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    SEISMIC STRATIGRAPHY

    By

    Prof. R. D. S. KushwahaDepartment of Geophysics

    Banaras Hindu UniversityVARANASI

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    Fig. 1: High- amplitude event indicating ahydrocarbon accumulation.

    The high amplitude event is produced by hydrocarbonaccumulation (Fig. 1) that has changed the properties of hostrock sufficiently to affect the amplitude of the reflection from it.

    Bright spots or direct hydrocarbon indicators, are nowregarded as a subset of seismic stratigraphy.

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    Fig. 2: Section showing out-building of a delta (AA),unconformities (BB, DD, and elsewhere) and stratigraphic

    features.

    Stratigraphic Pattern in Seismic Data

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    Among the more obvious stratigraphic features arestructural pattern as shown in Figs. 2-11. Fig. 2shows generally flat-lying reflections but a belt ofdipping events forms a progradational patternacross its centre (AA); this is interpreted asindicating original depositional dip in the out buildingof delta. Above this can be seen an unconformity

    BB, shown by the transgressive onlap of seismicreflections from the left. Still higher in the section (atabout CC), there is a strong change in reflectioncharacter. The reflections above CC are higher

    frequency, have less continuity, and are morewobbly than those below; the interpretation is thatthose above CC represent nonmarine deposition,those below represent marine

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    Fig. 3: Continental slope-shelf break in present sea floor andancestral shelf edges in the deeper reflections.

    Fig. 3 shows a present-day continental shelf edge. A number offormer shelf edges can be seen in the reflections at greaterdepths. The pattern is so dramatic that it is easy to pick periodsof mainly outbuilding, periods of mainly up-building, and periodsof uplift when the tops of patterns were being eroded.

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    Fig. 4: Reflection onlap onto an erosional surface (AA), indicative of a

    transgression.

    Transgression on to an unconformity is the obvious feature of Fig. 4.There is some thinning to the right of the onlapping units, indicatingslight subsidence and tilt to the left during their deposition.

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    Fig. 5: Portion of a migrated section over a tight fold. The shale unitvaries in thickness through the fold whereas the competent beds do

    not vary below. U indicates an unconformity.

    Fig. 5 is a migrated section of a tight fold. Some of the intervalsbetween reflections maintain constant thickness through the fold;these are the competent units. Other units thin and thicken; these arethe shales. Observations of the response of rocks to stress gives

    information about the nature of the rocks.

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    Fig. 6: Section across horseshoe Atoll in West Texas. R denotes the portion ofthe section that contains the reef (just left of center). The back-reef area offlat-lying, strong, continuous reflections is to the right. The fore-reef showing

    an entirely different progradational reflection pattern is to the left.

    Another set of fairly obvious pattern is shown in Fig. 6. The overall pattern is ofhorizontal reflectors but the zonesAA shows not only appreciable relief on itstop but several distinctly different reflection patterns within it. The no-reflectionzone in the centre is the Horseshoe Atoll reef, with back-reef facies to the right

    and fore-reef facies to the left.

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    Fig. 7: Phase section showing severalstratigraphic patterns. B is a unit thatthickens to the left (onlap at its base);A shows progradation from a source tothe right; C thins onto the structure; Dis fill in an ancient erosional channel;and U is an unconformity within amainly carbonate section.

    Fig. 7 shows several patternswith stratigraphic significance: aprogradational unit (A) showingthe sigmoidal pattern; a wedge of

    sediments (C) that is missingfrom the top of the structure,indicating that the structure waspositive and probably growing atthe time of their deposition; andan erosional channel (D),

    possibly the result of thechanneling around the positivefeature to the left. Much of thegrowth history can be worked outfrom seismic patterns, and in thisinstance the history is critical with

    respect to hydrocarbonaccumulation. The structure,incidentally, shows a bright spotand produces gas fromcarbonate porosity.

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    Fig. 8: The lithology of the unitAA changes at the shelf edge, over a reef (R).This is aphase display. There is a gas production from the pinchout of sands at the top of unitA, which show an oblique pattern. Unit B shows a sigmoidal pattern and its sedimentsare generally fine grained. C, D, and E are unconformities, as well as the top andbase of units A and B.

    The unitAA in Fig. 8 involves higher velocity rocks on the right third thanon the thicker section to the left, where the unit consists of clastic sandsand shales. This change creates change in the waveshape, and thechanges are emphasized by a polarity display.

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    Fig. 9: Phase section across a turbidite build up. The event at A is a flat-spotreflection from a gas/water contact. The anticlinal reversal (~2.4 s) belowthe turbidites must be real because velocity variation due to the turbiditescould not be great enough to produce a fictitious anomaly of this magnitude.

    Fig. 9 is a portion of phase display across a turbidite build-up. Phaseemphasizes relatively weak reflections such as those from turbidities. The flatevent at A is probably a flat spot from a gas-water contact; flat spots are one ofthe best hydrocarbon indicators.

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    Fig. 10: Change in reflection pattern allows the seismic section to be

    separated into seismic sequence units that differ in reflectioncharacter.

    Seismic Stratigraphy Subdivisions

    Seismic sections can often be subdivided into units that have commoncharacteristic but that differ from adjacent units. The separation of seismic datainto such units is the branch of seismic stratigraphy called seismic sequenceanalysis;and these separate units itself are almost naturally subdivided asshown in Fig. 10.

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    Fig. 11: The belt of oblique reflections (P) has produced velocityanomalies in the deeper section.

    The belt of oblique reflection pattern across the center of Fig. 11 separatesregular, fairly smooth reflection events above the belt from very irregularreflections below. The velocity of sediments that constitute the oblique portionvaries appreciably, and the variable travel time through this unit creates

    undulations and other irregularities in the deeper reflections. Higher velocitydistinguishes the portions of oblique reflection that are predominantlycarbonated.