a growth-faulted subbasin: Oligocene Lower FrioFormation, south Texas Gulf Coast: The University of Texas atAustin, Bu-reau of Economic Geology Report of Investigations 272.


itchum, R. M., J. B. Sangree, P. R. Vail, and W. W. Wornardt, 1993, Recog-nizing sequences and systems tracts from well logs, seismic data, and bio-stratigraphy: Examples from the late Cenozoic of the Gulf of Mexico, in P.Weimer and H. Posamentier, eds., Siliciclastic sequence stratigraphy: Re-cent developments and applications:AAPG Memoir 58, 163–197.

angree, J. B., P. R. Vail, and R. M. Mitchum Jr., 1990, A summary of explo-ration applications of sequence stratigraphy, in J. M. Armentrout and B. F.Perkins, eds., Sequence stratigraphy as an exploration tool — Conceptsand practices in the Gulf Coast: 11th Annual Research Conference, GulfCoast Section of Society of Economic and Palaeontologic Mineralogists,ExtendedAbstracts, 321–327.

ail, P. R., 1987, Seismic stratigraphy interpretation using sequence stratig-raphy: Part 1 — Seismic stratigraphy interpretation procedure, in A. W.Bally, ed., Atlas of seismic stratigraphy: AAPG Studies in Geology 27,2–14.

eng, H., M. M. Backus, K. T. Barrow, and N. Tyler, 1998a, Stratal slicing:Part I — Realistic 3D seismic model: Geophysics, 63, 502–513.

eng, H., S. C. Henry, and J. P. Riola, 1998b, Stratal slicing: Part II — Realseismic data: Geophysics, 63, 514–522.

eng, H., R. G. Loucks, and L. F. Brown, 2007, Mapping sediment-dispersalpatterns and associated systems tracts in fourth- and fifth-order sequencesusing seismic sedimentology; Example from Corpus Christi Bay, Texas:AAPG Bulletin, 91, 981–1003.


zeng etal 2008

Documents